Ok Guys, good chance I will get flamed here as it is my first post but maybe someone will answer this for once and all

1. I have lurked here for about 6 months
2. I searched but all the posts on SOA have lost of bullshit about i can climb this big rock or some great detail on how to do it
3. Have tried JU and other lesser boards with no luck

What I am looking for is the technical reason to back up what most people claim that SOA flexes better than SUA

Remove all the other factors that get done in a SOA such as longer shocks different springs more clearence etc

If you had just a chassis and a well designed SUA setup with no binding and max flex then used the exact same spring went to SOA would their be a change in flex I doubt it but would like a techincal answer to back what seems to be a popluar belief

here hoping I can get a real answer as this is the center of hard core tech

Originally posted by Aussie

here hoping I can get a real answer as this is the center of hard core tech

I'll point you in the right direction but I'm not into spoon feeding :flipoff2:

The compression force applied to a spring in SUA is a pull (pulling up on the spring since the axle is above it). The compression force applied to a spring in SOA is a push (pushing up on the spring because the axle is beneath it). Droop is a push for SUA and a pull for SOA.

The difference in force positioning/pivot origin should get you on the right track in your research.

How about picture proof?

Here is my CJ5 with stock springs (SUA), stock shocks, 2" BBL, Stock NT Axles with 33x12.5 TSL's (June 01): 523
http://archiegallup.homestead.com/files/PittMR04.jpg
http://archiegallup.homestead.com/files/PittMR05.jpg

Same CJ5, same springs (SOA), same shocks, 2" BBL, same front axle, Scout II D44 rear, same tires, same RTI ramp (June 02): 881
http://www.northeastonlinewheelers.org/yabbse/attachments/ramp01.jpg
http://www.northeastonlinewheelers.org/yabbse/attachments/ramp02.jpg

Proof enough for ya? :D

I think it has much to do with the arch in the spring. soa having typically flatter springs should give you even extension and compression travel where sua lift spring tend to give you good compression travel but poor extension travel.

As fare as going from no lift directly to soa you will see a huge difference as in the above picture, but I think all he really gained is about 5" of comp travel since he reused the same stock springs.
I actually think my stock yj springs flexed better than my 4" superlifts simply because the where much flatter.

In my experience, sua lift springs need to be very stiff to hold their shaft and because of that they tend to be max’ed out in the droop department in order to maintain their shape.

Does anyone else agree with my flat spring theory?

if not:flipoff2: in advance

Originally posted by SCORPION

Does anyone else agree with my flat spring theory?

if not:flipoff2: in advance

Nope. So :flipoff2: to you also.

You make a little sence, but the fact is that SUA arched springs will droop when enough force is apllied to them. This greater force is applied when the are placed in a SOA application. The greater force comes from what I elluded to earlier :D

My personal experience - once I went SOA it flexed much better. The ride's not as stiff. I don't know what technical aspect of the design allows this, although I've got a feeling it has alot to do with the spring arch.
Also, I no longer have a sway bar or track bars, and it drives almost as well as the SUA WITH the sway bar.

Originally posted by bigdude

You make a little sence, but the fact is that SUA arched springs will droop when enough force is apllied to them. This greater force is applied when the are placed in a SOA application. The greater force comes from what I elluded to earlier :D

exactly, thats my point, its tough to get enough down force on an arched spring to make it droop as much as a flater spring because it is almost already in its full droop position.

I agree with you, the soa setup has more leverage against the spring compared to sua simply becasue is it farther away from the main-eyes and shackles making the spring twist more easily.

revolver shackes are a great fix for sua setups because the alow additional droop aswell as spring twist, to bad they cost $$$

Originally posted by SCORPION

revolver shackes are a great fix for sua setups because the alow additional droop aswell as spring twist, to bad they cost $$$

Search on that. Myself and many others have posted about the downfalls of revolvers associated with the help they can provide on a ramp.

I under stand some of the down falls of the revolvers that dont get setup correctly, but there do adress some of the issues with sua high arched springs.........besides I dont want to start on revolvers becasue I dont use them.

i am surfing over from the zuk samurai side of things. the standard lift for a zuk is going spoa. when i did my spring over, i used my stock shocks and flipped the spring plates. this meant that i had stock damping and stock springs but the difference was spring over. my shackles were the same before and after and tires for a long time were the same before and after. the ride smoothed out. i got more axle wrap but overall i liked what i got. it flexed better and was much easier to drive on the twisty stuff. the samurai is an amazing vehicle to watch go from ground hugger to trail worthy with simple modifications. i am raping jeeps for their parts though.....i have cj front leafs in the rear and am working on the d-44s from mid 70's cherokee:flipoff2:


if you are debating wether or not to go spring over....no question, go spring over!! whenver i see a lifted yj or cj that isn't spring over...it makes me cringe!!:flipoff2: :flipoff2:

Originally posted by spencurai
whenver i see a lifted yj or cj that isn't spring over...it makes me cringe!!:flipoff2: :flipoff2:

Whenever i see a Zuk i cringe! :flipoff2: :D

Originally posted by Archie_G
How about picture proof?

Here is my CJ5 with stock springs (SUA), stock shocks, 2" BBL, Stock NT Axles with 33x12.5 TSL's (June 01): 523
http://archiegallup.homestead.com/files/PittMR04.jpg
http://archiegallup.homestead.com/files/PittMR05.jpg

Same CJ5, same springs (SOA), same shocks, 2" BBL, same front axle, Scout II D44 rear, same tires, same RTI ramp (June 02): 881
http://www.northeastonlinewheelers.org/yabbse/attachments/ramp01.jpg
http://www.northeastonlinewheelers.org/yabbse/attachments/ramp02.jpg

Proof enough for ya? :D

neat little series of the jeep but a wider axle WILL exert more leverage onto the leaf springs and get you more travel so there's essentially NO proof in your pics.:flipoff2:

Originally posted by bigdude


I'll point you in the right direction but I'm not into spoon feeding :flipoff2:

The compression force applied to a spring in SUA is a pull (pulling up on the spring since the axle is above it). The compression force applied to a spring in SOA is a push (pushing up on the spring because the axle is beneath it). Droop is a push for SUA and a pull for SOA.

The difference in force positioning/pivot origin should get you on the right track in your research.

bigdude

Sorry mate but im begging for a bit of techno spoon feeding here. Im just not getting the idea of this at all and like Aussie I have seen plenty of eveidence to show that spring overs do work and produce extra flex but also like Aussie I have yet to see a clear explanation why.

I run my vehicle sua the shackle angle is increased and there is a pivot at the fixed end of the spring. Plotting the geometry of the spring and shackle as it extends has lead me to the belief that for this particular arrangment (spring length, shackle length, shackle angle) I am only about 1" off the theoretical maximum arch that it can go to. You will accept that there is a point in the extension of the spring past which it is impossible to go because the shakle can no longer compensate for the shortening of the spring.

If this is indeed the case then how would a soa benifit me?????










By the way I dont agree with you about flat springs. I have reached the conclusion that the flater a spring is all other things remaining the same (spring length, shackle length, shackle angle) the more arch can be introduced to it before the shackle can no longer compensate for the reduction in length.


Putting my flame suit on now !!!


but after you have flamed me how about some spoon feeding and perhaps a diagram about this origin of force stuff.

Originally posted by gon2far


By the way I dont agree with you about flat springs. I have reached the conclusion that the flater a spring is all other things remaining the same (spring length, shackle length, shackle angle) the more arch can be introduced to it before the shackle can no longer compensate for the reduction in length.

Putting my flame suit on now !!!

but after you have flamed me how about some spoon feeding and perhaps a diagram about this origin of force stuff.

I didn't say that flat springs made no difference. My point was that the reason SOA flexes better is because there are different forces acting on the spring (SOA vs SUA, same spring/axle/tire SOA will flex more). Yes a longer/flatter spring can attain more of an arch when drooped, but the reason SOA flexes better IS NOT because "when people go SOA they use flatter springs". :shaking: as was mentioned earlier.

The force difference can be seen through geometric relationships and some physics. Scorpion touched on it earlier with his comment "the soa setup has more leverage against the spring compared to sua simply becasue is it farther away from the main-eyes and shackles". Take a look at weight location in each of the two set-ups. Also imagine axle center (pivot point) for a solid axle in each set-up. SUA pivot point is above the springs, SOA pivot point is below the springs. SUA weight ceter is above the springs, SOA weight center is below the springs. Are you starting to see how the differences pan out. There are different forces involved in each set-up.

Theoretically you may think your leaf can only droop to a certain arch. But as you apply a greater force to that main leaf (the only one connected to you to your frame) whose to say it maintains a perfect arch? Or maintains the same arch as the rest of the pack (obviously loosening spring clamps is notorious for aiding flex by letting leaves separate).

A canned example can be given to calculate a theoretical force difference. It would require math, BS thinking up a theoretical suspension/axle/tire combo (and weights/rates associated), and opening up physics/geometry books for formulas. I don't care to do this because it's too much work.

Originally posted by gon2far

By the way I dont agree with you about flat springs. I have reached the conclusion that the flater a spring is all other things remaining the same (spring length, shackle length, shackle angle) the more arch can be introduced to it before the shackle can no longer compensate for the reduction in length.

Forget about the shackles and shocks and focus on the mechanics of the springs.........

Does it make sence that a flater spring will flex more equally in any direction compared to a spring will alot of arch? I think so. and again its not the only thing going on here.

Then why the F will soa with stock spring flex better than sua with the same f'ing springs then? And why then does the same soa with stock springs flex better than a 4" sua lift? What the hell are you talking about?:flipoff2: :flipoff2: :flipoff2:

re-read the entire post then try to make an intelligent analysis

I also under stand that not all soa setups use flater springs and that is where the increase in leverage on the spring from the axle comes into play.

besides if you are running soa with 4" springs then you likely have full width axles and BIG tires, all that extra weight will do wonders for flex..........

As one who has done an soa without changing anything (tires, shocks, bumpstops, etc..) I'll explain the differences I have found. I used the stock springs from my '84 Scrambler. I didn't notice any difference in flex except that the bumpstops didn't come into play and inhibit the axle travel as much. However, if you're going to do a lift, you'll probably want bigger tires. This improves flex. You'll either go SUA with arched springs (need about 5 inches to compare to the gain from a SOA) or go SOA. But if you go with lift springs, they will be stiffer and not flex as well until broken in. Then they may sag. My personal favorite is Avalanche's 2" springs for SOA and then get out the welder. Hope this helps.

Reed

baylorboy has got it. All things being the same (ie. bumpstops, springs, shackles, etc...) the extra flex will come from the increased compression afforded by the SOA.

This says enough. It's the world famous 4" ProComp SUA lift:

Originally posted by Aussie
Remove all the other factors that get done in a SOA such as longer shocks different springs more clearence etc

If you had just a chassis and a well designed SUA setup with no binding and max flex then used the exact same spring went to SOA would their be a change in flex I doubt it but would like a techincal answer to back what seems to be a popluar belief

here hoping I can get a real answer as this is the center of hard core tech

You'll get the exact same flex because the forces on the spring are exactly the same (for the scenario given above). It makes no difference if the springs are over or under the axle. However, like others have said, you'll usually get more travel when going to SOA on a jeep because you'll usually get more compression travel.

Originally posted by harsanyj


You'll get the exact same flex because the forces on the spring are exactly the same (for the scenario given above). It makes no difference if the springs are over or under the axle. However, like others have said, you'll usually get more travel when going to SOA on a jeep because you'll usually get more compression travel.

Then why did I get more flex from my CJ, I kept the same springs, shocks, tires, no bumpstops (in either configuration), brakelines were a non-issue, etc....

The spring has more flex, explain why.

Generally speaking, and having done both, I think the PRIMARY (not only, but main) reasons you generally get more flex on a SOA Yj is:

1) You are generally comparing a SOA against a comparative amount of SUA lift (4 ~ 6 inches). In order to achieve that lift, they are effectively increasing the spring rate. This means that you are not comparing apples to apples because the SUA job is inherently using higher rate springs to accomplish the same amount of lift - these naturally require more force to flex as well.

2) The aforementioned post about a curved spring not wanting to curve more. In other words, I do believe that a flatter spring is usually flexier than a curved one.

3) I believe that for some reason the axle in SOA configuration also exerts more leverage on the spring, therefore more force is exerted for the same amount of deflection.

I have absolutely no scientific basis for these opinions, and I reserve the right to be completely absolutely 100% wrong. It's just my seats of the pants feel on the subject.

My objective experience, having done 4 inch SUA, 4 inch SUA with revolvers, and now SOA, is that SOA flexes the most, without question, in my experience.

Originally posted by Archie_G


Then why did I get more flex from my CJ, I kept the same springs, shocks, tires, no bumpstops (in either configuration), brakelines were a non-issue, etc....

The spring has more flex, explain why.

Sounds like something was limiting your travel before. Did you swap wider wheels or axles? Were your tires rubbing and limiting flex? Draw a free body diagram and you'll see that the force on the spring is exactly the same in either configuration.

Originally posted by Archie_G


Then why did I get more flex from my CJ, I kept the same springs, shocks, tires, no bumpstops (in either configuration), brakelines were a non-issue, etc....

The spring has more flex, explain why.

Because the smallest leaf is closest to the axle when the springs in an SOA configuration. While this does not change the forces acting on the spring it does change the leverage. There is also a difference in the spring rate of the springs in question (SUA lift and SOA of the same height). But then when you think about it if the spring behaved 100% identical SOA and SUA you'd have axle wrap with a stock YJ.

Thanks for your replies guys and there are some good points in most of them

I am not debating about the extra flex given due to bigger wheels that gives more weight to help droop or use used differnt springs or wider axles This to me is all part of a well set up package

So my thoughts on extra leaverage and I maybe off track but please correct me that why I am asking

Lets think about a simple senario of a ramp and I know a ramp is not the real world but it will reduce the factors invovled but still give a description of the forces

There is a vertical force applied to the wheel through its vertical axis as the wheel goes up the ramp the other wheel stay close to still ( not count rolling foward as I think there is little force on the springs in that movement some but little)

This vertical force is transfered to the axle that acts a leaver

The falcrum point is the tyre on the ground the lift point is the trye on the ramp lifting the end of the axle

with no springs chasis etc the axle will lift until it flips 360 deg

With springs in SUA the spring on the end that is lifting is istance x from the falcrum point and as such as y amount of force applied this is countered by how much resistance the spring provides which is a factor of length arch and spring rate

As in SOA the distance x and therefore force y dont change and the springs are mounted tight on the axle so they can be considered part of the same structure ( we have all seend what loose U bolts will do :) )

Mike u have elluded to a good point about the small spring close to the axle can you please explain further about how this will affect the forces involved I believe it would be the resitance force affected but how ??

So why does it flex more ???????????

remember we are not talking about using wider axles different springs bigger wheels or going to bumpstops


I just want to compare the 2 completely My thinking is that there are a lot of SUA setups that need work to get them to flex but the owners go SOA address all of the original problems and now say that SOA flexes better

I do not doubt that some of the limiting factors in SUA are easier to fix if you go SOA

Sorry this post is so long but hard to explian in a few words

Good topic. I have had many a discussion about this very subject. Recently I just went to SOA with the exact same setup all around (other then extending the wheelbase 3"). Don't get me wrong I love the setup, the clearance sure is nice, but I haven't noticed a great increase in flex. To me it seems like it flexes about the same. However in 2 trips my springs have sagged considerably (same springs I ran SUA for 2 yrs) so that makes me believe that they are getting flexed more. I just don't see how it could flex more with the same spring.........other then more uptravel:confused:

Originally posted by mike

But then when you think about it if the spring behaved 100% identical SOA and SUA you'd have axle wrap with a stock YJ.
Wrong! With an SUA setup under acceleration, you have the pinion trying to wrap upward, but the forward motion of the axle on TOP of the spring counteracts the torque applied. With an SOA under acceleration, you again have the pinion trying to wrap upward, but you now have the forward motion of the axle UNDER the spring ADDING to the torque applied, and you get the famous SOA axle wrap.

You can get some flex out of off-the-shelf SUA kits, you just have to use the hell out of them, hers some shots of my junk progressing over the last year or two, same springs, SUA, just more and more use. I'm just about to go SOA with 52" Chevy truck spring mutt packs because my 4" Superlifts are now 2" Superwhipped, they look like fettucine.

http://www.thunderbayoffroaders.rockcrawler.com/images/dune5.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/rock9.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/rock1.jpg
http://www.thunderbayoffroaders.rockcrawler.com/images/rkprt22.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/rkprt23.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/mounds05.jpg
http://www.thunderbayoffroaders.rockcrawler.com/images/rtiaaron1.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/rtiaaron2.jpghttp://www.thunderbayoffroaders.rockcrawler.com/images/parade1.jpg

I think you guys are over anal yzing this hole thing.:D Imagine you are looking at a side view of your rig with the SUA setup.

a..................... ___________
b................._______O________

Line a being the frame. Line b being your spring and axle. Notice the limited amount of up travel??


a............................ ___________
b.......................... _______________
c........................................O

Same thing only now it's a SOA . Notice the big gap between frame and spring??

The reason your springs are sagging is because they are now being flexed beyond their design. Lower the bump stops or beef up your spring pack and live with it. The fact that SOA can now take advantage of wheel travel that was previously occupied by the axle tube is the only reason that it flexes more IMHO.

BTW I have seen revolvers "unload" at inopportune times and cause the vehicles center of gravity to suddenly shift causing an end over end roll over.

Originally posted by withamc

Wrong! With an SUA setup under acceleration, you have the pinion trying to wrap upward, but the forward motion of the axle on TOP of the spring counteracts the torque applied. With an SOA under acceleration, you again have the pinion trying to wrap upward, but you now have the forward motion of the axle UNDER the spring ADDING to the torque applied, and you get the famous SOA axle wrap.

Amazing. You read my post then replied with "Wrong!" and then restate the same point. :rolleyes:


Now every one go play with levers and report back what you've found about the application of force.

Originally posted by mike



Now every one go play with levers and report back what you've found about the application of force.

Ok where did I go wrong in my description please let me know cause that is the reason I asked If I knew I would have answered the question when someone else asked it :flipoff2:

Hey folks

Instead of just making the point that you do know and we dont will one of you just set down and explain in straight easy language (with some force illustrations) just how all these levers pivots and forces make SOA flex better than SUA.





Please:rolleyes:





Because like Aussie im still not getting it

Ok, here is my stab at an explanation. Remember, in my case, the only thing (before and after SOA) that stopped the travle of the axle is the springs. No bumpstops, no fenders, etc....

From my assement, the axle under the spring allows more of a moment arm due to it being further away from the therotical 'center of the springs' (which is a line drawin between the spring eyes). In SUA, the axle is inline with the 'center of the springs'. In SOA, it is moved farther away by X (where X will equal the thickness of the axle tube + 2 times the thickness of the spring pack). This additional moment arm will excert more force on the spring per unit of force on the axle. (ie: the big breaker bar example). Therefore, SOA flexes better than SUA due to more force being applied to the spring to move it farther (either in compression or droop).

Originally posted by Archie_G
Ok, here is my stab at an explanation. Remember, in my case, the only thing (before and after SOA) that stopped the travle of the axle is the springs. No bumpstops, no fenders, etc....

From my assement, the axle under the spring allows more of a moment arm due to it being further away from the therotical 'center of the springs' (which is a line drawin between the spring eyes). In SUA, the axle is inline with the 'center of the springs'. In SOA, it is moved farther away by X (where X will equal the thickness of the axle tube + 2 times the thickness of the spring pack). This additional moment arm will excert more force on the spring per unit of force on the axle. (ie: the big breaker bar example). Therefore, SOA flexes better than SUA due to more force being applied to the spring to move it farther (either in compression or droop).

dat wut i be trian ter say (in my point # 3)

Finally a definate answer then thanks Archie-g

So a SOA flexes beter because the increased distance between the axle and the roll centre allows greater leverage.

That seams clear enough does anyone else care to back that up?








Just had a thought though. If the roll centre stays in one place IE. above the centre line of the axle at the height of a line drawn between the fixed spring mount and the upper shackle mount. Then the axle would also move further away from the roll centre with a SUA lift!

I suppose this would be offset by the fact that the greater arch in the springs would allow less flex anyway.

Originally posted by withamc

Wrong! With an SUA setup under acceleration, you have the pinion trying to wrap upward, but the forward motion of the axle on TOP of the spring counteracts the torque applied. With an SOA under acceleration, you again have the pinion trying to wrap upward, but you now have the forward motion of the axle UNDER the spring ADDING to the torque applied, and you get the famous SOA axle wrap.

THIS WAS THE BEST POINT MADE IN THIS THREAD SO FAR, EVEN THOUGH IT DOESNT ADDRESS FLEX.

Aussie_G
- I don't agree with what you are saying. A force on the spring can be thought of as either a push from the bottom or a pull from the top. Either way, the spring has to support the same force. It doesn't matter where the axle is in relation to the spring. If that did matter, then the larger the lift blocks you put in the rear, the worse your flex would be. And the closer your axle was to the center line of the spring (eye to eye), the better it would flex. But in reality, the forces exerted (neglecting axle wrap from the moment about the perch) can be interpreted as the same. Your thoughts?

Reed

My thoughts are that someone is going to have to nut up and crack open their college physics book. Start looking at COG, lever arms, etc, etc. That is what will be necessary if you want the truly technical explanantion with all the correct terminology.

I already called not it and my physics books are at my home not with me at work

Originally posted by SCORPION


I also think that you get most of the new flex of SOA simply from all that room to compress.

this topic is being hashed out on Pirate right now.

I stated on there that I though a flater spring would flex more than an arched one, and read the same point in this thread.

How about giving us a link to this other discussion?


Please:smokin:


I agree with you that a lot of the extra flex with a spring over must come from the ability to reverse arch the compressed spring into the place the axle used to be but surely this cant be all the answer other wise people wouldnt talk about extra leverage etc.

Originally posted by gon2far


How about giving us a link to this other discussion?


Please:smokin:


I agree with you that a lot of the extra flex with a spring over must come from the ability to reverse arch the compressed spring into the place the axle used to be but surely this cant be all the answer other wise people wouldnt talk about extra leverage etc.


This is what happens when you are reading/posting while at work.

:confused: :smokin: :D

http://www.northeastonlinewheelers.org/yabbse/index.php?board=4;action=display;threadid=2243

Here is the other discussion:
http://www.jeepsunlimited.com/forums/showthread.php?s=&threadid=264029

I dont know how much more physics I can add, but I will say that I think this same increase in force on the springs is the reason that lift blocks will cause your springs to sag quicker. I think its also is the force (moving away from the 'spring center) that will cause lift blocks to cause more axle wrap.

the axles have more leverage on the springs becuase you have lowered your fulcrum. Thats the way I understand it anyway.

Remeber Hooke's Law
F=kx,
where F is a force,
k is the spring stiffness,
and x is the displacement.

With this rather simple formula you can expalin your questions on SOA and SUA.

If F is held constanat and k is decreased then x will increase. Thus, most SOAs use springs with a samller k therefore there x increases,ie travel is increased. Most SUA have larger arches with a higher k thus x is less. :beer:

ok Nobody,

How does K decrease in SUA ??? remeber the discussion is the same springs

I am thinking that Archie G has made a good point but isnt the real falcrum point the wheel on the other end of the axle and k is the distance from the other end of the axle ie the distance from the hub to the spring perch on the lifted end of the axle

this stay constant in both setups and is regardless of the " centre of spring" theroy

The centre of spring theory may hold some truth in that it will change the resitance give to the force exherted and net us more flex

but exactly how will that change the "resitance" which is the sum of all the factors that affect a springs abillty to move ie length arch spring rate etc

try getting a rule and some cardboard cut the cardboard in a strip and bend it like a leaf spring now tape it to the rule near one end lift that end of the rule whlie the other end is stationary where is the falcrum point ????

if you are really keen try a pry bar and U bolt on a weight that is big enough to feel when u lift it

lock one end of the prybar under something solid and lift the end with the weight then roll the weight over to be on top and try again how hard is to lift both ???? I have not tried the last but know what I expect and is described above

k is not decreased, k is static, F is increased because of the lower fulcrum point.
I'm not an engineer, just a barefoot chassis rat.

hey kid wired

where is the falcrum point this will answer why it moves in the 2 setups

good question, I'm havent figured it out. the axle acts as a class 1 lever when you consider it as a whole or as a class 2 when you look at each spring independently.

class 1
R______E
......F


class 2
___R___E
F

resistance/fulcrum/effort (or lever)

in either case I beleive having the effort/lever/axle under the resistance/spring provides more leverage against the spring.

I aggree with your example class 2 if the axis is the axle that is what I described in 100 words and u put so well in a pic thank you.

not sure what you are trying to describe with your class 1 example maybe you can explain it bit more

maybe l8r, I'm bummed, I just got sack'd :(

sorry to hear kidwired

there is no worse feeling than getting sacked I hope it was not cause u were answering my question at work

In SUA, the axle is inline with the 'center of the springs'. In SOA, it is moved farther away by X (where X will equal the thickness of the axle tube + 2 times the thickness of the spring pack).-originally posted by ChadLloyd

__________________________________________________ _

I think you are right on the money Chad! That added to the extra distance that the spring is afforded to travel due to SOA=more travel from same spring.

This thread got me interested, so I actually went and pulled out my spring design handbooks and textbooks. I've designed alot of springs for my various employers in the last 6-7 years, and honestly, I didn't originally see how having the load a farther distance from the "center" or the leaf spring made any difference. Frankly, it DOESN'T make any difference. If the axle remains in the CENTER of the leaf spring both before and after, then whether the load to compress the spring comes from above the spring, or below the spring, is irrelevant. The load is still the same, the spring rate is unchanged, and therefore flex shouldn't be any different. Really, I can't even find a reference in any of the books regarding the "center" of a flat or laminated leaf spring.


Regardless of whether it's a flat spring (or single leaf), or a laminated spring, they both work the same way. The only difference is that the laminated (multi leaf) spring is a progressive rate spring, which gets stiffer as it compresses, since more leaves begin to act in resistance of the deflection.

To address the leverage argument. Assuming the springs are NOT moved laterally relative to the vehicle centerline, the leverage acting on those springs is unchanged, because the fulcrums and distances between those fulcrums are unchanged between SUA and SOA. The fact that the load is at a different VERTICAL height relative to the spring is irrelevant, because the lever arm is acted on IN the vertical direction. It doesn't make a difference how far the axle moves vertically during SOA, so long as the axle doesn't move horizontally during SOA.

To address the roll center argument. Roll center is just a term to represent instantaneous kinematics. Roll centers are DYNAMIC. A moment about the vehicle's roll center only exists when the vehicle is moving and turning at the same time. For all intents and purposes, when the vehicle is going straight, or not moving, the roll center doesn't exist. It's just a dynamic representation of the pivot point, which the body rotates around during cornering. If you are on a ramp, or rock, and the vehicle isn't moving, you don't have a roll center.

Here's an argument I will interject. The spring rate of the leaves in SUA and SOA will be the same, ASSUMING the spring perches are the same size before and after. Now, once a leaf spring is bolted to a rigid spring perch, it acts like two cantilever beam springs. Each cantilever beam is extending from the axle, one in front, and one in the rear. These *two springs* act in parallel, and therefore their individual rates are additive. Now, if the spring perch is shorter in the direction of the spring length, then ultimately, the two cantilever beams are longer, deflection is greater due to the extra distance on the lever arms.

The spring rate of a cantilever beam is:

k=3EI / L^3

where E is the modulus of elasticity, I is the area moment of inertia, and L is the length of the lever arm. If L is increased by way of a smaller spring perch, the spring rate drops by alot, since the rate is based on the length cubed.

Now, we can assume here that E and I are the same before and after SOA vs SUA. If the axle is centered on the spring, then the overall spring rate of the leaf spring mounted on the rigid spring perch is:

k = 6EI / L^3

say for example the spring perch decreased in size by 1 inch, making each cantilever beam 5% longer, then we would have:

k = 3EI/(1.05L)^3 + 3EI/(1.05L)^3

which boils down to:

k = 3EI/1.16L^3 + 3EI/1.16L^3

k = 2.59EI / L^3 + 2.59EI / L^3

k = 5.18EI / L^3

Shorter perch, longer lever arm, smaller spring rate. This would make a SOA flex more at the same load.

However, if the axle is moved forward or backward then L changes. If one L gets 20% longer, and the other 20% smaller, then we have:

k = 3EI/(.8L)^3 + 3EI/(1.2L)^3

which boils down to:

k = 3EI/.512L^3 + 3EI/1.728L^3

k = 5.85EI/L^3 + 1.73EI/L^3

k = 7.59EI / L^3

So, if the axle is moved relative to the center of the leaf spring, the overall spring rate of the leaf increases. This means more load per unit deflection. I really don't remember where I was going with this, I think I got off on a tangent. I'm really not even that old, only 30.

Anyway, reference:

Spring Designer's Handbook, written by Harold Carson, published by Marcel Dekker

Mechanics of Materials, 2nd edition, written by Beer and Johnston, published by McGraw Hill

Design Handbook, Engineering Guide to Spring Design, written and published by Associated Spring, a division of the Barnes Group Inc

Advanced Mechanics of Materials 5th edition, written by Boresi Schmidt and Sidebottom, published by John Wiley & Sons

Thanks Jared Rude.

That was a great answer.

Here is what I summed up out of it just to make sure I didnt miss read it

If the spring pearch stay the same size and the location of the axle is only changed to be ontop or on botom SOA /SUA and remain the same length from the end there is NO change

If you change the length of the pearch then flex will change but the change will be the same for SOA or SUA

That is why the pearch length can be a factor in wrap too I guess ??

Would this explain why OME springs dont have a constant curve they have a flat section along the botom they curve from the eye then flat the cure to the other eye. A OME guy told me this was to help on axle wrap

so does this mean that an offset spring will flex less than a spring of the same length that isnt offset?

OK. my BS contribution to this thread(since most of this is over my head anyway).

speaking from a consultants point of view, first we must determine if SOA REALLY DOES flex more than SUA. We must run a trial rather than try to picture this in our heads. the ways of a fool are right in his own mind. get together two completely identical jeeps with the only difference being one is SOA(remember that they must have equally used springs as this is also a factor in spring rate. kinda hard to come by two identical springs). flex them on a ramp or whatever and watch how the spring is actually being affected, not just how far the wheel moves. then you can measure the arched length of the springs(or whatever you scientific boys would do). maybe when you can SEE IT it will dawn on you just why it IS, or if it really IS at all.
if you cant figure it out...sit down, scratch your head, and give up. I know theres a perfectly good and simple reason why the sky is blue, but I just dont give a F**K.

If SOA flexes more then it flexes MORE!!!!!
I spent a half an hour reading this SH*T!:mad:

Originally posted by Jared Rude
This thread got me interested, so I actually went and pulled out my spring design handbooks and textbooks. I've designed alot of springs for my various employers in the last 6-7 years, and honestly, I didn't originally see how having the load a farther distance from the "center" or the leaf spring made any difference. Frankly, it DOESN'T make any difference. If the axle remains in the CENTER of the leaf spring both before and after, then whether the load to compress the spring comes from above the spring, or below the spring, is irrelevant. The load is still the same, the spring rate is unchanged, and therefore flex shouldn't be any different. Really, I can't even find a reference in any of the books regarding the "center" of a flat or laminated leaf spring.



If this is correct then the extra travel from a SOA must come from the simple fact that there is more room for up travel and the springs ability to reverse arch becomes part of the total amount of travel.

Kind of simple and it knocks on the head all the discussion about extra leverage etc. and where it comes from.

Thanks Jared

Originally posted by gon2far


If this is correct then the extra travel from a SOA must come from the simple fact that there is more room for up travel and the springs ability to reverse arch becomes part of the total amount of travel.

Kind of simple and it knocks on the head all the discussion about extra leverage etc. and where it comes from.

Thanks Jared

OR that most SUA setups are not working 100% ???

Still think it is easier to address so of the issues such as providing the room to flex via SOA

anyway thanks everyone it has been a learning experience

Originally posted by thenodnarb
so does this mean that an offset spring will flex less than a spring of the same length that isnt offset?

OK. my BS contribution to this thread(since most of this is over my head anyway).

speaking from a consultants point of view, first we must determine if SOA REALLY DOES flex more than SUA. We must run a trial rather than try to picture this in our heads. the ways of a fool are right in his own mind. get together two completely identical jeeps with the only difference being one is SOA(remember that they must have equally used springs as this is also a factor in spring rate. kinda hard to come by two identical springs). flex them on a ramp or whatever and watch how the spring is actually being affected, not just how far the wheel moves. then you can measure the arched length of the springs(or whatever you scientific boys would do). maybe when you can SEE IT it will dawn on you just why it IS, or if it really IS at all.
if you cant figure it out...sit down, scratch your head, and give up. I know theres a perfectly good and simple reason why the sky is blue, but I just dont give a F**K.

If SOA flexes more then it flexes MORE!!!!!
I spent a half an hour reading this SH*T!:mad:

Didn't Archie already do that? Same springs, same jeep, blah blah blah, totally different ramp score??

Originally posted by Aussie


OR that most SUA setups are not working 100% ???

Still think it is easier to address so of the issues such as providing the room to flex via SOA

anyway thanks everyone it has been a learning experience


I agree with you about the SUA not generaly being run at 100% efficency. My SUA set flexes like mad and this has always been my reason to question the assumption that SOA flexes better than SUA:smokin:

Jared, why did you use a cantilever beam? Seems to me that a cantilever beam represents a quarter elleptic setup.

Isn't a pinned-pinned beam with rollers on each end that are constrained vertically so that the ends only move horizontally a better representation?

I am not a spring designer.

Originally posted by mike


Amazing. You read my post then replied with "Wrong!" and then restate the same point. :rolleyes:


Now every one go play with levers and report back what you've found about the application of force.
Doh!!:eek: I went back and read it and you're right. At least SCORPION appreciated it :)

I used two cantilever beams because of the way the springs clamp to the axle perches. Effectively, the springs don't flex between the two side of the U bolt. If that is the case, then that potion of spring can't contrbute to the overall spring rate. Hence, two cantilever beams.

Jared, thanks, good answer. I see the point aout being clamped. :)

godddamn you guys are giving me a headache......BUT I LIKE IT........keep up the good work. i'm not far along enough in my studies to figure this out on my own......

Originally posted by TexasBlakeWFO
This says enough. It's the world famous 4" ProComp SUA lift:

Wow, OLD thread. I'm no longer SUA like previously stated in the thread. So here's a before and after pic of my SOA vs SUA. Big fawkin difference, never going back to SUA.



http://www.wfo-offroad.com/events/utopia/3-29-03/139386-R1-10.JPG

Thanks Blake for brining up this thread.

Originally posted by Kitty Cat
Thanks Blake for brining up this thread. So you gonna bring your junk over and let me SOA it? :evil:

When spring perches were mentioned that got me thinking...

So is it "safe" to believe that the longer the spring perch the less upward travel there will be? This seems true for me as I am running MORE 7" Anti-Wrap perches in the rear and as soon as my springs go flat the upward travel stops (no shocks still testing - and no body contact)...

so say I cut off 1" off each side and make them a normal 5" perch Would it be "safe" to assume I can go into a neg. arch easier??

Originally posted by TNToy
So you gonna bring your junk over and let me SOA it? :evil:

If you willing to do it. Then yes;)

Originally posted by Kitty Cat
Thanks Blake for brining up this thread.

I didn't bring it up, the post above me did. ;)

I just figured that since earlier in the thread it showed that I was SUA, that I'd update now that I'm SOA.

Good thread, Great post Jared. That post satisfies me that what I have always believed, same as the starter of the thread. The position of the axle, on top of or, below the spring makes no differance in the amount of flex of a given suspension. Until somebody can provide me with convincing arguements with the same type of documenting that Jared has provided, I'm going to accept this therory as gospel..

Now how about a new issue... Extened shackels reducing departure angle.
I don't see it . All other things remaing unchanged, extended shackels, to my way of thinking do not effect departure angle.. You still have the same amount of weight on the same springs, there fore your departure angle would not change... thoughts/arguements...09

Originally posted by Archie_G


Then why did I get more flex from my CJ, I kept the same springs, shocks, tires, no bumpstops (in either configuration), brakelines were a non-issue, etc....

The spring has more flex, explain why.

I'll make a stab at this:

Spring force is purely a function of deflection; the more you deflect it, the greater amount of energy is stored in the spring.

Most forces applied to vehicle leaf springs can be reduced to a collection of moments, that is, forces applied at various lengths (moment arms).

Fundamentally changing the length of an arm while applying the same force (as would occur with an SOA) changes the amount of moment applied, while different levels of deflection result.

Couple that with the different geometry created by an SOA would result in the generation of an apparent change in flex. Am I making sense here or blathering? :rolleyes:

One caveat: off-axis loading applied to a leaf spring does hanus things to the forces at play, like resulting inertia tensors, varied energy losses due to friction in rubbing leaves, shackles, etc. Like Big Dude said, doing it right requires analysis none of us can charge time to (hence it won't get done).

My $0.02 - Geesh

This thread is confusing because we are talking about two independant behaviors, flex and wrap.

First, suspension flex. Jared nailed this one nicely. We are just concerned with the vertical force as you gently roll across an obstacle or up a ramp.
SOA/SUA no difference (in theory).
springs closer to vehicle center, more flex
softer (lower k) springs flex better (same length)
longer springs flex better (same k)

Second, axle wrap. withAMC said this the best I have ever seen. In this case we are concerned with torque as in a hillclimb and there is a huge difference between SOA/SUA:
Originally posted by withamc

With an SUA setup under acceleration, you have the pinion trying to wrap upward, but the forward motion of the axle on TOP of the spring counteracts the torque applied. With an SOA under acceleration, you again have the pinion trying to wrap upward, but you now have the forward motion of the axle UNDER the spring ADDING to the torque applied, and you get the famous SOA axle wrap.

But real world wheeling is a combination of both these loads. I believe SOA flexes better because when you flex AND wrap the spring you get some plastic deformation near the perch. How many times have you heard "wasted my springs on one trip" or someting like that? The wrapped spring is already stretched to the limit on one side, then you flex it and the steel gives.

To restate: the wrap moves the load from one side of Jared's cantilever to the other. Normally, flex would be the same but the spring has gone from elastic to plastic deformation and flexes better but does not return to its original shape.

Originally posted by 4Bangler
http://www.thunderbayoffroaders.rockcrawler.com/images/rock1.jpg [/B]

Nice wheels :flipoff2: :D

Jared has the spring mount free body diagram correct, with a great analysis of the forces about the spring clamp.

The SUA/SOA comparison also needs to look at what forces are placed on the spring in each configuration. Resolve the forces experienced by the spring leaf pack into vertical and horizontal components, on the pinned side of the perch (the non pinned, shackle, side experiences only vertical forces by design). The cantilever spring analysis applies to the pinned side, but only to the vertical force component (the ideal level cantilever beam).

What is different, visually, to the typical SUA to SOA conversion (same leaf pack and arch, just a perch flip)? The elevation difference between the leaf end pin at the frame and the axle centerline (the two points defining the two force member of the pinned cantilever leaf half)? How does this alter how much of the weight is applied to the spring (vertical component), and how much to the rigid leaf (horizontal component)? Is there a difference?

Compare the force components on the spring in the two configurations, and remember the conservation of energy law: the energy stored must be equal if the displacement and spring rate are the same (Jared's analysis is confirmed, again, through independent physics ... but). If the SOA seems to flex more, then where does this energy go when the spring is mounted SUA? Does the same weight displacement (or ramp height position) really move the axle through the same spring displacement? Why the difference?

Another thing to consider is the perch length (again Jared hit the subject square on the head). The equations model mono-leaf systems, not a multi-leaf pack. They apply very well to a leaf pack in compression but they assume the extreme fiber stress is not decoupled at each stacked leaf, or the perch idealization (a stiff unyielding perch) is compromised with the flex of the closest leaf to the perch, and fail when applying extension forces (the leafs separate and now you have only the mainleaf working).

The SOA can place the perch closest to a rather thick overload leaf, essentially extending the perch lenght. How does this impact the flex? Should it make the SOA flex more (NO)?

SOA also places the perch further away from the leaf defining the rigid arm between the axle and the pinned frame mount. Think about what restrains the horizontal force component applied to the spring, and the leverage the resistance of the spring has about the axle centerline? What happens to the unloaded side to the axle (the side that is not driving up the ramp)? Should it make the SOA flex more (YES)?

What is the more dominant factor (the main leaf extension flex gain, or the overload leaf flex loss) in the SOA configuration?

So in summary, the spring force and displacement of the leaf pack (for a given load in compression) does not change from SUA to SOA (Jared's proof).

The geometry of initial conditions does change (forces horizontal and vertical at rest).

The influence of the leaf closest to the perch has an effect on the leaf packs spring compression performance (more or less perch extension effect).

The influence of the leaf closest (and farthest) from the perch has an effect on the leaf packs spring extension performance, as does the distance of the lever between the axle centerline and the main leaf.

So with all that has been observed, does the SOA gain enhanced performance due to any change in the compressive performance of the leaf pack (Jared's proof) or could the change be through geometry and the extension performance of the multi-leaf pack due to the change in mount (what may not have been considered)?

I'll leave the wrap issues (and containment methods) out of the discussion :) . This same leaf mount discussion went on for weeks, over a decade ago, on the original off-road e-mail forum sponsored by Stephen Roth (and most of the discussion participants are now driving linked buggies ;) ).

Happy Trails!

OK, without all the high tech physics crap, lets look at this with the KISS rule. First, we're comparing flex between soa and sua. Its all fine and good that we have all this spring analysis, but so far we havn't been comparing apples to apples. When you do soa you effectivley add 5" between the bumpstop and its contact point. You cant do this with a sua setup without contacting body pannels. Take your sua suspension and drop the fixed mount and shackle mount untill you have as much distance between the bumpstop and axle tube as you would with a soa and I'll bet they flex the same. Look at a stock YJ setup, it allows for about 2" of uptravel, no way for the springs to go into much of a negative arch. Add 5" of uptravel and there is your extra flex, its also why people waste their springs after a run or two (not counting spring wrap of course). Am I oversimplyfing? If so, tell me why. A spring is a spring, it doesnt care how its mounted, but it does care what its limits are set at, and you cant say it doesnt flex if you dont let it.


To simplify even more, so far we have been comparing soa with nearly free range of movement to sua with a limited range of movement and wondering why it doesnt do as well. Level the playing field before getting out all the physics books. :flipoff2:

It was my understanding that while a neg. arch isn't the best for a spring it wont do as much damage to it as the spring wrap caused by a SOA not setup correctly.

Meaning axle wrap will make a spring go bad faster than just going into a neg. arch. Combine the 2 and then after a couple runs they will go bad, but with a correctly made traction bar and neg. arch your springs will last a good while, and by that it all depends on how much you wheel.

I would estimate 1yr or so...

Originally posted by kwrangln
OK, without all the high tech physics crap, lets look at this with the KISS
<snip>
To simplify even more, so far we have been comparing soa with nearly free range of movement to sua with a limited range of movement and wondering why it doesnt do as well. Level the playing field before getting out all the physics books. :flipoff2:


I think that IS what Jared said: a leaf spring in compression does not change the applied force due to how it's mounted (my summary).

What I said: a multi-leaf spring in extension does change the applied force due to how it's mounted.

The travel limitations of a body or frame do not enter either isolated topic (spring pack compression or extension performance comparison).

The topic you mention is a valid discussion: take a SUA that is designed for equal compression travel and compare the performance to a SOA design (from a travel range and life cycle fatigue aspect)? What system performs better (not the simple flip over the existing SUA spring pack, but a designed system for the application)? What system better controls spring wrap?

The review may lead to an answer why the off-road endurance racers that must use leaf packs (but have the SUA/SOA option) are almost always ... ?

Happy Trails!

I purposly left spring wrap out of it, I think that was one of the "please ignore" parts of the origional post, but it does play a large part of the equation. I merely wanted to point out the obvious and possibly overlooked, that a level playing field was required before deciding which flexed better. Not knocking anyone who has the mathematical background to compute all the forces involved, but looking at it from a practical point of view.