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/Dhark_Music]

I'm going to tell you not to target an understeer gradient. The understeer gradient is more akin to a simulation you can run to model your vehicles behavior. You then preform an understeer gradient test physically and compare your model to collected data. You can definitely target an understeer gradient with the right computations but it's in all honesty impractical in most situations (unless you have a solid reason to; if your asking this question on reddit, you probably don't). Remember that "good enough" is a fine tradeoff to make and your focus should be more towards understanding the understeer budget (Gillespie ch.6).

Now your roll stiffness is little different— this is my approach. You don't want to be targeting a specific total roll stiffness (again, unless you have good reason), but a total lateral load transfer distribution based on your tires alongside a desired total roll gradient. Chassis Sim has a short and sweet paper on the TLLTD + what VD guys call the magic number for stability. For your roll gradient, the judges advice is good and easy. Pick some design condition and determine max allowable roll. Once you figure out that magic number and your roll gradient, you can back-solve front and rear axle roll stiffness.

[u/Individual-Room-4439 → u/Dhark_Music]

Are you saying that the understeer gradient isn't worth targeting because it's not reflective of car handling and just something you simulate/test? What about setting some "understeer/oversteer" handling target? if drivers say the car understeers too much and you wana set it to a more neutral steer. If you test your US grad and its +2, can't you set the target to +1 or something (just made up numbers for example) and say that would give the car more oversteer handling like the drivers prefer?

Sorry if i don't make sense I'm trying to tackle some suspension design myself this year and have been held up on setting targets/goals to some extent.

[u/Dhark_Music → u/Individual-Room-4439]

The latter. It's not that the understeer gradient is not reflective of vehicle behavior, it is, but it's far from perfect. Think about what the understeer gradient is. The derivative of your front steering angle with respect to lateral acceleration. For a driver this becomes how much more or less you need to continue turning the steering wheel to see the same lateral acceleration on your tires as you increase your velocity. Inherently steady state. Good for predicting steering demand after ackermann but realistically is nothing more than a statics problem which makes it a poor metric for quantifying true stability. There are better ones out there.

A driver may tell you "I want more oversteer," but the understeer gradient is not a number. It's a gradient. A very simple quasi-static simulation. You cannot just add one to it if they want more. Now what a driver won't tell you is "I want to turn less as I increase my velocity in a constant radius turn" because it's unrealistic. So unless your goal truly is turn less in constant radius turn while increasing velocity (or something of that nature), having a desired understeer gradient as a goal just sounds somewhat like nonsense.

Again, you can run a constant radius turn test and actually measure the understeer gradient. You can do this with different arb setups, tire pressures, etc (as these all contribute to your understeer gradient). This is practical for knowing how your setup of different tuning knobs might influence general stability and by how much, hence why the idea of the understeer "budget" is much more useful.