Bumpsteer FAQ

[JMortensen]

Bumpsteer FAQ

 

Bumpsteer is a condition where the toe changes as the suspension moves through its travel. It is generally not noticeable on street cars unless they are being driven "arrest-me-officer" fast, but is an important factor with regards to how a race car handles at the limit. Bumpsteer curves can be plotted out, and generally they follow the following rules: when the suspension droops the wheels toe in, and when it compresses the wheels toe out.

 

A car suffering from too much bumpsteer generally has the following symptoms: Bumps in the road hit by one wheel cause the car to jerk in one direction or the other even though the steering wheel does not move. When driving through a bumpy turn the wheel must be constantly moved back and forth in order to keep the car traveling in a continuous arc.

 

Why does my Z have bumpsteer in the first place?

 

Bumpsteer is engineered into most cars direct from the manufacturer. They use bumpsteer as a safety mechanism. The more the suspension compresses, the more the front tires toe out. When you turn very sharply and load the outside front suspension that tire toes out and makes the car want to take a straighter path. This will contribute to understeer and understeer is considered to be a safer and easier condition to control than oversteer.

 

What causes bumpsteer?

 

Bumpsteer is caused when the tie rod and control arm are different lengths, or sit at different angles, or a combination of the two. In a Z the tie rod and control arms are the same length from the factory, so the issue in the Z is that the two are at different angles.

 

How do I measure bumpsteer?

 

Bumpsteer can be measured with a bumpsteer gauge. Wasn’t that easy?

 

A bumpsteer gauge is basically a glorified pair of dial indicators. Usually a bumpsteer gauge comes with a steel or aluminum plate which gets strapped to the wheel. A stand sits next to the front tires that holds the dial indicators in contact with the plate. The suspension is moved up and down, and the front and back dial indicators are compared to measure the toe change. Unfortunately, you want to do this measurement right in the car’s normal ride height, and through a couple inches of suspension movement on either end of that ride height. To accurately measure the front springs and sway bar must be removed, then the car must be placed at ride height with relation to the suspension and moved up or down while monitoring the toe change on the dial indicators.

 

How do I “fix†bumpsteer?

 

There are many ways to fix bumpsteer. Probably the most common modification available is the bumpsteer spacer which can be purchased from just about any Z parts supplier. This is a spacer that fits between the bottom of the strut and the control arm, and “restores factory geometryâ€. It doesn't really restore factory geometry, and it really does not fix bumpsteer either. What it will do is return the ROLL CENTER to a position closer to stock on a lowered Z. What bumpsteer spacers really do is raise the roll center of the car which makes it have less body roll in turns with a given spring rate. The bumpsteer spacer does not eliminate bumpsteer.

 

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. 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 unless the pivot is moved to the correct height. In order to know what the correct height is you really need to measure on YOUR car, because a 30 year old car is going to vary chassis to chassis, plus some people want to run the "bumpsteer spacer" to raise the roll center AND fix the bumpsteer in addition to that.

 

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.

 

So far all of these methods have dealt with moving the inner or outer end of the control arm in relation to the tie rod. It is equally possible to move the tie rod.

 

One way to move the tie rod is to modify the steer knuckle that the outer tie rod attaches to so that it will accept a common bumpsteer spacer kit. These kits are very common in road racing and circle track racing, and generally replace the outer tie rod with a rod end, then use spacers to move the rod end up or down in relation to the steer knuckle. Cary (tube80z) has pointed out that a standard bridge ream can be used to ream out the hole in the steer knuckle. The bridge ream has the same taper as a Ford, so a Pinto bumpsteer spacer kit available from any circle track racing parts supplier will work once the steer knuckle has been modified. One issue with modifying bumpsteer on the tie rod end is clearance between the wheel and the tie rod. Sometimes there is not enough room to move the tie rod down far enough to get rid of the bumpsteer.

 

Another way to move the tie rod is to raise or lower the steering rack, which then raises or lowers the inner tie rod. This is not an easy option in a stock Z, but can be done with some fabrication if desired.

 

Where do I want the bumpsteer to be?

 

Opinions vary. Some like me prefer 0 bumpsteer, some like John Coffey prefer to tune the car by changing the bumpsteer and roll center characteristics by varying the thickness of bumpsteer spacer used. What is a fairly universal idea is that little toe out on compression makes the car more forgiving and toe in on compression is not a good thing.

 

Is bumpsteer worth messing with?

 

If you have to ask this question it probably isn’t worth it. 99% of street drivers will never notice bumpsteer or will consider it cured by the JTR mod or a bumpsteer spacer. For those that do drive hard enough to notice it can be very worthwhile.

 

Modifying your suspension is potentially dangerous and you do so at your own risk.

[randy 77zt]

i moved front suspension pivot point up 3/4" and out 3/4"/then installed gound control coil overs and camber plates.run 3.2 to 3.5 negative camber at the track and 1.2 on the street.i havent checked for toe change during suspension travel.the pivot points are behind the ball sockits on the rack.i straightend the outer tie rods bends so they are straight.

[BlackBeaut]

Attached is a set of diagrams that hopefully show the differences that using the so called bum steer spacers give over adjusting the LCA inner pivot.

 

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 diagrams. 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 toward stock location. 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.

 

Cheers,

Rob

[Guest rick_htm]

Is the pinto set up the same length and the same thread as a z car, or are other modifications required? I'm preping a z for scca e prod and this is one area I need to adress.

[JMortensen]

The pinto part is the taper that fits into the steer knuckle. It is not the same taper as the Z steer knuckle, so the steer knuckle would need to be reamed to the correct size. Once the reaming is done you can install the Pinto part, which leaves you with a 5/8" shaft hanging down below the steer knuckle. From there you can add spacers and then you would put a rod end on the bottom. The trick is getting a turnbuckle to attach the 5/8 rod end to the inner tie rod end.

 

I'm starting to work on this right now, and the ideas that have come up are to take a piece of hex aluminum and drill and tap both ends. The inside gets drilled and tapped with 14 x 1.5mm threads and the outer gets 5/8 - 18 threads. This looks to be the hard way to do it.

 

The easier way is to get a swedged 5/8" tie rod tube from Coleman Racing and modify it to work. This is done by cutting off the threaded portion of the stock Datsun tie rod end, cutting the steel tube from Coleman, and welding the two parts together. Credit goes to Cary (tube80z) for this idea. Modifying the tube is easier than cutting threads in the aluminum tube because the 240Z has left and right hand threaded inners.

 

You might also check this thread here which goes into these options in greater detail: http://forums.hybridz.org/showthread.php?t=111435

[Guest SS-Z31]

In reading all of this I started looking up more info on Ackerman steering geometries and found this that might be helpful.

 

http://www.eng-tips.com/viewthread.cfm?qid=117106&page=1

[tube80z]

In reading all of this I started looking up more info on Ackerman steering geometries and found this that might be helpful.

 

http://www.eng-tips.com/viewthread.cfm?qid=117106&page=1

 

This is far more useful http://www.clubracer.com.au/articles/ackerman.asp

 

Cary

[RUSSJZ-ZED]

I have a late model Subaru liberty power rack fitted, no bump steer.

Steering shop that shortened/modified my trackarms say that power steer will eliminate (most)bump steer . True or false?

[JMortensen]

False. Having power steering has no effect on bumpsteer whatsoever. Bumpsteer is created by steering geometry. Could be that by modifying the control arms they eliminated the bumpsteer.

[73GreeNMachine]

False. Having power steering has no effect on bumpsteer whatsoever.

 

Agreed. i have PS on mine as well, and i can still detect some of the bumpsteer when im taking freeway onramps hard.

[JMortensen]

I used to race with Jack Hidley and he is a very knowledgeable and helpful guy. Check out his .avi's showing bumpsteer in action.

 

http://forums.corral...296-post36.html

Edited April 19, 2011 by JMortensen

[mclark999]

When relocating the pivot point as described above by making a slot or by using the JTR relocation, is it necessary to reinforce the crossmember by adding a "doubler"? Is this model specific or is the thickness of the front crossmember the same for the 240, 260, and 280?

 

Additionally, as someone who has done little fabrication, what is the best way to measure for drilling the new hole on each side and getting everything lined up correctly?

 

Thanks,

 

Michael

[1 tuff z]

Jon, love to see some pics of your slotted lca mount holes if you have them.

[JMortensen]

Here you go.

[1 tuff z]

Thanks Jon, one more favor-can you post another but with the camera pulled back a bit so I have a better reference?

 

As always, thanks for sharing your in depth suspension knowledge!

[JMortensen]

Here you go.

[1 tuff z]

Thanks!

[CarolinaTZ]

I'm in the process of sectiong struts/rebuilding suspension. This will be a street/maybe sometime track car. I don't want to use camber plates but am looking into slotting/moving the LCA pivot to correct camber and/or roll center. On a car w/ this intended use, should I slot the pivot point or simply re-locate a single hole. Jon, are you using the stock pivot bolts or do you have to use a different bolt based on the thikness of the welded on backing plate? Thanks in advance...

[JMortensen]

My control arms have rod ends so I had to make spacers, and they're thicker than what you would have if you were using stock or poly bushings. There is no washer on the inside of the crossmember on my car.

 

If you wanted to keep poly bushings or not have to mess with spacers, then I would suggest that you drill out the old washers, drill the hole up 1/2", and then weld the washers back in the new spot. That should be pretty close to correcting the bumpsteer.

 

If you want it exact and you have to make spacers anyway, then I like my slotting method. Mine is a dedicated racecar. You'll have to weigh the modifications for yourself and see what you think is best or safest.

[CarolinaTZ]

That makes sense. I won't be using rod ends or employing any type of adjustment in my LCA....just poly bushings. I hoped to help correct bumpsteer/camber with the one act of moving the LCA pivot. I see that moving the pivot up, like yours, will help correct bumpsteer but you also have the rod ends to adjust camber. I won't have that option. That's why I was hoping there were some tried and true specs for slotting (I guess with some horizontal adjustment...not just vertical) that would give me both. Is the Ford F150 eccentric washer in the other thread the best way to go and/or do you do you know of some other slotting specs? Thanks again....