build diary

May 19, 2005: The transmission cover is off and now the failure is visible.
The ticking noise that alerted me to the problem turns out to have been part of the parking brake assembly bouncing off the pinion flange. CMC had reported a clicking noise and loss of drive and I'd never understood where it came from. The clicking, okay. The loss of drive is still a puzzle of course.

The failure itself was the mounting bracket on the transmission tunnel. The bolt holding the diff in place was only in contact with a small area of the bracket as this was too far forward. As the diff strained to move up and down, it tore the bracket apart. It's quite possible this was a slow-motion failure that only made itself known after the diff had lifted enough to contact the parking brake.

Solutions? I have three options right now. One is to copy what CMC ended up doing and build a tubular version of the PPF. Heikki has some photos of this on his site. It will have to wiggle around the edge of the transmission tunnel and driveshaft flange but it should be possible. It doesn't look compatible with a handbrake, though. One way to make this easier would be to put a plate on the side of the transmission tunnel instead of the tubes. This would open up 3/4" or so of width and I'd probably still be able to work out a way to attach the handbrake. I don't think any cars with this solution have turned a wheel yet although it certainly should work well.

Another option would be to copy the solution developed by BSI and used on the GRM car. Again, that car hasn't really been run hard. I'd be concerned with the welded tube coming off the back of the diff but it sure would make life easy. I suspect the weight would be less than the PPF solution as well.

The third option would be to simply reinforce the bracket at the nose of the diff. Instead of a thin piece of about 1/8" plate, I'd put in something stronger. It would also be a properly located hole instead of a half-engaged slot. An extra tube would be run straight down from the side of the bracket to ensure it was well supported - possibly a plate. This might be the easiest and lightest solution. Given the amount of abuse my original setup endured I believe this would probably work well - but I do have plans to increase the horsepower of the car at some point.

I've also decided to add some stiffer bushings in the side mounts for the diff. They are able to rock back and forth a fair bit. I might also bolt on a piece of C channel to hold the other end of the bolt in place. At the moment it won't move with the diff but that will change with stiffer bushings.

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