Racers, chassis experts, is this frame crossmember needed anymore?

B

Bee Jay

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Sep 11, 2008
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Lompoc, Ca.
I asked this over on the other forum also, but I think all the engineers are over here. Twin Turbo, what do you think?
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:
Corvette232.jpg
You can see the cross member in question in this picture:
Corvette229.jpg
You can also see the bolt in cross member for mounting the gas tank.
Corvette1202.jpg
Here is the Autopower four point roll bar from topside:
5def6373.jpg
 
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without it i don't see much support to keep the rear end from flexing the rails. if you wanted to save weight you might be able to replace it with some round tubing welded inbetween the rails.
 
The stock 80-82 frames don't have the crossmember either, but I welded one in mine (although I lightened it with large holes and I needed it for the upper control arms)

It did add some rigidity in combination with the cage but my cage is much more involved back there (a lot more triangulation and all)

I would keep it, if you cut it out it's going to be a bitch to clean up the frame to make it look good. If you're obsessed with weight you could rill some holes. Welding in a small diameter bar will not do much good, it will simply buckle the thin steel frame on the sides, or you have to weld some base plates on there that angle over the edge of the frame or weld the new bar so it sits right on the edge and a little over it so that it's not just welded to the inside of the frame.

You DID have to cut away the original crossmember pedestal mounts right? Where did you get the correct location for the batwing? I posted a diagram on CF a long time ago, in a thread by turtlevette I think it was. Had all the dims needed to locate the brackets.

Here's my "hole" job:

24856bea8665b4.jpg

24861978f2aa36.jpg

Not how it's not welded to the inside of the frame but actually across the edge, here's a bottom shot!

24861990fd20bf.jpg
 
while your welding and fabricating you should remount that shoulder harness lower, that mount is wrong and dangerous....look at the pics above, thats the correct way.
redvetracr
 
Twin_Turbo said:
The stock 80-82 frames don't have the crossmember either, but I welded one in mine (although I lightened it with large holes and I needed it for the upper control arms)

It did add some rigidity in combination with the cage but my cage is much more involved back there (a lot more triangulation and all)

I would keep it, if you cut it out it's going to be a bitch to clean up the frame to make it look good. If you're obsessed with weight you could rill some holes. Welding in a small diameter bar will not do much good, it will simply buckle the thin steel frame on the sides, or you have to weld some base plates on there that angle over the edge of the frame or weld the new bar so it sits right on the edge and a little over it so that it's not just welded to the inside of the frame.

You DID have to cut away the original crossmember pedestal mounts right? Where did you get the correct location for the batwing? I posted a diagram on CF a long time ago, in a thread by turtlevette I think it was. Had all the dims needed to locate the brackets.

Here's my "hole" job:

24856bea8665b4.jpg

24861978f2aa36.jpg

Not how it's not welded to the inside of the frame but actually across the edge, here's a bottom shot!

24861990fd20bf.jpg
Click to expand...

That crossmember isn't on the '80-82 frame? That explains a couple of things. Every picture I've studied of the batwing mounted had a clear shot of the batwing without a crossmember in the way. Also, you sent the locations for the brackets to me a couple of years ago, and I was wondering why all of the measurements referenced the bump stop bracket to the front, and the body mount bracket to the rear. No mention of that crossmember, which you can see, that I'm using as a measurement reference. My brackets and the mounting points will be slightly different from yours because I am also raising the diff 3/4". The Chevy Power book calls for 1", but there isn't room for the batwing to move up 1" without cutting my floor, and notching the batwing on the end or the frame. Can someone post a good picture of a "virgin" 80-82 frame in this area?
Bee Jay
PS. I've been meaning to weld in a crossbar to my rollbar to better mount my harness. I haven't gotten around to it yet. I don't race the car, I just use a lot of you racers ideas. Thanks.
 
I just looked on ebay...NO crossmember. personally I would weld something in behind the diff just to strengthen and triangulate the frame.
redvetracr
 
If you noticed an increase in stiffness by welding a 4 point roll bar in, that's a pretty good indication that you should keep the crossmember. Maybe put some holes in it if you are concerned with weight.



.
 
redvetracr said:
while your welding and fabricating you should remount that shoulder harness lower, that mount is wrong and dangerous....look at the pics above, thats the correct way.
redvetracr
Click to expand...

Howard is right on the belts, also keep the support, mine has an extra one installed. Remove weight somewhere else. No more soft drinks....lol

out of the CCR, hope this is helpful.

beltmounting.jpg
 
I didn't even notice that, they are right, all you need is a noose and you have your own hang man :)

Here's my virgin frame before all the madness :)

82frame.jpg
 
Bee Jay said:
Also, you sent the locations for the brackets to me a couple of years ago, and I was wondering why all of the measurements referenced the bump stop bracket to the front, and the body mount bracket to the rear. No mention of that crossmember, which you can see, that I'm using as a measurement reference. My brackets and the mounting points will be slightly different from yours because I am also raising the diff 3/4". The Chevy Power book calls for 1", but there isn't room for the batwing to move up 1" without cutting my floor, and notching the batwing on the end or the frame.
Click to expand...
Bee Jay/Twin Turbo, could you post those measurements? This is a swap that I am planning on doing eventually. I am going to raise the diff just like you are doing.:bounce:
 
Twin_Turbo said:
I didn't even notice that, they are right, all you need is a noose and you have your own hang man :)

Here's my virgin frame before all the madness :)

82frame.jpg
Click to expand...

Dang, lookie there. No crossmember behind the batwing. GM herself thought is was not needed. So, if I remove mine, my frame will be just like an 80-82 frame, but with a four point roll bar weded to the same area. I could remove the crossmember, then if I notice flex, I can fabricate a smaller, lighter tubular or square tube crossmember with a large angle end pad. Thanks everybody.
Bee Jay
PS Wasn't the stock shoulder harness higher than my shoulder?
 
I took it out! I was trying to measure accurately to help make templates for the batwing mounts, and the crossmember was all in the way. I couldn't help thinking how much it would be in the way when it was time for welding. I had some spare time this morning, and using a air chissel and sawsall, I took this useless crossmember out. The weight of the crossmember and the corner brackets that the old differential crossmember mounted to came to a grand total of................drum roll here................

20lbs.

So if I save 15lbs going to the aluminum batwing, and 20 lbs removing this crossmember, I should realize a total savings of 35lbs. Someone check my math. I'll take that, plus my rear end will be 3/4" higher. I'll post when I'm all done.
Bee Jay
 
No it does not. His rollbar is just a big hoop, there's no diagonal brace at all. It's basically a big U shape and without any triangulating it's very easy to deform and move the "legs" of the U shape relative to each other. This means that as the suspension gets loaded up one side of the frame may distort upwards and the other will not. The bolted in batwing will do very little to keep that from happening, same for the rollbar.
 
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