Brakes convert the kinetic energy of motion into heat, slowing your bike and heating the discs, calipers and, eventually, the air. Can that energy, instead of being wasted as heat, make you go faster? Can you store some of that energy and use it later to accelerate?
That's the purpose of the Kinetic Energy Recovery System, or KERS. This is big news in the Formula 1 car-racing world this year, but a KERS-equipped 125cc MotoGP bike beat F1 to the punch last year. That KTM, ridden in the final race by Tommy Koyama, had a motor/generator unit mounted to the gearbox and connected to the countershaft by a belt. Under braking, the unit became a generator that rapidly charged an exotic battery/capacitor. Then, under acceleration, the unit became a motor, helping to power the countershaft with 2 to 3 horsepower for up to 10 seconds.
That horsepower increase doesn't sound like a lot, but on a 125 that amounts to about a 5 percent gain. What's the weight penalty? It turns out that the KTM was under the minimum weight limit and had to carry ballast anyway, so the 8- to 10-pound weight of the system just replaced the dead weight of ballast.
We know more about KERS in publicity-conscious F1, where the organizers have made the new technology the poster-child of a greener and more eco-friendly race series. There, KERS is optional in '09 and will be mandatory in '10. F1 KERS is limited to accumulating and releasing 80 horsepower for about 6 seconds per lap. Energy is stored either in a high-performance battery/capacitor or in the form of kinetic energy in a small super-high-rpm flywheel. The driver has a button on his steering wheel that allows him to use the extra power when he chooses--typically to pass another car. Video graphics tell TV viewers when KERS is being used.
Like on the MotoGP bike, F1 KERS doesn't add to the weight of the cars, as the approximately 50- to 70-lb. system likewise replaces ballast. Estimates are that the cars are about .2 of a second per lap faster with KERS, but the benefit is deceptive--if there were no minimum weight requirement, heavier cars carrying KERS would almost certainly be slower than their non-KERS competitors.
Deception aside, KERS has also presented some problems. The extremely fast-charging and discharging batteries can overheat and cause fires or explosions, and flywheel systems can have spectacular failures as well. Crewmen have gotten severe shocks from touching the wrong parts. The "batteries"--which we would hardly recognize as such--have a super-short life of only about 2 hours. The biggest problem at this point, however, is expense: Organizers initiated the KERS plan in happier financial times. Now that the push is on to reduce the cost of racing, KERS is an expensive extra that they probably regret taking on.
MotoGP organizers have no interest in making racing more expensive, and in fact are trying to cut expenses any way they can. They reacted quickly to the news of KTM's technical innovation: The rules now say that bikes can only be powered by an internal-combustion engine, which effectively nixes KERS.
What about KERS on future streetbikes? As on the KTM, a motor/generator unit could be mounted on the engine, driven by the rear wheel, but another one could be mounted in the hub of the front wheel. As for batteries, you wouldn't want to replace yours every 2 hours, but if the charging rate wasn't as fast as in racing, more conventional batteries would work. Flywheel or hydraulic energy storage would be possible alternatives. The real answers about future KERS development will come from serious cost-benefit analysis. How much energy can be stored? How much power will that energy produce? What will be the savings in fuel? What about the performance benefits? And, perhaps most importantly, how much is it going to cost--in weight and dollars?
The relative benefits of KERS in racing have been masked by minimum weights, but on streetbikes the benefits and costs are going to stare you square in the face when you look at the sticker and the performance specs. KERS is still a squalling infant of a technology. It's too early to tell what we're going to see when it comes of age.