Originally published on Motorcycle News
WHERE do you start when you want to modify your bike? A bit of engine tuning first, maybe upgrade the brakes to suit the improved performance and as it goes along add rear sets and finish off with a spangly new paint job. But would you add lightweight wheels?
For most of us, light wheels are something of a luxury item. They cost a fortune, admittedly look good but surely they can’t make that much of a difference. After all, supersport and superstock racers don’t change them.
That’s only half the story, however. The racers don’t change their wheels because they can’t – the rules demand they use standard items. Rest assured, they’d change them if they could. The old supersport 600 class did allow them that luxury and they all made use of it. One of the main reasons for the new rules is cost – even standard wheels are expensive. The lighter you get, as a rule of thumb, the more you pay, so the danger would be that rich teams would have an easy advantage. Remember that racers do tend to crash more than your average road rider – they are pushing to the edge every time they ride after all – so the wheel bill could potentially be huge.
So why would teams who could afford it willingly meet that bill? Because reducing wheel weight has a host of advantages.
Until the arrival of the Honda FireBlade, road riders tended to pay more attention to outright power figures than the power to weight. Racers have always looked at weight as a prime way of gaining performance. If you cut the overall weight of the bike, you improve the power to weight ratio and gain on acceleration. The lightest standard wheels weigh around 12.25kg for the pair, on bikes like a Yamaha R1. Something like Dymag full carbon fiber wheels could bring that down to around 7.5kg for the pair, ready for tires to go on. That in itself is a valuable saving in racing, a world where even fuel is carefully measured to give just enough to finish the race, and riders diet to make sure they don’t carry surplus baggage.
For riders in the real world, there are other advantages to reducing wheel weight. Since wheels are attached to the bike on the end of the suspension, they form a major part of what is known as unsprung weight. Essentially, everything that sits on top of the suspension, including you, is sprung weight. Everything else – swingarm, forks lower, brakes and wheels – is unsprung. That weight still has to be controlled by the suspension, however, and makes it harder for it to do what it’s meant to be doing. If you imagine hitting a bump in the road, the wheel gets thrown up in the air and will try to keep going in its new direction. The suspension has to catch it, slow it down and keep the wheel on the ground while also preventing the bike from bucking around. Keeping the unsprung weight down reduces the inertia and gives the suspension an easier time, giving you an easier ride. The benefit of keeping both wheels firmly on the floor go further than just making for a more comfortable ride. Controlling the front wheel over bumps means you can brake more effectively and safely if the road surface is poor, as well as steer accurately. Keeping the back wheel planted means you can get good drive out of corners.
Wheels also rotate. OK, that’s pretty obvious. What isn’t so obvious is how bad rotating weight is. Cast your mind back to your school days. Did you ever do the experiment in physics where you held a spinning bicycle wheel and tried to turn it? It’s almost impossible to turn it without your arms being pulled all over the place. It’s the old equal and opposite reactions thing going on. When the wheels on your bike are spinning, the same thing happens. The heavier the wheels and the faster the spin, the worse the effect is. Slash the rotating mass and your bike will turn in quicker. In addition, getting the wheels to turn from a standstill, and slowing them again, is easier when they weigh less. That means better acceleration and braking at all speeds.
The final benefit only applies to the front end. The front wheel, as well as being unsprung rotating mass, is also steered mass. You have to shift that about to make the bike turn. At low speed even the heaviest front end won’t feel too bad – tire pressure would have a bigger effect then. But as the speed rises, again the rotational effect comes into play and makes it harder for you to steer. It’s one of those things you don’t notice much until you get the comparison, according to people who’ve made the change. Although you don’t turn the bars far to make the bike turn, the effort you put through them at speed is considerable. A lighter front wheel makes that easier on you. Both for road riders and racers, the benefit is in endurance – yours, not the bike’s. If you’re working hard to make the bike turn you’ll get tired. If it’s easier, you get off your bike feeling fresher than the rest of the competition.
One man who knows a lot about lightweight wheels is MCN columnist and technical boss at Kawsaki’s British Superbike team, Ray Stringer. As well as being in charge of putting the ZX-7RRs together, he is a former British Superbike privateer champion so he knows about wheels from both sides of the job. In his opinion, however, you have to be careful going for lightweight wheels – it’s not just a matter of bolting them on and going. He said: ” When the 600s used to be allowed lightweight wheels you’d see a lot of bikes suffering front end chatter. There may be suspension specialists who’d disagree with me, but in my opinion, you need more controlled compression damping to make them really work. If you’re going to use them on the road you really need to avoid potholes and anything that’ll make the suspension bottom out sharply as they aren’t as durable as the standard wheels. If you ride hard, though, you’ll notice the difference, mainly it’s the flickability at high speed. If you ride on tracks a lot you notice it most. ”