Preload and bump stops

It appeared there was some speculation or confusion elsewhere as to the impact of spring preload on handling. Bump-stops are related. Both are important parameters. Unfortunately I don't think we really appreciated what they all did when I was on FSAE.

Balance is essential to good race handling. Balance inevitably comes down to managing dynamic tire conditions. Load is one aspect.

I think most people get the basics. More front bar (or spring, or damper [momentarily]) = more lateral load transfer relative to the rear = more understeer. The opposite is true for adding rear stiffness.

If you were to put your race car on an SPMM, you could generate a plot similar to what's below... looking at lateral load transfer as a function of chassis roll angle. Rough indicator of cornering balance. Ideally you'd want load transfer as a function of lateral acceleration (Ay)... the two plots are related but not the same.


Anyway. This might be a typical baseline setup with no preload, not hitting bump stops, and linear installation ratios. The slopes, and difference in magnitude between front and rear load transfer are what's important. In this example it's pretty straight-forward. The front has a higher roll stiffness, takes a higher percentage of the load transfer, and all other things being equal (50/50 corner weights, same tires all around) the car would probably understeer slightly. The feel should be very progressive and predictable.

If you were to add a hell of a lot of front preload, it may look something more like this.


If your tires and suspension were infinitely rigid, that initial slope would likewise be almost vertical. The front suspension is locked. Since the tires have a spring rate and inevitably there's compliance in your suspension, the rate is just very high.

There was an article in Racecar Engineering that (I believe) claimed lots of front preload would help a car "cut in" on entry from extra heat into the outside front tire. I find that hard to believe. It's no different than having a stiff anti-roll bar that "switches off" after a given amount of displacement. There are other reasons I won't get into, and I can also vouch for SPMM plots.

Until I see instrumented track data that proves otherwise, to me, I'd think front preload will "numb" the handling on-center.

Bump stops I am damn sure of, and the way they behave backs up my thought on pre-load.


If you roll onto a bump stop in the rear, your rear roll rate is going to increase substantially, very quickly, and rapidly shift the balance to oversteer.

The point of all this being, unless you manage it all carefully, going overboard with preload and bump stops and what have you can make for really non-linear or bizzarre handling. If you don't need it, or can't justify the reasoning for it... why bother?