Quote:
Originally Posted by jenks
Sorry @MarkB , I don't see the logic in your posts at all... Are you saying this diff failure wouldn't have happened if the diff cage / chassis was further reinforced? Not trying to be offensive, I'm just not sure what you're trying to get at!
Can understand the need for more rigid mounting on the nose of diff but the overall cage / rear frame design and support of the diff wing has nothing to do with this failure. The tortional forces on the wing bushes are nothing compared to those on the nose mounts. Use the correct bush caps, mount the nose properly and diff won't dismember itself.
A more contructive input would be telling everybody where you think this additional triangulation needs to go (?)
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Rather than guess at the problem or assume someone else has solved the problem on their design. (westfield still have problems with their solidly mounted arangement.) lets try to understand the problem and then solve it!
Firstly the failure mode needs to be understood. Currently no one other than Jenks seems to have correctly identified what actually happened to cause the failure and so we make no progress in resolving the problem we just create a pile of broken diff carriers!
To understand what has happened we first need to understand how the power plant assembly in the donor vehicle works.
The engine and gearbox are rigidly bolted together and the powerplant frame is tightly fitted to the gearbox and the diff. This creates a complete unit which is mounted to the front subframe by two mounts on the engine which primarily are to control the engines verticle movement over bumps and reduce vibration transmitted to the passenger cabin and two rubber isolators on the diff carrier which are to isolate vibration. They provide some small resistance to rocking side to side motion but cars with a power plant assembly such as the MX5 have very little side to side movement under even the hardest accels due to the powerplant all being connected together and the torque forces largely canceling each other out. The torque reaction to the driven wheels is resisted as the diff cannot lift the engine/gearbox assembly attached at the other end of the powerplant frame and thus the diff is constrained totally in movement in this direction.
In mounting the diff in isolation the nose of the diff needs to be tightly constrained and herein lays the cause of the failure. The ex engine mount is being used in tension for which it was not designed and can therefore not sufficiently prevent the diff nose from moving vertically upward under drive as the diff reacts to the torque appplied at the rear wheels. The two mounts on the diff carrier were never designed to work in this way and the diecast ally case fails exactly as described.
This problem has a well trodden path and the solution lays in controling the diff nose. Triumph had the same problem in the 1950's when designing the triumph herald/spitfire and they resolved it by bolting a bracket to the diff nose and stiff mounts to the chassis and I would suggest a similar arangement with some sort of laser cut bracket that can fix to the mazda PPF mounting and via
suitable mounts to the chassis.
Hope this has helped people to understand the problem and enable all to work toward a solution.
BV
