Bump Steer Spacers

Would the alignment machine be able to tell you the degree of bump steer you had prior and after the shims?

Cap'n, if your '73 has a stock bushing in its rack, without a doubt it is shot. Stock bushings were made of a steel case lined with neoprene with a bonded plastic bushing rubbing the steel rack-gear, and most did not survive 15000 easy miles. Likely nothing is left except the steel case and a thin steel shim-washer. You can verify this by jacking up the right front tire, reaching up and grabbing the steering shaft and pushing up hard. If the steering rod moves up any amount near the aluminum rack-body, the bushing is worn or non-existent.

To clear everything out, the rack will need to be removed and almost completely disassembled, so a long rod can drive out the steel case and washer. Instructions are on www.Panteraplace.com website, or some vendors do a mail-order trade-in, supplying a rebuilt rack for your core. Call around.

About measuring bump steer. I do not have an alignment rack or the knowledge to do that.

After talking to Quickity, it needs rebuilding.

Your '73 should already have the shims installed from the factory. Certainly '74's do.


Measuring bump steer is a PITA. You need to move the suspension through it's entire travel to measure the changes in toe in/out.

Generally speaking this entails removing the shock and spring so that you have easy free access to do this.

I don't think it is possible to completely eliminate it. Just reduce it.

On the Pantera it is more significant with a bias ply tire like the original Arriva Goodyear where the walls don't flex much.

On a radial tire, the tire works by allowing the wall of the tire to flex. A controlled flex.

That flexing is going to be likely much much greater then the toe changes.

Also, by changing the spring rates and keeping the car away from maximum suspension travel, which is where the maximum toe change likely is, you also reduce the change.

Unless you are running a wide body car with 10 inch wide (or wider) front wheels, this is all insignificant.

If you are racing on a big track and entering turns at like 170 mph, then this becomes very significant because you are trying to hit a very specific line through the turn other wise you will spin the car.

Race cars use different tires, spring and shock rates which coordinate with this need to eliminate or reduce the toe change.

On a street car with radial tires you aren't even going to notice the toe change unless you hit a bad pot hole and the suspension suddenly drops 12" or so.

Measuring the toe change has nothing or very little to do with the machine...just the use of the platform maybe as a lift?



Most cars live with no problem with FRONT toe changes.

On an IRS car, toe change in the rear can be lethal. You need to maintain toe in there at the risk of life and death if you don't.

What will happen if you suddenly get rear toe out, the rear will steer and if you are lucky you will just spin the car and not hit anything or run off of the pavement into a soft shoulder and roll the car.


Irregardless, toe IN is always your friend.

quote:

Originally posted by PanteraDoug:...
Measuring bump steer is a PITA. You need to move the suspension through it's entire travel to measure the changes in toe in/out.
Generally speaking this entails removing the shock and spring so that you have easy free access to do this.
...

Just my keyboard mechanic thinking, if you had the car on alignment frame measuring toe, couldn't you lift and pull down the front end and measure the toe change versus +/-height change

It would be interesting to measure the steering rod angle as it is the variable you are wanting to change and there should be the angle where the reduction of toe change is optimized (probable need the lower A arm angle to compensate for different prefferances in ride height

If you had a way to safely compress the suspension you could measure some. Stock the springs are something like 500 # per inch of travel. You will never be able to compress the full amount of travel available to the suspension without the spring and shock.

Even if you could do that, you could not compress it the entire route. Only a few inches.

I don't know anyone who wants to do it that way. It is simply too dangerous.

Keep in mind that bump-steer happens when ONE wheel hits a bump and changes the toe-in setting for both wheels (because they're connected thru the steering rack).

Bump steer can be partially corrected by moving the whole rack down & fwd 10mm (5/16") or so, or by switching the steering arm connections from tie-rod ends to heim joints and using adjusting spacers. Rack movement requires longer rack-mount bolts to compensate for the spacers and both methods need realignment of the wheels afterwards. The idea is to level & center the steering rods relative to the ground.

Shimming the rack to reduce bump-steer also reduces Ackermann-steer, another toe-change handling parameter that some find important. Using adjustable-height Heim ends to minimize bump steer does not change Dallara's factory Ackermann settings. More info on this available.

One of the modifications that Shelby did on the 65 Mustang to make it a GT350 was to lower the location of the upper a-arms.

The ideal lowering on a 65-70 Mustang is 2".

The problem is that the lower ball joint would not tolerate that without being shimmed with a wedge shaped spacer.

Apparently neither Ford or Shelby wanted to pay for that.

The compromise was to lower it 1".


I have a '68. I went the radical departure of 2" AND used an angle ball joint spacer.

Now this definitely added handling capabilities in cornering since it put the tread more parallel to the ground as the car rolled in a corner. Good you say?

Now to the point, aka, the issue. By doing this a large amount of bumpsteer was installed into the car.

So much in fact that the car would change lanes, not an entire lane, but about a 1/2 a lane just cruising down the road and either tire hitting an "imperfection" in the pavement that the other hadn't.

If you ask me for what the definition of bump steer is, that's mine.

The car has since had bumpsteer modification done to it and along with radial tires, tracks very straight under every condition I could give it.


The Pantera, even the early one without the factory shim, is a pussycat. You might or you might not think that you felt something in the steering.

With the factory shim, I won't even spill my cup of cappuccino.


I don't disagree with Bosswrench at all with his statement BUT each corner must be able to travel close to it's travel limits while the other doesn't waiver at all.

That is VERY difficult to do, if not impossible since the suspension travels in arcs, not straight lines.


One way of reducing bump steer effect at speed is to dial in as much positive caster as you can.

This really would be very important with a wide body. Much more so than the original slab side design.

5 degrees of positive caster is going to deaden much of a bump steer effect right there without having to go to much further.

The problem with that is you will be at the point of needing power steering assist.

Solutions in one area rarely are done without causing drawbacks somewhere else.
Compromise is the term that comes to mind?

Some drivers don't even know what bump-steer is or that their car has it? They just accepted it, "whatever that was", as a fact of everyday life?

Remove the front springs and the process is easier.

Here I am again. I got my spacers for the bump steer. I jacked the car up, pulled the tires and crawled under. I started noodling out a plan of attack. I found the bolts that hold the caps in place. I started to loosen them and thought, wait a minute. How can the rack be moved anywhere with this brace in place?

quote:

Originally posted by PanteraDoug:
Your '73 should already have the shims installed from the factory. Certainly '74's do.


Measuring bump steer is a PITA. You need to move the suspension through it's entire travel to measure the changes in toe in/out.

Generally speaking this entails removing the shock and spring so that you have easy free access to do this.

I don't think it is possible to completely eliminate it. Just reduce it.

On the Pantera it is more significant with a bias ply tire like the original Arriva Goodyear where the walls don't flex much.

On a radial tire, the tire works by allowing the wall of the tire to flex. A controlled flex.

That flexing is going to be likely much much greater then the toe changes.

Also, by changing the spring rates and keeping the car away from maximum suspension travel, which is where the maximum toe change likely is, you also reduce the change.

Unless you are running a wide body car with 10 inch wide (or wider) front wheels, this is all insignificant.

If you are racing on a big track and entering turns at like 170 mph, then this becomes very significant because you are trying to hit a very specific line through the turn other wise you will spin the car.

Race cars use different tires, spring and shock rates which coordinate with this need to eliminate or reduce the toe change.

On a street car with radial tires you aren't even going to notice the toe change unless you hit a bad pot hole and the suspension suddenly drops 12" or so.

Measuring the toe change has nothing or very little to do with the machine...just the use of the platform maybe as a lift?



Most cars live with no problem with FRONT toe changes.

On an IRS car, toe change in the rear can be lethal. You need to maintain toe in there at the risk of life and death if you don't.

What will happen if you suddenly get rear toe out, the rear will steer and if you are lucky you will just spin the car and not hit anything or run off of the pavement into a soft shoulder and roll the car.


Irregardless, toe IN is always your friend.

I was able to achieve zero bump steer by modifying the width of the steering rack. It's not something many would do.


http://www.rc-tech.net/pantera1/bump/bump.htm


The bump steer shim basically moves the bump steer arc. It puts the arc in a position where bump see's far less change.



Instead of an immediate divergence it spends more time crossing the apex. The blue line represents the start point. It does make droop worse which is pretty much a non issue.

As far as Akerman, you're not going to change the rack enough for it to change much so that's a non issue. You can't really move the rack enough in a Pantera forward or back to even address Akerman.

quote:

How can the rack be moved anywhere with this brace in place?

To answer your question - yes, you will have to remove the brace to fit the spacers.
To refit the brace you will have to drill the brace, or the frame (that's what I did on my car)

When you add 10mm spacers under the rack, you lower it. So the Bill Stroppe rack-brace shown no longer fits. And because the rack is lowered, you need 10mm longer rack-mounting bolts, unless you cut 10mm off the step areas of the stock brackets where the 4 bolts seat. Finally, don't forget all this requires realigning the front wheels. Stock alignment specs assume 185-70x 15 bias-belted tires so if you're using wider radial tires, you get to experiment with toe-in, caster & camber to your liking.

With radial tires, 1/2 degree negative camber won't even show. You have to go with maximizing full flat tread contact in a conering situation. It might be a full.1 degree nevative or even a little more depending on the car roll allowed from the ant sway bar? How much camber you wind up with and how you get to it is up to you. A Shelby Mustang might wind up with -1-1/4 to -1-1/2 a Panter, depends on how much it rolls?

You need to mantain toe in. It has to be around 3/16, maybe 1/4"?

Castor? As much as you can get and still.steer the car? If you go over 5 degrees, count on needing power assisted stèering.

OK, I pulled the rack. I do not see where you can redrill the holes in the bracket/cross brace to fit. The bolts will not fit in the braces stock configuration. Interference problems. I would have to have a new one made or do some major modifications to it.

How important is this brace? I hate to drill new holes in the frame but that appears to be the simple solution.

quote:

Originally posted by captaintobeys:
OK, I pulled the rack. I do not see where you can redrill the holes in the bracket/cross brace to fit. The bolts will not fit in the braces stock configuration. Interference problems. I would have to have a new one made or do some major modifications to it.

How important is this brace? I hate to drill new holes in the frame but that appears to be the simple solution.

There may be an early (before) spacers, and late (after) spacers part for the brace?

I can tell you that my car did not have the spacers from the factory, I added them, and I reused the original bracket.

I don't remember there being an issue with it, or one that I could not deal with getting the bracket to fit back on?

I don't mean this as criticism against you, I mean it as moral support...you can do it. Go get 'em!