Motorcycle Suspension Part 2

www.london-motorcycle-museum.org

Rear Suspension

Part 2.

More words from Pete about suspension. This time at the back. A solid rear end is fine on smooth roads at low speeds as anyone who rides a bicycle will testify to however as the speed rises shocks increase and so does the level of stability. Even ace drag star Bill Bragg fitted suspension because Santa Pod was so bumpy. His first effort in 1960 was with Yellow Peril and with speeds around 145mph and he had rear suspension in 1961 for Scarlet Peril. Just as a side issue to go faster he fitted a supercharger in 1962 with Blue Peril and managed a standing quarter in 11.19 seconds nearly two seconds faster than his previous efforts.

Pete:
There are exceptions but in general it took a long time before designers got round to rear suspension. Many old sages thought that you would loose control if you put springs at the back. However common sense prevailed. It was pretty obvious that if you could keep the rear wheel in contact with the road instead of skipping over the bumps the tyre would grip better and the road holding of the motorbike would actually improve.

Without making a huge change to the rigid frame the larger manufacturers used what was called plunger suspension.

The rigid frame is what it says. Where the axel of the rear wheel is held in lugs fixed to the back end of the motorbike frame.

The Plunger sprung frame has the end lugs replaced by spring units rigidly attached to the rear frame. The component that holds the rear wheel spindle slides on guides and their movement is controlled by the springs contained within the units.

It took some of the shocks and bumps away from the back of the motorbike but it was difficult to damp the spring to maintain control. The other drawback is that the suspension is working in a straight line and this is contrary to the radial path of the rear chain.

As can be seen the chain is stretched at each end of the travel of the suspension. If the chain is run slack to prevent this there is a danger that it might jump the sprocket. Run too tight and both sprocket and chain wear is rapid.

A way of reducing this is to add a link at the plunger to work in a short radius. This was used by Ariel for a short period.

On a motorbike with shaft drive the shaft has to have a sliding component to stop tension in the drive damaging the gearing.

The answer for a chain drive motorbike is to mount the rear wheel on a moving arm with the pivot point as near to the same centre as the gearbox sprocket as possible. A Swinging Arm or Swing Arm.

The arm needs to be as strong as is necessary to provide rigidity between the wheel spindle and the pivot point. The wheel requires springing and damping. This can be done in a variety of ways. Either directly on the arm near the wheel or indirectly via a link.

Sometimes the structure used for the arm is reinforced. The earliest way of doing this was to fabricate a triangular structure. Advantage can be made of the reinforced structure and link to change the suspension rate.

 

Most modern motorbikes will have a reinforced swinging arm and a single spring and damper unit that can be adjusted to suit the rider.

	At the museum we see some of the early attempts at making the rear end softer. The earliest is on the 1923 Wooler  with plunger rear suspension. Much later is the 1937 New Imperial with a triangulated pivot and springs under the seat. A friction damper is also fitted  at the linking arms very much like on girder forks. There are a couple of Vincents that have something very similar from 1947. There is plunger rear suspension on the 1953 Norton Model 7. Other interesting bits I have come across and that bis a 1920's Indian with pivot fork and leaf spring rear suspension and the very interesting OEC with short arm plunger suspension  at the Norfolk Motorcycle Museum. And don't forget the Sprung Hub fitted to Triumphs around 1953 and 54.