The real deal on wheels

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LIGHTWEIGHT wheels are standard equipment on race bikes. But how much difference could they make to our own machines?

They are certainly more than an expensive pose. They can reduce the unsprung weight of a bike by as much as seven kilos (15lbs). This allows you to corner faster and also means the suspension doesn’t have to work so hard, in turn reducing the workload on the tyres. The downside, however, is reduced straight-line stability at high speed.

Firms such as Marvic, Dymag and Marchesini offer stylish, light aftermarket wheels. They look fantastic. They save weight. So why do we still have nagging doubts?

The problem is, most of us don’t spend all our time on smooth race tracks. We ride on butt-battering

B-roads and have come to believe these race-targeted wheels aren’t up to the task. Race tracks don’t have pot-holes or loose chippings, and racers don’t have to cope with ham-fisted mechanics gouging chunks out of their wheels whenever they change a tyre.

But the truth is that aftermarket wheels do have to go through a battery of tests to ensure they are tough enough for real-world conditions. They can cope with the TT, for example, and the punishment they encounter on the Island’s road surfaces is among the fiercest on earth.

A range of materials is used in lightweight wheels. The victorious V&M team which fielded David Jefferies on a Yamaha R1 and R6 at this year’s TT used magnesium wheels made by Wiltshire firm Dymag, but they plan to try even lighter carbon-fibre versions next year. Carbon-fibre isn’t that much lighter than magnesium, but because it’s stronger, less of it needs to be used, so the wheel ends up lighter.

All Dymag wheels – whether for road or race use – are tested to British standard AU50. Similar standards are in force throughout the world, and all manufacturers of wheels for use in road-going bikes, from the smallest workshop to Japanese giants like Honda, have to satisfy them.

British Standard AU50 involves a range of different tests. One is the radial fatigue test. This involves pressing the wheel, fitted with a tyre, against a rotating drum, which acts as an artificial road. The pressure holding the wheel against the drum is equivalent to two-and-a-quarter times the weight of a bike. If there is no evidence of cracks in the spokes after the drum has turned half a million times, the wheel has passed the test.

The cornering fatigue test involves mounting a wheel vertically on a lathe and then attaching a load arm to the hub. This pulls the wheel in one direction, mimicking the forces which act upon it when the bike corners. The wheel is then spun for half a million revolutions and the spokes are checked for any twisting relative to the rim before the wheel is passed.

For the impact test, the wheel is mounted vertically and a heavy

V-shaped object is dropped on to the tyre. This simulates the impact the wheel would undergo if you hit a

pot-hole or came down hard after a wheelie. Any distortion of the rim must still enable the tyre to retain 70 per cent of its pressure 30 seconds after the strike. In real-world conditions, this should enable you to retain enough control over the bike.

If a wheel passes these three tests, it is safe and legal for your road bike. But firms like Dymag often go further than the required British Standards tests and put their wheels through heat, cold and corrosion tests, too.

For the temperature tests, the wheels are subjected to the radial fatigue test but in temperatures of minus 10 degrees and at 70 degrees Centigrade. As an extra check for cracks and corrosion, the wheel is put through all the tests and then painted with a special dye which shows up any tiny flaws under

ultra-violet light.

Any new wheel under development is tested until it is destroyed, so Dymag knows exactly what its limits are – and they far exceed anything you’re likely to put it through on a Sunday run.

So next time you have any doubts about the strength of lightweight wheels on your road bike, rest assured. If aftermarket wheels can get David Jefferies round the Isle of Man they’ll get you round your favourite roundabout.

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MCN Staff

By MCN Staff