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.

Motorcycle Suspension Part 1

www.london-motorcycle-museum.org

Motorcycle Suspension.

Part 1 Front

Early on in the development of the motorised bicycle roads were unpaved, stone tracks at best or even muddy paths, much was learned from the development of bicycles and if you look up the history of bicycles all of the suspension systems around today were seen on bicycles by the end of the nineteenth century. Nothing is new, however the technology required to put some of the systems into operation on a motorcycle needed to be in place or invented or even as simple as a patent running out. For example take Leonardo da Vinci, he invented the differential where wheels on the same axle could run at different speeds. Not necessary on a horse drawn cart but four hundred years later necessary on cars. His ideas needed an application and the materials to make it work. I have been to many museums and looked at what manufacturers used to make motorcycles more comfortable. Some things like having tar on the roads that made a smooth surface slowed the progress. Undoubtedly suspension had become necessary as speeds rose. More on the front rather than the rear. Peter has put this together and is available at the Museum to read.

In the beginning motorcycles were developed from the ‘Safety Bicycle’ and had no suspension. The addition of the engine made the machine heavier and faster and so the shock from road bumps became more violent. This had several effects. Broken frames were common but worse, the rider could loose control and even be thrown off.

So the early motorcycle engineers tackled the front of the bike first. Mainly because that’s the end where you control from, but also because road shocks were felt through the handlebars. The constant jarring was really uncomfortable.

After a whole rash of experiments from different manufacturers the ‘GIRDER’ fork became almost universal. 

The ‘Girder Fork’ that holds the front wheel is suspended from the steering head by two parallel links. A spring between the links provides the suspension. The links swing in an arc and this motion is transferred to the wheel.

As the links need to be short for stiffness, it makes a sharp choppy action.

Non parallel links or links of different lengths can change the way the wheel behaves.

Motorcycles later used Telescopic forks but the early “Tele’s” were flexible so engineers tried different types of linked suspension to reduce the flexing.

By using stronger metals the flexing in the Telescopic Fork has been reduced. They can also be made to give a long travel so that Motocrossers can take big jumps without jarring the rider.

Now because they are visually more acceptable Telescopic Forks are used on nearly all large Motorcycle. There is another variation of the leading link fork and that is the Earles fork. On this the link is extended to pivot behind the wheel. This can be seen on the Around the World Triumph outfit of Richard and Mopsa English. The links are now able to form a fork that keeps the wheel in line with the motorbike. While in theory the wheel is under better control the unsprung weight on this suspension means the damping system has to work harder. The extra weight behind the steering axis tends to add a pendulum effect and can cause a wheel wobble.

More of Peter's views when we look at rear suspension.

Some interesting examples of early designs one is the plunger front suspension on our 1923 Wooler that I feel may have been more like riding a pogo stick as the sideways rigidity is only from the front spindle.

An even more intriguing set up is on this OEC that has a type of hub-centre steering along with suspension sliders within the upright tubes on the front fork assembly. This must also have been interesting to ride relying on the gyroscopic effect of the rotating wheel to maintain stability but still lacking sideways rigidity.

 On telescopic forks greater rigidity is obtained by using larger diameter tubes, compare the sizes of the EMC of 1947 to the B31 a few later across the isle from it and then look at any big Harley for comparison.

One question to ask is how does a motorcycle steer? We'll come to that after looking at suspension.