Cross Member Bump Steer Mod

[Twoeightnine]

Ok

I would like to have the bump steer mod performed on my cross member.

Problem is....I dont want to do it!

Any members out there that can do it for me?

I want it done right. Pros please.

[JMortensen]

Are you talking about redrilling the hole as per the JTR instructions, or adding a slot? Don't get too caught up in the precision of the location of the hole if you're just drilling a hole. The Z chassis wasn't all that precise to begin with. If you're 1/8" off on one side it isn't going to make a noticeable difference. The "hard" part is getting the washers off of the crossmember.

 

When I measured where my control arms were when I bump steered them with a slot, I found that I had only moved them up about 7/16", FWIW.

[Twoeightnine]

Yep.

Im at the point were it needs to be painted and thrown back in.

I was looking at it and felt that I would regret not doing the mod.

Sat it on the bench and contemplated how I would accurately place the holes.

After messing with some plumb bobs and laser levels, I actally lost conffidence in the possibility of a dead on placement. Especially inboard/outboard.

Knowing that many racers have done it in the past. And that some have a machine shop. Why try and reinvent the wheel.

And yes, the washer looks like it would be a bitch. Even with the 90 degree drill motor.

Shurely some one can perform the task better than I could.

 

Thanks J.

[Sam280Z]

Use a air hammer with a broad-faced chisel to pop off the washers.

[Twoeightnine]

Yeh, I keep forgetting about mans favorite removal method.....force. Or better yet....the hammer.

I used a large drill bit making shallow passes at the tack weld locations to remove all of the useless bracketry in the engine bay. One of the brackets I simply could not find the tack weld spots so I grabed the trusty vise grips and proceeded to wrestle the bastage off. When it popped off, it had pulled the welds through the firewall leaving really nice jagged holes.

But I realize that the cross member is much thicker.

 

So far I see there are no takers for the job. Im under the impression its not a big deal?

Maybe Im making too much of it?

When I was younger I worked for a time as a precision Machinist and have since then been engineering precision aircraft systems. Perhaps I should knock back the presision side of me. Is that it?

 

Im not really too easy to frighten, I just want them right.

 

J...what type of doubler did you use on your slotted system?

 

Thanks guys for the imput,

[JMortensen]

I used some 12 ga .105" thick plate.

 

I also went to 5/8" rod ends for the control arm so what I did was to strip the paint first. I used a cutoff wheel to cut through the spot welds as I haven't had any luck at all trying to chisel spot welds off, then I opened the stock hole up to 5/8. Then I set it on my workbench and used a square to mark the high end of the slot. Then I drilled that out progressively to 5/8", then I used a dremel with a cutoff wheel to connect the holes. I did the same thing on the plates, then welded the plates to the outsides of the crossmember.

 

I ran my old crossmember without any reinforcement at all, just washers on each side of the slot, but in retrospect that was pretty dumb and I wouldn't recommend it, even though I never had any trouble with it like that.

 

[Twoeightnine]

Thats a mean slot.

Surely you dont have the LCA all the way up there?

Just asking cause I have the JTR and supposedly it's what...7/8"?

 

Looks good however.

 

Well if I dont have a bite tomorow, I'll do it myself.

 

Many thanks,

[JMortensen]

Like I said I was running it 7/16" higher than stock and that stopped the bumpsteer. I originally slotted it as far as I could and I only needed to go up that far.

 

This time I'm going to run it all the way up and use a bumpsteer spacer setup on the tie rod to space the outside end down to compensate. That will give me a higher roll center and better camber gain. That's the plan anyway.

[Twoeightnine]

Got ya.

At least with the slot one can get bumpsteer to zero.

That was the basis of my concern was getting it right the first time.

But I see that getting it there "brings" an improvement regardless if it is dead nut perfect.

The JTR book kinda wanders around the exact measurement if I remember right. They even mention to bring the hole center out a bit to gain built in camber if one wished. Dont have it in front of me at the moment. Care to quote the exact point at which we could expect zero bumpsteer. I have seen 3/4, 7/8, 1, and your original 7/16 inches used.

Or is there even a real zero bumpsteer?

Or better yet, I'll read the FAQs! Hadn't thought of that!

 

Be kind...remember I was originally trying to get a Pro to do it!

 

Many thanks,

[JMortensen]

Got ya.

At least with the slot one can get bumpsteer to zero.

EXACTLY!!!

Care to quote the exact point at which we could expect zero bumpsteer. I have seen 3/4' date=' 7/8, 1, and your original 7/16 inches used.

Or is there even a real zero bumpsteer?

Or better yet, I'll read the FAQs! Hadn't thought of that! [/quote']

No, I don't care to, but I can tell you that on mine that number was 7/16. I was pretty surprised when I did it, because I was expecting 3/4 or 15/16, which were the two numbers I had seen.

 

It's pretty close to a real zero, I can tell you that. I moved my suspension through probably 4 inches of travel with no measurable bumpsteer (measured with dial indicators on the front and back of the brake rotor).

[Twoeightnine]

Very interesting.

I read the entire FAQ for bump steer.

Nice read.

I find your slot teqnique very appealing.

Your thoughts on a slotted mod, approximately one inch center to center, oval doubler inside factory thicknes, factory pivot. Adjust to zero, bolt it up?

[JMortensen]

Very interesting.

I read the entire FAQ for bump steer.

Nice read.

I find your slot teqnique very appealing.

Your thoughts on a slotted mod' date=' approximately one inch center to center, oval doubler inside factory thicknes, factory pivot. Adjust to zero, bolt it up?[/quote']

Yep, that's pretty much it. Easy to do and worked well for me. There are serrated plates that you can buy so that the adjustment can't slip, but I just torqued my LCA bolts every once in a while and they never moved on me.

 

To measure I used two dial indicators one on the front and one on the back of the rotor, took off both front springs and both end links, and set jack stands under the control arms. I then SLOWLY jacked the frame up and down until the toe remained constant on the dial indicators. The dial indicators will still move a lot due to the natural camber and track width changes, so what you do is set them both to 0 then jack the car up or down and you can watch the dials go around at the same rate.

[tube80z]

Jon, that's a really slow way to measure bumpsteer. It's much better to use a simple system of a heavy weight at the bottom, two cheap heims, and a simple fabricated H. The bottom legs of the H are hooked to the weight. You put a toe plate on the wheel and move it up and down the amount needed (I used spacers under the wheel to make this go quicker.

 

No dial indicators are used, you simply eyeball the gap front and back. Make a change and repeat. If you're really anal and want to meausre it you can use feeler guages. Not only is this cheap but you can do a corner in about a quarter of the time.

 

Cary

[JMortensen]

Eh, not something I mess with very often, and I already have the dial indicators and bases.

[BlackBeaut]

Any chance of a pic here Cary? I'm not really following what attaches to what, where the heims come in, or where you measure. So basically, I an't got a clue

 

Cheers,

Rob

[Twoeightnine]

I can understand Jons method as one can do both wheels at the same time and we would see full travel. One question. Do we need to check full travel while (with more than 100% increase in spring rate) the usefull suspension travel is at ride height to an inch and a half above and below?

 

Unless, of course the Dukes of Hazard borrow the car!

[tube80z]

Any chance of a pic here Cary? I'm not really following what attaches to what, where the heims come in, or where you measure.

 

Sure, attached is a diagram. I won't have a real pic until mine comes back. Credit has to go to Brabham. The design comes from the manual for the BT-18.

 

The other cool thing I need to build is flag alignment system.

 

Cary

[JMortensen]

Do we need to check full travel while (with more than 100% increase in spring rate) the usefull suspension travel is at ride height to an inch and a half above and below?

That's why you pull the springs. You want to check at the useful height. I would go from ride height down a couple inches at least.

 

Cary, I had something TOTALLY different in mind before I saw that picture. That actually does look pretty easy to make and easier to use.

[Twoeightnine]

That's why you pull the springs.

 

 

What I ment with respect to springs was, I have 225F/250R springs so the average suspension travel is greatly reduced. Also lowered 3/4 of an inch from stock. Wth that in mind, when adjusting for the prefered Zero Bump steer, should we consider a center datum as the static ride height plus or minus an inch and a half or just go for a full travel adjustment. This is considering that we will not have a perfect zero through out the full travel so would not we want to consider where the car starts from when sprung and resting.

[JMortensen]

You want to test the car where it is going to be driven. If it has 4" of droop that you're not going to be using, don't test there. So yes, test from ride height then down a couple inches.