View Full Version : bump steer

11-03-2007, 10:12 PM
I hate to split hairs, but we figure bump steer with the wheels pointed forward in a straight line. I understand this concept. However, once you put steering into play, like through a corner, all your steering points change. Won't a car with zero bumpsteer aquire changes in bumpsteer once body roll and turning radius are figured in? Does more ackerman angle make this worse or better?

11-06-2007, 12:28 AM

Norm Peterson
11-06-2007, 09:01 AM
Actually, I'm pretty sure that the various NASCAR teams test and tune with just that thought in mind. Probably including bump travel as well. Ultimately, bumpsteer is a 3-D problem involving the intersection of two arcs of different radii and is unlikely to be exactly zero at more than two suspension positions. Ackermann and caster do not remain constant with steering and/or bump travel either. So it makes some sense to tune for minimal bumpsteer at some point while cornering where you don't want the lateral tire forces varying very much unless/until you intend them to. It does help that conditions of 'toe' (using the SAE definition of toe as the amount(s) of steering at the steered wheels rather than the static alignment setting term) and bump (think nose dive under braking) are unlikely to vary as widely during oval-track racing as they would in a performance road car, road-race, or autocross environment.

I think you'd have to have a pretty good idea how much steering input you want to optimize bumpsteer for, and that up around the limits of lateral grip where a couple of hundredths or so of a lateral g becomes rather more important. Otherwise it would probably be better to set a curve with less bumpsteer in 'bump' than on the 'rebound' side, as the wheel in 'bump' should be carrying the greater load and would have more say in where the front end is going to go.


11-06-2007, 09:22 AM
Looking at a rear steer application with ackerman like a 1st gen camaro, the inside tire on the corner turns in more than the outside tire resulting in toe out during cornering. (one reason why I run toe out vs toe in- the car gets twitchy at the zero toe point.) http://www.auto-ware.com/setup/ack_rac.htm

Ref stock cars- I can recall setting up Busch cars years ago and we always had a bit of toe out in compression - the cars did handle better - part of the thought was to let the inside tire turn in sharper than the outside tire.

11-06-2007, 11:50 AM
This is something I like to see on these boards. Toe change during dynamic events (meaning something other than bump and rebound) is more important, in my opinion, than simple bumpsteer calcs. Sure, I set the steering geometry to have .020" or less (.010" in most cases) in bump/rebound, but I spend WAY more time analyzing what it does in a driving situation. In order for me to stamp "OK" on a suspension design, the toe change (as well as many other factors) must be satisfactory in these conditions:
1. Bump/rebound
2. Full acceleration
3. Full braking
4. Corner entry (some body roll, light braking)
5. Full cornering (full body roll, zero acceleration)
6. Corner exit (some body roll, moderate forward acceleration)

So the key is optimizing the numbers in all these driving situations, keeping in mind what a driver likes to feel. Do we want it to toe out, or toe in? Why? What is it going to feel like to an everyday street driver? What would a highly skilled driver want?

11-06-2007, 12:09 PM
matt, good info, what are the morrison 1st gen clip bump steer numbers, in normal bump and dynamic situations? also what other info canyou giver such as camber gain, instant center, roll center etc.