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Suspension Service

Tech Note 002

In the second Tech Notes article, given the topic of the first article, we feel that suspension service is worth touching on! Most of us have our bikes serviced regularly, oil and filter, brake fluid and coolant change. Right, lets spend a moment giving some thought to the suspension!

Why service your suspension? Probably the best way to start is to consider what your bike would be like to ride if it had no dampers. If your bike had no dampers, the suspension action would be close to that of a trampoline. Largely undamped movement of the suspension at the resonate frequency of the springs with regard to the sprung mass, with the only damping being from internal friction from the forks at the front and the suspension linkages and the dampers internal friction in the rear, the bike would be challenging to ride at best, we don't recommend trying it! So from this we can deduce that dampers do exactly that, they damp unwanted or undesirable movement, they do this by ultimately turning unwanted movement into heat. Remember that we can't destroy energy we can only transform it, and in this case we transform that unwanted energy into heat. All that heating and cooling takes a toll on the integrity of the suspension fluid over time!

When we think a little more about how suspension works, resistance or damping is ultimately developed when the fluid is squeezed through orifices that offer more and more resistance as flow increases. In some simpler forks this fixed orifice damping is still in use on a surprising number of machines, where the marketing and accounting departments have taken the ball point scalpel to the design, to make it fit a particular price point. On other, more complex designs, the most common design is the orifice controlled dynamically by a collection of stacked spring disks we call shims, thus offering a variable sized orifice dependent on load. Almost all adjustable front forks and almost all rear shocks have such an arrangement. Now we have to consider the mechanical degradation of the suspension fluid; we have to think about the shims closing against the face of the piston, trapping some of the fluid as they close, sometimes fracturing the long molecule chains that give suspension fluid its particular viscosity, and in the case of fixed orifice damping of being forced through small orifices often under considerable pressure and extreme velocities, again the molecular structure of the fluid comes under stress and breaks down over time.

When we think about how much work the suspension is doing, there are all kinds of figures thrown about for the number of compression cycles (which are almost always followed by a rebound cycle) per kilometer, a popular figure is 3,000 compression cycles for a road bike per kilometer seems reasonable. Have a think for a moment about how far your bike has travelled and multiply that by the number of cycles per kilometer (or if in miles multiply that by 1.6 to get you in the ball park) and it's very likely your suspension has quietly done a lot of work!

Then let's think about other factors as well. When servicing front forks, especially USD forks on a road bike, we often find a black sludge in side of the fork and a heavily contaminated suspension fluid, that in extreme cases is often quite "gritty". Any ideas as to what this mystery "addition" is? Brake Dust is the answer you are looking for and some road contaminants and likely a few bug parts for good measure! How does it get in there? Simple, the brake dust as it's ejected from the brakes under heavier brake applications, which can and does stick to clean dry fork legs (you may have noticed the build up of black / brown dust on the lower part of the fork tubes where the seals never sweep). The dust seals will knock off lightly attached contaminants, but the more tenaciously attached particles go right past the dust seals and past the main seals, doing neither of them any favours in the process and are eventually ground off by the lower fork bushings! Now, you dirt bike guys thinking that doesn't apply to me are largely right, as far as brake dust goes, but mud left to dry on the fork tubes suffers a similar fate and its not uncommon to pull a set of MX forks apart and find the fluid heavily contaminated with mud dissolved in to the fluid! Many road bikes suffer premature failure of the seals because the back sides of USD fork tubes, that is the side against the disk rotor, is not kept clean and the abrasive buildup of brake residue simply kills the forks seals! Road or Trail, the life of your seals and and the action of the forks can be greatly improved by keeping the fork tubes clean, and when clean and dry using a clean paper towel lightly dampened with WD40 or similar just to leave the slightest film behind on the inner fork tube, this will lubricate the rubber parts, resist the build up of contaminants and aid the dust seals in removing the troublesome build up from the fork legs. Not to mention also making the forks easier to keep clean in future. Also maintaining a reservoir of fresh seal grease like K-Tech FF Grease between the seal and dust seal will further improve the action of the your forks and extend the life of your seals. Road or Trail, contaminated abrasive suspension fluid is not the friend of suspension components at all, greatly accelerating wear to sliding parts, degrading suspension action and let's not forget that your suspension fluid is also required to be a high quality lubricant as well as being thermally stable and resistant to mechanical degradation! While the rear shock is much better protected, and it has seals that are highly loaded by the pressurized nature of the shock, the same thing goes on at a slower rate, contributing to the eventual failure of the shock seal.

Most OEM rear shocks have a dry bushing between the dust seal and the main seal of the shock, it is self lubricating and assisted greatly by a reservoir of grease between the bushing and dust seal. Its life is not infinite and quietly wears over the service life of the shock and eventually contributes to the eventual failure of the main seal. Keeping the shock shaft clean and moist does the same favours for your rear shock as it does for the front forks! Also because of the higher loadings and forces on a rear shock the thermal loading on the shock is much heavier than on the forks and the mechanical forces acting on the shock fluid are much higher than those acting on the forks, meaning that the fluid in the rear shock often tends to be of a higher quality than is often found in the front forks and let's not forget, there is only circa 200ml of fluid in the rear shock doing a lot of work indeed! Not trying to scare monger, but we all need to be mindful of the slow and relentless degradation of the mechanical parts in our shocks and forks and especially the fluid that is tortured both mechanically and thermally to provide damping.

Now comes the big question, how often should I have my suspension serviced? Giving a good answer is not so straight forward, many factors ultimately dictate the rate at which the fluids and other parts in your suspension degrades. The surface you are riding over, the kind of tyres you are using, the velocity of the bike, how much load its under and the design of the rear suspension and forks all have an impact on suspension component life! Many manufacturers of premium suspension components state 20,000Km of street use or a specific number of hours for competition machines that are worked hard and need to be right at their best!

What do we think? On some road machines with small rear shocks that are located close to exhaust pipes, the performance of the rear shock is often getting quite ordinary in as little as 10,000Km. Other machines go as much as 35,000Km before serious degradation of action. As a rule of thumb, if the exhaust is close to the rear shock with not much air flow, no more than 15,000Km; the additional thermal stress shortens the life of the fluid, if it's more conventionally placed, no more than 25,000Km.

The degradation of the suspension fluid is slow and insidious. Because you live with this slow but relentless performance decline, it's hard to know what you may have lost to wear and tear of the fluid and other components and of course while the suspension is in for service it's a great time to consider having the suspension made to fit you and your usage much better than the one-size-fits-all settings, the bike was sold with!!

Kerry Dukie

Dukic Performance

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