the Reatta Convertible cowl shake problem.....

[NYBobP]

This e-mail came today from one of the GM execs I contacted who worked on the Reatta program. I asked about the cowl shake in the '91's.....

<span style="font-style: italic">"the convert never had a cowl shake condition. There was a first order wheel/tire vibration concern in the rear, but the cowl area was more solid than a Corvette."</span>

Barney, Padget, Jim, et al; do you agree with this? Was the problem in the rear?

[Guest TheNewDamage]

"more solid than a corvette" is a tricky answer. I had a '91 and it had a noticable cowl shake at certain speeds. Especially with the targa roof panel off. Of course it was accentuated by having a single piece hood and fender assembly. I have not yet ridden in a Reatta 'vert so i cant say from personal experience. On the vettes you can see "brushed" areas on the cowl where the seal from the hood rubbed back and forth when it had "the shakes" as I called it, lol. I did a lot of performance driving in the mountains with other guys from Corvetteforum.com for a few years and so saw the cowl shake a lot.

[Guest F14CRAZY]

"Cowl shake" = condition in which the cowl, shakes.

I got thinking...'91 verts are rare due to the short model year and because a lot of them were scrapped before sale due to the shake. Did '90s have this? I know they do from driving them, but did the '91s suffer more than the '90s?

[jgmck]

I have wondered this same thing. What if anything did they change on the 91 ??????????????? I really like my 90 but I am disapointed with Buick over this and wish it could be corrected. I have a 91 Miata that I have never noticed this shake. Is this fairly common with the big GM's (Electra,DeVille,Eldorado,ect)?

[NYBobP]

FYI...I have not experienced this problem with any of the '91's I've owned. Has anyone noticed this vibration problem and had to have it repaired, or did they all have some kind of a factory- fix before they were retailed?

[Roadster90]

Hopefully this will continue and not only be an interesting , but informative thread. I have a 90 Roadster with 16" wheels (a stock and never altered in any way Select 60 with 39700k), and I only have cowl shake (and a little aggrafretting) when I hit BIG ripples, sunken man hole covers, or chug holes in the road....other than that....beautiful and comfortable ride.....

[Guest steveskyhawk]

For what it's worth, I was looking at a 91 vert today and there were tapered pins on the doors and matching recievers in the door jambs. I assume this is to make the body more rigid. Has anybody else seen these and if so do the 90 models have them as well? I finally saw the "triangular brace". I'll bet that ther can be added to earlier models if you can get them.

[Guest F14CRAZY]

steve, the '90 convertibles do have the door pins

[Guest Greg Ross]

I'm sure it's been written up here before about the road test routine that brought about all those Reattas finding their way to Vocational Schools around the US.

All the Verts were trucked to Florida to be road tested over a particular section of rough concrete. Thus divided in to 3 catagories;

Scrap, ie: donate to Educational programs, and contract the schools to scrap the vehicle when they served no further educational interest.,

Sell to GM employees, and finally retail

Sell to Dealers.

I would have understood this happened with both 90s' and 91s'

[Guest F14CRAZY]

Wonder why Florida? Michigan is known for some pretty rough roads

[jgmck]

Steve

Could you describe where this triangular brace is and/or better yet get us a picture of it on the car ?

[Guest steveskyhawk]

http://www.reatta.org/reader's%20Rides.htm

Go to this link and scroll down to the White 91 Canadian Vert where the hood is open. You can see a little of the "triangular brace".

[jgmck]

Thanks

The only thing that looks triangular to me....... is off of the strut tower on the passenger side. Is this what you mean? I think my 90 has a brace connecting the towers. I guess I have to go out too the garage and look.

[Ronnie]

Look over the battery.

[Richard S]

Is this a replacement for the long bars earlier cars have running from the strut tower to the front cross piece [not sure what that is called]? If so, why? I don't see any reason why the earlier long bars won't fit the 91 engine compartment. Do the triangular pieces work better?

[NTX5467]

I suspect that you'll find that many convertibles have a certain amount of "cowl shake", which I perceive to be evidenced by the rear view mirror shaking on rough pavement/roads. The first gen Reatta convertible I drove did it with the roof up. I took that as just part of the deal of a convertible not having quite as stiff of a body structure as a hardtop/sedan would have.

Over the years, convertibles and "factory" t-top cars have had additional wedges screwed into the door shell and door frames on the body. Kind of as "bumpers" to limit body flex "together" of sorts between the two parts. The pins near the door latches might have a similar function, but might also compliment the possibly different style of door lock on the particular vehicles they are used on.

The cowl area is one of the stiffest parts of the vehicle structure, typically. It would be hard for it to "shake", but the structure attached to the cowl might flex under certain conditions. Adding some stiffeners or such to the structure from the cowl area (forward, generally) might help reduce the offending harmonics and bending moments imposed from suspension reactions to road imperfections or other powertrain vibrations. Braces from the strut towers to the cowl might work, just as a cross brace between the strut towers (as many later GM vehicles used) might help too . . . not to forget a different tire choice (with a little more compliant sidewall, possibly) might help tame such conditions.

In the case of the flip-front Corvettes, there's quite a bit of real estate between the hinges for the front of the hood and where the latches are in the cowl area. While the cowl would remain "still", the outer ends of the frame horns (and related bumper items) would flex from suspension "workings", which means the front of the hood/fender item is moving too, in relationship to the stationary cowl area.

With respect to the way those front frame areas are designed to absorb impact energy, there will have to be a certain amount of compliance in them, rather than otherwise, so they'll collapse correctly to absorb/dissipate crash energies. Not that they are flimsy by any means, just that even a very minor amount of flex can multiply over the length of the hood until it reaches the latching points in the cowl area. Many factors at work here.

Just some thoughts and observations . . .

NTX5467

[Guest Ranger]

NTX,

No, the cowl shake hasn't been evidenced in my experience of owning two Reatta converts by a shaking mirror, the mirror is attached to the windshield which is a part of the cowl. I've owned several other converts that didn't have a cowel shake. It doesn't take a "rough" road to start the shake. Fastening the top to the windshield frame does not add much, if any structural integrigty to the convertible.

The non-Reatta convertibles were heavily framed cars, in fact the convertibles I've had in the past were heavier than the coupes/hardtops because of "beefed up frames".

I'd guess the reason for the cowl shake in the Reatta just may be that the only thing holding the heavy front end, including the cowl to the rest of the body is the floor pan and a very light frame/unibody.

In my experience, the cowl shake happens at a certain speed and tire balancing, rotating, and wheel and tire changes have not corrected the problem. I'd guess we have a weak point in the overall construction of the car.

So, in answer to your statement that "the cowl area is one of the stiffest parts of the vehicle structure, typically. It would be hard for it to shake", well Bucko, it does shake along with the whole front end of the car, but it's not a great issue with me, I just speed up or slow down and it's gone.

Don't know where you get your theories, but they lose something in translation to the written word.

Good Luck,

Jim

[Barney Eaton]

I copied this photo from Steves link of 12/24

It is not the best picture but it is a photo of a 1991 convertible engine compartment, showing the black triangular brace. The photo does not show the entire brace but notice that the black rod that all other Reattas have is missing. If you are having trouble finding the brace, it is above the battery.

[Guest NEMO]

The shake is like a duck walk,sort of like a waddle? hard to place,Its like the front wheels are pulling from one side then the other .More as if you were pulling a wagon with a rope on each side,causing it to rock very slitly,could be a lifting of one side then the other,twisting the floor pan.It all has to do with the front wheels pulling,one side then the other.Maybe if the tranny had a posi pull this would not happen.Both wheels pulling foward instead of only one. If you jack up the front wheels,put in drive,one wheel will not spin.It needs to pull from both wheels all the time .You do not get the shake when you put the car in neutral,rolling along same speed.evon while the car is waddling,it goes away.Try it!

[Guest Ranger]

Nemo,

Good point, I'll try shifting into neutral when I get back to the convert this spring, makes sense.

Jim

[jgmck]

Nemo

Interesting observation! That should translate to a rear wheel drive convert. not being subject to this problem.

[Guest NEMO]

With a positive drive,both drive wheels turning together, front or rear,you would not have twisting, effect on the car.

[NTX5467]

With respect to my "theory" of cowl stiffness, it comes from a statement (I read several years ago) of an engineer who stated that with the complexity of the cowl structure, it was one of the structures on a car which was the last one to be redesigned or changed (which also relates to production costs to change it also). Plus knowing that the front door hinges attach to the side of the cowl structure itself and that each of the welded joints in the structure make it a synergistically stronger structure. Other than just the "firewall", the cowl area houses the plenum area below the windshield (usually with curved sections rather than completely flat sections) which supply air to the hvac system, house and conceal the windshield wiper mechanisms, plus (as mentions) serves as the base structure to which the windshield attaches/seals to. Therefore, I concur with the engineer as to the complexity of that body area and its necessary stiffness to do what it does.

When you alter your speed over a bumpy or rough road, you are altering the frequency of the vibrations transmitted through the suspension to the body structure. If the transmitted frequency happens to coincide with the resonant requency of the body structure (or parts thereor), then "shake" can occur. In the '69 Camaro convertible, there were dampers placed at each corner of the body to dampen these harmonics.

These same suspension inputs will also cause some amount of flex in the body structure. In other unibody cars with separate front subframes (as in the first and second generation GM F-body), a common "fix" for drag racing is subframe connectors. When the first third gen F-body convertibles were finally built, they had visible cross braces for the floor pan. So, yes, convertibles generally need some heavier bracing or additional "framing" in the underbody area to add stiffness to that area of the vehicle--even on a body/frame construction vehicle.

All things considered, if the floorpan area is suspected as being the location of the flex, then some additional bracing in that underbody area might be easy enough to design and implement. Such additional structure would alter the resonant frequency of the body structure and possibly shift it to a different frequency which would not cause "the shake". If this was effective enough, it would be highly similar to the subframe connectors (mentioned) in effect and would make the convertible bodies last longer and be more enjoyable to drive.

Regards,

NTX5467

[padgett]

I dunno. Send me one (non-returnable) and I'll test it.

ps a real cowl shake, like a "speed wobble", is a scary thing. In the worst cases the steering wheel will jerk in you hand though rarely lasts more than a second or two. Railroad tracks were the worst for me.

pps the 66 Corvair convert in my garage has cocktail shakers in all for corners.