Vibration Cure for EML Sidecar
By Joseph C. Dille
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.
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.
Figure 1, Rear Body Mount comparison
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.
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
Figure 2, Detail of Front Body Mount
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.
Figure 3, Body Mount Load Calculation