BMW's development wheels tell all about their prototypes

With new models set to be unveiled next week, BMW gave us an exclusive look at their testing process and special sensors that festoon their bikes to get every detail from vibration figures, to axle loadings, to measuring riding style.

At the heart of all of this is a sensor called a strain gauge. A strain gauge works by converting changes in state of an object into a change of electrical resistance that can then be measured. When forces are applied to any object, stress and strain are the result. Stress is defined as the object’s internal resisting forces, and strain as the displacement and deformation that occurs.

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BMW use hundreds of strain gauges all over their test machines and they play a vital part in validating components while durability testing is being conducted. They even go so far as to manufacture their own test-spec wheels, packed with 80 separate gauges.

“The wheel measures all forces, longitudinal forces, lateral forces, braking moment, steering moment, roll moment, everything that is in real physics,” says Michael Heimrath, General Manager of Pre-Development.

“The strain gauges measure the deflection of wheel under load and from that we can work out so much.” Machined from solid aluminium and bolted together in two pieces, BMW say that the wheels cost €200,000 per pair to produce, but that they’ve already sold the patent to Harley-Davidson to use.

“A test wheel is 50% heavier. It does change the dynamics at little – it is like doing a 100-metre sprint in just normal shoes rather than sports shoes. But it is still good enough to measure the data. The bike might have a load on each wheel of 130kg, but the load you have in real life is about ten times that, so 5kg on the wheel doesn’t make any real any difference.

“The forces acting on the motorcycle are so impressive that in normal riding that weight of 130kg will reach 1500kg when you do a jump or hit a bump. The dynamic range is so large and high.

“Getting the figures is really important for the bike – as we need to know the working range of the loads all over the bike to enable us to calibrate laboratory test rigs. It is a must so that you can know how strong to make things. The truth is you can damage any part if you really want to, but we need to make things strong enough to enable the bike to be strong, but without being too heavy.

“Harley-Davidson have already paid us to use the wheels – they paid for the patent, but a set of wheels is €200,000. When you have three test bikes, all set up that’s almost €1,000,000, but we need to be at the forefront of measuring,” says Heimrath.

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