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Old 03-07-2003, 07:53 PM   #1 (permalink)
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TOTW: Four Link Suspensions

Welcome to the next installment of the

Topic of the Week


We did the three link, now it's time to do the four link suspension.

You know...

link placement
joint construction
squat & anti-squat (and I don't know diddly-squat)
etc.
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Old 03-08-2003, 07:02 AM   #2 (permalink)
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How about beginning a list of design elements that usually always work and should be suggested in most all 4 link setups?
Maybe starting with arm lengths?
Are the bottom arms usually always longer than the tops?
Should you usually always try have close to level top arms?
etc.,
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Old 03-08-2003, 08:26 AM   #3 (permalink)
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Also I am interestd in leraning more how to desgin the front 4 link...the rear seams pretty simple, but in the front you have to deal with steering issues. Another thing I would like to learn about is spring rates.
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Old 03-08-2003, 09:54 AM   #4 (permalink)
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Antisquat

I tried to draw up a decent picture to illustrate how to calculate antisquat. You don't need CAD to do it, a pad and paper work fine.

One absolutely necessary piece of info is the Center of Gravity of the vehicle, and that is not hard to figure out with the COG spreadsheet.

To figure it out:
1) Draw out a side view with the basics of your setup. Draw the tires, axles, and control arms. If find it easiest to draw a horizontal line through the COG.
2) figure out your instant center by drawing imaginary lines until they connect, that is your IC. That is the blue point in the pic.
3) Draw a line from the point on the ground below the centerline
of your rear axle through the instant center, it should look like the blue line.
4) Draw a line vertically through the front axle centerline. (not shown)
5) The point that your line drawn in step 3 and 4 intersect is your Point of Convergence (POC). This is the purple dot on the drawing.

The height of your POC divided by the height of your COG, multiplied by 100 is your Antisquat percentage. The green line on the diagram shows where 100% antisquat is; if your POC is above this line then your rear end will rise under acceleration, below this line and your rear will squat.

What's the perfect number? Who knows.
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Old 03-08-2003, 10:00 AM   #5 (permalink)
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Roll Axis

Here's a couple pics from the Thomas Gillespie book Fundamentals of Vehicle Dynamics that show how to calculate the roll axis of a 4-link.





The flatter the roll axis is, the less roll steer (aka rear steer) that will be present.

Hopefully, this will get the topic going now that the basics are covered.

[edit: attached pics rather than linking]
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Last edited by JohnnyJ; 03-05-2005 at 09:17 PM.
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Old 03-08-2003, 01:42 PM   #6 (permalink)
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Re: Antisquat

Quote:
Originally posted by JohnnyJ
The green line on the diagram shows where 100% antisquat is; if your POC is above this line then your rear end will rise under acceleration, below this line and your rear will squat.
I agree with all the tech you posted, sumed it up nicely how to figure your anti-squat and COG but the comment above does not seem to be accurate in the real world.

I have been researching and designing my 4 link rear for about 6 months and from what I gather from the people that are more experienced than me say that you will begin to have squat chararacteristics at around 50% anti squat in a rockcrawler. Is this true I am not sure but since we are all searching for the perfect number I highly recommend building adjustability into your design. Below is a CAD picture of my setup, I have anti-squat adjustability between 50% and 102% and up front I can adjust between 83% and 174%. These figures can be adjusted even further up or down by changing my ride height. I believe that in doing this I will be able to find the "perfect" anti-squat for MY rig!

Notice in the drawing the white circles on the brackets those are adjustment holes along with 2 holes in the frame for the upper control ARM up front.. My roll axis sits about level with the bottom of the frame!

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Old 03-08-2003, 07:00 PM   #7 (permalink)
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I love Mechanical Desktop for modeling. It is truly an amazing piece of software. Excellent topic that I know nothing about, but have always wondered. Keep up the good work guys..

TPIJeep, what's your wheelbase? What reference did you use while investigating this suspension type?

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Old 03-08-2003, 07:10 PM   #8 (permalink)
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Wheelbase at 102"

I learned 99% from the good people here and the almighty search button!
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Old 03-08-2003, 10:05 PM   #9 (permalink)
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Re: Re: Antisquat

Quote:
Originally posted by TPIJeep


I agree with all the tech you posted, sumed it up nicely how to figure your anti-squat and COG but the comment above does not seem to be accurate in the real world.

I kinda winced as I typed in that comment, and figured of all things it would bring out some sort of disagreement. I even intentionally left out any sort of comment about "acceptable" antisquat values for fear of throwing this thread off the deep end and turning it into a low squat vs high squat with strap holy war type debate.

I was over at friends today and I was joking around about needing a squat-o-meter, similar to those junky clinometers they sell through jc whipme that will help show you how close to tipping you are. (As if your ass-o-meter can't tell you that by your current pucker factor.)

It seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the window. I still believe that it should be taken into consideration, but I don't believe it's the holy grail of suspension design, but that's just my opinion. I think that your suggestion of making the level of AS adjustable is excellent, since different people like different amounts depending on the rig and environment they are in.

One thing that intrigues me about your statement is talk about AS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.

edit: TPIjeep - I am jealous of your autocad capabilities.. your 3D full renderings make my drawings look like I was a kid using crayons.

Last edited by JohnnyJ; 03-08-2003 at 10:09 PM.
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Old 03-09-2003, 09:18 AM   #10 (permalink)
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Re: Re: Re: Antisquat

Quote:
Originally posted by JohnnyJ

One thing that intrigues me about your statement is talk about AS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.

I said anti-squat for the front end but that is really anti-lift the flip of that on braking would be anti dive. I made a little adjustability up there incase it wanted to stand on the front bumper when braking. I would like to see is sorta plant the front tires on acceleration. There has been VERY LITTLE discussion on front link design Anti-Dive values most people say your limited to space so just build what you can and be happy with it.. Well if I am unhappy with it i would like a few options!


On with the topic at hand, Now that we have the Anti-Squat portion done with the conclusion that get it somewhere between 50% and 150% with adjustablitly we move on to another factor which is roll axis.

The diagrams really do make sense if you look at them long enough! You want your roll axis to be as high as possible and and flat as possible, This will make your more stable when off camber and help control rear steer. Link placement here is important, you can tweak your roll axis by moving the links inward or outward while keeping the height the same. Doing so will not affect your Anti-Squat values. If you cannot figure out the diagrams I am sure we can help to explain them.

Here is a pic of my roll axis(green line)
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Old 03-10-2003, 01:20 PM   #11 (permalink)
 
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dang - pretty good thread so far.

I'm curious to know how people are calculating their loads and determining tube dia/thk, joint size and bracket design. This is the area where a lack of knowledge could kill you (or someone else) - no pun intended.

Brian
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Old 03-10-2003, 01:53 PM   #12 (permalink)
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Quote:
Originally posted by NVR FNSH

I'm curious to know how people are calculating their loads and determining tube dia/thk, joint size and bracket design. Brian
Personally I think anything less then 3/4" heims on a link system is crazy. As far as calculating its a matter of choice, how much you want to spend, what size tires, rig weight and horsepower. I think the RE bearings will work well for most link systems as long as you have enough vertical link seperation on the axle. As far as materail size most heims and RE bearings can be made to go inside 1 and 1.5" ID tubing with an adapter or straight tapping the tube.

As far as the brackets go make sure you have lots of weld surface and get good penetration when welding.. in a double shear setup 1/4" thick should be more than enough... although I am building all my brackets out of 3/8" !!

I am running the 1" 1-1/4" shank heim joints with 2" .250" wall tubing, but my rig weighs 5k, has 40" boggers and 450 hp.

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Last edited by TPIJeep; 03-10-2003 at 03:03 PM.
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Old 03-10-2003, 02:22 PM   #13 (permalink)
 
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Arm length...
Uppers 37" 1.5" .25wall
Lowers 40" 1.25" .25 wall sleeved with 1.75 .25 wall
7/8" heims
Cherokee coils
I believe my AS to be at 103% but have not had it on the road yet. <2 months remaining.

Shorter uppers doesn't necessarily mean that the pinion rotates the wrong direction. Mine doesn't. It still depends on placement and the rotation through the arcs.

Here's a few pics...
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Old 03-10-2003, 02:24 PM   #14 (permalink)
 
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Old 03-11-2003, 05:55 AM   #15 (permalink)
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Re: Re: Re: Re: Antisquat

[QUOTE]Originally posted by JohnnyJ
[B]
... seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the wAS on the front end. I, too, do not claim to be the god of suspension, but I've not heard or read of doing AS calcs on the front end, but instead doing anti-dive with the brake proportion being taken into account.
[QUOTE]Originally posted by TPIJeep
[B]
I said anti-squat for the front end but that is really anti-lift the flip of that on braking would be anti dive. I made a little adjustability up there incase it wanted to stand on the front bumper when braking. I would like to see is sorta plant the front tires on acceleration. There has been VERY LITTLE discussion on front link design Anti-Dive values most people say your limited to space so just build what you can and be happy with it.. Well if I am unhappy with it i would like a few options!
On with the topic at hand, Now that we have the Anti-Squat portion done with the conclusion that get it somewhere between 50% and 150% with adjustablitly we move on to another factor which is roll axis. [/Bold]

I hardly think you have it all wrapped up and done fellas ...
How about
In tall rocks I'd like to see my back end rise and my front end squat when climbing up ... so I won't need to fill my front tires with water
Then when braking going down hill I'd like my rig to squat in the rear and rise in the front so it won't want to endo as soon as it hits bottom.

Last edited by Jaffer; 03-11-2003 at 05:58 AM.
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Old 03-11-2003, 09:06 AM   #16 (permalink)
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Re: Re: Re: Re: Re: Antisquat

[QUOTE]Originally posted by Jaffer
[B]
Quote:
Originally posted by JohnnyJ




I hardly think you have it all wrapped up and done fellas ...
How about
In tall rocks I'd like to see my back end rise and my front end squat when climbing up ... so I won't need to fill my front tires with water
Then when braking going down hill I'd like my rig to squat in the rear and rise in the front so it won't want to endo as soon as it hits bottom.
Sounds like you want a 64 with hydro word up player! peace & chicken grease ,I'm outa here gota bounce hommie, keep it on the down low, Ieeet
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Old 03-11-2003, 10:03 AM   #17 (permalink)
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Tpi what is the advantage of using 2 seperate links on the front drivers side couldn't the same thing be be done with say a Y on the drivers side i was just wondering if it may be done to help with adjustment or control. I was going to ask you how you plan on ataching your panhard bar to the axle when the pig is in the way or are you mounting it to the top of the pig by a rock ring support plate.
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Old 03-11-2003, 11:51 AM   #18 (permalink)
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Jaffer.... Thanks for the input, and how you offered so much tech on how to do that.... Hence why I built adjustability cause I don't know, nor think I have it all wrapped up..

Seabass..

Mad Mac,

I built seperate arms to allow some adjustability and so I can tweak my pinion angle if need be..

As far as the panhard goes take a look at this pic.. the mount IS above the pig..


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Old 03-11-2003, 12:14 PM   #19 (permalink)
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Tpi is there any reason for the upper panhard mount to be so long what is the distance from the top of the axle to bottom of the frame. I also wanted to know if keeping the panhard as flat as posible at rest had somthing to do with your design. Looks good so far though man keep it up looks to be you got learning autocad licked.

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Old 03-11-2003, 01:17 PM   #20 (permalink)
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Quote:
Originally posted by MAD MAC
Tpi is there any reason for the upper panhard mount to be so long what is the distance from the top of the axle to bottom of the frame. I also wanted to know if keeping the panhard as flat as posible at rest had somthing to do with your design.
You want your panhard bar to run at the same angle as your drag link to minimize bumpsteer, that is the reason for the long mount... that and to get it away from the upper control arm!
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Old 03-11-2003, 06:58 PM   #21 (permalink)
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Re: Re: Re: Re: Antisquat

Quote:
Originally posted by TPIJeep

The diagrams really do make sense if you look at them long enough! You want your roll axis to be as high as possible and and flat as possible, This will make your more stable when off camber and help control rear steer. Link placement here is important, you can tweak your roll axis by moving the links inward or outward while keeping the height the same. Doing so will not affect your Anti-Squat values. If you cannot figure out the diagrams I am sure we can help to explain them.

Great thread (I seldom have time to review the Jeep Forum). The comment about designing in a flat roll axis for stability is hard to dispute. The comment about the desire for a high roll axis needs to be tempered with the drawbacks of the design attribute (another debate like the anti-squat percentage question).

The roll axis height compared to the CG height determines the compliance of the suspension system during cornering and sidehills. A low roll axis will tend to allow chassis lean in cornering and on side hills. A high roll axis, close to the CG height will lose suspension compliance (make for a stiff ride) in cornering and impact vehicle control on side hills (minimal telegraphing of sprung chassis lean before the rollover limit is exceeded). Raise the roll axis above the CG elevation and you risk jacking of the suspension on side hills and cornering (folding the suspension under the chassis, time for the limit strap).

Paved track racers design the roll axis to be as low as possible (track height elevation) for suspension compliance to keep the tires in contact with the track surface at the traction limit, and employ anti-roll bars for chassis roll control. These racers do not exploit wheel travel (too much risk of tire traction patch misalignment) and demand the design to provide the suspension compliance during the short duration that high cornering and impact loads stress the tire contact patch.

The same low roll axis design on a long travel rock crawler without anti-sway bars will flop over on side hills. Fit the crawler with anti-sway bars and it will lose articulation. The crawler seldom experiences high tire stress, and the driver does not need the same level of feedback at the traction limit (the flotation tires already limit the traction). A crawler design can trade traction limit control for the stability gained by a higher roll axis. The compromise is to raise the roll axis high enough to balance the need for roll feedback and compliance with sidehill stability. Sometimes a light anti-sway bar (Anti-Rock bar) with a moderately low roll axis is a good compromise to allow the downhill suspension to work the suspension rather than the links on sidehill challenges.

Off-road endrance racers also do not need as much high speed cornering control at the traction limit (as a paved trach racer, the tires do not bite dirt as hard as pavement), and as a result can trade long wheel travel for roll compliance (more travel can allow for a higher roll axis, within reason, without risk of high speed suspension jacking over rough terrain). The same basic rules to maximize tire traction apply (same as paved track racing) with the compromise demanded of wild axle steer and misalignment (something that is impacted by long travel suspension).

Any thoughts?

Happy Trails!

Last edited by Ed A. Stevens; 03-11-2003 at 07:04 PM.
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Old 03-11-2003, 07:03 PM   #22 (permalink)
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I am thinking its a good thing that my COG height is within an inch of my roll axis! Good info Ed. Gives us novicies yet one more thing to worry about..
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Old 03-12-2003, 10:13 AM   #23 (permalink)
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Quote:
Originally posted by TPIJeep
Jaffer.... Thanks for the input, and how you offered so much tech on how to do that....
Seabass..
TPI, I have no formal education in physics, engineering, automotive design, machine nor metallergy and I must complement you on your Auto-Cad skills because I'd dearly love to possess some.
I'm just here trying to learn all I can on the dynamics of off-road chassis design. None of it is easy, most of it incomplete and a little that's just plain screwball IMHO.
Now, if you will stop being snotty and climb down off your high horse for a minute maybe you and others might explain in a little more detail some of the "tech" being shoveled around here...

First, I a little confused, or at least a little put off by a couple of your posts ...

[QUOTE]Originally posted by TPIJeep

The diagrams really do make sense if you look at them long enough! ... If you cannot figure out the diagrams I am sure we can help to explain them.
[QUOTE]
The diagrams don't need explaination but your reasoning sure does at times.

[QUOTE]Originally posted by TPIJeep
[B]
You want your panhard bar to run at the same angle as your drag link to minimize bumpsteer, that is the reason for the long mount... that and to get it away from the upper control arm![B][QUOTE]
Isn't the reason is because the panhard should be that the same angle AND LENGTH as the drag link?

Quote:
Originally posted by TPIJeep

I said anti-squat for the front end but that is really anti-lift the flip of that on braking would be anti dive. I made a little adjustability up there incase it wanted to stand on the front bumper when braking. I would like to see is sorta plant the front tires on acceleration.
[i]Say what? It seems here like you are now the one just wishing. Will drilling a couple of more near-by holes in your brackets help that much? Is that the tech?[i]

[QUOTE]Originally posted by TPIJeep

On with the topic at hand, Now that we have the Anti-Squat portion done with the conclusion that get it somewhere between 50% and 150% with adjustablitly we move on to another factor which is roll axis.
[QUOTE]
You are being more than just a little presumtuous, IMHO.


I still like this quote the best ...
JAF

Quote:
Originally posted by JohnnyJ

... seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the wAS on the front end.
So why are all of you doing your drawings and calculating your values on a flat surface?
It might tell you something if you plotted it all at 45 degrees
At least Ed Stephens knows enough to qualify the conditions of the intended terrain whilst describing suspensions ...
JAF


P.S. The above are all just thoughtfull little jabs, and food for thought, TPI.
Please don't take offence.
Especially just after scolding me.
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Old 03-12-2003, 11:09 AM   #24 (permalink)
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Quote:
Originally posted by Jaffer

So why are all of you doing your drawings and calculating your values on a flat surface?
It might tell you something if you plotted it all at 45 degrees
JAF


P.S. The above are all just thoughtfull little jabs, and food for thought, TPI.
Please don't take offence.
Especially just after scolding me.

Many race suspension designers have minimal formal Engineering education (some have to work hard, and some have a gift) so don't let it slow you down. As formalized technical expertise is applied to off-road suspension systems, everyone learns (more than only the shops getting paid to design and build). You do not have to be an Engineer to learn how to apply the technology (just be open minded and persistent).

I also wish I had maintained my CAD skills (and I had the education... before CRS kicked-in and I moved-on to other interests).

Your request for modeling the suspension benchmark changes (Anti-Squat%, Instant Center, Roll Axis, etc.) at different grade levels is a good exercise. The CAD skills may assist in the modeling (everything except the true dynamic weight transfer on the suspension -- field test time).

CAD skills may also help modeling the suspension at different travel or articulation limits. The change in AS% and roll axis in relation to the CG height, as the suspension cycles, will help you learn if the system is moving to a more stable configuration (or less stable) as the suspension works.

This modeling does not have to be dynamic (it would be nice, but way beyond my tools, maybe a poster with access to a University budget?).
Considerable insight can be gained by simply changing the static ride height in the model.

What does the AS% change to, after lowering (simulated loading) of the rear suspension a few inches? Rotate the model grade and calculate again. This is a crude indicator of how the system will change while climbing on (smooth) hills.

Raise the rear static height in the model and it is a crude simulation of decending (smooth) hills. Where is the AS%? How does it change as one tire gets stuffed, and the other extended?

Rotate the model (downhill) at the same time. Where is the Roll Axis in relation to the (now lower) CG? Is the roll axis so high that an turning off-camber decent may be unstable?

What is the front suspension system anti-dive% (is it high enough to possibly need a limit strap)?

There is considerable information to gain by modeling, and maybe someone with free time can draw the pictures?

Maybe someone else can also recommend a few other model scenarios?

Happy Trails!
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Old 03-12-2003, 12:56 PM   #25 (permalink)
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Quote:
Originally posted by Jaffer

I still like this quote the best ...
JAF
(then quote it right..)

Quote:
Originally posted by JohnnyJ

It seems that real world and book theory most collide on this subject more than any other. The problem is once you take the rig from level ground it seems that weight transfer throws the whole static calcuation away, and the lovely AS values calculated during design are thrown out the window. I still believe that it should be taken into consideration, but I don't believe it's the holy grail of suspension design, but that's just my opinion.
Quote:
Originally posted by Jaffer

So why are all of you doing your drawings and calculating your values on a flat surface?
It might tell you something if you plotted it all at 45 degrees
Because not all hills are not at 45 degrees? As I said in my quote "it should be taken into consideration, but I don't believe it's the holy grail of suspension design". By at least determining the static antisquat on level ground I know a little bit about how it should react. I can't imagine how long it would take to figure out AS in every real world condition, and then what would I do with that info?

Who pissed in your cherios?
JohnnyJ is offline   Reply With Quote Quick reply to this message
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