Update on the new C4 frame braces

[C2C3C4]

I just got back from Bloomington Gold where I picked up this set. It was designed by Gordon Killebrew and is being marketed by Jason Francis of Vette2Vette Corvette Parts!
I just did a very unscientific weigh in on the old bathroom scale and found them to weigh 19#.
The brackets bolt to existing holes/bolts at the front and rear of the 86-96 C4 frame. The Heim joints are tightened by turning the welded on bolt at the front of the rod with a wrench. This pulls the frame box together and eliminates cowl flex. This does not hang down as far as the convertible X brace and allows full access to the exhaust system and driveline. Installed pics are on a 1990 ZR1 and are of the prototype model.
I do not know the price at this time as I got them at a steep promotional discount. I will be submitting an article on my home garage install to Corvette Enthusiast Magazine.
Some members on a couple other forums are setting up a group purchase. The kit is painted with POR15.
Crud - something has changed on this site. There is a new way to upload photos and I have not figured it out. Gotta leave now. Will attempt to upload photos later today.

 

[catbert]

What ill does the frame brace cure?

 

[2VettesMike]

What ill does the frame brace cure?

Take the one piece roof off a C4 coupe and drive on a road with a few potholes and rough patches. The steering wheel and windshield frame shake side to side like someone is twisting the front of the car.

If it cures or reduces this I'll definitely order.

Thanks,

Mike

 

[boomdriver]

What ill does the frame brace cure?

The C-4 frame rails are HEAVILY dependent on the extensions for support...roof panel, rear end frame extenders etc. The ends of the rails like to twist around and roofs attached to pillars on the rails stop this.The roof panel completes a box that has support from all direction. Take that off, and the box cannot support itself under a load.

Mine is so bad without the roof that I cannot turn a corner with ANY power applied...none, or it'll wash out big-time. Sure, it has more torque than most cars but it is still like driving on ice when it was a stock powerplant.....now topless driving is impossible,.almost a safety issue.
The convertable underside brace helps but that also takes away nearly 1" of precious ground clearance, so thats not a good solution.
If this "kit" helps keep the frame rails aligned in stressful situations like cornering or rough roads so the suspension CAN do its job, it's worth its weight in gold....I just hope that its an affordable "at home" type of project.

I'm not ever going to forget the night that I looped my first c-4 going around one of those circular Ca on-ramps while topless doing about 60...a full 360 spin into the 1st lane of the freeway, car returned to its original course following the spin but scared me and my passenger half to death. That was many yrs ago when people were still learning C-4 handling characteristics....Now, I know better than to expect my corvette to "handle" without its roof inplace and bolted tight. Thats sad.

 

[catbert]

Glad I raced my C4s with the roof on. Truthfully, I never noticed anything worse than typical convertible cowl shake without the top.

 

[Rex S]

Basic C4 design never included the removable top but GM upper management insisted the car had to have it so now we have cowl shake w/roof off.

 

[Rogue]

I thought that the R&D Racing Targa Truss was suppose to cure the cowl shake with the Targa Top removed.

Targa-Truss, R-D Racing

 

[Ugolino]

The c4 is a perfectly safe car to drive without the roof panel, I have owned several c4s for many years without handling being degraded by a removed roof panel. The limits of the car have more to do with the driver than the car itself.

The C-4 frame rails are HEAVILY dependent on the extensions for support...roof panel, rear end frame extenders etc. The ends of the rails like to twist around and roofs attached to pillars on the rails stop this.The roof panel completes a box that has support from all direction. Take that off, and the box cannot support itself under a load.

Mine is so bad without the roof that I cannot turn a corner with ANY power applied...none, or it'll wash out big-time. Sure, it has more torque than most cars but it is still like driving on ice when it was a stock powerplant.....now topless driving is impossible,.almost a safety issue.
The convertable underside brace helps but that also takes away nearly 1" of precious ground clearance, so thats not a good solution.
If this "kit" helps keep the frame rails aligned in stressful situations like cornering or rough roads so the suspension CAN do its job, it's worth its weight in gold....I just hope that its an affordable "at home" type of project.

I'm not ever going to forget the night that I looped my first c-4 going around one of those circular Ca on-ramps while topless doing about 60...a full 360 spin into the 1st lane of the freeway, car returned to its original course following the spin but scared me and my passenger half to death. That was many yrs ago when people were still learning C-4 handling characteristics....Now, I know better than to expect my corvette to "handle" without its roof inplace and bolted tight. Thats sad.

 

[boomdriver]

The c4 is a perfectly safe car to drive without the roof panel, I have owned several c4s for many years without handling being degraded by a removed roof panel. The limits of the car have more to do with the driver than the car itself.

Really?

You'd be about the only one.

 

[catbert]

Really?

You'd be about the only one.

Nope, I successfully raced an 87 Z52 ROADSTER for two years without drama. That said you can't compare them to the new Vettes.

 

[boomdriver]

Nope, I successfully raced an 87 Z52 ROADSTER for two years without drama. That said you can't compare them to the new Vettes.

Note:

"You'd be about the only one".


But i do have a question...

Why is it always someone with a C-5 or 6, that always says C-4 complaints are soooo unfounded?

Fact IS, mine (factory Z-52 w/ faster springs, stiffer shocks)handles like crap without the top. So do all other coupes. Perhaps I just expect more.

Why did they (GM) redesign the chassis for the 5th gen I wonder? So a golf bag could be carried in the rear?


Just curious.

 

[93Rubie]

Actually the C5 is so much stiffer than the C4 it isn't funny. Improved in just about every way. As it should be. Our C4's are way better than any C3 in terms of structural stuffiness. Fact is with C4's the car IS stiffer with the top in and less without it. As far as it affecting handling I have never noticed much of a difference. Then again I drive mostly on the street but I drive hard. I mostly get more cowl shake and it may understeer just bit more. Why would you ever have the top off on a road course is anybodys guess.
​

 

[boomdriver]

Dude,
I agree.

Why someone would even take a competition course on without everything bolted in place.....must just be for show....sure ain't to win.

gosh..! I never realized until just now....There MIGHT BE a few that have never experienced their C4 Coupe getting loose or sliding out while still deep in the turn....or loosing it as they try to power out of a corner.

Then again, if they've never experienced that, i guess they were'nt near the "edge" yet and did'nt push the car enough to see the handling faults.

I became aware thru pushing the limits, but that was 23 yrs ago. I've only been performance corvette driving since 1885. Flaws I know....how to work with it, know more.

 

[Hib Halverson]

There seems to be misunderstanding about the C4 structure's stiffness and how that affects handing and safety.

On the safety issue, it was said earlier that the C4 coupe of safe to drive with the top off. Of course it is! With normal driving there is no safety issue with the C4 coupe with the top off.

As for handing...if we are talking about the handling when the car driven near it's limits...the "story" is far different.

The C4 structure's stiffness in torsion and bending is weak with the top bolted in place, much less when the roof is removed.

Anyone who does track day events, autocrosses or just drives hard on the street in a C4 coupe without a roll cage and says there is no difference is either unwilling or incapable of driving car close enough to the car's handing limits to feel the difference.

As for C4 convertibles...the problem is even worse. The structure in those cars is so weak that its frequency in torsion and bending approaches the its ride frequency. That's one reason the car shakes lot. It's, also one, reason C4 convertibles were never offered with Z51 or Z07, ie: the structure is so weak that when you start increasing roll stiffness (increased spring rates or stabilizer bar rates) you end up with the structure deflecting just as much or more than the roll stiffness increases. When the structure starts bending the car becomes unpredictable at the limit.

In motorsports events you'll never see anyone in a C4 convertible without a cage whose times are competitive unless it's a class for convertibles only or a class populated by poor drivers in coupes.

 

[93Rubie]

Dude,
I agree.

Why someone would even take a competition course on without everything bolted in place.....must just be for show....sure ain't to win.

gosh..! I never realized until just now....There MIGHT BE a few that have never experienced their C4 Coupe getting loose or sliding out while still deep in the turn....or loosing it as they try to power out of a corner.

Then again, if they've never experienced that, i guess they were'nt near the "edge" yet and did'nt push the car enough to see the handling faults.

I became aware thru pushing the limits, but that was 23 yrs ago. I've only been performance corvette driving since 1885. Flaws I know....how to work with it, know more.

Well, I have pushed my Vette to the limits both with top on and off. Limits are lower with top off than with it on. I have had the tires just starting to squeal in many a turn to I am at the tires limit. ASR has saved my BUTT more than once as I goosed it coming off a corner. Its worse with the top out, car seems more stable with it in, which makes sense.

As far as the C4 having handling faults sure it does, compared to newer Corvettes like the C5 and C6. It is still a good handling sports car even by today's standards but not exceptional just ok. Compare the C4 against an older Corvette like the C3 and you will quickly see just how much better the C4 is over the C3. I never really understand the reason why we compare our older C4's against newer Corvettes. Of course the newer ones will do everything better, the car is better. If it doesn't get better with each generation then someone is doing something wrong.

 

[93Rubie]

There seems to be misunderstanding about the C4 structure's stiffness and how that affects handing and safety.

On the safety issue, it was said earlier that the C4 coupe of safe to drive with the top off. Of course it is! With normal driving there is no safety issue with the C4 coupe with the top off.

As for handing...if we are talking about the handling when the car driven near it's limits...the "story" is far different.

The C4 structure's stiffness in torsion and bending is weak with the top bolted in place, much less when the roof is removed.

Anyone who does track day events, autocrosses or just drives hard on the street in a C4 coupe without a roll cage and says there is no difference is either unwilling or incapable of driving car close enough to the car's handing limits to feel the difference.

As for C4 convertibles...the problem is even worse. The structure in those cars is so weak that its frequency in torsion and bending approaches the its ride frequency. That's one reason the car shakes lot. It's, also one, reason C4 convertibles were never offered with Z51 or Z07, ie: the structure is so weak that when you start increasing roll stiffness (increased spring rates or stabilizer bar rates) you end up with the structure deflecting just as much or more than the roll stiffness increases. When the structure starts bending the car becomes unpredictable at the limit.

In motorsports events you'll never see anyone in a C4 convertible without a cage whose times are competitive unless it's a class for convertibles only or a class populated by poor drivers in coupes.

Hib, why didnt they make the car stiffer? I once read that David McCellean said that they lost a lot in the targa top, both stiffness wise and in added weight. Marketing had most to do with that. That being said, the C4 was created to be an open top car from the get go, why then isn't it stiffer. Or did they set a stiffness goal and once attained stop there? I'm sure ride quality had some to do with this. I also understand the car was designed using late 70's and early 80's technology, so...

 

[TedC]

I'm interested is learning more about these C4 support braces. I had the top off on my 1989 over the 4th and there is a big difference in body flex. Is the vette safe to drive with the top off? Sure it is. Just not sure these underbody supports can cure this body flex.

 

[WillC4]

Are these really worth it? I doubt that i'll reap the benefits of structural bracing, seeing that i havent pushed the car to its full handling limits (getting there though, one mountain turn at a time).
I mean, my car is due for a suspension overhaul, to which i am looking at kits offered by a couple of sponsors here with fresh shocks, leaf springs, struts, anti sway bars etc...
Since i purchased my C4 a year ago and 3,000 - 4,000 miles later, i never took the top off, this was due in part cause i am a bit lazy and dont feel like unscrewing the bolts, lifting the top off and storing it, when i can just jump in, start it up and go.

 

[MBDiagMan]

I can't answer whether the cross braces are worth the money, but I went through a lot of research several years back when I was preparing to do some Solo II competition with my 88.

I don't remember the reasons now, but the conclusion that I came to at the time was to build a camber brace and leave the top in place as opposed to adding the convertible bracing.

I built a camber brace at that time, and was impressed at how it made the car feel. If you look at the front, upper control arms, you can see that they are not well supported laterally. The camber brace gives the support so that the front of the car is stiffened to a point, but more importantly, the camber follows it's designed arc regardless of dips or other unevenness in the road.

The camber brace and targa top in place makes for a good handling car. Are the C5 and C6 superior in the handling department as compared to the C4? Of course they are. Does the C4 out handle any earlier Vette? Absolutely! I had a 67(sure wish I still had it) and compared to my C4, it was a FREAKY handling car. A lot of that, of course, was the small tire patch, but even so, it did strange things when throwing it around in the corners.

The big tire patch, 50/50 weight distribution and decent suspension of the C4, along with a torquey power plant, make the C4 about as much fun as anything you can buy for the same money IMHO.

 

[70b5rt]

I do not know the price at this time as I got them at a steep promotional discount. I will be submitting an article on my home garage install to Corvette Enthusiast Magazine.
Some members on a couple other forums are setting up a group purchase. The kit is painted with POR15.
Crud - something has changed on this site. There is a new way to upload photos and I have not figured it out. Gotta leave now. Will attempt to upload photos later today.

Please let us know how significant a difference the brace makes (especially with targa removed). Also, any pics and info on installation issues would be appreciated as well. Thanks.