Vibration Cure for EML Sidecar

By Joseph C. Dille
USCA #2054

I have a 1986 BMW K100 with an EML GT sidecar. For three years I did not realize how much the sidecar vibrated until I finally got a ride in it. Maybe I should have listened to my wife in the first place. I probably would have taken a ride earlier, but it took some time to train a sutiable pilot.

The sidecar body is supported above the frame by two rubber mounts in the front and three in the back. The stock mounts are too stiff, and allow the engine vibration to resonate with the body causing a noisy ride. The solution was to replace the rear mounts with softer ones and augment the front mounts with four isolators. Both items are made by the Lord company.

Figure 1, Rear Body Mount comparison
For the rear I used three J-3424-143 sandwich style mounts that will support 170 pounds each. These were almost identical in size to the stock mounts, but they were made of a softer rubber.

These mounts also had a unique concave end cap that made them soft in shear and firm in compression. The difference is shown in figure 1. This is important because the motor vibration is in the shear direction, but the weight is carried as compression. For the front I used four CB-1120-1 center bonded mounts. They are called center bonded because a steel tube is bonded to the inside of the mount. These will support 25 pounds each.

Figure 2, Detail of Front Body Mount

Installation was straight forward. The rear mounts required enlarging the mounting holes in the body and frame slightly to accommodate the 1/2" bolts instead of the 10mm bolts. To install the front mounts I had to enlarge the holes in the existing mounting bracket to 3/4". Figure 2 shows the details of how the mount fits between the body and the frame. A washer is used to compress the lower part of the mount so it expands.

Both new parts were made by Lord Industrial Products, Erie Pennsylvania, 1-800-458-0456. If you have a different sidecar I suggest that you call and ask for the industrial products catalog. It has a design guide and large selection of mounts with many load ratings and mounting options. I am confident that they make a mount that is suitable for any sidecar with a fiberglass body.

Figure 3, Body Mount Load Calculation

The load on each mounting point must be determined before selecting a mount. The procedure involves a few simple calculations and measurements. First estimate the loads that the sidecar frame will carry. These will be the passenger(s), luggage and the weight of the sidecar body. Then measure the horizontal distance between the front mounts and the other loads as shown in figure 3. Multiply the distance by the load and sum them together. Divide this number by the horizontal distance between the front and rear mounts. This will be the load on the rear mounts. In my case each rear mount will carry one third of the total rear mount load, so I divided the total load by three. The front mount load will be the difference between total body weight and the rear mount load. In the front I have four mounts so I divided the total front mount load by four to get the individual front mount load.

The price for the front mounts is $1.85 each and the rear mounts were $5.96 each. However, there is a catch, the Lord corporation has a $100 minimum order and a $50 per line item minimum. If you buy them, the project will cost you about four times too more than the list price. Large companies sometimes will ship small size samples if they think that your company might design their product into your product. If this is the case, the project will be quite affordable. The other possibility would be if an EML dealer purchased the minimum order and sold them to four customers. (Yes, this is a hint for BMW Motorrad.)

A test ride confirmed that these mounts provided a cure for the vibration in my sidecar. The windshield and trunk do not resonate at specific engine RPMs anymore. I wish everyone similar luck with their rigs.

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