Why it’s worth shelling out on a shock

IF you’re going to make any improvements to your bike, chances are the first things on your list are an aftermarket exhaust, Dynojet kit and K&N filter. These will sharpen the throttle response, fill out the mid-range and put right the compromises that manufacturers are forced to make to pass emissions laws.

You’ll be thinking about uprating the chassis soon after, and a top priority for many riders is a new rear shock – to improve feedback, get better straight-line stability, enhance grip, make the ride quality better, tighten up the handling… But it begs the question, what’s so different about a good quality aftermarket shock that means it does these things? After all, there are no emissions laws or other regulations restricting manufacturers with rear suspension.

There is one compromise, though, and that’s money. A decent aftermarket shock will set you back maybe £500, possibly a good deal more, and if that was added to the retail price of a bike it would simply become too expensive.

There are two main reasons an aftermarket shock (and we’re only talking about the more expensive, higher quality names like Ohlins, Penske and top-notch Showa kit here) is better than the one the bike came with.

First is the design of the damper. In a stock shock, the damping action is due to oil being squeezed through a range of tiny holes. Which is fine until the shock is worked hard, such as on high-speed, bumpy roads. Then the oil heats up and becomes thinner – so it takes less effort to force it through the holes. This is felt as fade, something you experience as the back of the bike bouncing up and down in an increasingly uncontrolled manner. Nasty.

It’s unusual for modern bikes to lose their damping completely, but most do exhibit noticeable fade after a hard run. Typical aftermarket shocks operate on a different principle. Instead of being forced through tiny holes, the oil is pushed through a stack of shims which are held down by a spring. This means the oil will only pass through when it reaches a sufficient pressure, determined by the strength of the spring. That, in turn, means the damping action is far less dependent on the viscosity or thickness of the oil, relying instead on its pressure. So the oil can become thinner without a significant effect on the damping quality of the shock.

On top of that, the oil itself in many aftermarket shocks has a viscosity modifier added, a chemical which reduces the thinning as the oil heats up.

The other major advantage of an aftermarket shock is that there’s more scope for making adjustments. If you’re an average sized rider, who rides in a typical manner on normal roads, your suspension should be fine on its standard settings. But lots of riders fall outside the norm, and then they find there’s a very limited range of adjustment on most original shocks.

For example, the compression damping adjuster – designed to control the rate at which the shock is squeezed as you ride over bumps – frequently has very little effect. And that’s if you have an adjuster to twiddle – some units allow no fine-tuning at all.

The rebound adjustment –there to regulate the speed at which the shock bounces back – often has a more noticeable effect but it’s still over a restricted range, whereas good aftermarket shocks are much more likely to be adjustable to your specific needs. Whether you load up your bike heavily for touring or spend every other weekend at a track day, this extra flexibility could make a big difference. It’s a case of you get what you pay for.

There are other advantages which aftermarket shocks can have over stock ones. Because they’re usually hand-made, the gas which is forced into the shock (to stop the oil foaming when it’s hot) can be pumped in at a higher and more consistent pressure – improving high-speed performance.

You’ll also find an aftermarket shock frequently allows more rear-wheel travel.

This makes a considerable improvement on the Honda VFR800 and Aprilia RSV Mille, whose shocks both hit the rubber bump stops after only about three inches of travel – which makes the back end kick up over a hard bump. An aftermarket one will allow the full five inches or so which the bikes’ designers intended, but which was lost by the end of the production line – probably to cut costs by sharing parts with other machines.

Some bikes have shocks which are just poorly designed. The ones on early Kawasaki ZXR750s, for example, had no piston or airbag separating the oil from the pressurised gas in the space above it, which meant the oil was very prone to foaming and completely losing its damping when worked hard.

But an aftermarket shock will also feel better simply because the original will have worn and become less effective (as many can do after as little as 5000 miles). The Yamaha YZF750SP shock was prone to its damping piston wearing through the inner lining of the shock after 10,000 miles, while many shocks suffer from corrosion on the central rod, which damages the seals and results in leaks.

Replacing your old shock with a new but otherwise identical one will immediately feel better but, chances are, fitting a high quality aftermarket shock will make a bigger and more lasting difference.

n Thanks to Maxton Engineering. Contact: 01928-740531.