Could this be a wheel balance issue? An alignment issue?

There you go! Have you had a front end alignment done also?

 

I dont know anything about that C4 front end, but to me even 500# springs seems high. I put 350 coil overs on my small block with AL heads, and i feel thats pretty good. Many years ago we had a bunch of Ford Superduty's when they first came out, they would do exactly the same thing. I forget what we did about them, but i do remember it being violent when it happened, and slowing down was what would make it stop. Most of the WWW says most of that was due to wore out ball joints, steering components, and the like. Take a really close look at that stuff before anything, i think. Search the WWW for "superduty death wooble" for fun, and see what you take away. a brief look there this morning, and alignment, and tire balance were actually pretty low on the list of causes, but of course, getting the wheels balanced is cheap, and probably a good start.

 

I doubt this relates at all to your situation but I had a steering wheel shake in my '56 that had me beat for a long time. Typically about 40mph and slightly down hill. Weird... I looked at stuff for a loing time and could not fix. Each time I found the wheel bearing loose and readjusted and the bearing felt fine then. That should have been the clue but i missed it ... the wheel bearing cup was moving in the rotor. New better quality rotors and bearings and that fixed it.

In my case it was a really bad wheel shake.

 

Side to side is normally a toe adjustment or too much split on L / R caster or camber.
Spring rate will not cause shake spring oscillations are controlled by shocks.
If a wheel balance issue & your vision is good you should be able to see the front tin moving up & down.

2 inexpensive fix options: Slow down on that road or select an alternate route.
Luck

 

Spring rate will not cause shake spring oscillations are controlled by shocks.

Yes but spring rate will affect what speed the shake occurs. 55 mph is pretty fast relative to most examples of this problem - which is consistent with the high spring rate.

This is probably a wheel balance issue.

 

could be tire condition, balance, front alignment, or a combo of the three.

 

Normally I would first suspect the wheels/tires, but in your case with such a high spring rate it could be chassis shake. The frames on these cars were designed for much softer springs/tires. Stiff sway bars make it shake even worse. It would be pretty easy to disconnect the ends links and give that a try, otherwise a different pair of front tires to eliminate those possibilities. I have C4 front/rear suspension on my car with stock traverse composite leaf springs and shocks. Stock Vette front sway bar, and no rear bar to decrease the chassis shudder, and it is rock steady at 80mph on concrete interstates, even though it rides plenty stiff on bumpy roads.

 

You already identified the problem. The C4 front end . Alignment coupled with tire issues, as these front ends are incredibly sensitive to issues related to out of spec set ups. First there are hundreds of threads on out of balance or improperly balanced tires. Due to the massive offset these front ends were designed around dynamic speed balancing is mandatory. This was the first car ever sold to have specific tires built for each corner. If Balancing is confirmed to be perfect and the tires are not out of round from extended bouts of sitting in one spot move on to mechanicals.

First confirm toe. (1/8" is the minimum I would run) and look at caster, as it is stated by you, stiff front springs may result in a higher then designed ride height that would actually increase while pushing 55 MPH of wind. As ride height increases your caster numbers will too.

A set of coil overs with adjustable ride height would be a great addition to your car.

 

With my setup I'm running 96 C4 Grand sport suspension front and rear with that is all stock crossmembers front a rear supporting the suspension. Running C4 spec sized 275/40-17 sticky Nitto NT01 tires on 9.5-wide wheels on all 4 corners. I have all of the equipment to mount/balance tires. I have the weight of the car near the Corvette stock weight, and the car is amazing to drive. It's like driving a really big Corvette, and more fun to drive than my C5 and many other new cars. Nothing more fun than flying through the dozen roundabouts we have all over town in a 68-year-old looking car.

 

About '94 & up C4 Corvette's got narrower front wheels IIRC they went to 8.5 width & a different size tire to reduce customer complaints of "hunting".
If you're running the standard C4 + 7 degrees of caster a C4 will hunt on uneven pavement = nature of the beast = live with it.
Luck

 

So, a few things.

The car has adjustable height coil-overs already. The suspension has been aligned to nearly stock C4 specs. I'll have to dig up the alignment sheet, but it was done right and I've not made any changes to it. I think all my steering components are fine, but I'll double check ball joints. When rebuilding the control arms, I seem to remember them being pretty tight so I left them alone. All bushings are new though. Rack is newish. Sway bar is a factory Z51 30mm unit with new sway bar bushings. Steering linkage has a new isolator in it as well. I'll have to check the tie rod ends...I don't recall replacing them, but I can't remember to be honest.

Tires are old Michelins PS2s I think, but probably have half their tread life left. The car does sit all winter so flat spots are a possibility.
Since I have the same tire/wheel combo all around, I'm gonna swap the fronts with the rears and see if that makes any difference.

I'm thinking this is a wheel balance issue too...I have a new set of front tires waiting to go on. Might not be the worst time to have them mounted. It's possible that the tires/wheels are out of balance, although I had them all done at the same time. Maybe a weight came off?

 

just out of curiosity what is the date code on the tires on the car now??

 

I'm sure they're around 10 years old, maybe a bit more. I'll have to go look. the weird thing is that it doesn't do this on asphalt...just that section of concrete highway.

 

Guys...I am 99% sure I found my problem.

I'm running OEM C7 Corvette wheels in perfect condition so I know they're not cheap out of balance aftermarket wheels.

However, in looking at my passenger side front wheel this evening, I'm seeing a patch where it looks like about 3 or 4 squares of those adhesive wheel balancing weights came off. The patch is not indisputable, but it's awfully suspicious given a few other things that have happened.

I had a power steering pump leak in my engine bay about a year ago that made a huge mess of everything. It was just everywhere...took me like 3 hours to clean it all up! And I had an oil leak at the mech fuel pump block off plate too. Hard to say which one got crap all over the wheels but I distinctly remember cleaning them both up, taking off the brake pads and cleaning the rotors up just to be sure the car would stop (there was nothing on the pads thankfully).

Anyway, on that pass front wheel, there's a long rectangular patch where it looks like (based on the absence of some dirt and mild staining) where there were more weights on my passenger side front wheel. In comparing the number of square weights on my driver's side front wheel...I see about 8 or 9 of them and on the pass side now, there are only 3. Not definitive evidence, but it makes me very suspicious that the pass front wheel is out of balance. If it is, that's most likely the cause of the shaking problem that happens at higher speeds.

I just never thought of the weights coming off...they may not have been stuck on very well to begin with knowing some car tech places these days, but in light of the two leaks I had, I can't blame them either.

I'm gonna pull the wheel off and take it in to have it's balance checked. Think I'll do them both actually.

I'll post up a report after I get the front wheels checked out.

 

If the tires are 10 yrs old as you indicated in your other thread, yes have the balance checked on the one with presumed missing weights. then put on the new tires you have. you are all ready at the tire shop. get it done.

 

i take a sharpie and out line those weights, so if they leave, at least i can see there used to be some there. usually there is some sticky residue, but this way i know for sure. if i get a wheel re-balanced, i make sure to clean the residue from the old weights.

 

At least if you replace the tires & rebalance you can eliminate tire wheel as an issue causing the shake.Not saying either is the cure but if changing the tires is planned why not now?

 

Yeah that's my plan. Like I said, I'm 99% sure this is the problem (i.e one wheel being out of balance). I finally got the heater core out of my brother's '67 Corvette (man, what a job...took me ~5 hours just to get it out!) so I'll take it and my front wheels/tires and my new tires off my '56 in this next week to have it all done at the same time. The local radiator shop is also a general repair/alignment shop.

One of my buddies will take my old tires...he uses them to grow newly planted trees...keeps the critters and Wyoming wind off of them!