ddahlgren

Hmmmm 20 year old new to the biz adheasive to bond steel to plastic how might they test it for bonding for decades??? Good point. I know when the 'drone ' sets in the whole rear trunk area vibrates.. An unbonded panel or the drone shakes it loose?
Dave

8valve

I would love to have a C4 in my shop for a week that Misbehaves with the Targa Top off. Everything we mention is speculation at this point. We really dont know. I think some of the Vettes displaying this problem have other serious problems causing this misbehaving. Sine 1959, I have been building serious off road trucks, suspension, diesels, bodies and mess'en with cars. Thats why I woudl love to have one in my shop to mess with.

8Vlave

Tonesc5

Every low mileage tight c4 ive had , drive like a dream when the top comes off steering wheel shakes its terrible

Hideki

When I take the roof off, my steering wheel shakes, the whole car feel less stiff by a wide margin. But I think the car looks better with the roof on, so I almost never take the damn thing off anyways.

primalurges

I own a 96 and drove it hard on smooth pavement and bumpy roads for months with the top off and was surprised at how stiff and solid feeling the car was because I heard many comments of rattles and things of that nature. The first time I pulled the top-off I had to pull over and pop the hood because I thought the suspension had come loose, this of course was only noticeable un bumpy roads at about 30 to 40 mph. I took the car on a fairly flat road at over 100 and didnt really feel anything. There is a definite difference and tire pressure and shocks may make it more noticeable from car to car but it has to be present in all. There has to be reason for the very being 300 lbs heavier, excluding the rear glass, and I'm sure its not the rag top.

massiveB

good thread, im gonna look into the crossmembers and braces, im liking that targa brace. meanwhile im getting her on the lift to check body mounting adhesive

Ratbagsmg

I have a '92 coupe with just under 37K on the clock. I found that when I replaced the original tires and shocks it made a BIG difference. Much less shake. Still a little just a bit on bumpy roads. From what I read on this topic, it really does vary from car to car.

8valve

I wish this forum was here in the early C4 Era. It would be interesting to read the archives on this topic....if such a topic even appeared. My feeling this is a age/usage issue that some cars develop. Anyone with access to recall info might be able to see if there were any recalls for this issue on the C4's, or any technical service bulletins issued.
8valve

Sue3a

Same thing is happening to my car

Sue3a

I had my roof off and the car was shaking all over the place. I took it to the shop and they said the tires were not balanced. They fixed that.... I thought that was the problem.. Got on highway and it was doing it again but not as bad.
The other day I took car to get windows tinted and I had to go threw Boston and went into a tunnel and the shaking stopped.. Thought that was odd.. But the air movement must have changed in the tunnel.
So after getting windows tinted.. The shop had to put top on... do to the tint.... On the way home the car ran great, no shaking... I'm guess it has something to do with the wind, and glass being off.. Must be throwing the cars balance off... It was very odd..

rocco16

The forces on the top put it into compression. The curved top is very stiff in compression. Also, the compressive forces acting on the top are not as great as you might think; the top's distance from the frame means the forces on it are diminished/it has plenty of leverage.
The top stiffens the body, that is unarguable.

rocco16

I think 6valve has hit it; these are 20 to 30 year old cars, and every car has a different history. Some are hardly driven off of super-smooth macadam. Some have suspension components that have been lovingly replaced and are like new. Some have rarely seen full-throttle, let alone been "abused".
And some have lead different lives.
This means some will still be tight and some will be loose.

Went

I'd buy a AFSB bar in a heart beat if I could find one. Mine is not as tight as I'd like without the top on.



Went

scottmxracer

I bought a 93 new, and I now have a very well maintained 96 CE with 27K. I can say that they both were tighter with the tops on than with them off. The wheel and window would shake with the top off on bumpy roads. I took the top off my 96 and took some measurements then jacked it up, nothing changed. I still decided to buy the Vette2Vette frame braces. I put them on lose then went down my road for a reminder of what shook and how much. I then started adjusting them, after about three adjustments I would say its 75 to 80% better. I remeasured the roof opening and found that it had opened up about a 1/6" my roof now drops right in.

Tom400CFI

Hmmmm. So the V2V bars "bent" the car some?

scottmxracer

My guess is that they put the car back where it was out of the factory. There were some marks one the top locating sockets that indicated that it had closed up a bit. FYI I have two journeymen's cards and a master motor cycles mechanics license so I do know a thing or two about how things work. I dont think bent is a good assumption they put the frame rails under tension to keep the sheet metal from flexing. If you take a car to a speed shop to get it to handle better one of the first things they do is weld up all the spot welded seams, the braces keep the spot welded sheet metal from flexing in a similar manner.

Tom400CFI

I beg to differ. I 'get it" how most things work pretty well too.

Welding up all the seams adds area of contact by closing up all the "holes" between spot welds. This allows the stampings to act as they were designed to; a monocoque. In practice the stamped pieces act differently, with spot welds that allow deformation between them. Welding doesn't put them in "compression", it doesn't change the shape or length in any meaningful way. All it does is increase the bonded surface area and allow the parts to act as they were designed to.

The bars put the front rail, rocker rail and rear wall spot welds into compression to increase stiffness. I don't have a "problem" with that, or doubt that it works for a subjective improvement (car "feels tighter").

My concern is the deformation of the points of contact at the spot welds. Why? Several people have posted that they installed the bars, tightened until snug and measured...then tightened to achieve the desired "feel", and measured again. I believe one person measured a 6mm difference. Where did the 6mm come from? Deformation of the spot welds at the ends of the rocker rails is the first thing that I think of. So, with that in mind, when I saw your post about measuring the hoop-to-windshield distance, before and after, it caught my attention. Something "bent" there...otherwise you wouldn't have seen a change in distance.

You're right that the car may have "sagged" over the years and you just brought it back into alignment, but I don't think that is what happened, because I don't think the C4 frame is so weak in bending, that it will "sag".

It's an interesting thing and it's cool that you did do some measuring before and after. As I've posted before; it sure would be cool if a structural engineer could weigh in on this, and shed some light as to what is really happening.

8valve

Its interesting to see this thread has come back to life. Back in 8/18/2010 I made post #28. Now 6 years later, just a update on my 95. When my 95 comes out of winter layup, the roof comes off. The only time I put the roof on is when I go on a trip and dont have storage room in the back for the top. After 6 years, I still see no difference with roof on or off, and I push it hard on the black top mountain roads. The only difference I see is my hat trying to blow off with the roof off.

I seriously suggest that those with this handling issue with the roof off have a problem with the car. First, I would get it to a quality alignment shop and have everything checked. Hopefully a good shop will have the knowledge to go past the alignment and know other things to look for that might cause this problem. The General did not build a Corvette that would act badly with the roof off. You have a problem, apparently quite a few have the same problem, whatever it is.

8Valve

94corvetteC4

Same with my 94. I have a side to side shake with the roof off. Put the roof back on and the car is solid. I have new tires, shocks and a four wheel alignment just done a few months ago. 105k on the odometer and the car was checked out by a friend who is a mechanic that I trust when I got the car last fall. He put it on a lift and checked it out front to back and said its clean and tight.


The side to side isn't that bad but it is annoying. Feels like bump steering that my fox body mustang suffered from except it jumps side to side constantly not just over bumps in a curve like the mustang.

itsalwayssomething

The first time that I took the top off my 95, I pulled over and walked around the car before I even got out of the neighborhood. It was a very noticeable difference.
Not undriveable by any means though.

FWIW- When the top is off for only a couple of days it goes back on easily. One time I left it off for the entire summer and it was a real bear to get back on in the fall.

Seems that verts would be worse without the halo and rear glass.