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Bush Mechanic bump steer test


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I'm in need of a bit of advice from those who know a lot more about bump steer than I do.

Visually the pivot points on my steering rack ends form a straight line between the pivot points on both my upper and lower suspension arms, so I know that I am somewhere in the ball park, but decided to seek a bit more confirmation by actually putting the suspension through its paces.

So in the absence of any specialist tools I've completed the following rudimentary test:

With my steering rack and all other front suspension components except for my shocks and springs in place, I have used a trolley jack under the lower suspension arm to raise the suspension from maximum droop to maximum compression. I did this test in turn on both the left and right hand sides, but not simultaneously. During each cycle on both sides I closely watched the steering shaft input spline (I don't currently have the steering column attached to the rack) and in all instances the input spline did not budge at all. Since I have already confirmed that there is no play in the steering rack itself, is it safe for me to assume that I am relatively in the clear from a bump steer point of view ?

Please feel free to shoot my simple theory down in flames .... I promise I wont be offended. 

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If it were me I would take the springs and shocks out (unless they are a locating member obvs), then I would clamp something long and straight to a hub and fix it dead ahead, then at a height halfway between full compression/droop.

If you don't do something like this then the hub can swivel unbeknownst to you and not turn the steering wheel.

Edit; I misread, ignore the removing springs and shocks bit.

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Have you seen the LVVTA bumpsteer docs on this page? https://www.lvvta.org.nz/documents.html#infosheets

There are two of them almost half way down.

I made a more budget version of this bumpsteer gauge. Make your pins 575mm apart so you get the same readings as you would with the LVVTA tool.

This lets you do one side at a time. The LVVTA device requires you to measure both sides simultaneously.

386140597_TheBump-Steertestjig(3)(Medium).JPG.499b0e357f965c95ed5766298cf18b30.JPG

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Pretty easy to do with even something as basic as 2 bits of square tube or angle iron clamped to the discs 

Doesnt have to be fancy, just consistent 

I've never done 2 sides independently so cant comment on how well that works, but if the rack is level and centered correctly it wouldn't be necessary to do individual sides separately  IMO 

The good thing about doing it as per the lvv info sheet is it gives you an idea of which way you need to move things if you have a problem 

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Thanks very much for the additional reading material and detail @cletus. Thanks also to @adoom for the pic of your test setup. @ThePoghas also made some good points. Thanks fellas, I really appreciate the info and advice. 

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After reading through the info sheets provided by @cletus and spurred on by his encouragement I spent a bit of time yesterday building a rudimentary replica of the LVVTA bump steer test kit. After setting my caster and camber this morning I cracked into some bump steer measurements. The results indicate that I am getting a maximum of 1.6 degrees of toe in under full compression and 2.0 degrees of toe out under maximum droop. Based on these results the LVVTA notes say that my tie rod is too high. I'm clueless as to how I can fix this, so would appreciate some guidance from those in the know.

Ta in advance.

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Looking at your pics in your build thread it may be easier to move the rack a bit.

You will be surprised at how little you need to move stuff to make a big difference, rack might only need to go up 10mm 

*edit* as adoom said 

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Thanks for the advice @cletus and @adoom

I really appreciate your guidance.

Yikes, I was hoping you weren't going to say that I need to lift the rack. I've got the engine out at the moment but from memory I've only got about 10 mm clearance between the top of the rack housing and my harmonic balancer and I can't raise the engine anymore without causing clearance issues between the top of the carby and the underside of my cab floor.

I'm thinking I should get the engine and gearbox back in and then see if I can tweak things a bit more.

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I've taken a few quick snaps for you Clint. These were taken with the suspension arms set at normal ride height. I really appreciate you taking a closer look for me. Shout if you need anything photo'ed in more detail.

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Ok couple of things. 

1. As Kyteler mentioned, the rack doesnt look straight, the angle on the rack end is greater on the passenger side, need to make sure it is level/same both sides

2. Make sure the tie rod ends are the same, and are tight in the steering arms so the pivot points are the same height/ distance from the steering arm on both sides 

3. Have you swapped the spindles around to make it front steer? One issue with that is depending on the geometry of the original suspension, that can stuff up the Ackerman angles and give it "anti Ackerman " which causes scrubbing on tight turns. I dunno what the rules for engineering are in Australia but you might want to talk to your engineer about that. I cant tell from the pics if that's going to be a problem or not but something to keep in mind. You can check it with your bump steer bars by making sure it toes out,  at full lock. Or have you fitted the later model spindles? 

4. If after all that you check it again and the tie rod ends are still too high, the good thing is that's the easiest problem to sort as you might be able to find a tie rod end that has a different height pivot, or convert to a spherical bearing/heim joint/rose joint with the appropriate length spacer, or you might be able to move the lower inner arm pivot point by re drilling the hole for the lower arm bolt 

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Thanks for the feedback fellas. I really appreciate the pointers. I will double check things to make sure that the rack and the rack ends are the same level on both sides.

Clint, in terms of the spindles and orientation of the steering arms I haven't changed anything. The L300 SD series front beam is setup as front steer from factory. All that I have done is replaced the SD series drag link steering with the rack and pinion from a newer generation L300.

Thanks also for the suggestions on how to address the tie rod position. Being so "space poor" I reckon that would be the way to go rather than trying to raise the rack.

Thanks again fellas. I'm kinda isolated out here and am so grateful that I have access to the wealth of knowledge that you and other old schoolers so generously share. 

 

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