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

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Cars that have to accelerate out of low speed turns and mostly operate at lower speeds, especially ones with high power like Justin Pawlak’s Mustang can benefit from careful consideration of their roll centers.

 

Non aero dependent hill climb cars like Rhys Millen’s Hyundai can also benefit from careful placement of roll centers.

 

The converse is true for a short roll couple.  If you have short roll couples, engineers say that you have a lot of geometric anti roll which has the same effect as stiffening the suspension, it increases weight transfer to the outside tire.  This can increase over or understeer depending on what end of the car you do it to.

 

Roll center placement all goes out the window on an aero dependent cars like the Nemo Evo due to the super high spring rates that cars like this must run.

 

The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. Although lowering the center of gravity and increasing the track width are the two most effective ways to reduce weight transfer, over lowering increases the roll couple and dynamic weight transfer.

 

This visually shows what happens to the roll moment on an over lowered car.  For cars that are extremely low, like some race and drift cars, the roll center must be corrected for this reason.  Check out the difference in roll center height vs CG height in lowered and non lowered examples.

 

This can cancel any steady state weight transfer advantage that lowering the center of gravity can have. The huge roll couple created by over-lowering will require an overly stiff suspension to control body movement. And when your suspension is too stiff it won’t absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground nnd you can’t drive fast if your tires aren’t on the ground.  This is called tire shock by us engineers.

 

Really high roll centers cause a jacking moment that can result in the car transferring so much weight that it can actually flip over.  This image is a visual representation of how that could happen.

 

A high roll center can cause the tires to jack and tuck under when cornering hard.  This jacking is very dangerous and is the reason why old VW bugs and pre 1964 Corvairs with swing axles had a disturbing tendency to flip. On most cars the ideal location for the roll center is 2-4 inches above the ground for the front suspension and 4-10 inches above ground for the rear suspension with the rear roll center higher than the front. This is so the car will transfer more weight on to the front of the car due to an increase in geometric anti roll giving a more predictable tendency to understeer at the absolute limit.  Most purpose built racecars are like this.

 

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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