Wednesday, 10 August 2011

How to Fit a Sidecar

Before fitting a sidecar check the motorcycle carefully. Any slight play in the wheel, swing arm, or steering bearings will cause low speed wobbles and will wear to a dangerous condition very quickly once the additional stress of a sidecar is added.

Take a look at the tyres, they do not need to be anything special but ideally they should be flattish on the top to get as much contact area as possible. Ribbed tyres really mess up a heavy fast combo and should never be used, if the front tyre has long lines in the tread going inline with the bike change it before the sidecar is fitted.

Fitting a sidecar, this is a rough guide only.
You will need a flat level area and plenty of time.

Sit on your motorcycle to see how much the suspension drops with your weight. Use ratchet straps to slightly compress the suspension so the motorcycle remains at this level when you get off.
If possible support the motorcycle securely with blocks of wood so it is on its wheels and upright.

The first measurement to look at is how much the sidecar axle is in front of the rear wheel axle of the motorcycle. This needs to be between 20 and 30 cm. Less axle lead (20 cm) will produce a sidecar outfit which turns quickly but is less stable at speed. It will also require slightly more toe in. More axle lead will produce a sidecar outfit that is slightly heavier to turn but it will be more stable at speed. It will also require less toe in.

Arrange the sidecar chassis on blocks of wood next to the motorcycle so it is level and the axle lead is correct. Loosely attach the two lower mounts. The front lower mount should be as low as possible and as far forward as possible, This can usually be attached to the main horizontal frame tube just before it turns to go up towards the headstock. The rear lower mount should be as far back as possible. This one often needs an additional subframe making to take the mounting point as far back on the motorcycle as possible.

You will need four bricks and two straight lengths of steel. Place the two straight edges on the bricks either side of the bike and sidecar. On the bike side measure accurately and adjust the straight edge so it is 25 mm away from the wheels rims. You should be able to measure this at four points. Rear of rear wheel, front of rear wheel, rear of front wheel and front of front wheel. Then do the same on the sidecar wheel but as you only have one wheel you will have to rely on two measurements. Finally measure the distance at right angles to the bike just behind the rear wheel and again just in front of the front wheel. This is the amount the vehicle toes in over the whole length. There are no hard and fast rules for this but a good starting point is 15 to 25 mm. So to be clear the distance between your two straight edges should be 15 to 25 mm greater at the back than it is at the front. (These measurements do assume that the wheel rim is the same width front and back even if the tyres are not the same size). If you had near 30 cm axle lead set the toe in close to 15mm and if you had near 20 cm axle lead set the toe in close to 25 mm.

Once the toe in is set and the lower mounts reasonably tight the lean out can be set. Loosely attach the two upper mounts. The front upper mount should be as high as possible. This is usually clamped round the frame near the headstock. The rear upper mount should be high up and as far back as possible. It is usually possible to clamp very close to the rear shock absorber top mount. The motorcycle must lean very slightly away from the sidecar. This must be measured with the steering straight ahead but the measurements are usually carried out on the rear wheel. Tie a washer to some cotton and hold the free end of the cotton near the top of the wheel so the weighted end hangs down on the outside of the wheel. Note where the cotton touches at the top and measure the gap between the cotton and the similar spot at the bottom of the wheel. Over the height of the wheel the lean out should be 4 to 6 mm. If the sidecar will be used most of the time with a heavy load this lean out should be increased to 8 to 10 mm.

Once the toe in and lean out are set. Tighten everything and remove the supports from the motorcycle. Release the ratchets used to compress the suspension. Once the body is fitted to the chassis and the wiring is complete take the outfit for a very short ride. Then check all the measurements again and check all the fittings are tight again.

Rough guide to fitting locations. The minimum requirement is four strong locations as described in this blog. The optional 5th fitting is desirable if possible. 

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