This page discusses the modifications I made to the motor/transmission mounts in my Toyota MR2 EV project. I did this for several reasons:
Why do you need motor mounts?
In any car, the engine and transmission are mounted to the frame of the car using vibration-absorbing rubber dampers. This helps isolate and damp the vibration from the engine and driveline, but it also allows the somewhat flexible frame of the car to move relative to the much stiffer engine and transmission assembly. Without shock mounts, the mounting points between the engine/transmisison and the frame of the car would quickly fatigue and crack or break, and the ride would be much less pleasant, with more noise and vibration.
In the case of an EV, the motor should not create any vibration to speak of. However, there might still be some vibration coming from other parts of the drivetrain (driveshafts, transmission, etc). And also, Issues with mounting the rigid motor/transmission against the more flexible chassis of the car still need to be considered. Therefore you still must use shock mounts, but they can be considerably stiffer than the ones used with the original engine.
Originally, the Toyota MR2 had four motor mounts: Two up top, at the either end of the engine/transmission assembly, and two at the middle of the assembly at the bottom. The bottom two counteract all the torque applied to the wheels of the car. The upper two simply balance the engine/transmission and carry some of the weight. Three of these mounts attached directly to the transmission housing, and the fourth attached to the engine. Without modification to the transmission mounts, I would only be able to fit six or seven batteries in the engine bay. With these relatively minor changes, I can get nine or ten batteries in.
I am modifying the motor and transmission mounts in the MR2 to provide clearance for batteries above the motor and transmission assembly. The clearance for batteries will be improved for two reasons: The first is that the whole transmission and motor assembly will be rotated down about the rear motor mount pivot point to increase clearance above the transmission to allow batteries to be installed above it. The second reason clearance will be improved is that with the installation of stiffer engine mounts, I will be able to eliminate one of the top mounts, and relocate the other to a lower position. The driver side motor mount frame bracket sticks up far enough to prevent installation of a battery above it. However it is rendered unnecessary with installation of stiffer mounts, so I can remove it. The passenger side mount and frame bracket will be moved down to increase clearance above it and to mate up with the back of the electric motor.
I basically copied the design of the original mounts, except that I used much stiffer rubber blocks. The design is basically two short cross sections of pipe, each about 7/8" deep, welded against either end of a washer of about the same outside diameter. This assembly has a rubber block glued into each end of it, and another section of 3/4" outside diameter pipe is pressed through the middle. The inner section of pipe is supported by the rubber blocks. Once in place with a washer on each side and a bolt through the middle, the rubber blocks will be held captive and will make for a good, solid vibration damping mount. This whole assembly is welded onto whatever bracket I need to make to make things fit together.
The pipe I used came out of a scrap metal bin from a local industrial supply shop. I had trouble finding rubber of the right size and shape, until I found a big hunk of old conveyor belt in the blackberries behind some industrial buildings near my neighborhood. It had probably been there for 20 years, but aside from some surface oxidization of the rubber it was in good shape. It was 1 1/8 inches thick and nylon reinforced. Perfect. I hacked a chunk out of it with a hand saw and hauled it home. Once Home, I used a hole saw to take "divots" out of it that were an interference fit for the inside of the large pipe. Viola! All the raw materials necessary to make new motor mounts!
Determining the new position for the motor and transmission was pretty ad hoc. By just playing around with the transmission and mounts when it was out of the car I determined I could flip the front center mounting bracket upside down. This would cause the whole transmission to tilt forward and effectively lower it. I did test fit the transmission with the bracket reversed (third image) To check for clearance. The driver side driveshaft is pretty close to a suspension bracket, but not close enough to be of concern. Other than that, no problem!
The first picture is of the chunk of conveyor belt and other raw materials gathered for making new engine mounts. The second is the new center rear mount, prior to painting and installing the rubber blocks. You can see the washer section in the middle that retains rubber sections and keeps the metal housing from sliding back and forth on the rubber blocks. The smaller loose section of pipe will get pressed through the middle of the rubber blocks. A mounting bolt goes through that. to hold whatever is supported by the mount. The third picture is from when I was test fitting the transmission (using the original mounts) to check for clearances with the front center mounting bracket reversed.
This mount consists of a vibration damper that bolts to the chassis, and supports the transmission via a bracket that is attaches to the bellhousing of the transmission. This mount is basically unchanged in design, except to replace the original rubber block with a much stiffer one that will allow more weight carrying capacity. This mount takes torque from the driveline whenever the car accelerates or decelerates.
The pictures to the side show the original mount and the modified one. A new shock mount made as described above has been welded on in place of the original one. The bracket that attaches the transmission to this mount will be unchanged.
The first image is of the stock rear mount. The second image is of the completed modified mount
This mount, like the rear center mount, takes torque from the driveline. Also like the rear center mount, its original, very flexible shock mount has been replaced with a much stiffer, stronger one. The transmission attaches to this mount via a bracket, just like the rear center mount. However, I am installing the transmission-to-mount bracket upside down. Doing this moves the mounting bracket hole up about four inches, which causes the whole transmission assembly to rotated down about the rear center mount (acting as a pivot point). Doing this causes the highest point on the transmission (the top of the bellhousing) to drop about 2.5 inches. This provides enough additional clearance to put a battery rack above it. The shift in position of the transmission is small enough however that there will be no issues with filling or draining oil from the transmission, or with driveshaft clearance. The differential is at the rear of the transmission, near the rear center engine mount. This means that the shift in position of the differential relative to the wheels is a fraction of an inch. Also, since the lowest point of the transmission is the bottom of the differential housing (which has hardly moved), ground clearance is not significantly affected.
The first image is the original front center mount. The second is of the modified mount. The third is a close-up showing the upside-down transmission-to-mount bracket. Reversing the position of this bracket caused the whole transmission assembly to drop several inches in the front, providing enough clearance to put batteries over the top of it.
The front and rear center mounts were modified from the original mounts. However, this one is totally custom as it supports the rear of the motor and will attach to a new mounting position on the frame of the MR2 that I will weld in to support it. The original chassis bracket (visible in the picture) is in a position to interfere with battery placement, so removed it and use bits of it to make the new frame bracket.
This one is the easiest one. Since the other three mounts are now much stronger and stiffer than before, this mount really becomes redundant. Three mounts are enough to prevent the transmission/motor assembly from moving since it will vibrate much less, and is about 100 pounds lighter overall than it was when the engine was installed. Therefore, I will be leaving this one out. I will remove the chassis bracket as it will interfere with battery placement.