Not another linked suspension question! Oh nooooooo! Before some smart ass tries to tell me to read the god of suspension thread, :flipoff2: been there done that.

I still need some help with roll axis. I know hot to figure it out (I think). You draw the suspension on 2 planes (top and side). On the top view, you extend out the lines where the suspension links would meet (both of the V's). Then you draw lines straight down to the side view. Go from the rear convergance meets one link, do the same for the other and whalla. Right?

So here's my questions. First, when you look at most linked suspensions (on trucks) the roll axis is at an angle with the front of the truck being the higher side. Do you want the roll axis to be parralel with the verticle center of gravity?

If so, how? Seems like to do that you need to make your top links very high which will either give you a lot of anit-squat or potential for axle wrap (because you would make the lower links higher as well).

Finally, what is roll center and what does it do?

-Jack

I have a printed copy from a guy who did some sort of thesis in school on calculating roll over. I'll see if I can find it on-line, I know it's somewhere cause I downloaded it. :D

Later

Found it. I don't know if this is what you are looking for or not....

Center of Gravity - Roll Over Angle (http://www.jeepaholics.com/tech/cog/)

roll axis and or roll center on a rockcrawler doesnt make that big of a difference build it and wheel it . i find for rockcrawlers people get to technical .

alan

Originally posted by fj junkie
roll axis and or roll center on a rockcrawler doesnt make that big of a difference build it and wheel it . i find for rockcrawlers people get to technical .

alan

How can you say that? Roll axis and roll center are very important in rockcrawling. If you rig has an extremely low roll axis, it will pitch all over the place evertime you get off camber. Thus causing it to roll over. I think the roll axis is one of the most important factors in deciding the design of a good link suspension. Its not something you can just say, oh well, it'll work, to. It's gotta be thought out.

I posted a pic of how I thought it worked on here before but didn't get much input...still don't know if my train of thought is correct or not but I'll post it again...

http://www.darksiderz.co.uk/randd/rollCG.jpg

so roll axis is the line where if you get the line past 90 degrees, you're going over and the roll center is the point at which your truck will rotate from side to side. Right?

Originally posted by jesselt


How can you say that? Roll axis and roll center are very important in rockcrawling. If you rig has an extremely low roll axis, it will pitch all over the place evertime you get off camber. Thus causing it to roll over. I think the roll axis is one of the most important factors in deciding the design of a good link suspension. Its not something you can just say, oh well, it'll work, to. It's gotta be thought out.

im not saying it isnt important , i realize what your saying but have you experienced the extreme at both ends correct and incorrect roll axis ? i have and the difference is not as drastic as you think .
i think that if your having a problem with the rig wanting to lean to much on a side hill your spring rate needs to be checked ..

all four link geometry is important . to me the most important thing to consider is keeping to much anti squat out of it .

i dont have an engneering degree but i have built enough of these things to know what happens with different link angles .
i know what works for good traction . anyone who has seen the four links i have built will tell you they work awesome . they dont win ramp contests but they hook up and work ..

shock angles and spring rate are more important to side hilling then roll axis that is just my opinion .

Okay FJ, let's see the resume (pics). I'm considering doing a 3 link for ease of fabrication and higher roll center.

BTW, I appreciate both of your inputs. You cannot make an informed decision without getting the information from both sides.

I've played with link geometry on a piece of paper and it seems that no matter what I do I cannot change the roll axis that much (the rear point) and the front seems to stay nice and high as long as I keep the lower link at a reasonable angle (which I will want to do to keep the anti-squat under control).

Keep up the info guys. There are a lot of people reading these threads and learning. I know there are a lot of threads on linked suspensions, but there are bits and pieces of new information that come out in all of them. And people are phrasing the information differently in the different threads which gives you a broader base to apply the knowledge. Thanks again...

-Jack

I have yet to see anywhere that suggests the rear roll higher then the front is a good thing.
would still love to see hard numbers on antisquat posted.
or
let us see how the math works out on all these no math link suspensions.

here are a few pix of my work . a couple years ago i spent the time to learn about four link geometry and in building them you just start to get the hang of things without all the numbers.
i always run carpenters string lines to postiton the links so i can put the instant center at the pilot bushing . thats as close to the cog i can guess . that is my first focus .
i build most link set up's with the lower links being 4" longer than the uppers .
i like the upper links to have an intersect point just behind the diff , yes i know it causes some rear steer but it works good .
i cant give you a mathmatical explanation why it works .
just after building these i found it works well in most all conditions.

there are certainly people on this board that have studied four link geometry and have a great understanding of the relation between all these terms . i have a good understanding also but some of the things i do go against what the numbers say .

peace

another shot

full view

i have to dig up some better pix

So FJ, why do you go 4 link and not 3? Also, do you have any side pics to go along with the rears? Good looking work!

-Jack

i have to dig up the side pix .

Not a good one, but here is one for ya Alan.

http://www.sonic.net/jkwaters/images/06-19-2002/729.jpg

And another..................

http://www.sonic.net/jkwaters/images/06-19-2002/730.jpg

thnx jk

i always run carpenters string lines to postiton the links so i can put the instant center at the pilot bushing . thats as close to the cog i can guess . that is my first focus .


so you have, or attempted to have, more then 100% antisquat?
if the IC is at the cg hieght the force vector from the rear contact patch through the IC would have to be way higher then cg hieght where it crosses front axle centreline.
though I think that is an error in the drawing and the force intersection hieght is the number with no relation to front axle centreline which would have you at 100%, still very high
of course there was a discussion (that I never finished reading) that was questioning the accuracy of this antisquat math (http://community.webshots.com/photo/19011079/30642696abWUZlmXsO)

this seems to contradict your statement

to me the most important thing to consider is keeping to much anti squat out of it .

which is why I asked for hard numbers as to what means "too much" as it is so subjective.
if everyone that claims they dont need math because their shitte works would lay out the math perhaps some advances could be made in the application of the math to our stuff.
then we all win

what im saying is i like the instant center to be no higher then the cog at ride height it's hard to get it to low but it's easy to get it to high .
seeing as you seem to be a mathmatician help us all out ..
im not gonna argue with you mj you bring up valid points i learned what i know from practical application . not from crunching numbers .

fj junkie,

To me it looks like you have way too much antisquat happening.

How much does this setup hop when spinning the rear wheels pointing up hill. Can you just throttle it and smoke up the rears with no bouncing or does it start to bounce if you spin the rears when your rig is pointed uphill??

I think that the most important thing is to get as little antisquat as possible. Cause if you build in some anti squat it may work well on the flat or pointing uphill at say 20 degrees (doesent hop as it is throttled) but as you drive on steeper and steeper stuff the rear springs start to unload (cause there is less weight on them) which gives you more antisquat (cause the rear is effectively at a higher lift) and sooner or later its going to start to hop.

There is nothing worse than a rear linked suspension that hops when throttled with wheelspin. And the bigger the tyre the harder it is to control. IMO any antisquat is a bad thing for driving in the rocks.

Sam

no wheel hop at all and there is no lift at all when the throttle is mashed . it is very neutral ..how can you tell from the pix posted that it has to much ? btw i dig your rover

Originally posted by fj junkie
no wheel hop at all and there is no lift at all when the throttle is mashed . it is very neutral ..how can you tell from the pix posted that it has to much ? btw i dig your rover

fj junkie,

Ah - my mistake, sorry about that. The pic that I was really looking at was the second last one and the angle of the links looked very steep but now I see in the last pic that the rig is crossed up (one coilover is extended and one compressed) and I was looking at the extended side.

Looking at the third picture (full view) I can now see that the angles of the links are really good (as opposed to my original thoughts based on the second last picture) they are very flat and converge slightly.

Back to the roll centre height question - what do you think of triangulating the links on the bottom of the diff (giving a very low roll centre) say on the front diff (where triangulating the uppers is hard to do. Having the roll cantre this low would have to be a bad thing??

Sam

what im saying is i like the instant center to be no higher then the cog at ride height it's hard to get it to low but it's easy to get it to high

so, less then or equal to 100% but not more.
I have never seen anyone ever suggest anything that high, 39-50% is the starting point that the books suggest for car.
the rear may get real light under braking with that high AS

Here are the definitions for roll center and roll axis

The roll center is the point which the axle rotates around. I is also the theoretica point where the unspring mass (axle, wheels, etc) connects to the sprung mass (engine, body, etc). So when the body rolls it rotates around the roll center for each axle. If you connect the front and rear roll centers you get the roll axis, This is the line which the body rotates around. There is also each axle (front and rear) has its own roll axis which can be used to adjust the oversteer/understeer (loose/push) balance of the car.

Some of you are wondering what the numbers are on some of these rigs where they just used a few rules of thumb. I have a program to calculate IC and antisquat and will gladly do it for anyone if they give me their numbers. That way you can see what these guys actually did.

Try and get your roll axis as parallel to the ground and as close to the COG as you can.

You can do that by either making the top links higher on the axle or the lower links lower on the chassis. Don't really use the COG as a target it is not really possible unless you have as much unsprung mass as you do sprung (Rockwells come to mind).

The farther away the COG is from your roll axis the more pucker factor you will get in off camber situations. If you were to have your roll axis in the same vertical plane as the COG you could lay the body over onto the tires with no big change in the location of the COG and so it would feel much more stable.

I too have noticed that most mass produced rigs have a roll center that is pointed upwards in the front, dunno why they do that but I know it is a PITA for me to not do it ona stock TJ chassis without killing my breakover angle but having the lower links that low.


Do a search on username PIG and read everything he has had to say, I too have been in this discussion quite a bit but I don't have nearly the knowledge he does.

Originally posted by JEEP_TJ_FREAK
Try and get your roll axis as parallel to the ground and as close to the COG as you can.


Yes, don't forget to take in consideration anti-squat. I have seen people set up their links perfectly for roll axis and placement near the cog. However, they then will get almost 0 antisquat. You want a little bit of it, its good for ya. Sounds like you got it down though TJ_Freak.

Mine's done now and been on some trails. I have very little roll steer, about 1/2 to 1/3rd what I had with long leaves and it's a bit different to drive. It wasn't a huge thing, just apparent that it steers less.
Here's a pic:
pic of links (http://www.offroaddesign.com/TopTruck%20K5.htm) You can get a rough idea of how they run. Lowers start just above the tube and run to the center of the crossmember under the t-case output. Uppers start about 8" higher and run along side the frame rail.

I need to re-calculate the anti squat around what's built compared to what I planned out but it should be around 70%. Doesn't jack up under power (front brakes locked, gassing it), no hop problems, behaves itself on hills, overall I'm really happy.

I have seen a similar vehicle with VERY high anti squat and it was a good thing he had a center limit strap because he was stretching it out every time he used the throttle.

Now, based on lots of research and building one suspension (take it for what it's worth), I recommend keeping the roll axis fairly level and keeping a moderate amount of anti-squat. That keeps the roll steer from getting wild and 50-100% anti-squat seems to be about what everyone is aiming for.

Couple observations:
Roll steer is only right 1/3 of the time. If it doesn't steer, 1/3rd of the time that's perfect, 1/3rd of the time you'll want it to steer one way, 1/3 of the time you want it to go the other way. Someone else here threw this out and my experience shows it true. BUT, a lot of roll steer will get you in trouble more often than not I think, just because it will move the vehicle more than you think.
Spring rates are really important.
Sooner or later, you just have to give up and build your best guess.

Not another linked suspension question! Oh nooooooo! Before some smart ass tries to tell me to read the god of suspension thread, :flipoff2: been there done that.

I still need some help with roll axis. I know hot to figure it out (I think). You draw the suspension on 2 planes (top and side). On the top view, you extend out the lines where the suspension links would meet (both of the V's). Then you draw lines straight down to the side view. Go from the rear convergance meets one link, do the same for the other and whalla. Right?

So here's my questions. First, when you look at most linked suspensions (on trucks) the roll axis is at an angle with the front of the truck being the higher side. Do you want the roll axis to be parralel with the verticle center of gravity?

If so, how? Seems like to do that you need to make your top links very high which will either give you a lot of anit-squat or potential for axle wrap (because you would make the lower links higher as well).

Finally, what is roll center and what does it do?

-Jack
theres no easy way to describe what the roll centre is but what it does is a bit easier, in short the roll centre will apply weight transfer to the controll arms instead of the springs, eg the higher the roll centre the more force is applied to the controll arms and less on the spring, the lower the roll centre the less force is applied to the controll arms and more on the springs, if you have a high roll centre you will get less body roll and the lower roll centre you will have more body roll on each car you will have to find a happy medium for track conditions that is a very breif run down on roll centre. as for the roll axis the roll axis i quite easy its the point or the imaginary line intersecting the front and rear roll centres, so once the front and rear roll centres are found from a side veiw draw a line starting from the rear roll centre to the front roll centre and this line is the roll axis, now if you want to say raise the roll axis at the front you will have to raise the front roll centre on another note this smart ass you speak of is this person on another forum:smokin: :smokin: