Bump steer and its effect

[Alonso]

Hey guys your going to get so sick of me but what can i say. Could you please give me a better understanding of what bumpsteer is and how it will work on a single seater double wishbone,pushrod double shock suspension.
Thanks Alonso

[RacingManiac]

the simplest explaination is that bumpsteer is the kinematic toe change throughout the suspension travel. If the toe link moves out of plane in the range of the wheel travel the link will either pull the wheel in or out as it moves, changing the toe. they can be designed in to provide some 4 wheel steering effect in the rear to give you some additional control for rear slipangle, and perhaps to create some stablizing effect as the car dive or squat to give you more stability during those events. Too much will again make the car difficult to drive as the wheel will change its toe if the corner was upset by some bumps or curbing on track. For our FSAE car we use to try to minimize it as much as possible as we design the kinematic layout. It is impossible to eliminate it completely because mechanical compliance will introduce some bumpsteer effect as well...

[Jersey Tom]

I would generally try to minimize bump- and roll-steer except under specific applications.

I'd want to the driver to steer the car, not the car to steer itself.

[ReubenG]

Bump steer (in laymans' terms) is the front wheels changing their steering angle as they move up and down. RacingManiac's explanation is pretty much spot on what I was taught. On a vehicle with lots of suspension travel it's fairly easy to spot - push the nose of the vehicle up and down and you will see the wheels toe in and out. I would hope that on a single seat open wheeler ( which has very little suspension travel), the bump steer would be barely visible.

On double wishbone suspension (doesn't matter whether it's push-rod or outboard shocks/springs) the steering arms are generally placed in the same plane as one of the wishbones - anywhere else and you will have lots of bump steer. If you line up all the hinge axes (upright / outer wishbone / outside steering arm and chassis mount / inside wishbone / track rod) you can get "zero" bump steer when the wheels point straight ahead - but the moment you turn the wheels and the suspension moves up or down there will be some bump steer due to the changing radii...

@ Alonso - I first encountered linkage modelling in 2nd year mechanical engineering, and it is fairly useful for describing suspensions with theoretically rigid linkages. If you have high school trigonometry and can work the trig functions in an Excel spreadsheet you can put together a 4-bar linkage model which will show you camber change, lateral scrub etc. Accounting for chassis roll is slightly higher grade than 2nd years do but so long as you assume rigid links it's not too bad.

[The_Man]

So if can can't avoid Bump Steer then I should have a rear suspension geometry such that there is toe out on bump.Right? Because while cornering then I would have an effective ackermann steering in the rear due to Weight transfer. Assuming the amount of toe out increases with the bump displacement. Also the car will be stabilized during accelerations as the rear will toe out during the squat.

Does this work this way? Or am I way off!

[RacingManiac]

So if can can't avoid Bump Steer then I should have a rear suspension geometry such that there is toe out on bump.Right? Because while cornering then I would have an effective ackermann steering in the rear due to Weight transfer. Assuming the amount of toe out increases with the bump displacement. Also the car will be stabilized during accelerations as the rear will toe out during the squat.

Does this work this way? Or am I way off!

That's about right idea wise, depends on what you want to do. You can also say to build in toe-in on bump if you think that you might have more compliance induced bumpsteer that toeout the wheel under acceleration. Engineered compliance/bumpsteer is common place I think in production vehicle due to the compliant bushings and such, and you design your kinematic to compensate. But I think they tend to be less desirable on racecars, at least to my understanding of it. As said though, you can indeed minimize bump steer, especially in the rear since all the points are fix(no steering rack movement), if you align all the pickup points and make your toe and a-arm inplane the bumpsteer can be minimized on design, and in which case the compliance will be the one you worry about...