Ok, so obviously a boatload of torque can, but i don't think we've got quite that much so heres the situation:

383 stroker M6 putting down 380 rwtq
4.10s moser 12 bolt
275 Firehawk sz50s (they're sticky, but no slicks)

Broke stock torque arm at track on Mothers day
Attachment to rear axle mangled and bent

Replaced with BMR standard TQ arm
Only street driven, maybe 1500 mi
Bottom of axle mount snapped, both top bolts missing! All 4 bolts were recently checked and retorqued during an oil change.

I didn't install it, but I'm assuming the angle was correct or really close because there were no vibrations.

I'm not jumping out and saying anything bad about bmr since this car has eaten two torque arms in ~ 2 months.

What could be causing this excessive stress back there?

Any suggestions appreciated.

-brent

 

Looks like the TQ Arm wasn't spec'd during the install for your application. I would look into a second opinion as to what's the right pinion angle for your application.

 

Thanks for the reply.
That was one of my concerns, but if the angle was off that bad wouldn't it cause a lot of vibes? I would have expected a U joint to pop before a bmr torque arm. Plus the angle on the stock one should have been ok and there are people running the stock one with that power, or was that an expected failure?

 

In my experience if you have too much negative pinion angle if it would will cause excessive u-joint wear but I have not had any vibration with up to 3-4 degrees of negative pinion angle. I have however experienced the vibration you mention at any positive pinion angle.

To be twisting up torque arms like that it sounds like your rear end is trying to roll somthing fierce. You might want to take a look at your lower control arms and check the bushings and attachment points to see if something there is loose.

How long has the 12 bolt been in there? Is there any way the torque arm failures could be related to the 12 bolt installation?

 

Thanks again, all the suggestions are appreciated.

I did look at the control arms breifly tonight to see if there was something amiss. Nothing obviously visible. The 12 bolt has been in there for awhile, and the problem jsut popped up. It has 1LE boxed control arms so rubbing bushings whichwill give more than the poly on the torque arm mounts. My thought was that any excessive flex in the control arms would be taken up by the front mount of the torque arm. The front mount can slip for and aft because it's just a Tube extended through a bushing. Sooo there shouldn't be too much stress from the rubber bushings deflecting and the torque arm should still be able to do it's job of countering the moment. I will look into that.

It raises another concen, maybe the tranny mount isn't up to snuff. The shifter doesn't flex under load though. I'll check that too along with the motor mounts but there are no symptoms up front.

This broke on the street! At the track with slicks i wouldn't have been as surprised.

How did hte upper bolts disappear? I know vibratiosn can cause wierd stuff like a bolt walking out, but this is still surprising.

I had recommended a TQ arm that mounted too the body and not the tranny but that was decided against because at the time 380 rwtq didn't sound like that much and the KBDDiamonds would have been in the way.

-brent

 

moser 12 bolts have been known to bend/break torque arms. because the bolts that hold the TA to the axle are known to come loose and break TAs, use some lock tight and you should be fine next time.

 

ok heres my story but the 12 bolt in 2 weeks later i broke my stock ta, bought a mac broke in like a week sent it back they sent me another one broke that like 2 days later so i bought a bmr extreme duty with bmr weld in sub frame connectors and the bmr anti-roll bar i figured i cant brake this. well the bolts kept comming loose in my 12 bolt i got on my car and shifted into second and i hear a bang so i drive it home very very slowly and the bracket that bolts onto the rear bent in half ordered a new one for 70 bucks broke that like 2 monthes later then got it welded like a week ago so we will see . oh and lictite doesnt do ****in ****

 

still comes loose with the lock tite? horrible. sell the 12 bolt and get a 10 bolt

 

yes i hate it im gonna weld the bolts in there

 

I got ragged on so much for purchasing a BMR TA cause the stock TA is supposed to be good for 11s and such.
I don,t have a stroker and I don,t shoot Nitrous but I think it was worth it.
Hmm, sounds like a 12 bolt issue.
Anyone with a 10 bolt and power adder experienced this?

 

Thanks frank and 93formula, thats exactly what i was looking for. Sound like what happened here. We thought we were definitly safe because they were retorqed at the oil change. Here's my course of action.

1) Double Check that the top rear bolt isn't bottoming out.
2) Reassemble with new BMR rear mount and Loctite.
3) Make a Shim Plate and a spare to put both bolts through and spot weld to the plate. The plate won't rotate since it's through both bolts and it will keep the powdercoating intact and be replacealbe by grinding off the weld without scuzzing up the surface.

Fun Fun Fun.
-brent

 

Your Moser 12 bolt is probably the problem. I broke 2 stock torque arms inside 2 weeks and a friend of mine that I race against all the time just broke his stocker on drag radials with a Moser as well. It's something about the housing and not just the bolts loosening up (although that can contribute to it).

What fixed it for me was getting a beefier torque arm (I have a BMR but not the extreme duty). Now these aftermarket torque arms have significantly thicker mounting flanges, which is good and bad. Good for TA strength, bad for thread engagement in the rear end housing because the bolts don't go as far in. What I did is get longer Grade 8 bolts and then tap the holes much deeper. If you feel inside the holes on the Moser 12-bolt housing the threads don't go very deep at all -- not nearly as deep as the holes. I don't know what Moser was thinking in using short bolts and shallow threads when there is all that space in there. So tap out the holes deeper and run longer bolts.

That solved my problems with eating torque arms.