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Suspension & Chassis

   A Bench Racer's Guide

Bruce Maus & Paul Jones
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  the small détente's felt every time you turn the small adjusters on either the forks or the shock absorber- usually every 45 to 180 degrees of movement, depending on the make. Typically these will have more effect on the high-speed damping than the low speed.

Compression Damping: the deliberate hydraulic resistance being used to control the upward force that is trying to compress the suspension. This resistance can include different systems to control low speed movements, such as brake dive (or 'sleeping policemen' on public roads) and high speed movements such as the little ripples that appear in the tarmac when really heavy vehicles brake for corners.

Rebound damping: the opposite of Compression, designed to control the natural desire of a spring to oscillate as it comes off load. Needs to be heavy enough to stop things banging around but light enough to let the wheel follow any bumps and ripples on the surface.

Offset: The fork triple clamps hold the fork legs typically between 25 and 40 mm away from the steering head. Offset can be adjusted, less offset will result in more trail and a bike that requires more effort to turn into a corner, the opposite is true. This can be adjusted either with different triple clamps or adjustable ones like those supplied by Harris and Spondon.

Oil level/airgap: the gap that remains over the top of the damping oil in the forks, typically a 100mm travel fork will have a 120mm, or thereabouts, airgap. As the fork compresses the air is forced into a smaller space. During the last 10mm or so of fork travel the air pressure adds enough additional force to the spring rate allow the rider to run a softer spring or damping set-up without bottoming the fork under severe braking or cornering loads.

Pitch: The movement of the bike to a new attitude; i.e. into a wheelie or a stoppie, the springs and damping seek to control pitch to ensure that the machine stays as stable as possible, and definitely with both wheels on the ground.

Preload: The deliberate pressure added to a spring to get the motorcycle to ride at the right height. In competition use preload is expressed as the amount a spring is compressed from its free length without any other load on it.  It is unusual to measure much more than 15mm preload or less than about 7mm on the spring (this giving about 25-30mm of sag when the rider is sitting on the bike in the garage, in riding kit and in his normal riding position). If the bike needs preload out side this range then a different weight spring is probably required. Remember however that all these numbers are guides, it is what the rider feels comfortable with at speed that matters

Sag: the amount of suspension travel that is used when you put the rider on the bike. Typically you would want about 30mm sag on both front and rear springs as a starting point with the rider on the machine in the normal riding attitude. You find the precise amount by feel, when combined with all the other things you can change this can take a long time to perfect – and will be different for every track. Static sag is the amount the bike settles on its suspension as a result of its own weight only, this can be between 10mm and zero at the rear and 15 to 10mm at the front.

Push:   the feel one gets when the front tyre is moving away from the bikes intended path because there is insufficient grip, or insufficient response to steering input to hold the bike on line (if it is ignored it is usually followed by another feeling - 'the crash').  The big question is why is it 'pushing' the front? Too much weight on the front? Or too little? You have to one way or the other and ask 'is it better or is it worse?'

Rake: The angle the frame holds the front forks at, can be adjusted by tipping the entire motorcycle, raising or dropping the front forks; raising or dropping the rear of the bike and, on a 748 or a 916/996 Ducati by turning an eccentric in the steering head.

Rising rate: a lever or series of levers connected to the suspension designed to increase the resistance as the suspension collapses. Not as popular as it was because of concerns on the effect the increase in load has on the tyre.

Springs: the things that hold the whole bike up, have to be changed to ensure (subject to all the damping and chassis changes) that there is a balanced force pushing down on each wheel at each corner of the racetrack. The springs are controlled by the compression and rebound damping circuits.  You would normally seek to fit the softest springs you can at the front, remembering that you want to keep the suspension from bottoming and the best compromise of suppleness and control.

Dynamic Centre of Gravity: The centre of gravity that occurs when the bike has settled on its springs under cornering loads.

                        Neil Spalding

When you can talk this lot effectively enough for long enough you will be able to call yourself a suspension expert. However when you can use these adjustments to get a bike without enough power round the circuit faster than the 'big thing with lots of power' then you are a suspension expert.

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