achieving ackerman geometry in front steer and minimizing bump-steer

[Ezio_Auditore]

hello!
i am designing the steering geometry of our car for the first time, and i decided on front steer (tie rod outer pivot ahead of the LBJ and below the wheel center). some topics and forums read, that to have zero bump-steer, the tie rod inner and outer pivots should coincide with the pivot points of the lower control arm (LBJ and chassis pivot) but in order to achieve 100% ackerman, the line of the steering arms must intersect on the rear axle, which pushes the tie rod outer joint further into the wheel. now in the front view, the outer tie rod pivot will not be coinciding with the LBJ hence causing bump steer. how do i arrive at a solution ?

also, while deciding the ackerman geometry, do i have to extend the line joining the outer tie rod pivot to the center of the rear axle or do i have to extend a line joining the tie rod outer pivot and the steering axis center point in the top view to the center of the rear axle?

any suggestions in this regard will be highly appreciated.....

regards...

[CameronBeaton]

100% ackerman - for when you really don't want to scrub your tyres on the driveway.

[Sormaz]

witty comments - for when someone shows no signs of independent thought

Ezio,
Try to find a way to quantify bump steer. OptimumK is great at this if you have access to it, but it is also not impossible to do in Excel or Matlab or your coding zone of comfort. Really any kinematic analysis software can do it, or you can just set up a model in CAD and back out results from there.

You are right that it is going to be very hard to achieve your ackermann requirements while maintaining perfect bump steer, but how much bump steer do you really have with your ackermann geo? It really shouldn't be much.

You mention that this is your first time working on steering, is it also your first design project in general? You will find that the majority of design work is identifying trade offs and making appropriate compromises. If you keep looking for the unique solution that is listed in the back of the book, you are going to be at it for a long time

[Ezio_Auditore]

thank u so much somraz for ur suggestions... is it preferrable that i go for rear steer instead of front to minimize the bump-steer while maintaining my ackermann percentage??

[Ezio_Auditore]

my simulations (in optimum k trial version) say that for a bump of 100 mm...the change in steer angle is 0.3 degrees, in the case of 100% ackermann geometry...any suggestions?

[BMEP]

Originally posted by Ezio_Auditore:
thank u so much somraz for ur suggestions... is it preferrable that i go for rear steer instead of front to minimize the bump-steer while maintaining my ackermann percentage??
Both work with some tinkering, I'd say it all really depends on your packaging constraints/preferences. Don't forget about full lock conditions with front steer as you might have to run "J-hook/non straight" tierods to clear the upright. And... the judges will slap your peepee for it

I'm a fan of rear steer due to the overall packaging and typically shorter tierod length reducing the slender member condition. With front steer, Rack extensions might have to be made to achieve the ackermann requirements and limited bump steer but again that all depends on the packaging constraights.

Its an iterative/comprimising condition to say the least!