Ever wondered how suspension works? We get the pros from K-Tech to talk us through it and tell you how to get the best from yours.
The mysteries of suspension are unravelled right here, right now, with the help of some rather clever people at K-Tech.
It can be very difficult to obtain that perfect suspension set-up. In racing, with so much adjustability, it can be a challenge to find the compromise between how the bike reacts into a corner, during a corner, and on the exit – and nearly every corner on a track would, ideally, need its own setting so finding the right compromise for all is never easy. In off-road racing and especially motocross, with ever-changing track conditions it’s even trickier.
On a stock road motorcycle, built to a specific budget yet designed for a huge range of rider weights (with the option to carry a pillion and luggage) getting it spot-on for everyone is, understandably, a near impossible task.
A bike manufacturer has a huge window when designing a standard road machine – from the physically smaller, lighter Asian market, to the often larger US owners. Will the rider be carrying luggage? What kind of road surface will they be on? A bike with too wide adjustment could, in the wrong hands, be set up to handle dangerously – give enough to suit a single Asian rider, and a couple who load their bike up excessively in the States could, theoretically, set the motorcycle to handle dangerously.
Due to the litigious times we live in, this could prove costly for manufacturers; hence an average is accommodated for. It’s this lack of real adjustability that contributes to many riders not believing they understand suspension – until you can actually feel what’s happening as you tweak it, it will always seem something of a dark art.
Good = well balanced
A lot of OE suspension is set up on the softer side, to be more comfortable. But good suspension, especially when designed around a specific rider, doesn’t have to be harsh. Aprilia’s RSV4, for example, sports an incredible setup from the box that remains sporty but pliant on the road, and razor sharp enough (for most) on track.
A bike that’s too soft on the front allows the forks to dive too far, changing the geometry and putting more weight over the front tyre, potentially overloading it. It’s all to do with how the bike sits on its suspension – often, firming a bike up will actually make it more comfortable. But it’s not as simple
as putting in a stiffer spring – the damping needs to match it. An aftermarket manufacturer like K-Tech can look at an individual rider’s circumstances and riding style to give them an ideal setup and advise them on the correct adjustments. So, let’s dive right in to what does what when it comes to suspension…
What does what?
Take a machine like 2016’s Yamaha Tracer 700 – it has no piston in the fork – it’s a ‘viscosity damping fork’ that is simply an inner tube with holes in the bottom going through an outer tube with oil in it.
Fitting a more advanced cartridge fork system doesn’t just introduce adjustability, it has better control of the oil, using a regulator valve that limits oil flow displaced as the inner piston moves. There’s also a shim stack fitted to the piston, which is basically several thin steel discs that can deflect under higher pressures, reducing the resistance of the piston in the oil by opening up holes machined into it (this is what allows the fork to be more compliant when hitting harsh bumps that cause the fork to move faster) – it’s basically the same in a rear shock. Good suspension will give more comfort, feedback and control.
A motorcycle works best as a balance, so it’s typically ideal to improve the front and rear of a bike, but people often tend to change the front first as it’s what they feel most. Suspension is simply the connection between the tyre and the vehicle, controlling sprung mass. The spring, be it in your fork or on your shock, controls the mass of the bike. The damping controls the speed at which the spring is allowed to move. Here’s what you need to know…
PRELOAD: The most misunderstood part of suspension. Increasing preload does not make a spring ‘stiffer’, it simply increases the ride height of the bike. However, it does affect the break-out force – the point at which it starts to move. If you have a 100lb/ inch spring, and you put one inch of preload on it, to make it move you have to add another 100lb.
Once it’s moving it feels the same, as the spring rate is unchanged, but this increased break-out force can make a bike feel more harsh as greater loads are transferred to the rider before the spring starts to react. Equally, if you add 50lb of luggage to a bike with a 100lb spring that is balanced to you as a solo rider, an additional half an inch of preload will even out the additional weight and maintain the correct static sag. Increasing the ride height can speed up the steering, but in order to do this without increasing the preload, many aftermarket shocks come with an adjuster that lengthens the shock without compressing the spring.
STATIC SAG: This is the amount the bike drops as the spring takes the weight. It’s the difference in height between the machine sitting naturally, and the bike being lifted to the point where the wheel is only just touching the floor. Sag allows the wheel to drop into dips in the road surface without taking the bike with it – it’s simply slack in the suspension. Picture a Dakar racer blasting across sand dunes – the bike stays level over dips while the wheels drop into them because of sag.
AIR GAP: Air acts as a spring and is generally also part of a fork’s characteristic. It’s simply the space between the top of the oil and the fork cap. Air can also be used to replace a spring altogether, as in K-Tech’s Bullit units designed for bikes like the Ducati Scrambler, Harleys, Indians and more. Like the oleo struts used in tanks and aircraft landing gear, these nitrogen-charged shocks give constant damping control over a wide range of loads and surfaces.
PROGRESSIVE SPRING: These are simply springs that get stiffer as they’re compressed further, making them return more force as the suspension increases its stroke.
PROGRESSIVE LINK: Also known as ‘rising rate’, the link joins the swingarm to the shock, and causes it to require more force to be moved as the suspension is compressed. As with a progressive spring, the principal is to allow a softer feel under normal conditions, without letting the suspension bottom out.
COMPRESSION DAMPING: Oil inside the fork or shock restricts movement to slow the speed at which the spring is compressed. Too little and the spring’s movement is uncontrolled, too much and it’ll create a harsh feel as the component can’t move smoothly.
REBOUND DAMPING: This controls how quickly the suspension can unload. Too little and the bike will bounce back from a compressed state. Too much and the suspension can ‘ratchet down’; imagine riding over a series of close bumps with too much rebound damping on the rear shock – the tail of the bike will never get time to return to its natural position, and get quickly lower until you’re kicked up the bum. This could be reduced by decreasing the amount it moves in the first place with more compression damping, but then the ride would be a lot harsher. Suspension is all about the balance of all the settings, so for the very best ride, the spring has to be
Ian Hutchinson’s won numerous TTs on K-Tech suspension.
If you ever feel the urge to strip your suspension, hopefully this pic makes you see sense.
Ask any racer and they’ll tell you setup is everything. Once this guy’s mastered scissors, the possibilities are endless.