Position of rack end / tie rod pivot points

[harry767]

Hi everyone!

I'm working on an RWD offroad vehicle with a suspension travel of 150mm. The suspension design (double wishbone) gives me a roll center position which is on the outside of the car (distance between upper & lower control arm pivots on the chassis is bigger than distance between the upper and lower ball joint centers on the wheel). The spread angle is about 9deg. The position of the linking point between steering arm & tie rod is also given (Ackermann geometry). I think that the problem is the different length of the control arms (lower 530mm, upper 330mm). Is it possible two find a correct tie rod length = position of the linking point rack end-tie rod?
Where can I find informations about this suspension geometry problem? I have to reduce the bump steer, actually the toe in is about 7deg!!!

Thanks!

 

[GregLocock]

Draw it at design and jounce and half way between, then fiddle with the end positions of the tie rod to get neutral steer (do that first, fine tune it later). If you look in the FAQ section there is a link to a program called wishbone.bas that will do this on your pc.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[damienmavis]

you've probably solved your problem, but if you havent heres an easy way if you have a simple CAD program. in front view draw the locations of your upper and lower inner and outer suspension pivots and your outer tierod pivot location (on the spindle)repeat this for rideheight full droop and full jounce positions- if the drawing gets clutered the only impotant outboard points are the positions of the outer tierod pivot. once done you will have three outer tierod pivot point locations on the screen. draw a circle with these three points occuring on its perimeter (there should be a command for this). The center of this circle is where your inner tierod pivot shoud be placed for zero bumpsteer in these three situations, check a few more positions to see how close they come to the original position. and if the corresponding bumpsteer is acceptable. Although I havent encountered this problem in practice, I think you could end up with a suspension where there is no inner tierod pivot location which would produce acceptably low bumpsteer. With the IC occuring outbord of the suspension you may have this problem. Another note the less grip you have such as offroad (dirt instead of asphalt), the less the effects of bumpsteer can be felt. I helped build an offroad vehicle for a competition in school several years ago where we screwed up on steering rack location and ended up with tons of bumpsteer. It was fine in the dirt but was scary on the pavement.

Damien

 

[GregLocock]

That works nicely if you have horizontal arm pivots

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

If the FVIC is always outboard of the tire, I wouldn't expect there to be a good general 2-D solution as the tierod arc is convex opposite to that which the steering arm end needs to follow for zero steer. A tall knuckle and longish tierods might get you closer by flattening out those arcs, but I think you need to look for a 3-D solution after including (and tinkering with) caster gain and its effect on steering arm position. GM apparently did.

Norm