Bumpsteer concerns

[Guest Anonymous]

The time of installing my front suspension is quickly approaching and with it comes many little worries about getting all the allignment just right...the first time. My concern at the moment is the alleviation of any bumpsteer characteristics that could be associated with poor suspension angles. How can I set everything up so there will be little -> no bumpsteer ? What would be the best position of the rack and pinion unit to cut down on toe angle change through the full suspension articulation? HEEELLLLLLLPPPP!!!!

 

SpencZ

MonsterZ Cont. http://members.xoom.com/SpencZ/index.htm

 

p.s. Pics will be up on thursday due to the lying b!$#@%^s at the photo place. Apparently they don't know how to count because they always tell me the pics will be done in 2 days and it always takes 3.

[Guest Anonymous]

Not sure if the following will help given your grand plan (totally new front end, yeah?), but:

 

I had similar probs; as I guess you know ANY Z suffers from bumpsteer, especially after lowering. The fix I used was to install bumpsteer spacers; in my case these are machined aluminium spacers, about 30 mm. high, that fit between the bottom of the strut and the ball joint. I'm also running adj. control arms & caster rods, and these have given me pretty good handling. Certainly the spacers reduced (not eliminated) the bumpsteer.

 

As to where from; I can give you an Australian reference, but I'm not sure that'll really help.....

[John Scott]

Monster Z man, check the blurb about bumpsteer in the JTR manual. They dispute the spacers and recommend relocating the lower control arm, up. Check the book for exact dimensions and details. Explains roll centers etc. . Don't know for sure 'cause I haven't tried it!

John

[pparaska]

I'm in total agreement with JTR on the bumpsteer spacers. I've written numerous emails to the IZCC list about this over the past ten years. I've studied the causes and design practices for eliminating bumpsteer in several books to research the topic. I've lower my 240Z with Motorsports Springs back in my L6 days and the bumpsteer was bad. I tried the bumpsteer spacers, but it didn't help much.

 

Here's the long and short of it:

 

The Z has a bumpsteer problem from the factory. The rack is too high in the car relative to the inner control arm (CA) pickup points. Or looking at it the other way, the CA pickup points are too low.

 

To have no bumpsteer, the line connecting the center of rotation of the ball joint and CA pickup point must be parallel to the line connecting the center of rotation of the inner and outer tie rod ends. The lines must also have the same length. The problem with the Z (240-280) is that it the lines are not parallel. To fix the problem, you need to do one of the following:

 

1) Raise the inner CA pickup point (3/4-7/8")

2) Lower the steering rack (3/4-7/8")

3) Raise the outer tierod end (3/4-7/8")

4) Lower the ball joint (3/4-7/8")

 

The "bumpsteer spacers" lower BOTH the outer tie rod end AND the ball joint, so they do nothing to fix the non-parallelism of the two "imaginary" lines discussed above.

 

What the "bumpsteer spacers" do on a lowered car is relocate the CA and tierod to near stock angles to the ground, restoring the static position of the suspension on the stock bumpsteer curve. This is a good thing, but you still have a bunch of bumpsteer, since the curve has not been changed for the better (made straight, or in other words, gotten rid of the suspension movement dependency on the toe.)

 

To fix the bumpsteer, you have to straighten out that curve, by doing just one of the above 4 choices.

 

The easy fix is the one JTR recommends - raising the CA pickup point 3/4-7/8". They say 7/8" totally eliminates the bumpsteer (making the curve of toe change versus suspension compression/extension be zero throughout). They say to raise it only 3/4" to leave a slight bit of bumpsteer to compensate for camber thrust. They also recommend moving the CA pickup point out by 1/4" to gain negative camber.

 

Another fix is to bore out the steering knuckle and put a bolt and spherical rod end on instead of the outer tie rod end.

 

 

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Pete Paraska - 73 540Z - Marathon Z Project - pparaska@tidalwave.net">pparaska@tidalwave.net -

[Guest Anonymous]

Pete...that is exactly the info I was looking for. Thanks

 

 

SpencZ

MonsterZ Cont.

http://members.xoom.com/SpencZ/index.htm

[DennisK]

I will be getting into the front suspension very soon, and there are a couple of things I'm not quite getting. As per JTR, I plan on relocating the control arm p/u point. Do I still need the bumpsteer spacers? I thought you do both, but I'm not sure. Also, as per Pete, lowering the rack is a fix. The spacers under the front crossmember lower the rack, which bolts to it. is this not enough? I plan on lowering as much as possible, and still be realistic.

[pparaska]

DennisK,

Lowering the rack BY ITSELF would be a fix, but you'd have to cut off the brackets from the crossmember and weld them back on in just the right place (adding material) and just the right attitude.

 

Lowering the crossmember lowers the rack and the CA pickup points. If you don't relocate the pickup points and just move the crossmember, the bumpsteer curve stays (mostly) the same, but you move the at rest point on the curve.

 

Actually, lowering the crossmember with the JTR crossmember spacers is the opposite of adding the "bumpsteer" spacers. (I hate that name, but everybody knows what you mean when you say/write it.) Lowering the crossmember would make the angle of the CA to the pavement move from a angled down from the CA pickup point to more horizontal or from horizontal to angled up from the CA point relative to the pavement. Adding the bumpsteer spacers has the opposite effect,

so they might be a good idea if you lower the crossmember.

 

Note: I've seen several V8Z converts not see the need, tunnel clearance wise, for the crossmember spacers. And as I've posted somewhere before, the crossmember spacers, coupled with the JTR trans mount are probably partially the reason for driveshaft u-joint angle problems and vibration under load. Funny how a discussion of bumpsteer gets you into driveshaft u-joint angles, no? Anyway, try leaving them out if you don't need them. You can install the crossmember spacers and install the engine/trans, check for clearance between the top of the bellhousing/tranny combo and the tunnel, and take them out if you find there us room to raise the engine 1/2" and you need addtional oilpan clearance and/or it would help with your driveline angles. BTW, I just ordered what looks to be a copy of the Competition Engineering Angle finder for doing driveshaft u-joint angle checks from http://www.mcmaster.com for under 12 dollars. (part number 20025A32, $11.55)

 

To tell you the truth, I'm not sure if the "bumpsteer" spacers are useful if you raise the CA point and leave out the crossmember spacers. I have the crossmember spacers out (to help get my driveshaft angles more optimal) and I put the bumpsteer spacers in. I also raised the CA pickup points so the effect should be (with a 1.5-2" lowering) that the CAs angle down at rest from the CA pickup points outward. If this angle is too severe, I'll remove them. I would think that the camber curve would get exagerated as the CA to pavement angle became too large, and that wouldn't be good if the car handled relatively flat. But I'm no racer and this is all theory. Any roadcourse/autox racers car to comment?

 

 

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Pete Paraska - 73 540Z - Marathon Z Project - pparaska@tidalwave.net">pparaska@tidalwave.net -