Trans cross member design help

[drdano]

I'm rebuilding the cross member for my TKO that is in my '62 Galaxie.  The mounting pad for the Tremec is back a few inches like the C6 vs the stock C-O-M that was in there at factory birth.  My first iteration of the cross member was built from 1.5"x3" .120 wall rectangular tubing and used the factory mounts, so it had to have a nice step back of a few inches and around an inch of drop to use the factory cross ears cut off the old unit.  See the construction pics of this attached.  This worked...ok.  Now I have new FPA Tri-Y headers that dump out right into the factory cross member mounts.  So, I cut them off the frame and intended to set them back a few inches so I can have a perfectly horizontal trans crossmember now with no built-in setback.

Ok, here is where I need some help.  I'm likely ditching the factory frame mounts and going to use something more compact since they are cut off anyway.  I need the cross member as high up and close to the floor pans as I can get it as I'll have 2.5" exhaust running under it now and maybe someday 3".  Not using the factory frame mounts I can now set the cross member as high up as I want and gain more exhaust clearance, which is great.  Also, no setback will be needed in the crossmember, again another plus.   The span is around 42" from framerail to framerail...pretty far.   I'm concerned that if I run the crossmember perfectly horizontal with the drop I need in the middle (around 3"), it may flex some, see the first sketch attached.  Do I gain any strength in vertical flexing at all from the second option where I set the frame mounts lower and kick up a section on each side for exhaust clearance?  See the second sketch for what I'm thinking there.

Thanks all, appreciate the feedback.

[BattlestarGalactic]

I can't see that size tubing flexing much at all even 42" in length.  But I think the zig zag version would be better just by looking at it because of the shorter sections of straight tube.  Consider most rear chassis crossmembers are 2 x 3 and put up with huge stresses.

I made my own crossmember for the wagon years back.  It is just 1 1\2" sq tubing with round tubes welded through the ends for bolts and two tabs welded to frame for bolts on each side.  Very simple, I can get a picture tomorrow at the races because the car is in the trailer currently.  Mine does flex, but I have a mid plate on the motor so it doesn't support much weight overall.

[My427stang]

I think either are overkill with that size box tubing, and go with whatever fits better and is easier to make.

If it runs stock style mounts, the mounts would tear long before you could put enough stress on that cross member to hurt it, and if it's all solid, the engine mounts take most of the rotational force.

Now, if you are trying to stiffen the frame in general, not looking at it as a transmission mount, then I would likely triangulate the ends to have braces go forward and backwards, but as a transmission cross member, those will be plenty strong

[cjshaker]

I can't see that size tubing flexing much at all even 42" in length.

Ditto. Looks pretty darn strong to me. And I agree with the zig zag design. With 1/8" thick material, that should be plenty stout!

[drdano]

Thanks for the feedback fellas, I'll probably do the second design only because I enjoy the tig welding and have a fairly decent rig for my porta-band that makes angled cuts much easier now.

I have the tube end mounts coming from an off road supplier this week that are bushed and sleeved 3" DOM with 1/2" mounting bolts.  Similar to what BattlestarGalactic mentioned, but with a insulated mount on each side.  I figured I'd fishmouth the tubing into the round DOM tubing and then have one large bolt per side to pull the crossmember.   

[drdano]

I was involved in a pretty serious motorcycle crash end of July, so finally getting well enough to get back to this.  I made a prototype crossmember from 1x2 with a .080 wall.  The focus was to see how making the cuts and bends would work without having to cut all the way through the tubing.  Trial and error, and a really useful youtube video helped make the right cuts https://www.youtube.com/watch?v=vFdmdC2zxZc.  I kept cutting too deep without the offset and had a nasty gap to fill, so this helped a lot.  So I ended up with a nice 42" span with a 1/2" drop and TIG welded it up.  This way it can tuck up just under the floor and give maximum exhaust clearance.  This material is definitely too thin though, when I stand on it it will deflect a 1/4".  I'm unsure how much actual weight would be on the trans mount, probably a few hundred?  So the next iteration will be 1x3 with a .120 wall.  I think that should be plenty stout.

[Rory428]

Sounds like the transmission crossmember in my Fairmont, is similar to Larry's. Just a straight length of 1 1/2" square tubing, with a pair of tabs on the frame rails, with a 3/8" bolt on each side. With the C6, and later on, the Jerico 4 speed, the trans mount studs just fit into holes drilled on the topside of the tubing. When I switched to the G Force G101A 4 speed a few years ago, just welded a plate sticking forward a couple of inches for the mount. Now, the crossmember in the Fairmont is less than 3 feet wide, but over 30 years of racing, most of that with 6000+ RPM clutch dumps, the crossmember is still showing no signs of fatigue or bending. I have the engine mounted solid, with a rubber trans mount.