Sidecar Design

This page shows the design for the motorcycle sidecar and gives some guidelines for producing a sidecar suitable for a given motorcycle.
3D CAD of Sidecar design
The chosen motorcycle was an R60/6 BMW. Not a hugely powerful machine and weighing 200kgs. So the design was decided to be a small lightweight sports chair, approximately 70kgs in weight in order to be appropriate for the bike. As a guide sidecar weight should be 1/3rd the motorcycle weight.

The chosen design was based heavily on 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.
The CAD for the proposed design is shown below; modelled with a 5ft 10" passenger

Drawing of sidecar concept
Width of the cab was decided to be 500mm with 1430mm of leg room.
These dimensions dictated 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.

Sidecar Frame Design.
Additional brace shown in magenta.

Suspension pivot parts shown in Green


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.

Suspension Design


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.

Bodywork Outline
These drawings show the proposed parts to be manufactured and folded by the laser 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.

Top body Panel cut and folded

Base panel folded and part rolled

Lower quarter panel
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 gave the final gore profile
Finally the part was unfolded to give the 2D profile of the required shape.

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 quater 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.

Calculating the angle of the lower quarter panel gore.

Here are the laser cut and folded panels  to the above design. 
Cut and Formed Panels