Suspension

I have decided I will design a race board, which will be purpose designed to be driven on a sealed race track in a controlled environment against a conceptual class of similar boards. I have decided this because riding a board provides little protection for the rider and to ride on the road amongst comparitivly heavy vehicles is too dangerous. I also investigated moped compliance issues and realised it would be impossible for me to create a design that complied with the law surrounding mopeds within the given timeframe. following these realisations I decided that research into suspension was necessary as it is a vital component on any vehicle. Vehicles traveling at speed are more sensitive to bumps because it is more difficult for energy moving in the direction of travel to suddenly change direction, especially in an arrhythmic fashion. Suspension helps to reduce impact and retain traction when going over rough terrain by pushing the wheels back down onto the road surface where they would otherwise be airborne.

There are multiple types and configurations of shock absorbers, I am aware that mono shock configurations are the lightest so I will investigate examples of this style. Because the rider will stand on my designs they will be able to absorb some shocks through their legs and also shift their weight more than on a bike which means that conventional problems may not apply and others will arise. Hence I have undertaken this research to gain awareness of possible problems and solutions.

Front shocks

Rake is the term used for what is probably the most important consideration in motorbike suspension, it is the angle off vertical of the front shock absorbers; less rake (shock absorbers closer to vertical) will give a bike less stability, but allow it to turn tighter. Front fork type suspension will normally compress during breaking because of the weight transferred toward the front, there are technologies to stop this happening to such as Honda’s “TRAC” system. They work by increasing the difficulty for oil to flow through the shock absorber which reduces its ability to compress. If the rake angle is small and the front shock absorbers compress enough this amount can become too minimal which can cause “headshaking”, where the front wheel and handlebars will uncontrollably shake from side to side if a steering damper is not used.

There are also systems designed to change the bikes geometry so that the angle of rake is increased as the shock absorber compresses, one such system is designed by a German company called German: A. Their system is specifically designed for use on mountain bikes, it only uses one small air shock absorber which is connected to a hinge system above the forks, the forks themselves do not compress. The lightest of their range weighs only 1.1kg, and has a 90mm travel range, this is several kilos lighter than a standard fork configuration and has a similar amount of travel, however it is limited to a weight loading of 95kg which is less than traditional forks. A similar set up to this could be appropriate for my design as the forces and weights involved are similar.

A more unusual looking front mono shock design is the Cannondale lefty, this system uses a strut with a tracked shock absorber built in. Due to the extra stress on the lone strut heavy reinforcement is needed to stop twisting, despite this they are lighter than a conventional fork set up, weighing around 1.2kg depending on the model.

Rear shocks

Rear shock absorber configurations do not need to be as considerate of changing bike geometry as those on the front because they follow the path set by the front wheel and they do not steer. Modern configurations normally use an “H” shaped swing arm in combination with a shock that is mounted close to the hinge via a pivoting lever at one end and attached to the frame at the other. A variant of this

Design is which is used by race bikes uses an “h” shaped swing arm, the wheel mounts to the end of the swing arm that only has one side. This is advantageous because the wheel can be changed more quickly, however because the wheel is only supported on one side the swing arm must have appropriate reinforcing to stop twisting which will increase weight.

Information and images from:

http://www.german-a.de/en/index.html

http://www.carbibles.com/suspension_bible_bikes.html

http://www.cannondale.com/CMS/Technology/10_HeadShok_Tech_Pages_CUSA.pdf