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.