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Lets discuss roll center in offroad.

Lets discuss roll center in offroad.

Old 03-29-2010, 06:01 PM
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Default Lets discuss roll center in offroad.

There is always a lot of talk around the track and on the forums about turnbuckle position settings... i.e, long turn buckles, which position on the inside, which position on the outside, etc.. etc.. But I get the feeling that not many really understand what is happening when the angle/position/length of the camberlink/turnbuckle is changed.

What it changes, is the roll center on that end of the car. What is roll center and what does it do? Well, the roll center of a car is a spot in space where by which the car will roll around, and is the only point in the car taht stays stationary as the car rolls.

It is determined by a series of virtual lines that are created, when the upper and lower control arms meet in space on the opposite side of the car, and how that point intersecs with the center contact patch of tires (see diagram in link below).

Roll center doesn't do anything by itself, how the car roll depends on the roll center point, AND the car's center of gravity. The farther away your roll center is from your center of gravity (either a higher CG, or lower RC), the more the car will roll in a corner. If your roll center is at the same point in space AS your center of gravity, the car won't roll at ALL in a corner. The more the car rolls, the more weight is placed on the outside tires, instead of the inside tires, and usually dynamic negative camber increases.

So to lower your roll center (increase roll), you increase the angle of your upper control link/turnbuckles (less horizontal, more vertical) which means a higher position on the outside and/or lower position on the inside.

My question to those in the know, is does more roll work in offroad, like it does in onroad? And that is, "more roll will add traction at that end of the car, but it will increase the time it takes for the car to change direction".

A great reference for roll center, (with great pictures), is here:
http://www.rc-truckncar-tuning.com/roll-center.html

My intention is to help people understand what it is they are actually doing, when they change their upper camber link adjustment settings.

Discussions?
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Old 03-29-2010, 06:05 PM
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haha, thats the same site i use for reference. i have another to suggest.

http://users.telenet.be/elvo/

this ones pretty good too. got some good articles on driving and etiquite.
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Old 03-30-2010, 05:43 PM
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This is a great topic... This is one of those things that most don't really talk about and just want someone to tell them how many washers to run under the camber link....
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Old 03-30-2010, 06:07 PM
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I just recently grasped the subtlety of this adjustment and am using it more to fine tune my buggy. Actually understanding what it does and why it affects the handling is primordial to properly using it. Not many people I know understand how it works (heck, can't blame them it's taken me a while to understand myself). But as in all things, I find it's worth the effort as once you do get it you can better integrate it into the other setting changes you consider while tuning!

Cheers Capn for making the effort for people!

Originally Posted by ChrisWolfson View Post
This is a great topic... This is one of those things that most don't really talk about and just want someone to tell them how many washers to run under the camber link....
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Old 03-30-2010, 06:41 PM
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Originally Posted by Cpt.America View Post
So to lower your roll center (increase roll), you increase the angle of your upper control link/turnbuckles (less horizontal, more vertical) which means a higher position on the outside and/or lower position on the inside.
Lowering the inner camber link point will raise roll center. "Less roll"

Raising the inner camber link point will lower roll center. "More roll"

I think any handling changes you feel from moving the upper camber link around are more associated with "Camber Gain" and not roll center. To really make roll center changes you can feel, you need to raise or lower the arm pivot point on the chassis.
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Old 03-30-2010, 07:01 PM
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Originally Posted by ShaunMac View Post
I think any handling changes you feel from moving the upper camber link around are more associated with "Camber Gain" and not roll center. To really make roll center changes you can feel, you need to raise or lower the arm pivot point on the chassis.
Im with Shaun camber gain is much more important. Roll Centers if you believe in them move like crazy with dive and roll anyway. I don't believe its the point about which the vehicle rolls either. It neglects many things...

The problem with roll centers is that two suspensions can have the same roll center but very different camber gain
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Old 03-30-2010, 07:28 PM
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Here's the way i look at it.


Short/angled camber link- That side of the car will snap quickly then stop rolling, So that side of the car will have more initial side bite but then not have as much traction after that.

Long/parallel- With a long parallel link, the car will not snap as hard on initial compression but it will continue to transfer weight until the spring/sway bar/etc. stop it. This will have less side bite initially but have more traction after that.

On the front of the car, adding washers under the inner ball stud or making the camber link longer will make the car have more mid corner off steering, while doing the opposite (shorter link with less washers) will have a lot of snap into the corner but not turn as well off.

Opposite for the Rear of the car, Short link with less washers will turn better mid corner off, As the long link with more inner washers will have less side bite (making the car want to step out while turning in) but have more forward bite.
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Old 03-31-2010, 12:25 AM
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that is why I love my Jammin SCRT10. I can actually play with roll centers etc. So I can adjust pivot angle heights etc.
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Old 03-31-2010, 12:32 AM
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That's classic. The guy posts about roll centers to show how much he knows and gets it backwards. Gotta love it.
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Old 03-31-2010, 01:05 AM
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Originally Posted by ShaunMac View Post
Lowering the inner camber link point will raise roll center. "Less roll"

Raising the inner camber link point will lower roll center. "More roll"

I think any handling changes you feel from moving the upper camber link around are more associated with "Camber Gain" and not roll center. To really make roll center changes you can feel, you need to raise or lower the arm pivot point on the chassis.
The direction the roll center moves, may actually reverse from this depending on the angle of the lower arm as well. On my 1/8 buggy front suspension, lowering the front inner pivot lowered the front roll center. Also as was mentioned, the roll center does move around quite a bit, especially with body roll. In full size cars, a guy who builds some of the fastest autocross cars told me that when he mods street cars, his goal is to make the roll center stay between the tires in roll. On most strut setups, the roll center moves well outside the car when it leans and this makes tuning very hit or miss.

In my case, I needed more front grip, so I went with lowering the inner front arm mounts. This increased the roll torque as the CG is above the roll center. This makes the outside rear tire have to work harder to resist the increase in roll torque, and the inside front tire gets a little easier time and can help more with cornering. The slight increase in camber gain on the front may have also helped with a tick more front grip, but the camber gain from the caster angle is far greater, so this change was trivial in my case.

If the roll centers are too close to the CG (sounds like a good idea to reduce lean) what you end up with is so little roll torque that you can't tune front rear balance with springs or sway bars, they need roll torque to function at all. But even if you have the roll axis lined up perfectly with the CG axis so there is zero roll torque in the suspension, the vehicle weight will still transfer to the outside tires and even traction roll the car if you have enough grip. The roll torque on the entire car is still the CG above the outer tire contact patches.

As an additional experiment, I raised my rear roll center a fair bit, and the car became very loose. The close roll center to GC cause all the cornering force on the back of the car to be resisted just by the outside rear tire without producing roll torque in the chassis, so the inside front got a great bite as the inside rear was basically floating off the surface. I had adjusted static camber to produice very close to the same camber while cornering. Adding some negative camber to the back did not help much, as it then was riding on just the extreme inside edeg of the inner rear tire. In the end I put the rear camber link back to stock, but if I ever need to make it loose, I have a good tuning tool there. There are a couple choices that should yeild a mor subtle change. I like trying a bigger change first off to make sure I see what it is doing, but I also realise, doing too big of a change can overshoot the ideal setting and give reverse results.

Roll center and camber gain has to be treated together. When I did my roll center change, I looked very carefully on how it made the camber change. My upper arm position coices are quite limited, and I had to make the front upper arm longer as I lowered the inner pivot. This reduced the change to the camber gain curve. I also have 3 caster choices. I have it at max now, upper arm as far back as possible. The resulting camber when I am sliding at full lock is very close to zero. My worn tires are nearly perfectly flat across their full width. The balance is now very good with just a little push to make it easier to hold on a delicat line.
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Old 03-31-2010, 01:18 AM
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Originally Posted by GSMnow View Post
The direction the roll center moves, may actually reverse from this depending on the angle of the lower arm as well. On my 1/8 buggy front suspension, lowering the front inner pivot lowered the front roll center. Also as was mentioned, the roll center does move around quite a bit, especially with body roll. In full size cars, a guy who builds some of the fastest autocross cars told me that when he mods street cars, his goal is to make the roll center stay between the tires in roll. On most strut setups, the roll center moves well outside the car when it leans and this makes tuning very hit or miss.

In my case, I needed more front grip, so I went with lowering the inner front arm mounts. This increased the roll torque as the CG is above the roll center. This makes the outside rear tire have to work harder to resist the increase in roll torque, and the inside front tire gets a little easier time and can help more with cornering. The slight increase in camber gain on the front may have also helped with a tick more front grip, but the camber gain from the caster angle is far greater, so this change was trivial in my case.

If the roll centers are too close to the CG (sounds like a good idea to reduce lean) what you end up with is so little roll torque that you can't tune front rear balance with springs or sway bars, they need roll torque to function at all. But even if you have the roll axis lined up perfectly with the CG axis so there is zero roll torque in the suspension, the vehicle weight will still transfer to the outside tires and even traction roll the car if you have enough grip. The roll torque on the entire car is still the CG above the outer tire contact patches.

As an additional experiment, I raised my rear roll center a fair bit, and the car became very loose. The close roll center to GC cause all the cornering force on the back of the car to be resisted just by the outside rear tire without producing roll torque in the chassis, so the inside front got a great bite as the inside rear was basically floating off the surface. I had adjusted static camber to produice very close to the same camber while cornering. Adding some negative camber to the back did not help much, as it then was riding on just the extreme inside edeg of the inner rear tire. In the end I put the rear camber link back to stock, but if I ever need to make it loose, I have a good tuning tool there. There are a couple choices that should yeild a mor subtle change. I like trying a bigger change first off to make sure I see what it is doing, but I also realise, doing too big of a change can overshoot the ideal setting and give reverse results.

Roll center and camber gain has to be treated together. When I did my roll center change, I looked very carefully on how it made the camber change. My upper arm position coices are quite limited, and I had to make the front upper arm longer as I lowered the inner pivot. This reduced the change to the camber gain curve. I also have 3 caster choices. I have it at max now, upper arm as far back as possible. The resulting camber when I am sliding at full lock is very close to zero. My worn tires are nearly perfectly flat across their full width. The balance is now very good with just a little push to make it easier to hold on a delicat line.
Wouldn't that change increase traction rolling. Increasing grip in the front increases traction rolling. Am I correct
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Old 03-31-2010, 11:46 AM
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Originally Posted by ShaunMac View Post
Lowering the inner camber link point will raise roll center. "Less roll"

Raising the inner camber link point will lower roll center. "More roll"
Yes, raising the roll center = less roll, but lowering the inner camber link point lowers the roll center, it doesn't raise it.
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Old 03-31-2010, 11:48 AM
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Originally Posted by rcgod View Post
That's classic. The guy posts about roll centers to show how much he knows and gets it backwards. Gotta love it.
Were here trying to learn about a difficult to grasp subject... maybe you can keep yourself out of the thread considering you wont have anything to add? great, thanks... we appreciate it.

And I didn't make to post to "show how much I know"... I made the post to start a discussion in hopes to spread the knowledge, correct and inhance things about roll center I DONT know, and discuss the best way to use roll center knowledge to improve the way our offroads drive.
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Old 03-31-2010, 12:38 PM
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I have done quite a few plots of roll center, and depending on the angle of the lower arm, it can go either way when you move only the upper arm, so it was not wrong, just too general. If the lower arm is parallel to the ground, he is right, the lower the inside of the front arm gets, the roll center does move up. I didn't expect this either, but try plotting it yourself.
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Old 03-31-2010, 12:53 PM
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Originally Posted by BuggyKing View Post
Wouldn't that change increase traction rolling. Increasing grip in the front increases traction rolling. Am I correct
My problem was too much understeer, I had poor front grip and this change gave me some much needed steering, especially at speed.

My buggy is a bit odd in that it has more rear grip than you want.

Rasing the roll centers at actually a more common cause of traction "trip" rolling. This is what the Corvair horror story was all about. The rear roll center was so high, the car would not lean, but the side load would cause the rear to jack up over the outside tire. As the ride height went up, the roll moment to the tire patch was like a catapult. The car felt rock solid due to the low lean until it was too late. The quickie fix on those cars and other swing axles like VW's back then was an "ANTI" anit roll bar, or Z-Bar. It would restrain the suspension from vertical movement while allowing roll.

You are correct in that too much front bite can cause a traction roll, but it is all relative. If you don't have enough traction, it will not roll. Ideally, we should tune the car to get the most grip we can, and when it has so much grip it roll sover on a 2G turn, then we back it off til it just barely starts to slip sideways before the inside tires come off the ground. Before I changed the front roll center, mine was lifting the insdie rear off the ground and it still would not get loose, just push. I could not add any more rear bar to help. The camber in the turn was about spot on. I even went to less rear toe in and a little positive camber in back, and yet the rear tire (only the outside one) would still not let loose as the front tires washed out wide. I wne down frm a 2.2mm to a 1.6 mm front sway bar, and the inside rear tire just flew higher off the ground. Lowering the front roll center was the change that made it turn.

In case your wondering, it was a nitro that I converted to brushless. It has the motor moved forward so I have a weight distribution of 52% rear, 48% front and has the same tires all around. I am still amazed how it can attain so much grip with the rear tires.
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