Know your motorbike

MOTORBIKES, or motorcycles, have a similarly varied selection of suspension systems as cars. On bikes, of course, you only have two wheels, so bike suspension systems tend to be a little more highly engineered because there is more at stake. By far the most common setup is the single rear coil-over shock system with either a regular double swing-arm or a single-sided swing-arm. At the front, telescopic forks are still the most prevalent. It’s surprising that there are still a large number of cruisers out there that are ‘hard-tail’ bikes – bikes where there is no suspension at the back. The wheel is simply axled straight on to the frame. This is a throwback to the very first motorbikes which were basically bicycles with an engine strapped to them. (In the 1920s, motorbike suspension consisted of the springs in the saddle and the air in the tyres.) One of the drawbacks of telescopic forks on a motorbike is their tendency to compress under braking, making the bike ‘dive’ forward. This is due mostly to the steering geometry of the average motorbike. When you brake, you’re slowing the forward motion of yourself and the motorbike. That forward force has to go somewhere, and that somewhere is the front suspension. Because the telescopic forks are at an angle to the frame, and consequently at an angle to the braking force, some of that forward force gets sent directly down the forks. Think back to your school physics. Force transmitted at an angle is equal to the main force multiplied by the cosine of the angle. Remember the rake on a motorbike is calculated from vertical. So the angle we want is actually 90° minus the rake – the complement of the angle. Conveniently, because sine and cosine are the inverse of each other, the cosine of one angle is the same as the sine of its complement. So for a bike with a rake angle of 25°, we can either use the cosine of its complement (65°) or the sine of the rake angle itself. Honda fired the first shot in the anti-dive war in 1969 with the introduction of its TRAC system (Torque Reactive Anti-Dive Control), but it wasn’t until the eighties that it became more mainstream. Anti-dive systems were typically linked to the brake hydraulic system, and is remembered best on the Kawasaki GPZ900R where it was introduced under the moniker AVDS - Automatic Variable Damping System. AVDS was a supplemental hydraulic cylinder mounted on the front of the fork legs which was connected to both the brake lines and the hydraulic fluid inside the telescopic forks. The idea was that as you applied the brakes, this unit would use the pressure in the brake line against a plunger to close a control valve. This valve restricted the flow of fork oil and thus stiffened the suspension. Stiffer suspension meant fewer dives. Anti-dive units mostly featured a dial adjuster on them, normally at the base. This was a way of affecting how much the anti-dive plunger moved, which meant the rider could make the anti-dive more or less severe. It all sounded good in principle but a lot of riders took a dislike to it because of its behaviour on bumpy roads. If you went to brake on a bumpy surface, the front suspension stiffened up and it became less like riding a motorbike and more like falling down stairs as all the road bumps and deformities were transmitted up the now-stiffened suspension into the frame of the bike, and consequently, the rider. The control valve would often stick closed resulting in permanently stiff suspension, which in turn would result in frequently blown-out oil seals. These “features” of anti-dive systems have since been ironed out and they tend to work maintenance-free now.

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Know your motorbike