Should I relocate the crossmember pivot points? (I have searched)

[tonycharger72]

jmortensen

 

I was reading through your bumpsteer faq and wondering about something, im copied and pasted some extracts,

 

"The next most common modification is the JTR crossmember modification or some variation of it. This involves redrilling the control arm hole up ¾†or 15/16" and out ¼†in order to reduce bumpsteer and add negative camber at the same time. Raising the inner control arm pivot is in essence the same thing as installing a ¾†bumpsteer spacer. Moving the pivot out ¼†will gain more negative camber, but it also exacerbates bumpsteer, because now the control arm length is now shorter than the tie rod length. Again, this approach will be helpful to the roll center, but will not “cure†bumpsteer."

 

"Now we’re into the not-so-common methods for dealing with bumpsteer. One method I have used is to slot the front crossmember. I did this by drilling a hole directly above the original control arm pivot hole then connecting the two holes with a cutoff wheel. I then measured the bumpsteer with a gauge and adjusted the pivot up until it was at 0. I've run my Z this way for more than 5 years, autoxing and doing track days on slicks and have not had the pivots move at all. "

 

 

If we use the JTR method, but dont move the pivot point out, ie........just move it up 3/4", arent you effectively doing what you say is your prefered method for eliminating bumpsteer?,

With the exception that your method is alot more exact, because you have an adjustable pivot point and a guage so you can get the LCA angle at a precise point,

 

I know i might be missing something here - but in essence, isnt this pretty much the same way of dealing with bumpsteer as the JTR method except its alot more precise???

 

Which means, that you could do this with bumpsteer spacers aswell, just play around with different thickness spacers untill you get the right one............

[pparaska]

I beg to differ on saying that raising the inner pivot point of the control arm by a certain amount (say 3/4") is the same as installing the misnamed "bumpsteer spacers" of the same thickness. I believe the following to be true.

 

(A) The "bumpsteer spacer" lower both the ball joint and the tie rod end by the same amount. What that does for a lowered car is return the control arm and tie rod angles to near where they are stock, and returns you to a similar at-rest area of the original bump steer curve.

 

( Raising the inner pivot for the control arm CAN bring the 2 lines that (1) connect the inner and outer control arm pivots and (2) connect the inner and outer tie rod pivots, into a more parallel relationship, which CHANGES the bump steer curve. Raising or lowering just the outer tie rod end with spacers can have the same effect.

 

The bumpsteer curve on the Z can be a good bit "curvy". If it's too bad, you'll feel it as bumpsteer as you go around a turn and hit elevation changes that move the wheels up and down. The "bumpsteer spacers" just move the location of the "at-rest" position in the existing bumpsteer curve.

 

Only the solutions in paragraph ( (as well as modifications to the location of the ball joint and steering rack height) can change the SHAPE of the bump steer curve. If a car has bad enough bumpsteer, this is what you would want to do, by making the bumpsteer curve have most likely less variation as the wheels move through their vertical travel.

[JMortensen]

I think you're right Pete. I'll change the FAQ.

[tonycharger72]

This might sound like a dumb question............

 

But what if you take a zed that has not been lowered, its at its stock ride height, then used bumpsteer spacers, would this not put the LCA at a similar angle to what you would achieve by doing the JTR mod???

[BlackBeaut]

It's taken me a while to get my head around this one, but I think this pic helps out. Seems to me that all this confusion has been caused by misnaming the spacers as 'bump steer' rather than 'roll centre'. I'm sure JohnC will agree there!

 

Now I should say that the pics greatly exagerate the angles etc. for clarity, and they might not bare any resemblance to the Zs geometry, but hopefully they show what's happening.

 

#3 and #4 are the important ones. In #3 we can see that adding in the spacer moves the LCA outer pivot point down and so LCA angle to a more OEM location, which should move the roll centre back up. HOWEVER the LCA and steering tie rod still remain unparallel so this has no effect on the existing bump steer.

 

In #4, the LCA inner pivot is moved up - and that's the only thing that's changed here. This has two effects, it returns the LCA angle to a more OEM location AND it causes the LCA and tie rod to become parallel which does have (a hopefully positive) effect on bump steer.

 

 

I could of course still be way wrong, and if so please tell me so I can adjust the diagrams so that they can be shown to future baffled people and hopefully clear things up quicker for them!

 

Cheers,

Rob

[JMortensen]

You should put that pic in the FAQ Rob. I don't know how I managed to confuse myself when I wrote the FAQ but I was just spouting it out from memory, and I wouldn't have gotten confused if I had been looking at that picture.

 

Tony, you could put bumpsteer spacers on a Z at stock height and what that will do is raise the roll center. Where to set the roll center is not a clear cut thing. The suspension gets acted on differently by lateral forces depending on the roll center. When the LCA goes past horizontal, then lateral forces try to COMPRESS the outside suspension, basically making body roll worse. If the LCA is totally horizontal then there are no jacking forces on the suspension. If the LCA points down then lateral forces try to raise the outer suspension. If it's really pointing down then lateral forces can tip the car over, a la jacked up Baja Bug. If you add the bumpsteer spacer to a stock Z then as Pete said you'd put the car into another part of the bumpsteer curve, and the curve works out to be pretty gradual in the middle but fairly extreme at the ends of the suspension's travel, so you might not like the results.

 

You can run the car with a roll center that is very low, but you need to run very heavy springs and shocks that can control those springs. For less body roll most agree that you want to run the control arms pointing "slightly down". If you're running softer springs, you'd want it pointing down a bit more. It is a trial and error thing, and you can fix the resultant body roll issues with more spring, more sway bar, or higher roll center. You aren't limited to one or the other.

[BlackBeaut]

You should put that pic in the FAQ Rob. I don't know how I managed to confuse myself when I wrote the FAQ but I was just spouting it out from memory, and I wouldn't have gotten confused if I had been looking at that picture.

 

Unfortunately I can't guarantee the web space it's hosted on, and I also cannot attach it to a post myself as I seem to only be able to attach weird letterbox sized images. However if yourself or one of the mods wants to grab that image and correctly attach it to the FAQ then I'm more than happy for that to happen, I'm certainly not worried about copyright on it!

 

Cheers,

Rob

[JMortensen]

Converted it to jpeg, tried to upload, unsuccessful. Not sure why...

[BlackBeaut]

Converted it to jpeg, tried to upload, unsuccessful. Not sure why...

 

Ah, got it to work as a jpeg, off I go to the FAQ!

 

OK, posted in FAQ, awaiting moderation.

 

Cheers,

Rob

[Here comes trouble]

Jon.. do you have any pics of your slotting modification ? Your instrucrions were quite detailed but I am only rememberig .000000005 % of what I read these days. I moved up my pivot point 15/16 ths on the Tomahawk project which required some modification to the lower control arm to ride that high. I removed the whole sub frame to relocate the pivot point. After completed with sectioned struts, lowered and tires the control arm (tranverse link) was positioned in a horizontal position close to stock. I would like to try your slotting method on the GTO project. I have some camber plates (Tomahawk project) for a later install since they woulfd drop the car another 2 inches.

[JMortensen]

No I don't have pics handy. I lost them a few years back in a hard drive crash. Not much to see though. Just a big slot instead of a hole. I did much the same thing you did previously. Took the crossmember out, drilled out the spot welds that hold the washers on, then drilled a new hole straight up from the original. Then just cut the slot between the holes with a cutoff wheel.

[tube80z]

( Raising the inner pivot for the control arm CAN bring the 2 lines that (1) connect the inner and outer control arm pivots and (2) connect the inner and outer tie rod pivots, into a more parallel relationship, which CHANGES the bump steer curve. Raising or lowering just the outer tie rod end with spacers can have the same effect.

 

If you plot bumpsteer every 0.1 inches you'll get a curve. It depends on the curve whether you change the inner pivot point or the outer pivot point when trying to minimize bumpsteer.

 

Cary

[tube80z]

Where to set the roll center is not a clear cut thing. The suspension gets acted on differently by lateral forces depending on the roll center. When the LCA goes past horizontal' date=' then lateral forces try to COMPRESS the outside suspension, basically making body roll worse. If the LCA is totally horizontal then there are no jacking forces on the suspension. If the LCA points down then lateral forces try to raise the outer suspension. If it's really pointing down then lateral forces can tip the car over, a la jacked up Baja Bug. If you add the bumpsteer spacer to a stock Z then as Pete said you'd put the car into another part of the bumpsteer curve, and the curve works out to be pretty gradual in the middle but fairly extreme at the ends of the suspension's travel, so you might not like the results.

 

You can run the car with a roll center that is very low, but you need to run very heavy springs and shocks that can control those springs. For less body roll most agree that you want to run the control arms pointing "slightly down". If you're running softer springs, you'd want it pointing down a bit more. It is a trial and error thing, and you can fix the resultant body roll issues with more spring, more sway bar, or higher roll center. You aren't limited to one or the other.[/quote']

 

There's a little confusion here. The suspension will always see jacking forces no matter what the angle is. Below ground geometric RCs have lower jacking forces and transfer weight at a slower rate than above ground RCs. Underground RCs will have a larger roll moment and this needs to be dealt with as Jon mentions.

 

The car doesn't rotate around the geometric RC. A more useful way of thinking about the problem is like lateral anti-dive/squat. Mark Ortiz and others have written a lot about this topic and how to think about it. The extreme case is the droop limiter (one of my favorite toys). When you limit droop and the inside srtut tops out the RC then literally becomes the inside contact patch of the tire, no matter where the geometric RC resides.

 

What you'll find in the case of a Z is that you have limited options. If you try and get one thing to work well you'll screw up others. There's no magic solution other than to try it and see what you like. If you have the means using much longer lower front control arms will help make things more stable.

 

I chose an underground front geometric RC for my car. It is an autox car and I did this to try and make it more stable. I run springs that are at least 400 lbs per corner.

 

Cary