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Weak link - front diff support


Z-Gad

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I still don't see them being the issue here, I mean, if the front member had been reinforced, would it have happened at all?

I believe if you had reinforce the area around the hole, the whole area would have torn out. I had a similar problem where I reinforced the trans crossmember (had to grind on the original for exhaust clearance), and it tore in half right at the edge of my reinforcing plate that was welded in.

 

The front mount/crossmember just isn't strong enough to take all of the abuse. It might work for a while, but it's not a good solution. When you leave soft bushings in the rear and bolt the front solid, the rear is no longer helping to control the diff, because you've isolated the front. So you have all of your engine's torque (600 ft/lbs?) x 1st gear reduction of say 3:1 x rear end ratio of 3.36:1. So basically you're putting 600 x 3 x 3.36 or 6048 lb/ft of torque to that bolt or if you want to be generous, the 8 sq/in of the front crossmember that makes contact with the solid mount. That crossmember is now expected to control both the twisting of the diff AND the lifting of the diff. You've got virtually no slippage going on, and the better you hook up the worse the shock load gets for the drivetrain.

 

I am not convinced that the AZC mustache bar and poly combo helps either. With poly bushings, the bushings still allow for twisting of the bar, which means you're still stressing the front mount. I think the best thing to do is to have some aluminum rear bushings made, and then use the AZC bar. Then you would have a bar that resists bending like a spring which the stocker is meant to do, and no bushings to deflect. Even better would be a "bar" that was made to resist torsional deflection. The tall flat shape of the mustache bar makes it like a leaf spring. Just changing the material to something not springy doesn't mean the shape is optimized...

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The crossmember has FAR MORE than enough strength to withstand the pulling force of the differential... It is fatigue that kills...

 

If you reinforce the center bolt hole in the crossmember then the next failures will be in the inner bushing mount holes...

 

If you reinfoce the entire piece and all the holes then the weld nuts will crack out of the chassis...

 

The problem here is that the differential is ROCK HARD... It wont bend.. It is very heavy.. and the mustach bar twists into an -S- shape under pressure... It will force the differential to move even if you weld the stock mustache bar to the chassis.. It is a spring...

 

You must replace the spring -or- allow the front of the differential to move...

 

The slight give in the poly mounted AZC mustache bar is nothing compared to the amount of flex the stock bar has...

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The crossmember has FAR MORE than enough strength to withstand the pulling force of the differential... It is fatigue that kills...

 

If you reinforce the center bolt hole in the crossmember then the next failures will be in the inner bushing mount holes...

 

If you reinfoce the entire piece and all the holes then the weld nuts will crack out of the chassis...

 

The problem here is that the differential is ROCK HARD... It wont bend.. It is very heavy.. and the mustach bar twists into an -S- shape under pressure... It will force the differential to move even if you weld the stock mustache bar to the chassis.. It is a spring...

 

You must replace the spring -or- allow the front of the differential to move...

 

The slight give in the poly mounted AZC mustache bar is nothing compared to the amount of flex the stock bar has...

I still feel that the bushings allow enough compliance that the diff rocking over and lifting the nose would still affect the front mount. I think we're going to have to agree to disagree, unless you can test diff mounts in the windtunnel too... :wink:

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Just an update... I apparently twisted the driver's side a-arm as well. After reassembling the rear end and rear suspension, I found that the driver's side rear wheel is cocked inward toward he front of the car something terrible. I can see the bend in the a-arm between the front and rear mounts on the a-arm... I'm going to be up late tonight trying to get another a-arm installed and all back together for test-n-tune Saturday night... wish me luck :-)

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More good news...

I installed a new a-arm, and the tow in is a little better, but still there. The strut tower assembly appears to have been bent as well. I have another strut assembly to try, and if this doesn't fix the wheel angle, I will have to assume that the frame has been tweeked. :-(

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Sorry to hear that Mike and I hope it's not the frame or unibody. I have a spare set of control arms and I have a pair of strut tubes that I'll be using for my coilover conversion but you're welcome to those as well. It sounds like you've got it covered but I thought I would offer.

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Finally I got some time to get this thing off the parts car...

 

Here are some pics of a unit that was mounted with an Aluminum solid front mount, Stock type mustache bar, with poly bushings on the mustache bar ears... Track car, no street miles, several seasons with less than 200 HP....

 

crackedcrossmembertop.jpg

 

 

crackedcrossmembertopleft.jpg

 

 

crackedcrossmembertopcenter.jpg

 

 

crackedcrossmembertopright.jpg

 

 

crackedcrossmemberdetail1.jpg

 

 

crackedcrossmemberdetail2.jpg

 

 

crackedcrossmemberdetail3.jpg

 

 

crackedcrossmemberdetail4.jpg

 

 

 

 

 

 

 

 

 

And the tricky thing is.... The bottom shows no signs of damage... even though the top was about to let go.. it already made weird creaking noises... and there seemed to be a lot of play in the driveline...

 

crackedcrossmemberbottom.jpg

 

 

I'll let the pictures speak for themselves...

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I was comparing the spectacular failure of the drag race mount vs. the low horsepower road race fractures(and impending failure)....

 

 

 

Drag race mount...

lateraltearing1.jpg

 

lateraltearing2.jpg

 

 

 

and then the lateral cracking of the road race fractures near the center bolt hole...

 

lateralcracks.jpg

 

 

 

 

 

 

 

 

 

Just for fun...

 

Some nice cropped close ups of the rest of the mounting hole fractures...

 

 

stressfractures.jpg

 

stressfractures2.jpg

 

 

 

 

One in the middle just for fun

 

stressfractures3.jpg

 

 

 

 

 

...

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Very interesting findings!!

Thank you for following up no this thread.

I would think that over several seasons a track car would stress/wear other parts quicker as well due to the side loads of lateral G's. I wouldn't be surprised if much of the spider cracking on the bar was due to the body flexing ...

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Very interesting findings!!

Thank you for following up no this thread.

I would think that over several seasons a track car would stress/wear other parts quicker as well due to the side loads of lateral G's. I wouldn't be surprised if much of the spider cracking on the bar was due to the body flexing ...

I'm not buying the body flexing argument. This has been seen enough times to draw a correlation between the solid front mount and the damage to the crossmember. Plus, it's kind of a strange coincidence that all the damage is to the top of the crossmember. I'm pretty sure the damage is from the solid front and soft rear mounts.

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I am not saying that the mount didn't have anything to do with it, but a dedicated track car sees a lot more body roll and flexing than a street car will. Parts will fatigue quicker, whether that be a diff crossmember, brakes, transmissions, stub axles, etc. Under those extreme conditions, that cannot be denied.

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Hmmm...

 

Obviously the track car will see more cycles of repeated stresses... but keep in mind that the loading is much lower for the track car...

 

 

The chassis flexing IS FAR more pronounced on the drag strip... The Dragster will see body flex that the track car will never achieve...

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The chassis flexing IS FAR more pronounced on the drag strip... The Dragster will see body flex that the track car will never achieve...

 

Maybe so, maybe not. The loads into the chassis are different for the two sports so the types of chassis flex are different.

 

Road Racing

 

Pulling 1.2gs in a corner means the outside suspension is seeing 3,000lbs of lateral load at the outside suspension LCAs and 2,500 lbs vertical at the outside strut towers. In addition you have typical acceleration loads of 1323 ft. lbs (200 ft. lbs. x 1.61 third gear x 4.11 axle ratio) into the driveline.

 

Drag Racing

 

Launching a powerful drag car puts 6006 ft. lbs of torque through the tires into the driveline (500 ft. lbs. x 3.08 first gear x 3.90axle ratio) plus 2,500lbs vertical at the two rear strut towers.

 

The road racing chassis sees large lateral laods and small driveline loads. The drag racing chasssis sees large driveline loads and small (hopefully!) lateral loads. I think the main difference is that drag racing imparts large shock loads.

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