Frame stiffening with trans crossmember

[vette427sbc]

Gathering parts and ideas for a T56 swap this winter... I have a welded in factory crossmember now (4speed). I was thinking of ditching the whole crossmember in favor of a large tubular X-brace to stiffen up the frame a bit and also serve as the trans crossmember. I am yet to actually get under the car and look at obstructions, but I dont remember anything being too low or in the way. It also has to bolt in... I have sidepipes, so exhaust clearance is not an issue.
Will it stiffen up the frame enough to be worth it?

 

[mrvette]

My '72 vert was a muncie car, and so I cut the cross support on about 4" either side from the rails....then got a buddy to weld some 2" channel on the top rear of the loose section, and same for under side, and in back so it would slide to the rear, held in place by a fore/aft bolt so could not slide rearward....

been there some years now, rough roads, etc never found any performance problems, or rattles.....

SO some years later I got the lower loops for the exhaust pipes but out, on a keyhole through the lower frame, and a mini lattice structure built into the top loop and close the horozontal structure of the cross piece.....

so I can drop my entire exhaust system out with 6 bolts, on the ground and outta there.....

GOD, I'M lazy.........:hissyfit::nuts:

 

[SuperBuickGuy]

Just thinking out loud here:

I keep worrying that the point where the frame goes up and over the rear diff is the major weak point/flex point on the frame.... probably just overthinking it. But that would worry me more than the cross member. I lift my car on the straight frame rails, can still open my doors without hanging. I jack up the car side-to-side in the middle of the frame rail - still can open the doors.... I guess the only thing in my mind that cross member would be instrumental in doing is protecting you in an accident by tying both rails together (doubles the strength).

You got me thinking, maybe I'll run a door bar tomorrow before I put the interior back in my car....

 

[clutchdust]

I'm not so sure it will really be possible with a size that would give you any benefit. The reason I think is because the floorpan sits in between the frame rails and is specifically shaped to clear the original cross member. Putting an 'X' brace in there would either require using tubing too small to be beneficial or the brace to hang down too low under the car.

My experience tells me that the main problem with our frames is their relatively single plane design. By that I mean that for all intents and purposes, the frames might as well be a flat, two dimensional structure. The result is they're pretty good in shear, but pretty horrible in buckle and torsion resistance.

Now I've been contemplating a tunnel stiffening addition similar to the c5/c6 backbone. In the stock configuration there's not a great deal of room in which to work but bringing your structure up should increase the buckle resistance while boxing it in (just think of a full length driveshaft loop that goes all the way to the bellhousing) should add some torsion resistance. In the back top, just above the pinion yoke, I plan to run lateral supports to the top of the framerail kick-ups and diagonals from that point back to the differential crossmember. In the front, I intend to tie it directly into my SFI bellhousing and then laterals and diagonals from there to the forward framerails.

But this is all top secret, so don't tell anyone.

 

[vette427sbc]

Just thinking out loud here:

I keep worrying that the point where the frame goes up and over the rear diff is the major weak point/flex point on the frame.... probably just overthinking it. But that would worry me more than the cross member. I lift my car on the straight frame rails, can still open my doors without hanging. I jack up the car side-to-side in the middle of the frame rail - still can open the doors.... I guess the only thing in my mind that cross member would be instrumental in doing is protecting you in an accident by tying both rails together (doubles the strength).

You got me thinking, maybe I'll run a door bar tomorrow before I put the interior back in my car....

Ive always thought the same thing... I have been running roll bar ideas through my head recently where the main hoop attaches to the frame before the kickup and the rear bars brace the kickup section. Seems like it could be a little intrusive and no better than just running some low profile door bars

I'm not so sure it will really be possible with a size that would give you any benefit. The reason I think is because the floorpan sits in between the frame rails and is specifically shaped to clear the original cross member. Putting an 'X' brace in there would either require using tubing too small to be beneficial or the brace to hang down too low under the car.

My experience tells me that the main problem with our frames is their relatively single plane design. By that I mean that for all intents and purposes, the frames might as well be a flat, two dimensional structure. The result is they're pretty good in shear, but pretty horrible in buckle and torsion resistance.

Now I've been contemplating a tunnel stiffening addition similar to the c5/c6 backbone. In the stock configuration there's not a great deal of room in which to work but bringing your structure up should increase the buckle resistance while boxing it in (just think of a full length driveshaft loop that goes all the way to the bellhousing) should add some torsion resistance. In the back top, just above the pinion yoke, I plan to run lateral supports to the top of the framerail kick-ups and diagonals from that point back to the differential crossmember. In the front, I intend to tie it directly into my SFI bellhousing and then laterals and diagonals from there to the forward framerails.

But this is all top secret, so don't tell anyone.

Id be interested to see how you are going to fit anything besides the trans in that tunnel without major interior intrusion. I do like the idea though...
The stock crossmember sticks down an inch or so below the frame rails anyway, so I dont mind if the X brace sticks down the same amount. Im going to crawl underneath tomorrow and see what kind of room I have to work with.

 

[SuperBuickGuy]

I never got to the point to look at doing the bars today.... maybe I'll diagram it out tomorrow night...

I'm not sure you'd fit a vertical bar before the rise in the frame.... it's right at the back of the bottom of the seat. When I built my roll bar, I welded it to the top of that rise...

 

[clutchdust]

You're right, there isn't a lot of room. However, I have had my car up on stands and spent enough time under it that the Mrs worried about my mental health. In stock configuration, there isn't that much room for running steel tubing. But what I'm thinking of doing is using sheet metal and boxing it in. If I measure correctly, there's plenty of room to fit sheet metal between the trans and the tunnel. There's all kinds of room between the shaft and tunnel though.

For me, I actually plan on cutting the tunnel and raising it up a few inches. I see not only no point in it being as low as it is, but that it could be made more functional both in the interior and under the car if the tunnel were about three inches taller from the dash back to the cargo compartment. It should be the same height as the cargo compartment and that's what I'm going for.

But for what you're looking for, I think you would see an advantage if you were to make rectangular boxes from the trans crossmember to the rear frame crossmember. Rather than cutting the entire crossmember out, just cut out the center section that has the mount and make that part removable. Then box it in over the transmission tailshaft and run braces back to the kick-ups. Then, rather than put a roll cage in the back to tighten up the rear rails, jump off the rear crossmember, go under the pumpkin and then box in the frame from under. You could use smaller tube and get pretty good results without too much additional weight and interference.

 

[SuperBuickGuy]

I'm having trouble envisioning what you're planning. With the C3, the rails are at the outside of the car and the body is mounted on rubber - how would stiffening the center help the car's rigidity when the chassis and body are independent?

Your idea could work with solid body-mounts; but I couldn't imagine the fiberglass body panels would enjoy the additional vibrations....

 

[rtj]

Will it stiffen up the frame enough to be worth it?

A lot of after market frames for oem bodie use a large x brace between the frame rails.

Anytime you can triangulate a structure, it is beneficial (in general).

 

[SuperBuickGuy]

Will it stiffen up the frame enough to be worth it?

A lot of after market frames for oem bodie use a large x brace between the frame rails.

Anytime you can triangulate a structure, it is beneficial (in general).

I understand the general rule, but there does come a point where the weight gained not only takes away any gain but actually makes you even slower than you were before the X brace

 

[427Swede]

I am by no means an expert on frame modification or strengthening, but I can not really see any improvement in torque rigidity by adding X bars on the underside of the car? The reinforcement will basically be in one plane only and that will not do any good.
Now, if we could manufacture the X members with a bit of height we would achieve much more torque rigidity. Can not see how that should be done in a C3. However, the X members will add some shear resistance in the frame but that is a different story.

A very simple reinforcement that can be done easily is to fully box the rear kick up braces. This will introduce a box design to the braces. I did this on my frame, have not seen it done before. Dont know if it adds much strength but it is too simple not to do, while you´re at it !

Just my input on the subject

 

[rtj]

Will it stiffen up the frame enough to be worth it?

A lot of after market frames for oem bodie use a large x brace between the frame rails.

Anytime you can triangulate a structure, it is beneficial (in general).

I understand the general rule, but there does come a point where the weight gained not only takes away any gain but actually makes you even slower than you were before the X brace

My car was like pretzel with the big block until I built a cage (granted it is not the best workmanship). I know it's out of plane but the difference was huge.

Look at frames from some of the aftermarket companies. They use an x.

Turbo84 is on this forum, he built some models. Maybe he'll comment.

 

[clutchdust]

I'm no CAD guy so this is just a rough sketch I did in MSPaint. It's ugly but I think you can kind of get an idea of what I'm considering. Of course, mine will be a bit more extensive because I am planning on raising my transmission tunnel. But in the end, I believe the results will be an order of magnitude stiffer frame.

 

[rtj]

I'm no CAD guy so this is just a rough sketch I did in MSPaint. It's ugly but I think you can kind of get an idea of what I'm considering. Of course, mine will be a bit more extensive because I am planning on raising my transmission tunnel. But in the end, I believe the results will be an order of magnitude stiffer frame.

Yes, double frame rails is a good idea.

Some of the drag cars also incorporate a full driveshaft tube. That could add stiffness if done right. Hard to do and keep the driveshaft removable.

 

[73c34me]

I cut the Trans. X member approx. 4" from each frame rail, capped the ends then
Maxe a removable beam above the exhaust. Then I triangulated this from the rear cross member and tied into the fore frame rails. Simple really, a pic would be so much better. Also have a 6 point roll bar tied into this.

 

[BBShark]

I am by no means an expert on frame modification or strengthening, but I can not really see any improvement in torque rigidity by adding X bars on the underside of the car? The reinforcement will basically be in one plane only and that will not do any good.
Now, if we could manufacture the X members with a bit of height we would achieve much more torque rigidity. Can not see how that should be done in a C3. However, the X members will add some shear resistance in the frame but that is a different story.

A very simple reinforcement that can be done easily is to fully box the rear kick up braces. This will introduce a box design to the braces. I did this on my frame, have not seen it done before. Dont know if it adds much strength but it is too simple not to do, while you´re at it !

Just my input on the subject

I really like what you did with boxing the rear kickup braces.

The X brace does seem as though it would be marginally effective for a C3. However, take a look at the X brace that was adapted to the C4 when the convertible was introduced. It doesn't look like it would do anything and yet I heard at the time that torsional rigidity was improved 20%.

 

[BBShark]

I'm no CAD guy so this is just a rough sketch I did in MSPaint. It's ugly but I think you can kind of get an idea of what I'm considering. Of course, mine will be a bit more extensive because I am planning on raising my transmission tunnel. But in the end, I believe the results will be an order of magnitude stiffer frame.

The tunnel reinforcement does look like an opportunity to strengthen and raising the tunnel probably isn't that big of a deal. If you do this, post lots of pix.

 

[clutchdust]

Don't worry, I'm already planning on a couple gigs of pics. But in all likelihood, by the time I actually get around to doing it, you guys could probably teleport in and see it live! :nuts:

 

[71vettewi]

What else

I have done this on my own and I was thinking about bonding c6 floors in, rebuilding close to a C6 that I just rebuilt from a wreck. Lots of good ideas.
Tying in the front is as important as the rear.

 

[84rzv500r]

If my opinions change over time thats good...

Best mod to my car was preping the frame to Power Service manual specs and the 14pt roll cage...

number two was ditching the stock iron calipers for wilwoods

number three Driveline... engine trans rear sticky tires

number four Rack and Pinion steering

number five very comfy ultrashield seats


The stock c3 frame is a flexy flyer and couple that with the general age of the fleet and you have a recipe for disaster when it comes to handling and structural failures

:devil: