Motorcycles have featured front suspension for over a century, with BMW being the first manufacturer to produce a motorcycle with hydraulically damped telescopic forks in 1935. Currently, a majority of motorcycles use telescopic forks for the front suspension. The telescopic front fork is made of fork bodies, which are the shock absorbers that contain coil springs inside an oil-sealed enclosure, into which the solid fork tubes, or stanchions, slide to provide damping action.
Motorcycle suspension serves dual purposes – contributing to the vehicle's handling and braking, as well as providing safety and comfort by isolating road noise, bumps and vibration. There have been various iterations of two-wheeler front suspension over the years, from Telelever and Duolever forks (used by BMW), right up to single-sided front set-up and front swingarms with hub-centre steering. However, telescopic suspension systems have survived the longest, and seem to be the most practical solution for motorcycle front suspension system.
Within telescopic front suspension systems, the traditional setup was with the stanchions being mounted to the top, and the fork bodies at the axle. But starting in the 1980s, motorcycle racing teams changed the orientation of the front fork by inverting it, resulting in the 'Upside-Down Fork' or the 'Inverted Fork.' In this setup, the bigger, fork bodies are mounted to the triple clamp and the stanchions are at the bottom, clamping on to the front axle. The inside of USD front forks, however, is almost exactly similar to that of traditional telescopic suspension. Each fork body is essentially made up of a piston, valves, cylinder and side pipe.
THE USD DIFFERENCE
The unsprung weight (wheels and part of the suspension itself) on a motorcycle negatively affects its handling capabilities. Turning of the handlebar creates a large amount of twisting pressure on the point at which the suspension system is fitted on the triple clamp. The dimensions of the fork, including the stanchions and the fork bodies, are designed to ensure that the amount of force that can be handled. The setup of USD forks results in lower unsprung weight on a motorcycle, since the lighter components of the suspension are placed at the bottom.
USD forks provide a higher level of rigidity, since the fork bodies are mounted higher up at the triple clamp, at the point where the highest level of force is exerted while riding. This rigidity results in a lesser degree of suspension flex under hard riding, hence improving stability and cornering prowess, which is especially relevant for high-performance bikes. While USD forks provide these advantages of stability, we'll note here that performance advantages can be felt more in extreme riding situations (for example, on a racetrack) rather than everyday riding or commuting. This USD fork system comes with its own set of challenges as well, with costs being higher. USD forks, especially adjustable ones, are also more complex than traditional forks and require more skill for maintenance.
As mentioned earlier, USD forks offer significant advantages for high-performance bikes and their usage might not make a big difference to the riding dynamics on smaller, commuter-focussed machines. USD forks on higher-end bikes offer multiple types of adjustability, for compression damping, rebound damping and preload.
The springs in motorcycle suspension are always under compression even when fully extended, and preload is used to adjust the initial position of the suspension with the weight of the motorcycle and rider combined. Preload adjustment on motorcycles is generally made by changing air pressure inside the forks, using valves to add or release air from the fork. Higher air pressure gives more preload and vice versa.
Front forks also act as hydraulic dampers, with changes to the weight of the fork oil altering the damping rate. Telescopic forks offer external adjustments for damping, which is the simpler method. The cartridge fork is a more sophisticated approach, which uses internal cartridges with a valving system. Different types of USD forks, including 'Big Piston' forks and Cartridge type forks are now also offered by various suppliers with semi-active electronic control, which allows a rider to adjust damping at the press of a button. Such suspension, in some cases, can also automatically adapt to terrain by optimising damping characteristics in real time. Such systems are already being offered by OEMs like Ducati and BMW on their streetbikes.
Cartridge type forks provide high rigidity and stable damping force, while Big Piston front forks feature increased surface area for improved damper efficiency. Forks that separate compression and rebound damping (one function for each 'leg' of the fork) offer greater adjustability and are often found on high-performance superbikes. These can also be made to work in conjunction with the 'riding modes' provided on some bikes these days, with front fork damping being optimised by the bike's on-board computers in accordance with the chosen mode. Typically, 'street' and 'comfort' modes see softer damping, while things are firmed up for 'sport' and 'track' modes for better high-speed controllability.
THE INDIAN SCENARIO
In India, domestically produced motorcycles featuring USD front suspension include the KTM Duke and RC series, and the Mahindra Mojo. The upcoming BMW G 310 R will also feature USD forks, which shows that this suspension setup is becoming more mainstream, at least when it comes to motorcycles having some sporting intent. In India, twin rear shocks on bikes have given way to monoshock set-ups on many bikes, and in the same vein, traditional front forks might increasingly be replaced with USD forks.
One factor that may hinder the adoption of adjustable USD forks is the level of riders' knowledge and ability to understand and implement changes. All riders are not likely to know, or understand, the effect that compression/rebound damping adjustments make to a bike's ride and handling qualities. Changes, if made by an inexperienced rider who doesn't understand these functions, could actually have an undesired effect on the bike, and might even spoil ride and handling to a large extent. Still, with the relentless march of technology, the Indian two-wheeler market might still see more widespread adoption of USD forks in the years to come.
TEXT: Naveen Arul