How to build a motorcycle sidecar step by step

There are tips on producing a proportioned design suitable for a given motorcycle. As a guide sidecar weight should be 1/3rd the motorcycle weight. The model here is for R60/6 BMW.

The chosen design borrowed heavily from the classic Steib designs. These sidecars from Germany featured 8 segment “zeppelin” style bodies inside an external hoop frame. Originals were made from steel and were typically finished in black or drab olive paint.

 

SIDE CAR DESIGN

Design shown below is modelled with a 5ft 10″ passenger.

Width of the cab is 500mm with 1430mm of leg room.
These dimensions dictate the width of the frame. The wheel track would be on the smaller side at 1200mm without being too narrow.
Sidecar wheel lead was set to a nominal 250mm, but would be adjustable using the mounting points to the motorcycle. Likewise toe-in and bike lean would be adjusted the same way.
The frame featured a compact suspension arm with horizontal shock absorber and the ride height was dictated by a chosen wheel of 18″ diameter.

 

The frame

The Frame was designed to consist of two main hoops curved around the same radius; one at the front of the sidecar and one over the top which would also act as a grab rail.

At the back a straight joining piece would be used between the frame sides.

Each side contained an S-bend partly as a styling feature and also to give a shape to house the suspension design.

A later addition was to add another loop under the chassis to provide a mounting point at the back of the frame and also to strengthen the suspension pivot point.

The suspension pivots were made from 6mm thick steel plate, cut on the mill as  pairs to ensure symmetry.

 

Suspension

The chosen suspension design was to have a horizontal shock absorber driven via a bell crank from the wheel. The pivot for the swinging arm would use roller bearings.

The arm was designed to fit the chosen wheel and axle assembly and to make sure the tyre was clear of the frame.

The swinging arm was made from 3mm thick 30mm box section and 6mm plate steel parts, plus some turned steel components.

 

Bodywork

The sidecar bodywork was to have an octagonal nose section merging to a square section at the back. To make manufacture simpler, only 2D curves were used so that no panel beating would be needed.
Also the top three sections would stop at the passenger opening so that in effect, it would only be required to merge the 5 lower front sections to into the 3 back sections

The plan was to produce drawings for the parts that would be laser cut and folded to make assembly easier and with less welding required.

Material was to be 1.5mm thick aluminium.

These drawings show the proposed parts to be manufactured and folded by the cutter. They were unable to do the rolled nose sections.

The top panel was the largest part and the most complicated. The lower panel would be folded with tabs to join the sides together.

The smaller lower quarter panels were the simplest to cut but would require twisting to get them to fit. These panels would be the key to translating the 8 sided front, to the 4 sided back.

 

Gores
The front of the sidecar was basically hemispherical and made from 8 profiled segments. Segments of this type are called gores. Gores are 2D profiles which can be combined to make approximations of 3D bodies.

The profile of these gores was calculated using CAD in the sequence shown here.

First a blank of the correct material thickness was created.

 

Next the blank part was bent at the correct radius for the nose of the sidecar.

In this case the inside radius was 250mm less the thickness of the top and bottom sheets.

The part was now cut by viewing from the front and cutting with a vee shaped tool as shown.

The cutting tool started from the outside edge of the part and the Vee shape in the centre would be symmetrical about the centreline of the part. The height of the Vee was enough to reach the end of the bend just formed.

As expected the angle at the tip of the vee was 45° meaning the 8 gores would make the complete 360° at the nose.

This cut below gave the final gore profile

Finally the part was unfolded to give the 2D profile of the required shape shown below.

In this case the gore was not of a constant radius so it was not possible to dimension it. A DXF file was provide to the laser cutter so that they could take the profile straight from the CAD.

All 8 gores would be the same profile.

 

Another point worth noting is that although the gores on the lower quarter panels were of the same profile as the rest; they had to be angled from the centreline of the panel to be geometrically correct once fitted.

The twisting of the panel would mean that only one corner of the panel would be 90°. The others would need to be worked out.
The angles were calculated by looking at the edge lengths of the neighbouring panels and drawing two intersecting circles. The 205mm was the width of all the gores.

 

Below are the laser cut and folded panels to the above design.

 

BUILDING THE FRAME

The frame was made from 38mm diameter tube with a 2mm wall thickness.

The ‘S’ bends and front ring-rolled hoop had to be made in separate parts and joined with an inner sleeve.

The parts were notched on the milling machine with a 38mm cutter and tack welded in place as per the design document.

 

The swinging arm was machined from 30mm steel box section with a 3mm wall thickness. This was combined with some turned parts for the axle housing and swinging arm bearings.
6mm plate was used to create the forked end for the shock absorber and the web between the arms.
The assembly was tack welded in situ to ensure alignment; and then TIG welded by a professional welder.

The photo below shows the frame after assembly to the bike. The wiring loom can be seen threaded through the cable holders and the lights were assembled too.

 

 

SIDECAR ATTACHMENT

The plan was to use a 4 point mounting system, fairly typical for a medium sized motorcycle with a light sidecar.
The lower rear mounting point would be a ball joint allowing angle adjustment in both the horizontal and vertical planes. This would enable Toe-in and Lean-out to be adjusted.
The lower front mounting would be designed to have a horizontal adjustment so that Toe-in could be adjusted without affecting lean or sidecar height.
The top two mountings would be standard adjustable struts to complete the set up. Below gives more details on these parts.

This top view of a typical 4 point mounting system illustrates an important point.
The lower mountings are shown in green and the upper ones in Purple.

The lower joints can be parallel. This is OK and makes adjusting the bike easier.

However the upper links should be at an angle. This triangulation helps the sidecar and bike stay rigid without relying on the friction of clamps to keep everything in place.

 

Lower Rear Mounting

The lower rear mounting used a second hand ball joint. This was a sidecar specific part from an old bike and so was strong enough for the task. It was also lockable which would help hold everything rigid once angles had been set.

This was the only mounting that was not cross braced at the motorcycle frame.

The mounting was attached to the passenger foot peg at the back and the rear engine bolt at the front. The material used was 10mm thick steel flat and the offset between these two strip parts was accommodated by adding a steel tube which also took the shank of the ball joint.

The frame clamp would be able to slide and pivot on the sidecar frame. This would enable sidecar wheel lead to be varied slightly and also the sidecar height to be set to ensure the chair was level.

The ball joint on the bike side would allow the bike tilt and toe-in to be set.

 

Lower Front Mounting

The lower front mounting was designed to be length adjustable so the toe-in could be set; and height adjustable so that the sidecar could be levelled.

A Y-shaped part was made from 25mm thick walled steel tube. This was braced and welded into a rigid assembly.

This design allowed the link to clear the cylinder head and also gave a nice horizontal adjustment facility.
The threaded rod was a fine pitched 16mm metric thread.

Where the front mounting clamped to the sidecar frame a simple steel clamp was used. This was bored and sliced in the same way to the rear clamp part. The two clamp halves were joined with some cap head screws and another homemade eye bolt.

The clamp was able to slide on the sidecar frame and also pivot on the eye bolt giving enough degrees of freedom for adjustment of toe-in.

 

Top front Mounting

The top front mounting was a flat bar positioned just under the fuel tank on the front of the bike frame. This was a 10mm think piece of steel bar with two clamps welded at the correct angle to suit the frame. These parts were welded in-situ to provide an exact fit.

The bike horn had to be moved to fit the clamp in this area. So the clamp bar was drilled to create and new mounting for the horn.

The end of the clamp bar was drilled 14mm to take an eye bolt.

 

Upper rear mounting

A bracket was designed to sit above the battery, just under the seat so that the sidecar could be attached whilst still retaining the side panel on the bike.

This bracket was made from 10mm steel flat and was welded in-situ to create an exact fit on the bike. The rear subframe bolts were used to hold the cross brace which was then fitted with an arm extending out from under the seat to take an eye bolt.

The two top struts were made from fine pitched 16mm metric threaded rod (16 x 1.5mm).

The clevis’ were threaded onto the threaded rod and then pinned in place.

The body of each strut was made from thick walled 25mm diameter tube with a threaded boss welded in one end and a clevis on the other.

Each top link was attached to the sidecar frame at a fixed point. This joint was a through hole drilled in the frame to take an eye bolt and then re-enforced using a shape steel plate top and bottom.

The plate parts had been machined using a boring bar.

Eye bolt

 

Sidecar Electrics

The law required that the sidecar be fitted with indicators, front and rear marker lights and a brake light.

In this project, the front marker light would be a spotlight which would run with a pilot light most of the time but would operate as a spot light when the motorcycle main beam was used.

The connections for the lights were taken from under the seat.

The wires to the rear light were cut and a 6-way connector soldered into place.

A separate, thicker earth wire was taken straight back to the battery to give the sidecar a good earth.

Self amalgamating tape was used to create a wiring loom from the individual wires.

The loom was threaded to the outside of the frame via short lengths of steel tube welded to the frame and upper rear mounting.

The wire was then routed inside the frame to reach the front and rear lights.

Silicon tubing was found to be the best tool to post down the frame tubes to pull wires through.

An earthing point was added to the inner suspension plate by drilling and tapping a 6mm hole. This formed a union to join the earth wires and connect them back to the earth terminal on the bike battery.

 

 

FINISHING THE SIDECART

Here is a description of the finishing touches added to the sidecar including construction of the seat and a luggage rack for the boot.

The front and rear edges of the passenger area were finished with a self adhesive trim strip.

This satin black plastic U-channel came with some impact adhesive already inside to hold its position.

The floor, sides and boot of the cabin were covered with some automotive carpet.

This was cut and stuck in place with contact glue.

The seat was a simple plywood bench upholstered.

The seat was the covered covered and cushioned

A luggage rack was made for the back panel of the sidecar. The rack was made from 10mm steel tube. The drawing for the rack is shown below.

4 matching stand-offs were machined to hold the rack to the back panel of the sidecar. These were simple feet turned from steel bar and tapped M6x1mm in the centre.

An additional rail was added to the bottom edge of the rack to help keep cargo secure.

 

 

 

Source: Steve’s Workshop