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Tuning out bump steer

Tuning out bump steer

Tuning out bump steer

I've only just started playing with bump steer so please excuse my basic knowledge. I'm designing my suspension geometry almost from scratch and I've got my suspension geometry in Solidworks and Lotus Suspension Analyser (nice and simple software). Minimising bump steer is one of my objectives for my 4x4 buggy as I'm seeing that other off roaders suffer from it quite badly.

As I see it so far: There's limited space inside of the wheel to adjust the position of the steering rod end at the hub. So I need to move around my steering rack to a position that eliminates bump steer. But moving the steering rack end of my steering rod requires that I have a custom steering rack length? So I can't just use any old steering rack from my donor car or the junk yard. So what do most people do in this situation? Do I need to custom order a steering rack to the right length? That is probably going to get costly then.

I know I also need to look at the Ackerman geometry and steering ratio.

Thanks for the help as always!

RE: Tuning out bump steer

Ya gotta do what ya gotta do.

Moving the rack up and down (which is what primarily affects bump steer) should not affect the required length of it.

It isn't necessary to get it "perfect" towards the extreme ends of the suspension travel, and most vehicles don't have it "perfect". It should be as good as you can get it when it is at nominal ride height and for the little bit above and below that (whatever range you foresee for normal suspension movement).

Toe-out on bump is the direction that leads towards roll understeer. You don't want roll oversteer, so if you are going to have known imperfection, try to make it err in that direction.

Bear in mind the fore/aft compliance effects as well. The foreseeable direction that your control arm deflects when it gets a whack from the front from hitting a bump should be parallelogram with your tie rod so that it doesn't affect the steering.

I spoke to someone who had a (rare) FMV motor home as a race-bike haul vehicle, last summer. When he rebuilt and restored it, he redesigned the steering completely because the stock geometry was so far off. In stock form, he said that after you turned the steering into a corner, you then had to unwind the steering by more than the amount of your initial steering input, which made it absolutely terrifying to drive above 80 km/h. That's roll oversteer for you ...

RE: Tuning out bump steer

I'm surprised more people don't use a travel bar on their rack to set whatever length is needed. I agree with Brian that bump steer is most critical at speeds where your wheels won't be turned sharply.

RE: Tuning out bump steer

To a first approximation

your ITR needs to be on the line joining the LCAU and UCAI in front view.

Your OTR needs to be on the line joining LCAO to UCAO.

Then the lines joining OTR to ITR, UCAI to UCAO and LCAO to LCAI should all meet at one point.

That gets you in the ballpark. Now you need to start calculating bump steer, and fine tuning OTR and ITR, including ackerman. Move ITR and OTR inboard or outboard together to maintain bump steer while adjusting for rack length, or use rack extenders, or don't use a rack.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Tuning out bump steer

And depending on how high you mount your rack, bump steer can go out of adjustment every time you re-set camber and castor.

Also, only center link type steering with idler arms' axis parallel to the spindles (steering knuckles) axis in both planes can truly replicate spindle motions at the end of the steering arms for theoretically no bumpsteer in any wheel position.

For entertainment: Early cars were designed so bumpsteer would pull them to the right on bump and extension to avoid head-on collisions. Only in the US of course.

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