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Motor Mounts - Still need them?

By David Brandt
January 5, 2004

In response to the questions: "When you guys make an adapter for your electric motor, do you keep the rubber motor mount or do you eliminate them? Are they there for vibration or torque supression?"

I think we are forgetting what the original purpose of motor mounts is. It's not just vibration and noise isolation. It includes the following (and can probably serve more functions):

  1. Support the weight of the motor/transmission, both while at rest and while pulling vertical G's for an instant while you traverse that nasty pothole you didn't see.
  2. Minimize vibration and noise transmission (remember ICE's vibrate a lot more than electric motors).
  3. Continue to secure the motor/transmission under high torque conditions. Both RWD and FWD motor/transmission combinations can rotate a LOT under torque. This condition is still true with an electric motor, and perhaps more so (especially for you drag racers).

All these can occur in any combination. While some things are minimized with an EV (like driveline vibration), the rest still have to be taken into account. In any car, the entire driveline flexes under loads from potholes, etc., driveline torque, inertial loads induced from the car manuvering, etc. In the original driveline, the ICE/transmission was very heavy (lots of inertia), so the whole thing was allowed to "float" on the motor mounts. Since the other end of the driveline, at the wheels, was flexible anyway, it made sense.

In an EV, things CAN be significantly lighter and smoother, reducing (but not eliminating) the need for extensive flexibility and isolation. But since most of us drive conversions and keep the transmissions, they usually wind up being only a LITTLE lighter (speaking in regards to the motor/transmission combination only). Plus, there is the convenience factor. That is, the mounts are there anyway, they are very convenient, and we may as well use them.

The flexibility of motor mounts is mandatory if you intend to apply a lot of torgue to the wheels (even anti-torgue rods for RWD vehicles incorporate a bushing in the tie rod end that is isolated with a rubber compound). It is still desirable for everyday street use EV's. If you don't want to use them, then the vehicle need to be designed or modified for the method of attachment you wish to use. I would imagine some modified CV joint designs with a bit more rotational travel (and some translational travel, too) would be required.

Food for thought, anyway.

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