Gentlemen, thank you for these pages & pages of discussion. A long time ago I took off (and still have) a drag link from a SCCA AP Corvette built in the 70s. The drag link has had the two tie rod holes moved upward about 1/2". Nice job too.
So what did that achieve.
Thanks Scott

 

We would love to see a picture of that!
Did they bend the entire link, or just add holes on top?
Anyway that is one way to get rid of the bumps steer.

 

2* Caster Bump Block Results

I am getting closer to a terrific bump steer curve.
I was hoping for a linear curve, with a bolt on centerplate, but that turned out to be extremely difficult.
The use of bump blocks generates a strong curve. That makes the final adjustment ride height sensitive, to get the flatter portion of the curve to center at ride height. But even at 1.5" movement either way, there is not a lot of bump steer.

I had to replace my garage heater before I could continue. It has been cold out there! With the prolonged cold snap it was almost impossible to find a replacement. It has been decades here since the snow stayed on the ground for this many weeks in NJ. 3 or even 4 weeks now of nasty cold. 64* in the garage today!






As I was changing different block sizes I had to come up with a new way of keeping track of my measurements. This is graphed as total drop from the bottom of the steering arm, to the center of the tie rod joint. These are all heim here.

The Dark blue curve above had only .013" toe-out on bump, and .003" on rebound. At 1.0" movement. Terrific results. We are there!
1.5" is about the maximum suspension movement you would ever get, and it still stays under .030" there.
That is good enough for a race car, and I dare anyone to say they can "feel" that little bump.
Another .050 or .100" shim might make it even more centered.

My 0.70" bump block had a bump stud, the center of the heim joint was 1.68" below the steering arm. It had a .30" shim above it to get it there.
So this was basically the same as a 1.0" block with no shim.

Not coincidentally, that is the thickness of the old VBP block. (Surprise!)
However there was not any information about them as to what alignment setting to use them with, what the actual bump curve was, or whether you needed a bump stud or not. Now we know.

And my block is even thinner at 0.7", to allow for use of the bump stud, which allows fine adjustments.
Realize a block this thickness is only going to work for 2* caster.

I will finalize the final thickness, so that it works with either an normal tie rod end joint, or a bump stud. And allow for enough adjustment for when someone lowers the car. And check the left side of the car also. Murphy's Law says they will not measure the same.

Then I will contact someone about having these produced.

4* caster and 6* caster alignment settings will require different height blocks than these, but they will be shorter.


My prototype wood 0.70" bump block, with bump stud, with a .30" shim, at just the right drop, for 2* caster.

 

Amazing work and results Leigh!

 

The math on this one says if you raise the inner tie rod end by exactly 1/2" (as mentioned in the GM Powerbook), and run 2* Caster, and run a production tie rod end, which drops about 1.2" below the steering arm, you should wind up with near the same Blue bump curve as above, with under .010" bump. Do you think maybe Duntov's engineers knew what they were doing? LOL

Now how do you raise a hole on a centerlink, that is 1.5" thick, and the hole is .5" in diameter, when you only need to raise the hole 1/2"????
Cut off another round section and weld it in??? Boy would I love to see how they did that!
The metal around the current hole is about 1/2" thick.
So maybe take a pac-man style bite out of the current centerlink, and weld the new hole in?

 

Since you used the word "math" here Leigh, I just want to verify something. Your dial indicator measures inches and is sitting at 14" off the axle, correct? So let's say I use 0.050" as kind of a "dead band" zone to various "really good" bump steer curves that fall within that ->(-0.050" - +0.050" )<- zone.

That calculates out to plus or minus two tenths of a degree. A really small angle in the sense of being able to steer a car to that level of accuracy I think*.

Have you ever wiggled your steering arms and seen what the excursion is in all the slop in the linkage? Like maybe fixing one end solidly and then wiggle the dial side (I think that's the driver's side).

I would suspect anything within the say 0.050" limits is as good as things could get mechanically given everything else, and that is not even considering what tires do or don't do.

_____________________
*The "math" I haven't done is to relate that angle to an actual thrust on the front end. I might take a stab at that if you can verify what I just wrote.

 

Interesting thought.
Everything feels pretty solid, but I may just have to try to measure any "deflection"

 

2* Caster effect on ride height

A little more progress today.
This was basically a test to see what effect changes in ride height have.

I fine-tuned the bump steer at my Z=2.5 ride height to .007" in bump and .013" in rebound. (red curve)
It is almost exactly the same, above and below the center, or my chosen ride height.
It is about as perfect as I can get it.

At this level a the thinnest .03" bump stud shim moves the curve around.
That is how close you have to get it, because of the curve we are stuck with.

The blue line is the same as yesterday. It is centered at a -1.0" rebound level or 1.0" higher ride height than red
The red line is slightly improved. It centers at ride height.
And the orange line is too far, but useful if you lower the car. It is centered at +1.0" bump.

If you lower or raise the ride height of the car 1.0", to a different ride height, you need to add or subtract a .03" or .06" bump shim.
But then you would wind up with the same bump curve at +1" or -1" suspension travel.

The bottom line is changes in ride height do have an effect, (because of the short tie-rod) but very small shim changes can correct it.


Imagine you lowered the car 1.0", now the +1.0" bump horizontal line becomes your new ride height.
You would want to use the orange bump curve, by adding a .07" bump shim.
Then you would have almost no change above and below that 1.0" ride height.

 

4* Caster Final Results:

My new measuring system is really helping me speed this up now.
I nailed this one on the 2nd try.
A 4* Caster setting drops the steering arm 1/4" below the 2* position.
It should come as no surprise I need less shims, to keep the tie rod at the same spot.
With the correct size block, I think one block can cover both alignment settings.
And then the bump stud shims can make up the .25" difference.

These are measured:

Bump is +.006 up and + .011 down
Not quite under .010" darn it! LOL
It worked out to be almost exactly 1/4" less drop than the 2* setting. Heck my caster setting might not even be that accurate!
This was with the 0.70" bump block, and a bump stud with a .16" shim, for a total drop of 1.54"

 

4* Caster with no Bump Block

I got annoyed with all those strong C-shaped bump curves.
And tried to get a linear one.
Finally did

.001" and .004" bump!!!
Yes at one inch travel!
And since it is linear ride height would have zero effect on the bump.

But I really cheated to get this "linear" bump curve

The bump stud does not really drop the tie rod end enough.
It is not that much longer than a regular rod end.
A regular tie rod end drops the "pivot joint" 1.2"
The Bump Stud with all of it's .7" of shims, only drops the pivot joint 1.5"
It can basically adjust the heim joint either .35" up or .35" down from a fixed tie rod end.

It's just not long enough.
I looked allover for a longer one and could not find one.
So I made one.

Since the bump stud could only drop it 1.5", I made one out of hardware bolts that could go another 1/4" A 1.69" drop was the sweet spot.

I would rather have a perfectly linear bump curve.
If only there was some way I could make something like this.
I wonder if a machine shop cam turn down a long 5/8" grade 8 bolt, put threads on both ends, put the taper joint on one end, and turn it into a longer bump stud?

 

Here is a thought. Have you looked at the Howe Racing tie rod ends? Duntov Classics makes bump steer blocks that have a 5/8" threaded hole rather than the tapered pin one, and Howe has a really long stud option. If you look at my post #136 above, you will see I am using the Howe tie rod end. Works great with easy adjustments.

 

Now that's what I am talking about!
That could work.
It would require drilling the steering arm to 5/8" hole though.
I am attempting to make a "bolt-on" part that requires zero mods.

I have the Duntov blocks and they are 1.25" tall. For the life of me I can not figure out why. They must have other suspension mods in place as well for those to work.
For stock a-arms the block should be a max of 3/4" tall. And even then the curve is very strong.
No block at all with the stock 16" tie-rod gives a perfectly linear curve. Why change it?

I am the kind of guy who if I can't find what I want, I'll just make it.

If I could make a bump stud like this Howe one that I do have, but it needs to be about 1/4" to 1/2" taller, it would work terrific, with no bump block at all, and give the straight vertical bump line above. And slide right into the stock steering arm.

Why couldn't I take a long 5/8" bolt and cut it down, make the slide-in 4.8* taper, and put another 7/16" or 1/2" thread on the top of it?
Even if I need to have it heat treated afterwards?
Is this crazy?


Howe bump stud, disassembled. And 2 extra plain bolts


Allstar makes 3 variations of bump studs. Only the one on the left has the taper we need. The other two are Pinto taper studs . But the one on the right is the length we need. Also note the diagram has a typo. The stud on the left has a 1" per foot taper, not 1-1/2" like the other two. I tried the longer 286 one, it didn't work. The Pinto studs have a 7* taper, 1-1/2" per foot. But it is 4.3" tall vs 3.3". 4.0" long might be ideal for our cars, even 3.75". A stock tie rod end is already 3.3" tall, so it is not that much different.


This is a Duntov Racing steering arm where they drilled both .5" taper holes out to 5/8" dia for bolts. I would rather not have to drill out the steering arm, if possible. More guys would like the result if it was just a bolt on. So I need to try to keep the .5" taper fit.

 

Just a thought Leigh.
There are many forum members who professionally race vintage C3 Corvettes.
Maybe go to the those forum sections and ask what those guys have done to get their front suspension and bump steer dialed in.

 

I've been following this all along. Here's what I'm taking away so far,
Every different castor angle and every different ride height will require different height settings of the tie rod end to get the bump dialed out.
What if my ride height is slightly higher or lower than these tests? What if you have 4 1/2 degrees castor on one side and 5 degrees on the other side?
How does the guy at home under the shade tree get this all figured out?
I can see why the factory never got it perfect.
Perhaps in the end you will have all the answers. For now, I'm watching.

 

Good comments guys.

The bump blocks and the raised centerlink were both common racing mods. They also would not hesitate to drill the steering arm and add a long 5/8" bolt.
I have tested those. They all work, in certain circumstances. There is no one-size-fits-all answer.

IMO being able to fix this fix without altering the steering arms means more guys are likely to try them.

Neither of the first two work well on a stock a-arm suspension. Most racers would also have other mods like taller spindles, taller ball joints, etc. I have those also, but have not even tested them yet.

If I can keep the bump steer response linear, that removes the ride height issue from the equation. And makes it much easier for someone to apply the "fix" without running their own bump curve. I just checked tire sidewall clearance tonight, and the longer bump stud still has plenty of clearance.

The bump blocks are much easier to make, but give a pronounced C curve, which makes the bump correction very ride-height sensitive. Not as desirable of a result.

A 1/2* difference in caster has very little effect on the bump curve. 2* is a 1/4" height difference, so 1/2* is only 1/16" height or shim difference. Getting the shim or arm height to within +/- 1/16" (.062') seems to be the key for good results. You only need to nail it to .030" if you want amazing results. .030" would require your own bump gauge. .06" or .1" may not.

There are a lot of combinations. But I like doing the research, and the required mental exercise.
Hopefully this will all be useful to others in the end!

In the end I see no reason why a chart could not be develeoped.
If you have "X" caster, use this parts combo, etc.
There should be very little need for a customer to run his own bump steer curve unless he just wanted to get it really close.

 

6* Caster with no Bump Block

More final results!
No bump block, just a bump stud.
The currently available Howe style bump stud almost makes it perfect.
If I could drop it just a little more it gets even better.
But that would mean a longer bump stud.


The green line is the Howe style bump stud with all .70" of shims, for a 1.50" total drop.
The bump steer is a very low +.010 & -.011, at 1.0" up or down.
Because it is linear, ride height will make no difference.
I improved it just slightly, with a little more lowering to 1.55", but that was my home made bolt method. Bump dropped to -.004 & +.003. But that extra .050" shim just will not fit on the Howe bump stud.

The currently available bump stud is the easy choice here!

 

Leigh,
Can you please post a part number and picture for the Howe bump stud you are referring to?
If it is better than using the stud that came with my Van Steel bump steer kit, I would like to purchase a set of the Howe bump studs, especially if there is no modifications needed to the oem parts.
Thanks again for all your work on this!

 

OCB
I am using the Van Steel Bump stud.
To me it looks exactly the same as a Howe Bump Stud.
I have two different length tie rods that I am using, depending if I use the bump block or not. They vary by 1"
IIRC one is 10.5" and one is 11.5"

 

If you manage to summarize all this, that is an important point. Knowing what I did required drilling out my arms, I kept my originals and bought another set. Still have the originals.

 

Perfect!
Wow, I don’t even need to buy anything.
Things are looking up!