Motorcycle Front Suspension Systems
There has been a slow but interesting evolution of engineering approaches to motorcycle suspension. Early machines simply had no suspension as the wheels were rigidly mounted on the frame and had solid front forks much like a bicycle.
The first stages of suspension development came at the front of the bikes. The riding characteristics of rigid front suspension were so hard at speed that something simply had to be done. Thus was born the "springer" front end. This system and variations of it endured even past the World War II era as the standard approach to front suspension on motorcycles. You will still see this occasionally on ol skool choppers.
Another old design is the Earles type or leading-link front suspension. This design was developed in Europe, and its most famous user is BMW. The leading link is a strong, smooth-riding front end though a bit slow in handling and awkward looking. Perhaps the aesthetics of modern forks prompted BMW to offer the more modern slider forks on current models although the Bavarian engineers have kept experimenting with variations on the Earles theme on some recent models.
One of the most significant developments in front suspension was the slider or telescopic front forks. This unit showed itself on foreign machines before Harley-Davidson and Indian bothered to change over to the better system. As a result, the light, quick-handling British machines with telescopic forks severely challenged American manufacturers for stateside sales. Soon the American manufacturers switched over to the telescopic units.
The early telescopic forks had short external springs usually protected by a rubber accordion gaiter or cover. Also, they only dampened on compression and would often rebound with a noisy "clunk." Even with such drawbacks, they were vastly superior to the springer front ends.
Double-Acting and Internal Spring Telescopic Front Forks
The current state of the art in front suspension sees the nearly universal use of the double-acting internal spring telescopic front forks that are also known as Ceriani type, after the Italian company largely responsible for their popularity. This fork has a long, completely enclosed spring, a double-acting dampening device, and wipers rather than rubber accordion boots. Since this type of front suspension is by far the most popular and has been so for almost 70 years, let's examine it in detail.
The typical front fork consists of the upper tube or station, the lower leg or slider, a coil spring, dampening devices, O-rings and seals, vents and plugs. The design and purpose of each of these items is explained briefly.
The upper or stationary tube section is clamped securely in the fork clamps or triple tree. This forms a secure base for fork operation and establishes the angle between the forks and the road. The top of these tubes are fitted with a threaded plug through which the fork may be filled with fluid. This plug often contains a spring-loaded ball check valve to prevent bursting the slider seals.
A long, progressively wound coil spring is fitted inside the tubes and provides most of the suspension action of the forks.
The dampening action of the front forks is similar to that of a double-acting shock absorber. Different manufacturers use various approaches to the problem, but generally the dampening action utilizes a series of ports or orifices and check valves through which fluid is forced to pass.
The lower leg contains the fork seals, bushings, drain plug, and axle attachments. This lower leg, or slider, must fit the upper leg very precisely or fluid will escape and suspension action will be hindered.
There are also all sorts of front suspension variants, such as telelever, duolever, single sided, etc. but this is a Hub, not a suspension encyclopaedia so we must move on.