Practical limit to bump steer adjuster length? 4

[Mr168]

In a road-racing only situation (I know, this is a "bring me a rock" question with respect to variables, so hoping someone has feedback of collective experiences or "conservative" values), is there a practical limit on the length of bump steer adjusters, i.e. lowering the connecting point of the outer tie rods at the spindle? This is a strut front end car, with a K-member which serves as engine, rack, and lower control arm mounting point.

Current situation is that steering rack interference requires relocation of the rack down almost exactly 1.5" due to engine clearance issues. Oil pan is already at bare minimum required internally for rod/crank clearance, rules prohibit modification of the firewall/tunnel to create the required engine and transmission clearance. Consequently, I'm looking at requiring 1.5" of "relocation" at the outer tie rods to maintain that geometry, and was hoping to do so using bump steer spacers and heims in lieu of the factory ball joints, but am questioning whether that is within the realm of "typical" for most sub-3000 lbs race cars, or if bending (or any other negative side-effects that don't immediately come to mind) become a concern at that length.

If that is of "questionable" height for the bump steer adjusters/hardware, I have no qualms over modifying the spindles to lower/gusset the spindle arm to reduce the amount of spacer necessary, but that is obviously a more time-consuming and labor-intensive approach, and consequently, I would prefer to avoid. I'd also like to avoid the route of shortening the rack if at all possible, as this would create and even greater clearance issue due to the large, semi-circular cast shape in this particular rack housing which acts as a stop/support for the rack bushings/clamshell mounting brackets.

Thanks in advance for thoughts/feedback/experiences.

 

[GregLocock]

You'll be introducing compliance into the steering system, as the arm from the OTR to the spindle will twist more. If it's a steel part that may not matter, but if it is forged aluminium then I'd guess you might not like it.

Personally I think you'll get away with it tho I've never had more than 1/4" of spacers at the OTR. You may be able to find some guidance on whether the taper in the steering arm can cope with the additional writhing moment caused by the increased offset.

Cheers

Greg Locock


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[Mongrel]

Perhaps I'm misunderstanding, but - why do you need to lower the outer tierod ends? Just lower the rack, mount it securely, and shorten the tierods by the appropriate length. Or does this create a fouling problem? I can't see the tierod angle changing a lot, unless they were already on a tight angle to begin with?

 

[BrianPetersen]

^ Doing the above - lowering the inner tie-rod ends without also lowering the outer tie-rod ends - will change the angle of the tie-rods with respect to the control arms, which will cause bump-steer.

It doesn't take much steer angle of the wheels to start doing bad, bad things to handling and stability.

Operating on the assumption that this is a front-drive car of the usual layout, with the steering rack behind the drivetrain ("rear-steer"), then lowering the rack without correcting the tie-rod angle will cause toe-out with bump travel, and that will lead to substantial roll understeer.

 

[Mongrel]

After thinking about it - yes, I can see how that would work. Appreciate your time in correcting me!

 

[Mr168]

Brian - in this case, it's a front steer configuration, but bump steer is still the offending party as noted.

Greg - appreciate the feedback. They are factory steel spindles. Drilling out the taper to accept a full-length adjuster with a double locknut configuration is an option in this case, but that is a point I had previously not considered.

 

[Mr168]

One additional thought - drift cars frequently use an offset inner tie rod adapter which locates the inner tie rod pivot point forward of the original location to correct over-center conditions as their increased steering angles. It may be possible to lower the rack, but fabricate an inner tie rod adapter that moves the pivot point up instead of forward, at the risk of losing a minimal amount of steering travel (I already rub the inner rail at full lock anyways).

Any thoughts as to the merit of this idea, as opposed to relocating the outer tie rods down? Splitting the difference is also a possibility.

 

[GregLocock]

I've seen that done. I'm not a huge fan, bending the rack is not a good idea, the mesh of the rakc and the pinion is a delicate thing.

Cheers

Greg Locock


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[BUGGAR]

I may be facing a similar situation in something I am building and I am planning on using center link steering. This lets me use a center link that fits through tight chassis parts and I can raise or lower, lengthen or shorten the link for tuning bump steer. I plan on running a rack and pinion above, where it will fit, and linking it to the center link. Kind of like using a travel bar on the r & p.

 

[Mr168]

Thanks, Greg, hadn't considered the mesh on the R&P. After tearing apart the front end last night and removing the rack mounts, it looks like I can get the necessary clearance by relocating the rack mounts outboard (gives me an additional 1/2" by itself), lowering the rack within the limits of the K-member (another 1/2" - 5/8"), and moving the engine mounts up about 1/2". Splitting the difference should let me get by with a much shorter bump steer adjuster and no impact to the firewall/trans tunnel as there would have been if I had to move it the 1.5" needed.

Buggar - not sure if you've watched Project Binky, but they had to get very creative to address bump steer on their Celica-to-Mini swap. May be worth watching to get some ideas if you haven't seen it already.

 

[GregLocock]

Incidentally if you do start bending your rack (elastically) then the yoke adjustment screw will let you ease up on any binding, at some cost in on centre precision.

Cheers

Greg Locock


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[cibachrome]

... and the high risk of "jumping a tooth".