An opportunity to get your brain into gears

Published: 01 May 2000

HOWLING along your favourite stretch of road, you snick down two gears as you brake into a corner, turn on a well-planned line, open the throttle, change up and surge away.

Your memories of the road will all be about how well you judged the corners, used forward thinking to avoid hazards and how you enjoyed the engine’s power delivery.

What the gearbox has just gone through won’t even have crossed your mind. Yet without a slick-changing gearbox and carefully selected ratios, your riding would be a far less dynamic and satisfying experience.

In some ways the gearbox is like the quiet bloke in the corner of a busy office. He’s easily forgotten because he sits there quietly and efficiently doing his work, he rarely needs any attention, he seldom fails and he does his job without drama.

The fact that all that is the case with a gearbox is tribute to the teams that design them. Even when they are being asked to deal with 150bhp surging through a few small pinions – as they increasingly are in the latest superbikes – all is unruffled calm on the exterior.

Surely gearbox design has had to be given a radical overhaul in the last few decades, just to keep up with the demands of spiralling power and torque delivered by each new generation of bikes?

The fact is, the basic design has remained unchanged for about 70 years. Extra gears to suit engines with today’s narrower powerbands have been added – and that’s really the biggest change.

In theory, if we used engines with powerbands as wide as Vanessa Feltz’s butt – with slogging torque from tickover to maximum revs – we could make do with a single gear. Veteran machines from about a century ago did without. Even the high-speed Vincent Black Shadow only had four gears 50 years ago.

Today’s superbikes can accelerate from a standstill to more than the legal limit in first gear. But we want more. So the gears are there to take us to a maximum speed step by step.

Because of the low gearing in the first few gears, the amount of torque reaching the road is very high. Towards the end of the torque curve, it is time for the engine to step up to another gear.

The designer must make sure that when the rider changes up, the gearing puts the engine back into the meat of the torque curve to maintain acceleration. For example, in a high-performance bike it is possible to change from first to second gear as high as 75mph – making use of the huge torque in low gears. The change would be set at the point at which torque is just about to start falling.

Changing up means the engine speed falls back on to peak torque, so it’s ready to haul away hard again in second. The process continues, making the most of the torque available in each gear before reaching the optimum point to change up and back into the meat of thepeak torque.

If you tune a bike to raise revs and extend the torque curve, you’ll get a higher top speed. But tuning can also result in an extended gap between gears, especially if your bike has a five-speed box. In cases like this, a specialist tuner such as PDQ (01628-667644) can offer a conversion to a six-gear box. One example is to give a five-speed Yamaha FZR1000 the six-speed box from an FZR750.

Production racers tuned for more power need the overall ratios to be raised to turn the extra power into extra speed. This is done by altering the final drive sprockets of the gearbox and rear wheel.

Sometimes the first gear ratio is raised by using new gearbox pinions and bringing the ratio closer to second gear for more seamless acceleration.

But changing gearbox internals is only for very serious performance freaks. Usually it’s enough just to tinker with the final drive ratio.

Inside, the gearbox is crammed with cogs and is called " constant mesh " because all the gears are always spinning, awaiting selection (see graphic, bottom of page 32).

This type of box is also called an all-indirect box, because the power is fed from the clutch on to the input shaft, and the drive always goes out via the output shaft on the opposite side. Earlier gearboxes had a direct drive top gear, with input and output on the same shaft, and all other gears working indirectly.

You can trace how the power is routed through the gears in the cutaway engine, below. Gears are changed by selectors which slide the pinions from side-to-side, and these engage and disengage with adjacent pinions by means of " dogs " , which are like square ended fingers.

The key to a good gearchange is having a positive engagement of the dogs. If the edges of the dogs become rounded they may fail to snick into place, resulting in a slip back to the previous gear – or a false neutral.

One way to avoid that is to use dogs which have been " undercut. " These are common in close-ratio gear clusters. In these the dogs are angled at one or two degrees to stop them slipping out.

Keep your gearbox sweet using a quality engine oil and changing the filter regularly and you can play tunes with your engine all day, safe in the knowledge that the box will always work well.