From what I've gleaned so far, the lowering of the tie-rod helps with bump steer, and the inboard mounting of the tie rod changes the steering angle. Again, while this alignment may have an "ideal" (at or about where the line from the ball-joints, through the steering arm, meet at the rear differential), I haven't found it listed anywhere. I guess I'll have to measure everything when I install the new front suspension.

 

Yes Ackerman makes the lightly loaded inside tire turn "more" to better match a cornering radius.
However I believe it is Dynamic, and changes as you turn the steering. IIRC I believe it only perfectly matches the corner radius at one chosen speed.
It generates slight over or understeer at other speeds. But it is a lightly loaded tire.

 

Here is the tie-rod and it's effect on bump steer.

As you lower the car, all 3 inner points move down. Their instant centers will likely not move at the same speed. As you add positive caster, the tie rod end only moves down. You correct the tie-rod end position in all directions until it lines up as shown. The Howe style adjusters handle the up/down correction. The Bump steer block also moves it inward, but a set amount.

Interestingly, this scenario also works:


This is a bump-out scenario. The two instant centers do not match.


I bumped-steered my Camaro years ago.
I will do the C3 as well, I just do not have the steering linkage installed just yet.

With a good clear front-end picture at ride height, a lot of this could be graphed / estimated on the PC.

When I do this, I will start with a pic, and make estimates. And then confirm everything with actual measurements.
If someone has a picture now we could get started.

 

Ok you really have my curiosity up now.
I just remembered Ignatz's measurements from 2011. He measured the Ackerman angle before and after the Guldstrand blocks.
https://yacc-by-cliff.blogspot.com/2...1_archive.html
Basically the stock Corvette had very little ackerman steering. The lines coincide "far" behind the car. This would force the inside tire to have large slip angles to negotiate a turn (drag).
The bump steer blocks move each tie-rod end inward 1-3/8" and change the lines to coincide under the gas tank, which is much closer to research recommended length of under the rear axle.
This would create far less drag from the inside lightly loaded tire, as it's turn radius almost matches the turn. Before it would have been pointing too "straight" ahead inducing both "toe-in" and oversteer steering to the outside of the turn.
Like Staley said "it felt much more sure-footed in the turns" aka: predictable.
So the Ackerman change is a good thing.


Ackerman angle with VBP / Guldstrand bump steer blocks


What remains to be seen is what all this does to the bump steer.

 

OK courtesy of Ignatz we now have a rear view picture.
Here is my first attempt to analyze the tie rod length.

The Blue Lines represent the ideal length of the tie rod.
The red line is the current tie rod length. It is far too short. The VBP bump steer blocks shorten it 1-3/8" and make it even worse than stock.
Any of the yellow or green lines would be a correct length. Notice that the stock steering arm location is very close to the blue line, and a better position than the VBP block. It is the inner joint position on the drag link that is far too outward than desireable. A different drag link, with more inbound joint location, and a tie-rod end location outside of the stock steering knuckle, would be much more ideal.

Conclusion:
The VBP block actually makes the bump steer worse then stock, by shortening the tie rod length.
They would introduce a strong curve to the toe-change. Since their radius length is much shorter than the lower a-arm.
This would explain the pronounced curve achieved by both Jason Staley and this VBP chart.

However the VBP bump steer block also changes the angle of the tie rod, and I am assuming it coincides better with the IC of the A-arms.
Because the tilt of this bump steer graph also changes. And the very center, if located properly, at ride height, is almost flat.
Hence near zero bump steer for 1" of suspension movement. That is the most important area anyway. Beyond 1" the short tie-rod makes a sharp arc to the bump steer curve. Generating new drag links and steering arms to get an ideal toe-rod length is extremely difficult, but would yield an absolutely straight purple bump-steer line, even at 2-3-4" of deflection. These are the kinds of changes made by Nascar, race cars, Hotchkis and DSE.



This chart provided by VBP? shows the pronounced "arc" to the purple bump-steer line caused by the too short of a tie-rod.
If the tie-rod were exactly the correct length the purple line would be straight, not arc'd.
Interestingly Jason Staley got almost the exact same results. It almost? looks like the same graph from a couple posts back.
And both of them also had 1" longer upper ball joints which will move the A-arm instant center.

Next up is figuring out the tie-rod angle.

 

So here is the tie-rod angle.
It does not come close to the A-Arm Instant center, so there is significant bump steer. But this is at full droop.
The red line is the actual tie-rod angle, and the yellow would be an ideal angle.

HOWEVER there are many significant issues with this picture.

• First and most importantly, it is at FULL DROOP, and we do not care about that at all. This pic needs to be at normal ride height. We know the IC moves around as the suspension "strokes". We also know we are not going to be correct at full bump or droop because of the short tie-rod.
• I can not see the locations of the ball joints clearly enough, so the blue lines could be off.
• Thirdly I do not know if Ignatz used a 1" longer upper ball joint or not, and I can not see that ball joint, so I "guessed"
• Lastly his measured results were much better than this graph shows.

So until I can get a much better pic, we are at a standstill.

 

Here is a video I did with the Suspension Analyzer program 1-2 years ago.
Focus on the red and blue dots movement. Those are the A-arm Instant Centers and look how much they move around as the suspension moves. It is going to be pretty hard for the tie-rod angle to keep up with that movement. They have a lot of vertical movement as well as horizontal. IIRC this model has the stock car moving up & down 2" in each direction, as well as rolling 3 degrees in a turn.


The second video has a modified suspension, 1 " lowered car, and a 1" longer upper ball joint to change the upper A-Arm angle.


These Instant centers are much lower and require a different tie rod angle.
Plus it is nice that they do not move around as much. And vertically they barely move. YAY. They should be easier to "catch" LOL
The modified car only moves 1" up/down and rolls only 1 degree in a turn, because of stiffer springs/bars.

And this discussion is not even talking about what is happening to the camber changes going on with the poor tires!

So the "VBP/Guldstrand Bump Steer Blocks" definately help in a couple ways.
But can we improve on them as well? Or get most of the results without drilling the steering arms? Very likely.

 

I'm really glad I started this thread, even if I don't have much to contribute. This is GREST info. Thanks!

 

Wow, thank you for digging that blog up. I have a better picture.





Yes it is at full droop but the mechanical dimensions don't change. This aspect is pretty direct as a reference. You can just see the extended upper ball joint. Given I have plotted the red line as shown in this picture, I would say that I have it at the point everything got tightened up but it clearly isn't here. The car is on the ground right now and the easiest thing I could do would be to measure the tie rod length and maybe the sleeve for reference. In the interest you've all shown in this subject, that is the best I could do for now. let me know.

 

Wow yes, much better pic. And you do have the extended ball joint.
But for these purposes it does not help unless the suspension is at normal ride height.
All 3 links have different size arcs, and their angles change at different speeds as you go up/down.
I just wish I could get to my frame right now, and take some pics, but it is pretty buried.
It will be 2-3 months til I get here, but I will.
This analysis helped me enough that I will be buying or building bump steer blocks to test, and longer upper ball joints.

 

If you have a part number for the extended ball joint, please post it here! I have a new set of SPC UCAs (with stock C3 ball joint bolt pattern), Global West LCAs (for semi coil-overs, with 2" drop), and a car on jackstands with no engine. There's no better time to get some measurements!

I can even 3D print test blocks of various sizes.

 

I've been reading this as it goes along. Very interesting reading. One day you guys are going to work this out so guys like me with a stock front suspension will benefit.

 

You could do some of this with a compass, but I agree it would be a chore. The upper control arm is unfortunately missing from any decent view in my photo library. For 'bikespace' here is my setup from SPC back when. I don't know what they are offering now.

 

I've had the Guldstrand blocks on my 68 since I bought it in 84 (pretty sure, even had Guldstrand stickers on the first convertible top it came with). Not sure what the previous owner was doing but it had some quite solid sway bar bushings (Nylon? Stiffer than poly), and also for the long rear bolts. i changed the front sway bar to poly and the rear mounts to rubber. I still have what appear to be Red Nylon (or some solid plastic-type?) bushings for the front control arms. I've pondered changing out those control arm bushings but it's a lot of work and they work.... car has always been a little "harsh" but I've softened it some.

If someone wants pictures, or measurements of the Guldstrand blocks let me know. I'll get off my **** and take some.

 

You are correct about using shorter tie-rods and the kit I purchased came with new shorter ones along with the bump steer blocks.

It's been too many years for me to remember the exact ride height I have my car set at, but I set it by that graph I previously shown. That is data I measured on my car and I set the car's ride height at "0" on that curve. The VIP curve is one from the Vette Improvement Program article from Corvette Fever by John Greenwood. To make sure I was measuring bump steer correctly I plotted my stock curve with the one from the article before I made the changes to the front suspension and measured the new curve.

The caster is more than stock but not terribly aggressive. Think its about 4 degrees if I recall correctly. I used SPC's upper control arms to set the caster when I assembled them. These upper control arms also allowed me to use longer upper ball joints (note grey spacer below ball joint boot on top of spindle) to improve the geometry and modified the camber curve.






Just noticed that ignatz and I did a lot of the same mods. Time is flying bye. I did most of this stuff ~15 years ago.

 

https://howeracing.com/collections/h...cap-fits-k6034

 

That is the one I remember from way back.
7 different lengths to choose from!

 

Hey Jason
Any chance you remember where you got your bump steer block setup from?
I can't read what it says on the rod.

 

Just found an old discussion from 2003.
Norvalwilhelm had this to say about why bump steer is bad:

• "Bump steer is when the wheels turn themselves without an input from you. As the car becomes air born like flying over a hill the front tires point inward or tow in really bad, even 1 to 2 inches. When the car comes down whatever wheel has the most load can cause the car to shoot in that new direction causing you to loose control
• The opposite happens when you come down a hill , hit the bottom of a gulley and start back up the other side. The suspension compresseds, the tires point out but in this case both tires are loaded heavily and one doesn't tend to steer the car.
• The worst case is comming down a hill, at a high rate of speed, there is also a small creek in the bottom of the gulley with a bridge, so you are flying down the hill, hit the bridge the front end compresses, bump out, the car goes over the bridge causing the suspension to unload, extreme bump in, the car hits the other side of the bridge and the front suspension is once again compressed as you start up the other side, extreme change from toe out to toe in agian and whatever tire is loaded the heaviest will start to drive the car in that direction.
• I have been there, done that and almost had a bad crash because of it.
• NO more. My bump steer is almost gone and I can do that same hill/bridge at 100 mph without any feedback.
• Nice feeling when the suspension goes through it's travel without any darting of the car."

He pretty much sums up my opinion of it. My daily driver, a Caddy CTS, had it's suspension tuned on the Nurburgring and I swear it has absolutely zero bump steer. Even with my racing experience I can not feel it. It is just absolutely unflappable. Yeah it's the "Caddy that zigs" and I have been known to drift it. I will not rest with this C3 until I massage the "nervousness" out of it.

Their 2003 discussion also was about some very serious mods to control bump steer:
https://www.corvetteforum.com/forums...er-blocks.html

Such as:

• Lengthening the tie-rods 1-1/2" to get them the correct length.
• Lowering the outer end 1/2" lower than the Guldstrand blocks.
• Raising the inner end 1/2" with some drag link mods.
• Raising both the steering box and the idler arm, to raise the drag link.

I will share with you that some 40 years ago, we bent both the pitman arm and the idler arm in my pro-solo Camaro to correct the bump steer.

Many solutions to fix the "bump" issue exist, but first we have to graph it to see how bad it is, and then tackle the mods one step at a time.
GM says it is as high as 0.140" (first 1" bump), and these guys said they got theirs down to .015". That's 90% less!

 

.

Not to be mean spirited here or hijack this excellent discussion, but there's always camber thrust. Possibly that is the irreducible limit? Hoosier A7's specified camber above 2 degrees as I remember. I think my car was set to around 2.5 with zero toe for best performance and still is despite conversion to more streetable tires.. It's cold and wet here and I haven't bothered to put it up on a lift and get to more sensible settings, but that's on my list. As of now my car is still a little nervous.