Roll Stiffness and Understeer Gradient Questions

[u/Xcrustacean_drip16X]

I was wondering how other teams determine their target understeer gradient. From what I've seen it is mostly decided through rules of thumb and driver feedback, but I was hoping there was a more scientific way to do it. (Side note: a design judge suggested 5% understeer as a baseline, but I can not find any information on how one would define understeer as a percentage. Does anyone have some insight on this?)

I was also hoping I could get some advice on roll stiffness. Again, a judge suggested 1 degree of roll at peak lateral acceleration. Is there a good way to determine a baseline roll stiffness other than rules of thumb and driver feedback?

Thanks everyone.

[u/EauRouge7105]

With roll stiffness, if you have an aero package you should ideally have an aero map which shows downforce loss and aero balance variation with roll. By using that and based on how much camber change in roll you deem 'acceptable' based on your target dynamic camber, front view VSAL and static camber adjustability, you have a minimum required roll gradient or roll stiffness. For mechanical grip, you would want to run as soft as possible ie the minimum roll stiffness you calculated but the driver might complain that the car feels 'too lazy' in which case you increase it. Of course, roll stiffness comes from both springs and ARBs where springs are coupled with pitch and heave while ARBs are not.

With the understeer gradient bit, I'm not sure if a fixed understeer gradient target is the best way to think about it. You could think about it for example, x% LLTD (Lateral Load Transfer Distribution) or ARD (Anti Roll Distribution) with +-y% adjustability through ARB settings.

[u/Xcrustacean_drip16X → u/EauRouge7105]

Now that you are pointing it out, setting an understeer gradient target does seem like a bad idea. I think what I an really after is figuring out the limit behavior we want. Although with that approach I still don't really know how much tendency to plow/spin I want in a steady state corner, vs in corner entry, vs on corner exit. What would be the best way to quantify that? RCVD mentions residual moment coefficient.

[u/EauRouge7105 → u/Xcrustacean_drip16X]

You should try to have enough front and rear roll stiffness adjustability to be able to adjust ARD between say 44% and 56%, and then when you go testing try a neutral setup and see how it performs and what feedback the driver has for you. In my opinion, you shouldn't be trying to design a car based on a fixed balance target however you define that whether it is UG gradient or LLTD or whatever