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This is a little long, but worth it if you’re interested in the topic. A nice addition to chuck’s explanations and so we’re clear, this is pulled directly from the Yamaha Racing Tech Corner

Part 1

Mention better performance and most racers head straight for the aftermarket exhaust catalogue and hot-up engine parts micro-fiche. Horsepower is often considered the be-all and end-all to going faster, but there’s no point having the extra power if it can’t be taken full advantage of. This is where suspension comes into the equation, but when you take a look at all those adjusters, where’s the best place to start. But it needn’t be that difficult. The only question is where do we start? Do we discuss the variety of front forks available, from non-cartridge to cartridge, and of the two types do we tackle inverted, conventional or telelever? And that’s just for starters.

Or should we turn to the rear shock list, which includes linkage monoshock, cantilever, a monoarm with single shock, underslung shock, twin shocks or separate damper and spring units – the list just keeps going on, and on…

To simplify things a little we’ll concentrate on the most popular front-end option for sportsbikes, which are the cartridge type front forks. Although they look reasonably similar from the outside forks are quite a complex component of a racing motorcycle and can vary greatly internally. Especially when you consider that there are a pair of coil springs, which can range in strength and length; there’s also compression and rebound damping to consider, which is affected by the type of valving used; and then there are the number and size of the shims used. Add to that the fork oil viscosity and you’ve got an open can of worms. Lets start with the fork springs. It’s like the foundation of the suspension unit; if the bike isn’t suspended correctly then everything else is going to be compromised. The spring is what supports the weight of the rider and the bike, nothing else. The choice of spring is determined by the weight of the bike, the weight of the rider, the weight bias of the bike and the riding position, as well as the type of riding it is intended for.

With so many variables, the rider being the biggest, ‘guessing’ which is the correct choice of spring to use is not an option. It’s for this reason standard suspension offerings found on production bikes can often be slightly off the mark. It’s not to say the manufacturer-set suspenders are incorrectly dialed in, they’re just set to suit a wide range of uses, and an average sized rider, and can therefore be a compromise. In racing it is a different story. The possibilities of spring strengths, lengths and widths are infinite. Obviously the fork body they will be confined to will determine the length and width of the springs, while the numerous above-mentioned conditions determines the strength. To differentiate between one spring and another the stiffness is represented in kilograms per millimeter. For example a 1kg-mm spring will compress 1mm for every 1kg of pressure it is put under. Put it under 2kg and a spring will compress 2mm, 3kg will result in a 3mm difference between its free length and its compressed length, and so on.

What we have here is known as a constant-rate spring where every kg has the same effect on the spring throughout its length. Constant-rate springs are the most common used in performance bikes and competition. But there are also multi-rate-springs, which have a fine pitch and a coarser pitch. One part of the spring may have a rate of 1kg-mm while the next stage of the same spring compresses at 2kg-mm.

One reason the multi-rate spring isn’t used in competition is it doesn’t absorb energy in as linear fashion as the constant-rate spring. The softer-rate of the spring will naturally compress before the stiffer-rate, but it’s the transition between the two that is the problem for competition use. Yet multi-rate springs are popular in sportstourers because of the softer ride they provide over road bumps, while still offering the firmness to prevent the forks bottoming out over potholes and such.

Another spring variation is the progressive-rate spring, which offers a ‘progressive’ transition from the softer to the harder part of the spring. To keep things simple we’ll concentrate on the characteristics of a constant-rate-spring.

A good spring rate uses around 30-percent of the total suspension travel under the total weight of the bike and rider – also known in suspender speak as One-‘G’ sag. It is very important that the spring is able to support the weight of the rider and bike. This way the available wheel travel is optimised, while keeping the geometry balanced. To check the One-’G’ sag – which generally equates to about 25 to 20mm for most racebikes – first the suspension is extended to its full length so that there is no weight on the forks; then the length of the fork’s inner tube is measured. Now with the rider on the bike, crouched in the riding position, the difference between the two measurements is evaluated.

For those bikes fitted with a preload adjuster, generally found on top of each fork leg, fine-tuning the sag can be carried out by winding the adjuster in or out. Increasing the preload will lift the front end, resulting in a reduced sag measurement, while decreasing the preload will lower the front end and increase the amount of sag.

The preload adjuster is not a tool to stiffen or soften the front end – remembering that with a constant-rate-spring the spring’s strength is set. Compressing the spring with the preload adjuster will not change its rate either; whether the preload is all the way in or out the spring rate remains constant.

The next thing to consider then is controlling the movement of the forks… see the second installment in the suspension story ‘Suspension tech feature two’.

Part 2

Continuing on from the opening ‘Suspension tech feature one – well sprung’ we now move on to the next part of the suspension mystery – damping. Although the correct choice of spring is important to suspend the bike there is also a need to govern its movement, otherwise it will continue to bounce for some period after having absorbed a bump.

Just as a steering damper prevents the handlebars from slapping about, front fork damping prevents the front of the bike from continually bouncing up and down. To do this two damping units are used – even if there are no external adjusters on the bike there is still internal valving used for compression and rebound damping in the fork legs. The rebound controls the rate the spring returns to its free length, while the compression damping controls the rate at which the spring compresses under load.

The damping rates are controlled by two separate damping valves which are located in the one damping rod. There is one of these rods in each fork leg. Although it’s still possible to get forks with the rebound in one leg and the compression in the other, it’s not very common these days. A damping valve works by controlling the flow of oil, which it is submerged in. Reduce the resistance for the fluid and the softer the damping will be. This in turn determines what rate the fork is able to move through its stroke.

The compression-damping unit is located at the base of the damping rod while the rebound-damping unit is located within the damping rod – also known as the travelling valve. It acts much like a piston sliding up and down the damping rod cylinder. These two valves, rebound and compression, determine the type of feel the forks provide – whether the bike crashes sharply over the bumps, absorb them or bounces over them. This feel hinges on a number of factors. First there is the actual damping valve. It looks much like a piston used in an engine, only smaller (around 15mm in diameter), with a range of holes punched through it. The size of these holes can provide a soft feel if they’re large (less resistance), or harsh feel if the holes are small, due to the increased resistance. The trick is to find a compromise that will be firm enough under heavy braking, while offering the ability to soak up the wide ranger of bumps it will be confronted with. The valve itself gives the overall feel, but the fine-tuning of the fork-fluid flow, which in turn affects the wheel axle travel, is done using flexible shims. Shims are like thin flat washers that can vary in thickness by as little as a thousandth of an inch, and can also vary in diameter. These shims are stacked one atop another, up to as many as 10 or so, and are then placed over the centre of the valve – covering the valve holes.

When the fork moves up, at which time it is working the compression valve; or down, controlled by the rebound valve; the fork oil forces its way through that particular valve holes and past the shims by bending the outer edge of those shims. The stronger the resistance the firmer the damping and the slower the movement of the suspension. Any problems with suspension patter, wallow, chatter, harshness or suspension dive are damping related. If the damping isn’t quite right, adjusting it can become quite tedious if the bike doesn’t have any external damping adjusters. In that instance it would be necessary to dismantle the forks every time a change was needed – very frustrating, with the fine increments available, especially while under the pressure of qualifying for a race.

It’s for this reason most sportsbikes, and all race bike suspension boast external damping adjustment – rebound, compression or both. Unless the internal damping is really off the mark, the fine-tuning capabilities of adjustable units will allow the rider to dial in the suspension to suit their riding style and the conditions of the track/road.

To understand the range of adjustments better, imagine a piece of string two metres long. This represents the internal valving options available on a particular suspension package – one end is the extreme harsh end of the scale, while at the other is the soft. Choose a point along the string and this will be your damping valve and shim stack, the 10cm either side is the range of adjustment available with the external adjusters.

But there is more. We’ve covered the valving and the shim stack, and that the amount of resistance they provide the fork fluid determines what the damping rate will be. But then so will changing the fluid’s viscosity – the thicker the oil, the greater the resistance, the slower the damping rate.

Oil rates vary from what is known as five-weight upwards, generally increasing in 5W increments – eg. 10, 15 and 20-weight. The lower the figure the thinner the oil. Often race teams will use the lightest fork oil possible because it provides a more consistent viscosity when heated. Thick oil begins to break down quicker, becoming thinner as heat increases. But as with the spring, the valves and the shim stack, there is more to fork oil than just that…