Bumpsteer, again

[Guest Anonymous]

If bumpsteer is toe change caused by non-parallel control arm/tie rod geometry, it seems that the proper point to raise the CA pivot point is where these two lines ARE in parallel. Now, that may be 3/4", or 7/8", or some other absolute value, but if I set up the front end to achieve this particular goal, have I improved handling? Since I have neither the time nor the money to make the front end adjustable in 3 planes, I need to figure out what is the best guess, maximum bang per buck. Thanks.

 

BTW, please see my post about the Maxima/810 I.D. issue - need some help there, too.

 

[ April 30, 2001: Message edited by: jpd280z ]

[pparaska]

John, TimZ posted that each Z car is different, as far as the correct height to get (near) zero bump steer, and I agree, to some extent. My caveat to that is that this is a production tolerance stack up and there is a distribution of errors that make it such that some number like 7/8" for raising the CA pickup point is probably right.

 

I'd shoot for 3/4" of 7/8" and then convert the tie rods to heim joints and do the fine tuning there, I guess.

[Guest scca]

that is close... but what i have been told is that you want a slight angle downward not "totally" flat -- the reason is so that as the car enters a corner the suspension will compress and its at this point you want it parallel..

[Guest Anonymous]

Pete, Mike --

 

Thanks for your ideas. It seems like 3/4 - 7/8 is "close", though maybe not perfect on all cars. Still, if i can get an 80% improvement in the bumpsteer curve with a good execution of a decent guess, I call that a win. Mike, what downward angle are you referring to? Control arm? Hmmmm...raising the pivot ought to work in favor of that. Are any of the popular spacers either a good alternative, or an add-on that gets better results from the same mod? I seem to recall something about raising the roll center with some spacers. Thanks again, guys.

[Guest scca]

as pete said each car is different.

a-arm spacers or moving upper pivot point.

 

as an example my car with 16" wheels doesnt use spacers and it is pretty flat.

 

i dont think every car needs the mod..

 

and pete is correct in that the fine tuning should be done with tie rod spacers at the steering knuckle..

 

first get your car with suspension at the ride height you want and then go into correcting the geometry after you can see how much correction it may need.

[jeromio]

This is me, chiming in...

 

My car definately had a bump steer problem. I'm not a racer, and not very knowledgeable about suspension stuff in general, but I know that there is a slow sweeper on my drive home with an unfriendly bump in it. Before, when I hit that bump, I had to be prepared, because the car was gonna change direction on me.

 

I had done some reading of my own and saw 1" and 7/8" as the relocation number. I didn't try and do any measuring on my specific car - didn't seem like any kind of straight forward way of actually measuring the degree of non-parallelism between the steering link and the arm. So, what I did was split the difference between the 2 numbers I'd seen and came up with 15/16".

 

It also seemed logical (and I'd also seen people discuss it on this forum), to move the pivot out at the same time. I moved mine 1/4" out on both sides. I did this to increase the camber on the fronts. But it also made the mod much easier since if the pivot hole was moved straight up, there wouldn't be much room left for the arm. As it was, I had to grind off some of the metal on the inner part of the arm.

 

The results? The bumpy sweeper is no longer a problem. But Caveat Emptor - if you've seen another thread that I started here, I am experiencing some twitchiness at speeds over 100MPH. There were a whole lot of other changes to the car at the same time though, any one of which could be contributing to this sensation.

[blueovalz]

Just to add a bit more confusion to the string (feeling kinda mean today). Parallelism is not the only issue here. the lateral spacing of the pivot points is important too. Even if the rods are perfectly parallel, the swing arcs will be different from the steering rod swing arcs if the rod's pivot point is more inboard or outboard from the CA pivot point. I ran into this with the Ford rack. I was limited as to how close I could get to perfect (no such thing here), but turned out pretty good with a 1.20" spacer between the rod and the steering arm.

[johnc]

Check out this thread from last January: http://www.hybridz.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=6&t=000344

 

A few points:

 

1. Bumpsteer is not always a bad thing.

2. Total elimination of bumpsteer is probably not possible (or desirable).

3. Removing bumpsteer affects other suspension geometries (you don't get something for nothing).

[pparaska]

JohnC, thanks for digging that up!

 

Terry, everything I've points to two conditions for elimination of bump steer:

 

1) Parallel lines connecting the pivots of:

a) the tie rods, and

the inner pivot and ball joint

 

2) The lines connecting the pivots must be of the same length. (Same effective length of the tie rod and control arm.)

 

So if you keep the CA pivots in the same lateral location, and use the same steering knuckles, yeah, a wider or shorter rack will force the effective length of the tie rod to be shorter or longer than stock, respectively - causing a change in the bumpsteer curve.

 

BTW, Al (alsil), what is the difference between the inner tie rod end pivots on the 240Z and the Mustang rack you used?

[Guest Anonymous]

Guys --

 

(Long Post)

 

Thanks for all the info. Now, if youse could just agree on what is the proper course of action, that'd be nice, or maybe miraculous. Just kidding - if we don't air it out, it doesn't get tested.

 

OK, let's talk about my situation.....'78 280Z, street car, but sees about 6 - 8 track weekends a year. GC coilovers all around, C/C plates front (-3.5 each side), springs are 175F/200R, Tokico 5-ways all around, Addco 1" sway bars F & R. Eccentric poly bushings on inner CA pivot, turned to the 11:00(L) and 1:00® position to try and reduce bump steer (since I got the neg. camber with the plates). 15 x 7 aluminum wheels with 225/50-15 track tires (Kuhmo's, Toyo, Yoko's - whatever shows rubber, not fabric), ZX tranny and 3.9 rear end (open), Toy 4 pots up front (unsprung weight?), lowered about like Dan B's 240Z (low, but not radical - BAD driveways cause a drag or scrape, speedbumps are a slow-motion event). Got the picture? Good.......

 

I have had a long standing battle with understeer. I can trail brake like there's no tomorrow (rear disc conversion might affect that tactic) and power understeer is immediate and dramatic, even though the car is down on power after 280K miles. Making positive changes on the cheap are my stock in trade, so a couple of things come to mind.

 

1) I might be bottoming out the front suspension. The CO's helped a lot, but didn't solve the problem. I have my doubts though - my decidedly low-lateral-G street tires exhibit the same tendency, even though the suspension is not loaded as much. However, if (have not jumped to conclusion - all opinions are still valid) I move the inner CA pivot up, which lowers the CM, and thereby the car, then I could crank up on the adjusters to gain some suspension travel. Maybe, if it is a good idea for geometry.

 

2) Higher spring rates might help all of the problems - eliminate bottoming, reduce bumpsteer with limited suspension travel, reduce body roll, pulverize kidney stones, etc. Might happen regardless of other choices. I'm thinkin' 250F/300R?

 

3) Spacers might help by getting the OEM bumpsteer back and raising RC.

 

Someone said that the MSA BS spacers (appropriate name?) are not the right size or shape - what IS the right size and shape, and who sells 'em? Anyone have pics and a write-up on heim jointing the TRE's?

 

Better yet, why don't you guys come on over - I'll supply the beer, pizza, and tools/parts while we figure this beast out. Can't make it this weekend? Post a reply so your voice is heard.

 

Thanks,

[pparaska]

John, raising or lowering the CA pickup point has no effect on the ride height, suspension travel, etc. Only the strut located stuff (C/C plates, springs, coilovers) do that. The bumpsteer spacers don't lower it either. I took your statement that raising the CA pivot lowered the car - is that what you meant?

 

I think you have to balance the amount you lower the car with the CA pickup point location change, and the "bumpsteer" spacer thickness. This is really all a juggling act to get the CA to a near stock angle relative to the horizontal when the car is at ride height. Maybe a bit more or less of an angle is advantageous, I don't know. But if you lower the car, the CA starts to become horizontal (maybe goes up from the pivot points if you lower enough?) and you need to reverse that by either the spacers of moving the pivot point.

 

The deal is that raising the pivot point not only changes the CA angle (and the roll center and camber curve as well - as do the "bumpsteer spacers"), but it also changes the bumpsteer curve, for the better.

[TimZ]

Ohhhh man... now you dun gone and did it...

You got me talking abut bumpsteer again...

 

quote:

---

Originally posted by jpd280z:

1) I might be bottoming out the front suspension. The CO's helped a lot, but didn't solve the problem. I have my doubts though - my decidedly low-lateral-G street tires exhibit the same tendency, even though the suspension is not loaded as much. However, if (have not jumped to conclusion - all opinions are still valid) I move the inner CA pivot up, which lowers the CM, and thereby the car, then I could crank up on the adjusters to gain some suspension travel. Maybe, if it is a good idea for geometry.

---

 

Did you section the struts when you added the coilovers? Better question - when the car is sitting at it's nominal ride height, is the shock near the center of it's travel? How much travel do you have left in the compression direction? If you are near the middle, your suspension should not be bottoming.

 

Also, raising the CA pivot point will have almost no effect on ride height whatsoever. Your ride weight is determined by the distance between the bottom of the tire and the upper spring perch. I said almost, because changing the pivot point will have a small effect on the angle bewteen these two points - actually this will probably raise the ride height imperceptibly.

 

I agree that your setup does not sound like it should have a high degree of understeer. One thing that comes to mind - when you say "trail brake like there's no tomorrow", are you describing braking as hard as you want when entering the turn? Forgive me if you already know all this, but inducing oversteer by trail braking is a very subtle technique. There is a relatively narrow range of braking force where this is effective. The trick is to shift the weight forward just enough that you unload the rears. If you go too far, the postive effect of the weight transfer is negated by the fact that the longitudinal braking force uses up too much of the tire's available force, leaving little or no reserve for generating lateral force(i.e. you end up with understeer). Remember - any given tire has a maximum total force that it can exert in any direction - if you are using all of that force for braking, then there is nothing left for cornering.

 

Sorry if you already knew this, but I've seen people make this mistake many times.

 

quote

2) Higher spring rates might help all of the problems - eliminate bottoming, reduce bumpsteer with limited suspension travel, reduce body roll, pulverize kidney stones, etc. Might happen regardless of other choices. I'm thinkin' 250F/300R?

 

If your roads are relatively smooth, then this will work pretty well. On bumpy roads, the car will quickly turn into a handfull, though.

 

quote:

---

3) Spacers might help by getting the OEM bumpsteer back and raising RC.

 

Someone said that the MSA BS spacers (appropriate name?) are not the right size or shape - what IS the right size and shape, and who sells 'em? Anyone have pics and a write-up on heim jointing the TRE's?

---

 

Those spacers are where my experience started with all of this. When I installed them, the actually made my bumpsteer WORSE. I measured the bumpsteer curves before and after, and was pretty much shocked at the results. BTW, I'm talking about single-side toe change on the order of 1 INCH through the suspension's range of travel. Probably not something you'd want to live with. I'm not saying yours will definitely turn out that bad, but I'm not saying it won't either...

 

As far as what is the right size and shape - your guess is as good as mine - sorry.

 

I don't have any pictures of my tie rods, but the basic fabrication is pretty simple:

 

Get a length of steel rod, and drill and tap it on either end to accept the rack end and the heim joint. You would probably want to use one of the high misalignment types of heim joint, to help ensure that the joint doesn't bind through the full travel. I had to put a bend in mine, so that the joint was centered in its travel at the nominal ride height.

 

Next, drill the taper out of the steering knuckle, and use a long grade 5 or 8 bolt to connect the knuckle to the heim joint. As I recall, I used a 5/8" heim joint and bolt. You can then adjust the bumpsteer by adding different sized spacers between the heim joint and the knuckle. I used 5/8" id steel tubing cut to different lengths for the spacers.

 

Now, I mentioned this in my post before, but I will reiterrate - changing the CA inner pivot point does have some positive effects -namely correcting the camber gain curve on a lowered vehicle (camber gain is the amount that the camber changes with suspension travel). Mike was spot on above when he mentioned needing a small amount of droop in the control arm at your nominal ride height. If you think of the control arm describing an arc as it pivots, you will see that if it is flat at the nominal height, any suspension travel will decrease the camber. The reason for adding some droop is to cause the camber to increase for the few degrees of roll instead, which generally helps the tire's grip.

 

For this reason, I do believe that the CA pivot mod is a good thing to do, just don't expect it to zero out your bump steer curve.

As I said before - it might improve it. It might make it worse. The only way to know for sure is to measure it.

 

One more thing - I made my bumpsteer guage out of a couple of pieces of scrap plywood, a hinge, a couple of machine screws, and a cheap plastic caliper with a depth gauge. It's not like you have to spend a fortune to do this. My procedure was basically this:

 

1) Remove the springs, and reassemble the suspension

 

2) Place a couple of 4x4's under each tire, in order to allow a jack to fit under the crossmember at full compression.

 

3) Start off with the suspension at full compression (on the bump stops).

 

4) Setup my plywood gauge, so that the two screws (spaced the diameter of my wheel apart) are an equal distance from the leading and trailing edges of the wheel rim, respectively. The wheel will move around a bit with suspension travel, so you don't want the screws to be touching the wheel.

 

5) Use the jack to raise the car 0.5" at a time. Each time, use the depth gauge on the caliper to measure the amount that the distance from each screw to the rim has changed. The difference will be roughly the amount of toe change.

 

6) Plot the results for the full travel of the suspension, and you now have a plot of your bumpsteer curve.

 

Yes, it's a pain in the ass and time consuming, but it's not particularly expensive, and not rocket science.

 

 

Okay - I'll shut up, now...

 

...For a while

 

[ April 30, 2001: Message edited by: TimZ ]

[Guest Anonymous]

posted by TimZ:

[QB]Ohhhh man... now you dun gone and did it...

You got me talking abut bumpsteer again...

 

Ah, Tim, that was my intent -- sorry.

 

Those spacers are where my experience started with all of this. When I installed them, the actually made my bumpsteer WORSE.

 

OK, spacers are OUT! So much for the quick fix.

 

As far as what is the right size and shape - your guess is as good as mine - sorry.

 

If my guess was a good as yours, this thread wouldn't be so long....

 

changing the CA inner pivot point does have some positive effects -namely correcting the camber gain curve on a lowered vehicle

 

OK, sounds like something that might help - did I mention the beer and pizza?

 

For this reason, I do believe that the CA pivot mod is a good thing to do, just don't expect it to zero out your bump steer curve.

 

Not expecting zero BS, but an improvement is an improvement. I might try your BS measurement trick - could this be done by measuring the front and rear track width of the tires at different heights? You know, groove to groove?

 

One thing that comes to mind - when you say "trail brake like there's no tomorrow", are you describing braking as hard as you want when entering the turn? Forgive me if you already know all this, but inducing oversteer by trail braking is a very subtle technique. There is a relatively narrow range of

braking force where this is effective. The trick is to shift the weight forward just enough that you unload the rears. If you go too far, the postive effect of the weight transfer is negated by the fact that the longitudinal braking force uses up too much of the tire's available force, leaving little

or no reserve for generating lateral force(i.e. you end up with understeer). Remember - any given tire has a maximum total force that it can exert in any direction - if you are using all of that force for braking, then there is nothing left for cornering.

 

Well, maybe I exaggerated just a tad - I use the brakes to plant the front end as I enter a curve, then ease off the brakes so that I can carry some momentum and not lock up the fronts or wipe the tail out. The rear end just follows me wherever I go - the front end needs a lot of discipline to get it to point where I command. To answer another part of your question, I have about 2" of compression travel available - maybe not enough with those springs. All struts are standard length - I may have to section them to get the compression I need, or go to the 250/300 set-up which should reduce/eliminate bottoming and, having a greater differential between F and R rates, help with some rotation.

 

Thanks again.

[jeromio]

Well, I'm actually in Durham (over by the School of Math & Science). You're welcome to come by and take a ride in my car to get some feeling for what my particular CA pivot mod (and a host of others) has accomplished.

 

Just drop me an email...

 

[ May 01, 2001: Message edited by: jeromio ]

[johnc]

Forget bumpsteer for now. It seems that the REAL problem is understeer. Let's come up with a list of things to work on to correct that problem, using the existing hardware on your car.

 

1. Driving Style.

 

As TimZ inferred, this is absolutely the #1 cause of understeer in ANY car. Entering a corner with the brakes on fire and then throwing in an armful of steering will unstick the front tires instantly. I'm not saying that's how you're driving the car, but I suggest that you try a session at your next track event without any trail braking. Entering a corner, do all of your braking in a straight line and then smoothly turn in. Personally I don't trail brake my Z at all.

 

2. Ride Height.

 

Most people don't realize how important this is on a Z given its limited suspension travel. With the spring rates and shocks that you have the car can bottom pretty easily. To start, shoot for 5" at the front of the rocker panel and 5 1/2" at the rear. This rake will place a little more weight over the front giving you better grip and turn in.

 

3. Camber.

 

3.5 degrees negative on the front is great. Don't go any more. Run 2.5 to 3 degrees negative on the rear.

 

4. Toe.

 

Toe out in front will help turn in but increase understeer. Try 0 toe in front as a starting point. Make sure you have the stock toe measurements in the rear. Unless you are running rims over 8" wide in the back you can't run more than 1/4 to 3/16 toe in in the rear.

 

5. Caster.

 

As close to 6 degrees positive in front as you can get with the camber plates, bushings, etc. that you have.

 

6. Tire pressures.

 

30 psi front cold and 30 psi rear cold on the Kumhos. If you have a tire pyrometer try for temps betweem 150 and 180 event across the face. If you don't have a pyrometer, bleed off air pressure to about 35 psi when they are hot.

 

I suggest you forget about any hardware changes until you've established a good baseline using the stuff I mention above. Once you've got the car as balanced as possible using alignment and tire pressures then start looking at hardware.

 

BTW... all of this assumes the hardware on the car is in perfect condition. Look for any asymetry to identify a bum shock, etc.

[TimZ]

quote:

---

Originally posted by jpd280z:

OK, spacers are OUT! So much for the quick fix.

 

(ME)

For this reason, I do believe that the CA pivot mod is a good thing to do, just don't expect it to zero out your bump steer curve.

 

(jpd280Z)

Not expecting zero BS, but an improvement is an improvement. I might try your BS measurement trick - could this be done by measuring the front and rear track width of the tires at different heights? You know, groove to groove?

---

 

Well, here's the thing... the pivot point mod does essentially the same thing to your suspension geometry as the spacers, so there is the distinct possibility that it won't be an improvement at all.

 

As far as just measuring the toe from groove to groove, yes you can do it this way, if you want to get a feel for whether you have a problem, or whether things have gotten better or worse. I would highly recommend doing at least this, if you do the pivot point mod. If you decide that you want to dial out the bumpsteer, you will need to do each side individually.

 

That said, as JohnC pointed out, I probably did get a bit carried away with the bumpsteer discussion - I just think that this area could use some pointed discussion.

 

I completely agree with John that you should try to find out what's going on with the understeer in your current setup before making such major changes. Try the changes that John already listed out - those are all excellent advice.

[DavyZ]

Are we still coming over for beer and pizza?

[Guest Anonymous]

Originally posted by DavyZ:

Are we still coming over for beer and pizza?

 

Yeah, sure, just wipe yer feet if you go inside. In the garage, all's fair.