The Ultimate Guide to Suspension and Handling: Part 9 It’s All in the Geometry – The Roll Center

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This Nissan Sentra obviously needs some help!

 

This B15 Sentra has had some help.  The outer ball joint has been replaced with a spherical bearing and a long spacer to the spindle.  This drops the outer pivot and raises the roll center even in a radically lowered car.

 

In Dai Yoshihara’s MPTCC championship winning B14 we replaced the Nissan ball joints with some much longer Chrysler parts to lower the pivot.  This required some fabrication as Chrysler ball joints don’t just drop into a Sentra.

 

If you are lucky, Whiteline makes longer balljoints and bump steer correction tie rods for your car and you don’t have to fabricate anything!

 

If you have a Nissan S chassis, Driftworks makes some knuckles that lower the car, correct bumpsteer and fix the roll center.

 

Nissan S chassis guys are lucky.  This car has an SPL lower arm the corrects the roll center as well as a Hariguchi knuckle that lowers the car and corrects the roll center.  Parts like these are how some Formula D cars can be so low and still work.

 

SPL has a rear lower arm that corrects the roll center on the outboard pivot.

 

The roll center can be adjusted on cars like the 240Z-300ZX, 240SX, AE86, Late model Celicas and probably others by using custom fabricated control arms or spacers to adjust the pivot locations of the control arms. SPL, Whiteline, Perrin and a few others make kits to do this for many cars. Remember, if you can adjust the roll center you can lower the car and still reduce the roll couple and lower the center of gravity effectively at the same time.

Adjusting the roll center is a little known but effective way to change you car’s dynamic balance by reducing roll couple and weight transfer. It can do this just as well as springs and swaybars within limits. But remember it is just as important to remember that over lowering a car will create more problems than it solves, unless you can afford the esoteric parts and fabrication to compensate for the geometry issues the over lowering causes.

 

Until next time, happy Motoring!

 

Be sure to check out the rest of the Ultimate Guide to Suspension & Handling series

 

19 comments

  1. If i understood correctly, you want the roll centres close to the centres of gravity, but always below them? I looked at the rear axle multilink setup on my car and it looks like the roll centre, unloaded, is somewhere just above the boot floor. Im guessing the centre of gravity is somewhere there aswell. I have lowered the car about an inch at back because it got very light and skiddish under 0.8+ G braking, now i think the roll centre was going above the centre of gravity. Anyway, problem completely gone with lower and stiffer oem springs. No body roll in the car to speak off, so will not lower the front axle as the control arms are about horizontal already.

    1. You don’t, necessarily want the roll centers close to the CG and certainly not above them because that would induce jacking. Also checking the roll center unloaded is not very useful, it should be checked at ride height.

  2. Unloaded meant without load in the car, it was most certainly at ride height. The car is an alfa romeo giulietta, with a rear setup very close or even identical to a 2013 dodge dart. All the links on the body are higher than on the wheel side and just inches below the boot floor. I ordered the car with a dynamic chassis setup which had slightly stiffer and higher springs at the back. Cars with a heavy glass roof get the same springs to correct their ride height and attitude, but mine being lighter got very dynamic indeed. The front however seems very well sorted

  3. Ok, I looked at the Dart rear suspension and the roll center probably isn’t that high. It looks to be about 8-10″ off the ground on the Dart or just under the midline of the wheel.

    1. I ll try to work out the lines in detail from upper and lower mount, and from center of tyre contact next time its on a 4 post lift. Today i went around a roundabout a few times and between 1 and 1.05G lateral the rear right tyre starts squeeling. I can keep it there indefinitely and ESC doesnt quick in but i would have expected the squeel to start at the front. The rear tyres are new and the fronts are nearly worn so that will have an effect aswell. I will investigate the rear camber and toe settings next to see if i can raise the G limit some. Dont want to mess with it much as its very tolerant in changing direction right now. was thinking to modify the front lower control arms for a bit more track width and caster/camber but will leave it for now or just add a little caster as it would be easiest. Love to see your work here though Mike. Thanks for your time!

  4. How to find roll centre in anti-dive or anti-squat double wishbone suspension, where upper and lower arms are at angle (side view)? Thanks

    1. The roll center isn’t anti-dive or anti-squat and isn’t related. I think you need to read the article because I am not sure you understand what it is.

  5. Hi mike, could you suggest a way (if any) of raising the rear roll centre on a multi link rear suspension, the likes of that found on the mk5 golf?

    1. There a lot of ways to do it but I suggest not trying to mess with this unless you really know what you are doing and even why you want to do it.

  6. Amazing info and very concise and understandable, many many thanks. But, uh….typo? Second to last paragraph says: “If the space between the two lines is less in the front of the car, with an upward sloping Mike axis, the car will tend to understeer …….. If the distance between the lines is greater at the front and less in the rear, the car will understeer…” Should not the last example say “oversteer” and not say “understeer” again?

  7. Roll moment depends on both distance og cg to roll center and distance of roll center to ground .If you lower roll center the distance of roll center feom cg increases whereas distance from ground increases.So will the roll moment increase or decrease?

  8. I lowered a vehicle with McPherson strut by 35 mm does it mean that the roll centre is closer to the C.G now as it isn’t an extreme lowering?

  9. Thank you for the reply mike !
    So if I reduced body roll of a vehicle using stiffer lowering springs, did the body roll decrease because of decrease in load or weight transfer in vehicle? Or is it just because of reduced roll centre to C.G distance?

    Thanks in advance

    1. Mostly because of the stiffer springs. It’s hard to say about the roll couple height without actually measuring whats going on.

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