Todays measurements are in!
The cardboard centerlink "plate" was more than strong & stable enough to get some good measurements


This is the tough 2* caster setting that needs the most correction

Still using the bump stud to move the outer end up & down a little bit. 3 variations shown.
But Bump-Out (at 1.0 inch compression) was reduced from the original .185" to .009"
The curve with the .016" shim has .015" toe-in on compression, and .006" on rebound.
With a slight shim adjustment that could probably be evened out.

Woo-hoo!

Now I just need to go figure out why the curve came back......

 

In this Motor Trend / Global West article with their new aluminum spindles they rave about getting the bump down to .015" at 1" compression, on a 65 GTO.
So I think we are there.
Trying to make it even better might be pointless.

https://www.motortrend.com/how-to/16...u-need-to-know

This video is pretty good too.

 

Note that the VanSteel steering arm moves the outer tie rod attachment both down and inboard. That should significantly change the bump steer characteristics (for the better) and provide faster steering. I think having a custom steering arm built that would fit the OE C-3 spindle, but incorporate the geometry changes of the new VanSteel piece would be fairly easy and would probably be less expensive than fabricating a more complicated and heavy drag link.

 

A BIG THANK YOU to Leigh1322 for his large outlay of time devoted to this front suspension research.

I have been following this link closely as I have made suspension changes to my 1978 C3 that I have owned since the car was new.
I auto-crossed the car from 1979 through 1986 with mostly “Stock” settings except for Goodyear NCT 255/60 x15” tires, Koni Shocks,
bigger GM sway bars (1-1/8” FT / 9/16” Rear), one coil cut from the front springs & longer rear spring bolts, thus slightly lowering the car.
With these settings the car handled well enough to be slightly competitive in local SCCA events 30 + years ago.
The car also handled well at higher speeds on the highway.

The car now has had major changes to the engine-transmission drive systems. The car has been lowered more
( Front “Z” Height is now ½” vs. stock height) The upper control arm cross shafts have been slotted thus allowing added caster settings.
( All 1978’s were Power-Steer Cars – thus the tie-rod steering arms are in the PS holes )
CASTER setting of 5* positive degrees ( Slotted Upper A-Arm Cross Shafts )
CAMBER 1/2 degree Positive. ( FRONT Tires & REAR Tires )
TOE-IN 1/8” ( FRONT & REAR )

The stock type front springs (450 # ) have been cut to achieve a lowered front “Z Height” of approximately 1/2 inch.
Upper A-Arm Spread-Bar Brace with Heim Joint ends.
The rear spring is a VBP 390 # Single leaf fiberglass spring.
The rear trailing arms are VanSteel Offset with “Johnny-Joint” front bushings (No Rubber)
The rear Stut-Rods are Global-West Heim-Joint end Rods.
The wheels are 17 x 9” with zero offset & 255/50 x 17” tires all 4-corners.

This setup is VERY Twitchy ( Evil-Handling Car) at 75+ MPH on rough interstate highways.

Lee’s research shows a setting using only 2-degrees of CASTER plus tie rod blocks may reduce toe steer upon compression?
These changes that Lee has found may cure my lowered suspension twitchy handling?
Again, the “Z” Height on my lowered cut spring front-suspension is less than 1-inch.
Specifications for Z height:
Stock Z height = 1.95” / F-41 & FE7 = 1.35” / Chev. Power Race Manual = 1" to 1.25”
Front Fender Height = Approx. 26.5" / Rear Fender Height = Approx. 27.25"
I hope to make some changes & add a careful alignment later when the garage is warmer.

AGAIN A BIG THANK YOU TOO LEE

 

I am still planning more testing. Pondering which new centerplate re-positioning to try next.

Silver 7t8 I get it.
On paper it looks like you put on all the right parts, but it's still twitchy. Very annoying.

Our car has 4" of tire scrub radius. Kingpin axis to tire center. Nothing we can do about that. The tires will give strong feedback.
But that makes any bump-steer multiply that kick-back to the driver's hands.
Wide tires do the same.
And changing the wheel offset to the outside also does the same, by increasing the scrub radius.

5* caster should help get rid of some of the bump-steer.
But if you look back and find the 4* or 6* PS chart, I believe you still have very significant amounts of bump steer. Like .075" at 1" compression.
I would add the Van-Steel or Howe bump steer studs and drop the outer tie rod ends as as far down as they will go..
That will help cut it in half.
Leave the 5* caster, do not go back to 2*, that would raise the steering arm, wrong direction.

If you are still not happy, go for tubular upper a-arms, and try 6 or 7* caster, to lower the steering arm even more.

I am trying to come up with a centerplate solution, that will give us even more adjustability.


One more thing I would check is to make sure you have not over-lowered the car.
Check your front lower rubber bump stop clearance.
If you have lowered the car to the race car Z height, you will need to cut or remove the lower bump stops, so that you have some suspension travel before hitting them. Not doing so will give you evil handling.

I also assume you are measuring your front Z height to the bottom of the ball joint stud, try 3/4" higher to the bottom of the spindle. Then you Z=1/2" turns into Z=1.25", which is the race car spec, and that seems to jive with your fender height. About 1.5" lower than stock. Regardless the upper bump stops need to go. You need a minimum of 1.5" at the bump stop itself, for ~2.0" travel at the wheel. The powerbook says cut the lowers to 0.6" height or remove them. I do not recommend lowering the car beyond the race specs, there just is not enough suspension travel.

And BTW, lowering it aggravates the bump steer issue, sometimes dramatically.

Front wheel travel specs vs Z height:


There are eight Z heights, depending if you have 300lbs in the front seats, or not. And then the race one, which is 1.00 to 1.25"
Stock fender height (P) is almost 28" (27.9") with no one in the car.
Your quoted fender fender heights seem almost dead-on for an F41 setup, just a little lower in front, like 1/4".

 

Shoot me some pics and some rear suspension specs also. Height etc.
It is possible, if you lowered the car a lot, you have moved the roll center heights too much.

 

It has been really cold in the garage this past week. 29*
Hard to get motivated.
Got my heater out there fixed, and good to go now, 57*

In the meantime I was doing a lot of reading, and pondering my centerplate results.
They were not what I expected. At all. (That is why we test.)

It finally dawned on me the reason these two curves flipped left & right.
The original tie-rod was 16" and had a straight line.
Both of these are curved, and 2 different directions.
Why?
Tie-rod length.
The bump block caused the tie-rod to shorten by 1.25", (14.75") and yield the blue curve. The tie-rod was too short.
When I built my centerplate, I did not use the bump block, but I moved the inner point 1.25" to the inside, making the tie-rod too long, at 17.5"

So I listened to what the car wanted.
It wanted the stock length tie-rod. Of 16.0"
Duntov knew what he was doing and this car had a highly unusual perfectly linear bump curve, to start with.

It also means another Internet Myth BUSTED
Every how-to correct bump steer thing I have read online, said this pic below would give me the correct tie-rod length.

Every article said my tie-rod length had to be on this line.


So I made my tie-rod length match that line. AND IT DIDN'T WORK!

I realized I was better off with a stock 16.0" tie-rod length.

So using the 16" tie-rod I was able to measure these two linear bump curves.
The blue line is absolutely stock OEM. A whole lot of toe-out going on.
I could not lower the outer enough to straighten it, so I raised the inner instead.
The green curve was tonite when I moved the 16" inner tie-rod directly above the current inner location, by 1.5" up. Way too much toe-in.
Exactly opposite the above.

I realized I needed something about half-way in between.
Or .5 to 3/4 to 1" up on the inner.
I raised the inner .875" above normal and got this bump curve.

FINALLY!
Almost perfectly straight up, and nearly. linear.

Only .005" bump up and .008" bump down. WOO-HOO!

Now obviously my mock-ups today were very temporary. I just have to figure out how to build it this way.....


I raised the idler arm up as far as it would go, and raised the centerlink 0.875" It worked!

 

I have run into this issue when designing my own suspensions - generally when converting from rear steer to front steer, which requires entirely different steering arms. Because most steering arms are designed to introduce some Ackerman, the outer tie rod end does not fall on a line between the upper and lower ball joints. This is true of the C-3, and even more so with bump steer blocks. As you point out, in order to retain the proper tie rod length, the inboard tie rod end must be moved inboard. Or, in the case of front steer, the inner tie rod end must be moved outboard. On front steer setups I adjust by choosing a different rack length. The following article has a pretty good discussion: https://www.hotrod.com/how-to/ctrp-1...teer-explained

 

2 degree Caster Finale

To summarize what is going on at the factory 2* Caster setting and my efforts to correct it:

Stock Bump Curve:

A completely stock std suspension car will roll up to 2.0" in a corner. This gives bump steer over 0.250", on each wheel.
Modern 450-550# springs and stiffer sway bars will cut this bump steer in half, just by limiting body roll at max G to 1.0"
Stiffer roll:

Pros: less roll, less bump, quicker response everywhere. .360" bump at 2" goes to .185" at 1 inch, all stock, no mods
Cons: still twitchy when 1 wheel hits a bump

Solution #1:
Add an adjustable bump steer stud


A shim adjustable bump stud, installed on the steering arm end, can drop the outer tie rod end down roughly 1/2" from a stock joint.
This reduce bump steer at 1" from .135" or .185" down to .080" or .104" (MS, PS)

Pros:
Better. Much better.
And it is an easy bolt-on. No permanent mods.
Cons:
But as you can see this is still not enough correction, to get it below .010"
.080" or .104" will still cause twitchy feel on one wheel bumps

The outer tie rod end really needs to be lowered 1.30" for std ratio, or 1.8" for fast ratio, to get a vertical bump curve, starting with the arm at a 0 caster / vertical spindle position.
Moving to 2* caster provides a .25" of the necessary arm lowering.
The bump stud can provide another .40"
We still need about an inch more lowering.
Another .52" (std ratio) or 1.0" for fast ratio.

Solution 2:
Install a long 5/8" bolt instead of the short bump stud. Add the heim joint and spacers to that. No one makes a bump stud that is that long, that will fit our 7* taper, in our steering arm joint connections. You would need to drill out the steering arm to 5/8" diameter, and use a standard grade 8 bolt. I am not crazy about doing it this way because that would be a very long bolt, and it would have a lot of leverage on the steering arm, which could lead to flexing, or fatigue and/or failure. And it means modifying stock parts. And there could be tire sidewall interference issues. It would not be a bolt-on. But once you space the tie rod heim end down enough, you could have .a terrific bump curve. You could wind up with this and bump under .008" :

This curve was measured with the bump stud in the neutral position.

Pros:
Could be a linear bump curve.
Cons:
Permanent drilling mod to stock steering arms.
Bolt needs to be 0.90" or 1.4" long, below the s. arm, strength issues unknown.


Solution 3:
Raise the inner joint 80% of the required lowering of the outer joint. The objective is to get the angle of the tie rod correct, and for it to intersect the instant center (IC) of the upper and lower a-arms, which is out near 80". Raising the inner joint works just as well as lowering the outer. With the outer bump stud still in use, we still need about 1/2" or 1.0" of downward correction (outer), or .40" or .80" of upward correction on the inner. I confirmed this by making the above chart measurements with the inner raised .875"

There is a big issue with this. I do not see how to make a bolt on part. The centerlink itself is 1.50" thick at the inner joint location, and the least I could move that joint upward with a plate is 1.50" Which is too much correction. So the bolt-on centerplate idea will not work here.

There is another way to move the centerlink upward, and that is to raise the whole thing, by altering the idler arm and the steering box. I do not see a way to bolt these 2 items 1/2" higher. But both the idler arm and the pitman arm could be heated and bent upwards the required 1/2". These are both bolt-on parts. So it may be feasible to offer modified parts. Torch heating them to red hot would ruin their heat treating, since both parts are forged. I would have to investigate whether they would need to be re-heat treated. I have to admit, I bent my arms in the '80s, and never had them heat treated again. Maybe I was just young and stupid. LOL

Pros:
Perfect linear bump curve
Cons:
Can't figure out how to manufacture, too many things in the way vs where it needs to be

BTW I found this quote from Duntov, in the GM Performance Book about corvette chassis mods. And they ran 2* caster back in the day. It supports my measurements. They back then. Surprise.



Solution 4:
Use the bump steer blocks. This is the only simple way to lower the outer tie rod joint the necessary 1.30" (std) or 1.80" (fast) Guldstrand originally made these block in 3 thicknesses. They are not hard to make. They do require drilling out the steering arm hole taper to fit standard bolts, so that bolt on part needs to be modified. Duntov Racing does currently manufacture and sell these in the 1.25" drop version, complete with steering arms. The bump stud should still be used for fine adjustment. One negative that this mod does, is it shortens the tie-rod by 1.25", from 16.0" oem to 14.75" This is not ideal, because it introduces a significant curve into the bump graph, like this:

The oem bump numbers, at 1.0", are .185" toe out in bump, and -.215" or toe in, in rebound.
The improved numbers are -.024" or toe in in bump, and .037" toe out in rebound.
Vastly improved. Not perfect, but pretty darn good.

I would suggest still using the bump stud, and juggling it's height a little bit, just to even out the curve and make it even in bump and rebound.
Realize this is at stock ride height. Z=2.5"
With a curve to the line, lowering the car would put you in a different portion of the curve at rest, and adjusting the outer spacer will be necessary.
But if you look at the curve, lowering the car 1.0" might just put the car in the sweet spot of the curve.
Just bolting them on would give a good improvement.
But the curve could be fine-tuned, with the spacers.
Raising or lowering the ride height would require a new alignment to correct the static toe-in setting. 1/16" toe is .062"

Pros:
Simplest, easiest and inexpensive bolt-on.
Bump steer down from.185" to .024"
Cons:
Not linear. Sensitive to lowering. Not under .010", but very close.
Permanent drilling mod to steering arms.

 

Did not know that, I'll have to see what I have. I bought mine from VBP. But I don't understand that I have a position choice, as the blocks use both holes.

 

I'm glad you are following along.
Guess I slipped up!
I'll fix that.
Thanks.

 

Delete

 

Leigh,
I went back and looked at the Van Steel steering arms that mfain purchased.
Wouldn’t the Van Steel steering arms work better than installing blocks?
They lower the connection location of the tie rod and they don’t shorten the length between the connection points.
In addition, they appear to be aluminum which should remove some additional weight.
I'm learning as you go so you can scold me if needed!

 

Sorry Greg, but the Van Steel arms won't bolt up to the stock C-3 spindle. They are made to sit against a flat surface, and the OEM piece has a ridge on the back that requires a "bow" in the steering arm between the bolt holes. Van Steel might be approachable to build compatible steering arms if there is sufficient demand, but I suspect they would rather sell the arms with their spindle. It is a nice piece. I will have to use custom steering arms for my application since I am converting to a front steer arrangement.

 

In a critical application like the steering arm which will bear the entire thrust of the car, without a bending analysis, I want a piece of steel there. The weight savings is insignificant. And AL and FE have equivalent strength only when they weigh about the same.

I’ve been in and out of this thread and only cursorily thinking about this.

Back in the day when I was building my car to be a pretty good autoX’r, my thinking was that I wanted my car NOT to contribute UNWANTED steering input. My understanding at the time wasn’t as sophisticated as what you are doing here Leigh. I wanted the car up off the lift but not without taking advantage of the situation. The bump steer blocks got me to the good enough point that I could move on. And also it looked to me that they fixed the Ackerman situation.

I want to make two comments here where you are now.

1). I think that a car in a turn that has some body angle roll should have a Bump/Rebound curve that say Toes-In in Bump and Toes-Out in Rebound, as in solution 4, keeping approximately the same total toe. That is because the roll angle puts one side in Bump and the other side in Rebound. That sounds right to me, but is it? Do I have that polarity sense right from your graphs? If I do, that is unlike solution 2 say, which adds Toe-In in both Bump and Rebound. Wish I had measured total toe then.

2) No more AutoX for me, but CA’s roads suck, so any unwanted steering input makes driving more unpleasant, and I still want neutrality for bumps just on one side of the car or the other to even things out. On an aside here to this, I have lots of caster so I have lots of turn-in. Unfortunately I also have lots of camber, 2.4 degrees which I haven’t set back to something more sensible. So I have lots of camber thrust and I have to get after that when some warmer weather appears. My God it gets down into the 40’s some nights. What a wusss you might say, but I’d have to leave the C8 out, and rodents around here seek comfortable places to sleep those chilly nights.

3) Yeah, I said two, but I still want to make a turning diameter measurement and post that to see what this bump steer change has done to that, if anything

 

[QUOTE=ignatz;1609430504]In a critical application like the steering arm which will bear the entire thrust of the car, without a bending analysis, I want a piece of steel there. The weight savings is insignificant. And AL and FE have equivalent strength only when they weigh about the same.[QUOTE]

Almost all modern cars use aluminum spindles and steering arms. I have mine built out of 7075 aluminum, and they are plenty strong for a race car with huge front slicks. The only thing I question about the Van Steel arms is the minimal meat around the tie rod mounting point.

 

[QUOTE=mfain;1609430637][QUOTE=ignatz;1609430504]In a critical application like the steering arm which will bear the entire thrust of the car, without a bending analysis, I want a piece of steel there. The weight savings is insignificant. And AL and FE have equivalent strength only when they weigh about the same.All the other kids are doing it didn't work with my Dad. I still want to know somebody has done some math.

 

My goal is to have a good handling street car with limited bump steer on our horrible California highways
I may go play on an auto-X track some time just for fun, but mostly street driving.

 

If the toe-out in bump is the same as the toe-in in rebound, they would cancel out in a turn, as one side goes up and one down.

The biggest issue is during a one wheeled bump, either going straight, or in a turn, and you suddenly gain and then lose 1/8" of toe, or more. It feels exactly like someone jerked on the wheel.
When it is bad enough, the car will have time to move left/right a foot or so. Very nerve wracking. Our car has considerable tire thrust, like 4 inches, so this magnifies the feedback to the steering wheel. I once drove a car that hopped 18 inches right, and then 18 inches left, repeatedly. That was just absolutely terrifying.

Norval said he had a dip near his house that nearly put him in the ditch a few times. Then he fixed his bump steer, on his c3, and it was fine. And he was a college engineering professor. But I do not think he has been on here in a while.

On the aluminum steering arms, they would be expensive, but much less so than a steel forging. The bump blocks would be much more affordable. There is really only about 140lbs of pressure on them during normal driving. ~7# at the steering wheel, times the 20:1 steering ratio. Any big lateral 1000# tire traction thrust would be handled by the lower ball joint. And that shaft there is only a 1/2" diameter forgings. Almost none of that force would be transmitted to the steering linkage. An arm would just need to be stiffer than that, so the driver doesn't feel any flex. Somewhere between 65 & 70 Duntov made the centerlink a larger diameter, after racing driver feedback. My 72 version is a 1" diameter. I heard the earlier ones were thinner. That is a very long shaft vs the 7" steering arm.

I think it may make sense to pursue the bump blocks, and see how equal I can get the bump. 2*, 4* and 6* caster will all need a different height block/correction.
Much like Guldstrand developed years ago. Someone sent me this recently:

 

Today's measurements using the 1.25" drop bump block.
Just to find out how sensitive the shims are on the bump stud.
They make a lot of difference!

Basically .020" less bump for every 0.1" less shim. At 1.0" bump.

This bump block seems too large when used with the bump stud & hiem joint end.
Tomorrow I'll measure it with a normal non-adjustable tie rod end.
That may be similar to the green line, or maybe not. I have been surprised many times doing this.
Looks like I need to move it up another 2 or 3 tenths.