shrike

iTrader: (1)

Hello,
It is probably not going to be a challenging question but I would like to know if you think my crossmember with the modification I did is strong enough.

The car is a 1987 Cadillac brougham into which I am putting a 4.8 vortec engine with a 4L60E transmission so it will be pretty much stock, IF ever it might have a cam and headers so it won't see more than 300 -320 horses. But I doubt it I will do that.

So what I did to the crossmember is to weld a 2"x2"x1/4" tube in order to reach the mounting point on the tail of the transmission. This moved the mounting point about 4 inches towards the front of the car. The old position of the bolt is shown with a green rectangle and the old bracket is shown in red. The blue recangle shows the new position of the bolt.

But I am now worried this will induce a torque on the 4x10mm bolts that attach the cross member to the frame of the car. Do you think the bolts are strong enough or will they shear? Or even worse, will the frame be damaged? The distance between the bolts shown on the second of the pics below is about 2.5 inches

What is the scale of the forces involved. I know there is the weight of the transmission which gets multiplied maybe by 3-4 times when you go over bumps.
What is the force that is applied when the car accelerates? I really have no clue and could not find the info on the net.


Thanks

zipster

iTrader: (5)

The twisting force on the cross member seems like it would break the bolts on the frame rails. Why not move the cross member forward 4 inches and use the original mounting pad? Just drill new holes in the frame. And, maybe upgrading the bolts in the frame.

01cherryreds10

iTrader: (9)

Because if its anything like the metric gm cars, the trans crossmember isn't straight, and has a pretty large curve going forward from the trans to the left frame rail, if that makes sense.

I think for a mock up it works very well, but the final product may want some extra strength for street conditions. My chromoly tubular crossmember has much more flexibility than my fathers g-force boxed trans crossmember.

shrike

iTrader: (1)

Thanks guys,

I don`t want to drill the frame. If I really have no choice I probably will but it would really be the last thing I`d like to do.

The cross member does not have a big curve on the left side. The old attachment point was about in-line or more towards the front of the car as the first two bolts from the front of the car that attach the crossmember to the frame.


Can anyone give me a number, even approximate, on the force with which the transmission is pulling upwards upon acceleration? I will try to calculate what is the resulting force being transfered to the bolts.

gofastwclass

iTrader: (2)

Um, don't take this the wrong way but you're thinking too hard about this.

What you have done will work and it will work for a long time (BTDT years ago). The bolts are plenty strong and won't shear if they are not rusted. What may (and probably will) happen is you will have an additional harmonic vibration at certain speeds you can't quite trace. The lever you created will twist the trans cross member ever so slightly at highway speeds and induce a small bit if vibration if your pinion angle or drive shaft isn't quite right.

Personally I would fabricate a cross member from 1x2 .095 steel or something similar and make it as straight across as possible with clearance for exhaust. I would also box the "C" section of the frame in the middle. Having owned several GM cars of similar frame design, those two things and the power plant upgrade in progress are some of the best things you can do for one of those cars.

shrike

iTrader: (1)

Thanks,
So I will keep it like this until it runs and then I will figure it out if I can tough it for 1 year or should make another one. Maybe I can find some stronger bolts, but in that case can I keep the nuts or should I change them too?

I see what you mean now about the harmonic vibration. If the end of the driveshaft orbits at the transmission end I understand how this can happen.

I am also working on getting the pinion angle to match the transmission angle. I hope I have enough space so the driveshaft won't touch the floor. But that will be a chapter a bit farther down the road.

I think I have an idea now how to do a crossmember with tubing. But I suck at welding. I can cut it though and bring it to be weld by someone else. That's what I did with the actual one.

WHat would be involved in order to box the frame? I was thinking to do it, or rather have it done but the body should be lifted right? If I ever do that at least I will do it right and also paint the frame but that is a big job that I could only see happen in 5 years, maybe!

gofastwclass

iTrader: (2)

Honestly I wouldn't worry about it. Replace the hardware if you like but the factory stuff is quite heavy duty. If I were to replace it I would use grade 8 (US) or grade 10.9 (metric) nuts and bolts in the same size as factory.

Welding is a necessary skill in the fabrication business.

Boxing the frame is easy if the body is off, but with the body on things get more complicated. One needs to be fairly skilled to do the work with the body on but it can be done. In a pinch you could lift the body up a few inches for easier access. A four inch strip of 11 ga. works great for the middle section which is all you really need to worry about. Honestly if welding isn't your strong suit, taking it to someone would be much better.

shrike

iTrader: (1)

I would have loved to be able to weld. I would weld stuff but nothing structural.
Anyways, for sure I will bring it to someone else to box the frame. The more i think the "easier" it sounds. But I want to finish the engine swap, use it 2-3 years and maybe attempt that.

If I can find someone who can do it with the body on the frame I will probably do it before.

BelAir1000

iTrader: (1)

Isn't there a third hole in the frame? Can you move it forward and use the forward two hole instead of the rear two?
My '68 Bel Air wagon had two mounting positions for the cross member. I'm guessing one for the TH350, Th400 or powerglide or maybe the 4-speed.
I am using the rear holes because the front position is too far forward. I also made a plate to adapt the extra 3 inches to work with the 6L80E I am using.
While my modification is not quite as extreme as yours, I will had a 400hp L92 and a 4:1 first gear with the 6L80E. So the forces involved should be greater than yours. I'm a mechanical engineer and not the least bit worried about it. The crossmember is thick tubular steel and it runs from frame rail to frame rail.

shrike

iTrader: (1)

Yes there seems to be a third hole in the picture. I was also thinking about it and will take a look next time I get under the car . I got to remove the crossmember one more time and drill a hole.

So If there is an extra hole more forward on each side of the frame, whenever I will make a new crossmember I will use these holes to mount it.

terryr

It would take thousands of pounds of force to snap the bolts, or probably tens of thousands.