Hello all
I have a 1979 Corvette with VBP gymkhana springs in the front and back
I also installed 17x10 rims with 315 tires in the rear 295 in the front (with the help of some huge wheel adapters)
The tires are Yokohoma and the rims and tires were originally from a guy that road raced a C4 so they have very aggressive tread.
Every time I go over a bump, and on some days this is worse than on others, the car jumps, sometimes a half to a full foot in one direction or the other and it can be scary as hell on really bumpy roads
The steering system has recently been rebuilt, new power steering valve and center link, but the worm gear and the pump have remained the same, it also has been aligned.
I have tried to get rid of the bump steer by installing a custom kit that drops the tie rods where they connect to the center link, and it has helped some, but not enough, Ill try to post pictures of the setup a little later, I am just wondering if anyone has any other suggestions of what could help or if I should just buy steeroids and be done with it?
Did I just get too aggressive with the suspension/tire setup for it to be street driveable? I'm ok with it being really stiff, but the jumping is just unsafe.

 

I noticed this behavior in my car when i drove it too, and it had no special tires. I think Norval had something for this, it involved a lengthened spindle and relocated pick up points of the steering rods. Sorry but that's all I can tell about it.

 

I bet your worn steering box would add to it. It does on my car

Stay away from the vendors on here though. My "rebuilt" one was junk

Contact GTR1999

 

Does the front jump?
Or is it the rear??

 

i believe that with the bump steer kits you want to lower the tie-rod were it connects to the steering knuckle, not the center link

 

Check your t/a bolts to see if they come loose,
Happened to me ,it felt like what you described.
The ones in the front bolt where the shims go.

 

Well it feels like the front, but not sure, hard to tell
The TA's are done for, the shims are rusted into place and the whole thing needs to be replaced, I know the rear left has bad alignment, but I did not want to touch it because it looked thoroughly nasty, we tried to start by taking off the shock mounts and couldn't even do that and it ended really bad (long story...)
I thought that it would be best to leave it alone and that the jumpiness was coming from the front, could it really all be caused by the one misaligned TA? I know it doesn't help but it seems odd to me that that would be the cause of it.

Yeah it is attached at the steering knuckle, sorry I just forgot the proper terminology for where I attached it

 

From your description of the rear suspension I'd guess that's where the problem is originating. My 72 used to dart to the left whenever I would hit the accelerator hard. Turned out that not only were the trailing arm bushings worn out but the driver side yoke in the differential had no retaining clip so the whole T.A. was moving in and out.



Rick B.

 

Try a couple of Noval's old bump steer posts.

http://forums.corvetteforum.com/showthread.php?t=706899
http://forums.corvetteforum.com/showthread.php?t=735791
http://forums.corvetteforum.com/showthread.php?t=587470
http://forums.corvetteforum.com/showthread.php?t=606627
http://forums.corvetteforum.com/showthread.php?t=750371

Ton's of great info.

 

i would bet if you fix the t/a you will solve to problem....jmo

 

You are most likely experiencing a combination of things. I agree that the rear sounds like it is contributing a lot to the problem. From experience I can tell you that incorrect rear toe and sloppy T/A bushings make for a very twitchy and unsettled car. Slop in the steering is probably also contributing but don't forget that wide tires really like to track ruts in the road and they also tend to go their own way over bumps regardless of bump steer (although that can contribute). Adding a lot more positive caster will help the tracking and stability problem and perhaps adding some more toe-in. What are your current alignment settings on the front?

What I'm saying is that there are other contributing factors that are probably influencing the stability of your car more than bump steer is at this point. A good test of bump steer is to find a dip in a road, drive the car over it and without touching the steering wheel observe what it does. If the nose moves according to the suspension movement then it is bump steer. For instance, as you feel the suspension compress the nose moves left then as the suspension extends the nose moves right you have a good indication of bump steer. A good sized dip or bump that will allow the suspension to cycle up-down-and back to normal is a good test to see if the nose moves left-right-left and then runs straight after.

 

This "bump steer" problem is most likely not a basic geometry issue (other than the stated mis-alignment). Address the TA's, alignment and anything else not in proper order, then see where you are.



BTW, the bump steer blocks actually serve to increase Ackerman rather than reducing it, for anyone who may get confused about that particular side issue. They are a good sound mod any handling enthusiast should have on their things to do list.

 

I know Norval hates them, but what are others opinions on the VBP bumpsteer kit? I may want to tackle this project later this year, but don't know if I want to get as involved as Norval, I just want it better.

 

Vintage racers used them going 200 mph

That works for me

I want to put them in also

 

The only way to do bumpsteer is to measure it before and after any changes. Otherwise you migh as well not do anything since you don't know if you made it better or worse.

Take the spring out and measure the toe change, make a change and measure again.

Keith