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      12-22-2012, 02:43 PM   #33
beta
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Drives: 2006 M Roadster
Join Date: Mar 2011
Location: CA

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And here is some more info!

If you're on the fence on this group buy, you should really join up. If we sell a lot of engine mounts they're more likely to make us a crossmember which has a good chance of fixing our shifting issues and the 1->2 grind.

Quote:
Basic geometry of the Z4 mount system.

The Z4 has a effectively a three point mount system (even though it uses 4 mounts) The front two mounts are hydraulic. The damping fluid is glycol and this is used to isolate low frequency high amplitude vibration that occurs around idle. This is often referred to as idle shake. At higher frequencies the damping is de-coupled and the rubber does the damping and control of movement. The engine mounts have a similar stiffness in both the vertical and horizontal directions and it is the front mounts that control accelerating, braking and corner forces as well as engine torque and weight. They are not fail safe and engine repeated high engine load, or overrun can put one mount in tension (torque overcoming the static engine weight) which eventually causes the rubber to split.

The gearbox mounts use a "bobbin" type rubber mount. These are stiff in compression and soft in shear. They are arranged in a "V" formation which means they are in both shear and compression at the same time the two mounts effectively make a single support. This is a classic and well proven arrangement. Transverse and vertical forces are supported extremely well but the mounts do not take any accelerating and braking because this is the shear direction where they are the softest. The also are very soft in the roll direction so torsional vibration is not transmitted into gearbox crossmember and into the car. These mounts are also not fail safe and if the rubber or rubber/metal bond fails the mounts will pull apart.

Poly mounts copy the geometry and have the same stiffness ratio. In effect you get none of the benefits of a stiffer mount and all of the disadvantages of poly in vibration transmission (poly has poor resilience so transmits much more vibration).
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Last edited by beta; 12-23-2012 at 05:18 PM..
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