It’s not how much weight you lose, it’s where you lose it

Published: 01 May 2000

AT the launch of the TT600, one of the features Triumph’s engineers were keen to highlight was the bike’s exceptionally light wheels.

They are the lightest in the 600 supersport class. Triumph even went to the considerable lengths of working with the supplier of the TT600’s front forks, Japanese company Kayaba, to design a new set of damping internals made from aluminium instead of the usual steel – saving 500g in the process. Half a kilogram may not sound a lot, but when it’s lost from certain areas it can make a big difference.

Shaving mass from parts of the chassis which are classified as " unsprung " has a very significant effect on the handling of a bike.

We’re talking about those components of the chassis which aren’t separated from the road surface via the suspension springs. That includes both wheels – which are clearly resting directly on the ground. It also includes components attached to them, such as mudguards and brakes.

At the front of the bike, the bottom halves of the fork legs (with their internal damper units) are attached to the wheel and move with it and so are unsprung.

At the back, the swingarm and chain (or drive shaft) are attached to the wheel. But only half of these count as unsprung weight because their other ends are on the " sprung " part of the bike.

When the motorcycle is rolling, it’s the unsprung components which follow the road surface exactly. When the front wheel hits a bump it has to follow its contour, rising up and over it.

The difficulty in trying to get a wheel to move upwards over a bump and then down again very rapidly is that you have to overcome the wheel’s inertia. Sir Isaac Newton had something to say about this in his first law of motion: An object which is moving will keep moving in a straight line unless an external force causes it to change speed or direction. For the motorcycle’s front wheel, this means that if it starts moving upwards because a bump in the road has just kicked it in that direction, it wants to keep moving in an upwards direction for ever. So when it gets to the peak of the bump it tries to keep going upwards. Eventually, gravity would pull it down again, but far too slowly to be of any use. Instead, the wheel is forced back again by the suspension system, with springs constantly pushing the wheel downwards.

This is where unsprung weight comes in. The heavier the wheel, the more inertia it has, which means it resists harder when being pushed back down on to the road. Heavy wheels are more likely to bounce off the top of bumps in the road than lighter ones, which can respond more rapidly to the force of the suspension.

Lighter wheels are able to follow the surface of a road more closely, which means the tyres stay in touch with the road surface more and the bike has better grip.

This is also the main reason Triumph sought to minimise the weight of the TT600’s fork internals. Reducing the total weight of all those parts at the front end which are not supported by springs allows the front end to respond more rapidly to changes in the road surface.

And there are other important benefits to reducing unsprung weight. A rotating wheel generates considerable gyroscopic forces which, like Newton’s moving object, resist changes in direction. With less weight being spun round at any given speed, the gyro forces are lower and the bike requires less force from the rider to change direction rapidly, improving its cornering.

Lighter wheels with lower inertia also have the added advantage of requiring less force to get them spinning from a stationary start or to stop them spinning when they are in motion. In other words, they will allow more rapid acceleration and braking under a given engine or braking force.

This whole package of performance benefits of reducing unsprung weight is why magnesium wheels are so common on race bikes, even when all the other major components might be aluminium or steel. For a given strength, magnesium is lighter than aluminium, and although it is more costly and difficult to work with and less durable, the benefits of lighter wheels are worth all the expense.

If you were to lose a couple of kilograms from the sprung regions you would hardly notice it. Lose the same amount from your wheels and the handling would be transformed.