KNOW YOUR SUSPENSION
Without those bouncy bits at both ends of your bike, riding your beloved steed would be a right pain in the arse – literally. We chatted to JWR’s suspension supremo James Wood to get a better grasp of how they work and what goes wrong…
FB: How crucial are the right weight springs?
JW: Fitting the right springs to your bike is crucial. Too soft and the bike will squat, causing it to run wide. Too hard and the bike with feel vague, or chatter. Either way, it won’t be a nice experience. Manufacturers often err on the side of being overly sprung with the standard springs fitted. So chances are you’re not getting the best from your bike, as balancing the bike on the springs is essential. It is the foundation for a good setup.
There are standardised charts available which will identify which springs are best suited to your weight. If you want to invest any money in your bike’s handling, start here. FB: If you change your spring rate will it affect your damping? JW: Quite often, a bike’s damping will have been set to compensate for an ill-weighted
spring. For example, excessive compression could have been used to support a light spring, so this would need backing off to suit the new stiffer spring weight.
Likewise, the suspension’s static sag will be wrong too, so you’ll need to adjust the preload front and rear to achieve the bike’s optimum balance. FB: How long do shock absorbers last?
JW: It’s hard to predict a shock absorber’s life. Like anything, it all comes down to how well something’s been looked after. A bike with 50,000 miles on the clock might not need a service, whereas one that’s only done 10,000 could be desperate for an overhaul. The worst killer of shock absorbers is poor maintenance. We’re not talking a strip down every summer, but a good clean on a regular basis can really help to extend a shock’s life. Especially if you’ve been
riding your bike on salty or gritty roads. Left unclean, debris slowly gets forced from beneath the bump rubber, up the shaft, past the dust and oil seals and into the main oil chamber.
Once inside, it can cause no end of trouble, often wedging itself between shim stacks and completely corrupting your bike’s damping. FB: Why is nitrogen used?
JW: Nitrogen is used to stop cavitation. Most rear shocks will feature a nitrogen sack to keep equal pressure between the oil and the piston, both sides of the shims. Essentially, it’s there to ensure optimum damping performance. A problem of old was that there would be an air gap before the piston and shims came into contact with the oil.
This meant the initial damping performance would be compromised because the air would be compressed, before the piston made contact with the oil’s surface; translating as a soft and lethargic initial feel to the stroke. For this reason, it pays to stay on top of your shock absorber’s nitrogen pressure. A typical shock will contain
around 10-12 bar, which will escape over time. Once the pressure goes down, cavitation will take place. FB: Why are air gaps important?
JW: Air gaps are used to create a progressive feel. If you were to fill a fork full with oil, it would hydraulic lock under compression. Oil can’t be compressed, but air can, which is why an air gap is so essential. As a fork dives down, the air that locates above the oil gets squeezed onto it by the piston – gradually increasing the level of resistance. The key role of an air gap is to control the end of the stroke, when a fork is most compressed. If you’re trying to achieve a more supported feel on a fork’s down stroke, reducing the air gap by adding more oil will help to achieve this. FB: How often should you change a unit’s oil? JW: The frequency of oil changes depends on the bike and its usage. If you take a motocross bike or top level road race machine, the oil would probably get changed two to three times a
year, whereas a standard road bike’s could last decades without ever really needing to be replaced. The reason for this is that the demands are different, along with the shaft speeds, which will run quicker on a competition bike. While oil doesn’t really have a life, the faster pace makes the units’ oil more prone to contamination and drawing air, which would impede performance. On the roads, you possibly would never notice the difference, so the reality is a road bike’s oil might only ever need changing when an oil seal goes. FB: What goes wrong the most? JW: Suspension bushes are the most common worn internal parts; especially on the fork legs. It’s not so usual on shocks, mostly because the shaft’s not long enough to replicate the pressure applied by the long fork stanchions. Even so, they don’t tend to go all that often and it’s quite hard to tell when they have without removing the units and checking for excessive play. The only other tell-tale sign is seeping oil because, once the bush goes, the excessive play can damage the oil seal, causing it to leak.
FB: Why do fork seals blow?
JW: Fork seals go because they’re faced with the elements. Dust seals do a good job of protecting the oil seals, but micro-sized particles can still find a way up past the oil seal. Once there they can damage the important seal or force its lip from the surface of the stanchion; causing oil to seep out. Another big killer of oil seals is damage to the chrome-plate on the stanchions. A stone chip is quite typical and it’ll often leave a lip that’ll tear the seal.
Sometimes you can remove the burr with emery cloth and an oil stone, unless the score is too deep. The rear shock is better protected, so stone chips on the shaft are virtually unheard of. Keeping the shock clean though is more of a challenge. Never spray a shock absorber with any oily spray, because if you do dirt will stick to the oil and it won’t take long before the debris gets up behind the seals and absolutely mullers the unit.