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MG MGF Technical - Lower Engine Mount in Polyurethane

During the course of finding an engine bay squeak it was evident that the lower engine mount/shackle moves a great deal under torque. Had it moved less, a hose clip would not have got close enough to bodywork to cause that intermittent squeak.

So I've been looking at the lower engine movement, wondering how to reduce it. Perhaps the shackle rubbers are softening with use and loosing potency? New ones are silly money.

The large rubber mount at the subframe end of the lower shackle is moulded in a softish rubber material in a maner that allows movement in left-right directions whilst absorbing shock front-back. Its metal bush is bonded to the rubber and the bush is nipped tight into the subframe via the through bolt at 100nm. I deduce the rubber is molded with cut aways to allow left-right movement; the engine and the subframe are not always in the same plane with each other during work.

Looking at commercially available poly replacement mounts, they are like two tophats and in hard polyurethane. This "upgrade" would seem to be ideal to reduce front-to-back engine movement, however hard poly is not very compliant so vibrations and torque shock might transmit to the car. Also the poly material is tight in the housing as well as the subframe so left-right movement would not be possible. Using this type of poly replacement would strain, and possibly fracture, the entire lower mount/shackle. Actually a standard mount/shackle doesn't naturally line up exactly with it's smaller partner, you have to move it a bit to connect the pair - hence the l/r compliance in the large rubber. Fitting a poly loaded mount won't be easy. For these reasons commerically availabl poly is not an option I want to follow - it's too hard and doesn't have a much needed knuckle function.

I've experimented with making my own polyurethane mount insert from shore60 grade material, which is harder than rubber but softer than commercially made poly mounts (they at shore70 or more). My poly mount insert is cast only into the metalwork in one go - so not a top hat design and thus doesn't have excess material outside the mount. The internal metal bush protrudes 8mm each side of the poly and is not stuck to the poly. Consequently the mount slides along the metal bush permitting left-right movement. It self centres between engine and subframe as the engine moves. Torque shocks and vibrations are nicely damped by the shore60 poly material, which is compliant without being too soft.

So far so good - the car does not judder on slow take up of drive. Snap gearshifts up/down the box do not cause a lurch as before. No increase in vibrations, a smoother drive and there's no squeak! I have not yet fitted the lower mount with shore60 as I seem to be OK with the standard item in place.

Job done - but I have only done a few miles of testing, so might have to come back to the forum to report any issues.

You can buy polyurethane to self mix and mold a mount like I did. It comes in two pots and can be purchased in various hardness/elasticity grades in the "shore" scale. The higher the shore number, the harder the finished material. Once the chemicals are mixed you have 5 minutes to pour it before it goes off. Within 2 hours it can be demounted from a mold and takes 5 days to achieve max strength.

Jeff
J Lennon

This thread was discussed on 10/09/2011

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