Mechanically Speaking: Modern motor mounts engineered to ‘have your cake and eat it too’

Tell a customer they have worn out engine mounts and they usually react in one of two ways: fear or disbelief.

Tell a customer they have worn out engine mounts and they usually react in one of two ways.  Many will freak out in fear that their engine is going to drop right onto the road.  The other reaction is complete disbelief.  In the second case the owner assumes the engine mounts must be good because the engine is still where it belongs.  Under the hood.

Engineering a motor mount system is a very important step in vehicle design.  Engine mounts have two basic purposes.  Number one is to locate the engine in the vehicle in the correct position to allow propulsion of the vehicle.  Number two is to isolate the engine from the driver and passengers from the inherent vibration of the engine.

On one hand these mounts must be stiff to prevent the engine from twisting too far under load.  Too much twist and drive shafts start to be driven at high angles.  When this happens your vehicle will exhibit vibrations and or clunks and clanks.  On the other hand, overly stiff mounts will transfer engine vibration throughout the vehicle causing buzzing steering wheels, dashboards and pedals.  Enough to drive you crazy.

Race cars have stiff motor mounts.  Step on the gas hard and a race car motor will apply all its power to the chassis immediately and directly.  Not one iota of energy will be wasted flexing a soft rubber mount.  Your father’s Oldsmobile Delta 98 Regency, on the other hand, was designed to isolate the driver completely from anything going on under the hood.

They did an excellent job.  That Olds was a great cruiser but not that happy in the corners.

The modern day motor mount has been engineered to “have your cake and eat it too” so to speak.  It has not escaped technology.  In almost every case the mount is made up of metal with rubber bonded to it.  The metal parts of it are designed to not let the engine leave its location if the rubber breaks free from the metal.  That’s right; a worn out or broken motor mount will still hold the engine in place.  It will just do it very poorly with zero isolation and a lot of clunking and banging.

The choice and design of the rubber portion will be soft enough to filter out vibration yet stiff enough to prevent too much engine movement.  Many motor mounts now have an added element.  Typically a liquid.  The liquid can really absorb the fine vibration while allowing the rubber to be a little stiffer for holding the engine in place.

The technology has not stopped at liquid filled.  Engineers had to get some electronics involved and maybe a computer or two.  So step on the gas hard and the motor mounts get firm by either closing valves within the liquid portion but when you are just cruising more valves are open to let every last vibration be dampened out.  Now you can have your Oldsmobile for cruising with Porsche like response in the corners.

As with all technology these sophisticated  mounts come at a cost.  And as with many parts on your vehicle, in time they do fail.  During my recent Honda timing belt replacement I noticed a liquid stain inside my engine compartment.  Was it engine coolant? No!  Was it engine oil? No!  Was it power steering fluid?  No!  What could it be?  You guessed it.  Motor mount oil!   That mount set me back a couple hundred bucks but ohhh that V6 is smooth.

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. He writes every other Thursday.

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