Bee Jay

I'm in the middle of installing my aluminum batwing diff cover, crossmember. There is a frame crossmember just behind where I will be mounting the batwing brackets. This crossmember seems to do nothing. Nothing is bolted to it since I don't carry a spare anymore, and I think it may not be needed anymore since I am running a welded in four point roll bar. The roll bar really made a significant difference, a difference I could easily feel, in chassis stiffness. Just behind this crossmember is a bolt in steel crossmember that is used to support the gas tank. At the end of the frame, I removed the heavy battering ram crossmember and replaced with an aluminum angle bar. If this crossmember is not doing anything but adding weight, I'd like to cut it out. What do you experts think?
Bee Jay
You can see where the four point roll bar is welded in in this picture:

You can see the cross member in question in this picture:

You can also see the bolt in cross member for mounting the gas tank.

Here is the Autopower four point roll bar from topside:

birdsmith

Actually, that crossmember is a structural part of the frame....while removing it probably wouldn't cause you to lose more rigidity than you gained by installing the rollbar, it is still part of the frame. Its initial primary function was not to hang the spare; it was to lend strength to the aft part of the frame. Additionaly, it probably doesn't weigh more than about 15 lbs. ...most of us could probably give up a couple cheeseburgers a week and recoup that. A C3 Vette's body is what contributes the bulk of 'dead' weight to the overall whole of the car, so chopping a 15-lb. crossmember out of the car's primary structure won't help you that much IMHO. You could possibly remove it and then replace it with something that's a little lighter and more elegant, i.e., tied into the frame horns on two sides instead of just the bottom. And no, I'm not an engineer, but I have been employed as a depot-level aircraft structural mechanic for many years, and also did a few years of vintage racing with a car that I built myself......hopefully that at least qualifies me to offer an opinion (and please don't remind me of that tired old '70's saw about "what opinions are like").

Bee Jay

Thanks for answering Birdsmith. I'm an aerospace engineer and I appreciate your insight. I just found out on another Corvette Forum that this crossmember is not even included on 80-82 frames. That is even without a four point roll bar like mine. With the bolt in crossmember holding the tank, and the crossmember at the end of the frame, I guess even Chevy decided that this crossmember was useless. 15lbs is significant, that is all that I'm saving by going to the aluminum batwing. No way am I giving up my Bacon cheeseburgers. I like the tubular replacement idea.
Bee Jay

turtlevette

That's what i was going to say. I think its mostly dead weight. I think one reason why they were able to take a lot of weight out of the 80-82. You have to take it out to get significant weight savings on the swap. My batwing and aluminum diff are still sitting in the garage as is a Rodeck aluminum block.

Please take lots and lots of pictures as you progress.

SH-60B

Good job on the roll bar. It's out of the way and stiffens the frame, outstanding. There are some guys on here that don't have a clue when it comes to simple fabrication that works, I hope they view this thread so they can see. As mentioned before, keep the pics coming.

V-Twin

It's not in the way, keep it. If you want to remove weight, drill a bunch of large holes in it. My brother aded a crossmember like that to his frame and I don't think he added it for ***** and giggles.

tfi racing

I say cut it out.Weld in a smaller tube of your choice and hang the fuel tank off of it as well and lose the bolt in strap.Anything behind the bat wing is just along for the ride and not contributing much.While you are at it,consider replacing the rear part of the frame and putting in a real fuel cell,lower and further forward will help.The purists will cry,but who cares,it is your car after all!

RippoDippo

Seat Belt set looks like a gallows. You need to have restraint points located below shoulder height, think it's 2-3 inches? Think I read that on the Skybolt plans

63mako

Ther is no way I would remove that. The weight is at a low center of gravity and it adds a lot of rigidity to your frame when you launch. The 80 to 82 didn't have much HP out of the factory to worry about twisting the rear on a hard launch

TheSkunkWorks

While the argument that the x-member can be safely removed is certainly valid, as the '80 & up batwing chassis never had it, IMHO you'd likely notice a reduction of chassis stiffness if you remove it altogether. My vote would be to either lighten it or to fab up an improved replacement.

Have you thought about raising the diff height during this project?

turtlevette

the instructions that came with my 5 point belts said to provide attachment points above the shoulders to avoid compression of the spine in a collision. They provided an angle i can't remember. I guess i'll have to find the instructions when you write back and say i'm full of crap.

TheSkunkWorks

Upon reflection, I must admit that typical attachment point heights did move from below the shoulders to roughly level with them in the various cars I've raced over the years, and that I haven't installed any racing belts myself in a good long time. And, as there's been advances in thinking about safety since then, I'm in no good position to argue with you here. However, if Bee Jay's install isn't exactly "by the book", I think we can agree that this is not a place where you fudge things. All that said, not to disappoint, but I guess I'll have to wait for a better opportunity to tell you how full of it you are...

68/70Vette

Could you say where you got the brackets to attach the batwing to the frame?

Thanks

Bee Jay

A friend is making brackets for me. And yes, I am raising the diff 3/4". That is all I could go, at 7/8" the batwing will be up against the floor and the frame rails.
Bee Jay

TheSkunkWorks

Good call.

Maine Vette

Well, I'm not a racer or chassis expert but I say leave it in.

I see two major differences between your new crossmember and the old one - the old one mimics a 'fixed' connection at each end whereas the new one is what's termed a 'pinned' connection. The original connection can transfer rotational and torsional forces from one frame rail to the next whereas the new one cannot.

Imagine you were standing along side your frame with a car jack and, with the jack, tried jacking up your car by placing the jack right next to the crossmember connection. The connection used by the old crossmember would be very effective at resisting differential deflection between the two frame rails and would provide for a relatively rigid framing system - it would resist the tendancy for one frame rail to lift up and not the other. The new crossmember alone would not be effective in this sense and would allow one frame rail to lift without the other - the cross frame would simply pivot about the bolted connection. Now instead of thinking of lifting the frame with a car jack, think of the forces transferred from your tires to the new crossmember, and then to the frame when you take a corner at high speed.

With only the new crossmember in place the net result would be that each frame rail would act more independantly rather than together as a sytem. The more frame action you have, the stronger and more rigid your frame will be.

Like I said above, I say leave it in. And yes, you could cut holes in the old crossmember but I view that as a lot of work for relatively little benefit. Plus the holes would need to be located properly to avoid adversly affecting the structural integrity of the crossmember.

Good luck with whatever you decide.
Tim

Bee Jay

I uderstand what you are saying and I would agree if this crossmember was necessary. But GM stopped putting this crossmember in with the '80 models. Even GM ditched this crossmember. Why did they do that?
Bee Jay

68/70Vette

With the relatively weak all-aluminum differential housing and low engine horsepower, they probably assumed that 80-82 customers would only stress the frame to boulevard/highway cruising performance standards.

69427

I'm inclined to agree with 68/70. It seems reasonable that the philosophy of the Corvette engineering group at that particular time was to get weight out of the cars to improve mileage/acceleration, even at the loss of some frame structural integrity that essentially would only present itself under high horsepower/high speed circumstances, unlikely given the rating of the production engine.

Maine Vette

I tend to agree.

I'm not intimately familar with 80-82 frames, but I wonder if GM might have strengthened the side rails when they removed the crossmember to reduce frame flex.