IT USED TO BE EASY

Years ago, full suspension, indexed gears and V brakes didn’t exist and bike set-up was a doddle. Buy a bike and ride it. The only problem was that in those early days, bikes didn’t actually work very well, and magazines were obsessed with things like ‘handlebar buzz’, subtle nuances in tyre compound and weld type minutely altering ride quality.

Then suspension arrived. Rockshox, Offroad and Manitou suspended the front of bikes, and everything went mental. Cannondale added new bottom brackets halfway up the seat-tube and used them to mount a swingarm on. They flexed so much the front mech would rub on the rear tyre. Suspension on bikes these days has such an air of simplicity that it’s puzzling why it’s taken us so long to get there. Over-simplicity in early bikes, with corners cut on bearing and shock materials, made bikes that stank like the dogs they were. These days we’re used to coil-over shocks, metallic rolling-element bearings and the like. Everything’s getting much better. One reason things have moved on so much is set-up. Early bikes used stiff springs and binary damping circuitry, and it was all just guesswork really. Early Rockshox forks remained locked out unless you hit a big rock. Compare that to the modern breed of low-stiction fork that’ll sag under the weight of the unloaded bike. As softer spring rates and damping settings have come into play suspension has been adopted by more people. Suspension requires a learning process to get the most out of it though. No more sprinting up hills, or nose-diving off rocks. Now fast riding is all about smooth power and wheelying off drop-offs.

The best suspension bikes gobble up the trail without worrying the pilot. Any obstacles smaller than the amount of travel your bike has can be ridden over without a thought. Anything larger than the available travel can be ridden over with far less weight shift than you might expect. All this is a bit surprising at first. It’s often only when you have an unplanned crash, or take a line you didn’t expect to, that you find the upper limits of your suspension. Hitting foot-high rock steps square on is possible on some of the longer travel forks – but you’d never believe it until you’ve seen it. We’re not saying you should go out and crash your bike to determine whether your suspension is working properly. A full suspension bike might not necessarily be more comfortable, but you’ll end up riding harder trails faster until the bumps you’re getting through the bars feel the same. To achieve this level of performance, a bike needs to be set up properly. Some bikes can be set up ‘more properly’ than others. And herein lies a problem.

The process of transferring power from your legs to get the back wheel turning is difficult enough without having to move the back wheel up and down as you ride over over bumps. Simply having enough gears and getting those gears to work in muddy conditions is remarkable enough – look at the trouble Shimano are having with their nine-speed system – but having it all happen with up to eight inches of travel is quite a feat, though it’s achievable. Many manufacturers claim that their suspension systems isolate rider pedalling action from the suspension movement, but it’s a hopeless cause. All powered vehicles react differently whether being driven or decelerated. Putting a spring at each end affects everything. And, besides, who says it’s bad? By placing your pivots cleverly, you can affect the way the suspension moves when the power is applied. If the pivot point of the bike is above the chainline, then the suspension will be extended under power. If the chainline is above the pivot, then the suspension will be compressed. Some bikes with pivots below the chainline use four-bar linkages – other pivots within the rear triangle – to modify the path that the wheel axle travels through, thereby modifying where the ‘effective’ swingarm ‘effectively’ pivots from. These instantaneous centres of rotation follow the same rules as real pivot points, it’s just harder to spot where they are. But it’s possible – see page 58. Then there are two different designs: conventional and Unified Rear Triangle (URT). URT bikes have the bottom bracket (and cranks, chainrings and other gubbins) all mounted on the swingarm. The pivot point can be close to the bottom bracket, around the bottom bracket, or a long way from the bottom bracket, depending on the model of bike. All these things have an effect too. Does your head hurt yet?

But why should that matter?

The location of the swingarm pivots has an effect on the suspension set-up. How the swingarm pivots and how the shock compresses have an effect. You don’t need to be an expert to set your bike up right for you, but it helps to have a number of things clear in your mind. As we said before, a bike with a pivot point below the chainline compresses the shock when
the power goes down. Add to this, the other force that’s being put into the suspension – your legs flapping up and down – and the bike will bounce down the trail. A bike with a pivot point above the chainline can, in the best case, eliminate bob by countering the force that your legs moving downwards exert on the suspension with an upward force generated by chain tension. Match these properly and you’ll have a rear end that doesn’t move under pedalling pressure, within a limited range of gearing. The problem is that suspension designs can’t yet detect what gear you’re in, and so though a gear combination of middle ring/large sprocket might give the same gearing as a small chainring/other sprocket, the way that the suspension reacts to that combination is very different. Because the chainline is lower, the resultant force on the shock will alter. This increases the further away the chain is from the pivot point. To get the bike to work properly for you, the amount of spring rate, type of spring and amount of damping all need to be tailored to your weight, riding style and the type of riding.

You may imagine that suspension manufacturers spend huge amounts of cash and resources in tuning suspension for different people. After all, only boy racers shell out for new springs and shocks on their cars – normal men in the street don’t have to set their cars up for their weight, do they? The difference is that a bike weighs a fraction of what the rider does,
and it’s one of the lightest suspended vehicles. The result is that a small change in rider weight requires a significant change in suspension set-up, and a similarly small change in rider weight positioning can also affect suspension performance. So not only do you have to take into account how much you weigh, but also how you ride and how your bike is set up – small wonder manufacturers don’t offer pre-tuned bikes for sale.

Everyone knows what a spring is, and thankfully, the bike industry is, to date, free from the complexities of torsion bars and leaf springs. We use ‘simple’ coil springs – actually wound-up torsion bars, but let’s not worry about that – and air springs. Coil springs can be wound to give a continuous rate – add a force and they compress a given amount, double that force and they double their compression. Or they can be progressively wound – double the force and they compress 75 per cent of the initial amount, for example. Air springs are progressive too. Setting the spring rate is the most important aspect of suspension performance. Without the correct spring rate, the dampers will have to support the machine’s weight, leading
to a lumbering ride with frequent bottom-outs and the possibility of blown seals.

If you’re the owner of a bike with a steel-sprung shock, you might be feeling pretty smug. You’ve heard that air shocks aren’t as plush as coil-over ones, and you’re happy that you’re getting a better ride. But what happens when the spring in your coil-over shock isn’t right? Air shock users simply add more air whereas you’ve got to buy a whole new spring. If your spring doesn’t give you the sag you require it’s time for a new spring. But how much stiffer do you need the spring to be? Spring rates are dependent on many things – rider weight, on-bike position, the shock length, the leverage ratio of the bike, the type of riding you’ll be doing… all complex stuff.

To this end, we’ve calculated various different things through and graphs accompanying this article will give you an excellent starting point to set up your suspension with the correct spring rate. You’ll already know the travel of your shock, from the above sums, but you’ll also need to measure the travel at the rear wheel. This can be calculated reasonably accurately by measuring from the centre of the rear wheel to a point on the back of the saddle at full extension and full compression. From there, it’s a matter of looking at the graphs (left). Because spring rates are shock-dependent, look at the graph that gives you the length of shock you have and compare it to the amount of travel your bike has. Read off, along the bottom, your weight (we’ve allowed 10lb for clothing) and see where the lines intersect. This gives you the spring rate you need, which you’ll have to take in increments of 50lb to your local shop who’ll order you up a spring to suit. Of course, if you’ve an air spring, you’re laughing. Simply pump more air in and it’ll be sorted. All you’ve got to do is fiddle with the pressure and get the sag right. Simple!

Damping next: the damping has to be as light as possible. As we’ve already mentioned, the fact that the bike is so light when compared to the rider means that any additional force slowing down the movement of the suspension will hinder the operation of the suspension. Damping circuitry is designed to control the movement of the shocks, but should be kept a minimum. Compression damping is hardly needed by light riders, and only needs to be increased
with heavier riders. There’s no hard and fast rules for applying damping – if your shock will let you change it anyhow. But if you’re still bottoming the bike out despite the correct spring rate, then you’re going to need more damping. Either dial it in on the shock, or send it away for a service. Rebound damping governs how fast the shock returns. It too should be set as light as possible. Too much rebound damping causes the back end to pack down over repeated bumps, but more often makes the bike feel sluggish at the back. Wind off rebound damping until the bike’s kicking up at the back over big hits and then wind it on a little more.

Firstly, check you’ve got a linkage bike in the true sense of the word. Some models use a linkage to drive the shock via a swingarm. Others mount the rear wheel on part of a true four-bar linkage. You can spot this by looking at the bit of the swingarm that connects the wheel to the frame. Is there a link between the rear wheel axle and the main pivot? If yes, then you’re looking at a four-bar linkage design where the rear wheel doesn’t pivot about a simple arc around the main pivot – it follows a ‘modified path’, modified by the addition of the extra linkage. Think Spirograph, but with straight lines rather than circles… Find the centre point of the modified arc, then project lines through the top and bottom arms of the four-bar link. Where they cross is the instantaneous pivot point. This point will move, as the suspension compresses. Where it is relative to the chainline will provide the same outcome as if it was a real pivot point. But why a moving pivot point? It allows precise control of the axle path of the rear wheel to keep chain tension exactly as they want. However, most virtual pivot points provide an axle path very close to that which most single-pivot bikes are settling for. Strange.

There are things that can be done to that help you tweak and tune your suspension easily. Things that you can do to make your bike work better if it’s like one we’re describing. Keep them as your armoury of shock-setting knowledge, and use them as you see fit.

Because of the way a URT works, the spring rate you need when you’re standing up is different to that when you’re sat down. When you’re sat, most of your weight is going through your saddle, into the main frame. When you’re stood up, all your weight is going through the pedals onto the swingarm. When you stand up on a URT bike, the shock extends, as there’s less weight bearing on it from the main frame. Setting the spring rate so that there’s the right amount of sag when you’re stood just off the saddle means that you get a great dual-purpose ride. When you’re sat down, meandering along, you’ll be using lots of the travel. Stand up, the bike stiffens and you’ll be able to take those big hits too. The bike should just bottom out when you slam your weight down on the saddle as you’re going along. URTs are different beasts, and work well when set up in this softer manner, as opposed to setting up ‘normally’ and then becoming harder when you stand.

Lots of bikes out there have different holes to mount the shock in. It allows you to change the rate of the shock and the amount of travel. Generally, dropping the back of the shock increases travel and dropping the front of the shock changes the rate. A rate change means that the shock will tend to compress more easily in the early part of the stroke, stiffening up at the end. This can make the bike feel a little wooden – particularly with an air-shocked bike as an air spring is itself a rising rate system. By dropping the shock down (or raising
the front up) you can set the shock up to tend to be a falling rate system, counteracting the rising rate of the shock, giving a more comfortable straight rate bike. We’ve used this trick to good effect on a few SID-shocked bikes and got a plusher feel.

If your shock has no rebound damping adjustment and your spring is a little soft, then wind on some preload and get something approaching the ride you need. It’s not perfect, it’s not the answer to all your problems, but it will give you a better ride.

This last trick – winding on preload to speed up rebound – is one way that a stiffer spring can make a bike feel softer. If the shock is returning faster, with a stiffer spring or one under more compression, then the shock will be able to react faster. If you’re not getting the ride you want then add preload and see where it gets you.

When a suspension system is correctly damped, you’ll feel ‘one and a half overshoots’. Hitting a simple bump, like a kerb (either up or down) you’ll feel the suspension compress, then extend, then compress to its median level. If you feel one thud then it’s wrong, you should feel down-up-down. If you can actually feel, or see yourself bouncing multiple times after hitting an obstacle, then you’ve not got enough rebound damping wound on.

Of course, as we’ve mentioned at the top of this article, because the chain tension changes with pedalling load – or in the case of URTs, the weight distribution changes – the bike will bob. Low pivots do it, all URTs do it. What this means is you’ll have to crank on more rebound damping than you need for correct suspension action. Either that or pop the sea-sickness tablets.

Though no one uses them for the reason they were invented any more – to smooth out the pedal stroke – we’ve used oval chainrings to tame bounce on a couple of suspension bikes. They work by changing the chain tension and chainline throughout the pedal stroke, and can totally eliminate bob and actually increase traction throughout the stroke. We’re not pursuing it because of the chop and change of test bikes and the nice way Shimano chainrings shift. Anyone care to take up the cause and cut us in on a tidy share of the profit?

JARGON BUSTER

Adding a loading onto the spring in its static position to create a force against suspension movement. Doesn’t increase the spring ‘weight’ but does increase the load needed to start it moving. Normally done by winding collar (rear coil shocks) or top cap adjuster (forks) down onto spring.

Negative spring

A spring that pushes the shock back past its static position when unloaded. Creates a plusher feel and allows wheel to extend quickly into depressions for better traction.

Spring weight

The force needed to compress a given spring one inch.

Spring rate

The change in effective weight of the spring as leverage within the suspension system changes under compression. A rising rate system will increase its ‘weight’ as it compresses – often described as ‘ramping up’ or ‘progressive’. A straight rate will remain constant, and a falling rate will decrease. Coil springs can be wound specifically to a rising, straight or falling rate, while air shocks will always be rising rate.

Leverage

The ratio between the travel of the rear wheel and the movement of the shock.

Sag

The amount of travel used up when the rider sits on the bike in a static position. Again allows the suspension to extend into depressions, increasing traction and creating a far more fluid, plush ride. We recommend around 20-25 per cent sag for XC work and up to 50 per
cent for downhill traction.

Damping

A resistance applied to the shock’s movement to prevent pogo stick lunacy. Can be tuned at manufacture in elastomer springs or provided by parts of the shock moving through oil. Compression damping is the resistance against the compression (impact) of the spring to prevent it diving too fast. Rebound damping controls the return of the spring to stop it throwing you from the bike.

Linkage

An extra link in the suspension system used to take structural stress away from the shock. A four-bar linkage can be used to alter the path of the rear wheel under compression. Also allows the shock to be repositioned around the frame, and the leverage and rate of the shock to be modified to designers’ needs.

Air/oil

Air spring damped with oil.

Coil/oil

Coil spring damped with oil.

Elastomer

Chewy, springy rubber available in different weights.

Piggy back

A shock where the damping reservoir is mounted on a mount above the shock.

Remote reservoir

The damping oil or air spring chamber is linked to another frame-mounted chamber to increase the volume and decrease progressiveness.