Hoop It Up: Sportbike Wheel Comparison

Moment of Inertia
How we measured it and why it's important
We're all familiar with Newton's second law, in practice if not in theory, and for objects moving in a straight line, the concept is fairly easy to grasp: If the object is lighter, it will accelerate easier. Once the object is spinning around an axis, however, things get a bit tricky.

We've discussed the concept in an earlier Art and Science (April 2001, online at www.sportrider.com/0402), but here's a quick recap: For spinning objects, the characteristic that relates force and acceleration is Moment of Inertia (MoI). Expressed in pound-inches squared (lb. in2), an object's MoI depends not only on its mass, but also on how far away that mass is from the axis it spins about-in our case, the wheel's axle. As an example of how the distribution of mass is just as crucial as the mass itself, during our test of Buell's XB9R Firebolt ("Occam's Razor," Oct. '02), we found that, while the Buell's front-wheel assembly was 4.5 pounds lighter than that of a Yamaha YZF-R6, their MoIs were identical-the huge weight savings is offset by the Buell's larger single disc and heavier rim area.

Moment of Inertia is important for two reasons. First, a wheel with a lower MoI requires less force to spin up at a certain rotational acceleration, and this can even be seen on an inertia-type dyno, which will read more horsepower for a given bike with a rear wheel that has a lower MoI. Second, MoI determines a wheel's gyroscopic characteristics. A lower MoI reduces the gyroscopic effects, making it easier to turn the spinning wheel from side to side-for the front wheel, this makes turning the bars easier, and for both wheels this makes the bike lean quicker.

John Bradley's The Racing Motorcycle: A Technical Guide for Constructors (Euro Spares, 415/665-3363, www.eurospaces.com) details a method for measuring a wheel's MoI, which we employed for our test. Each wheel was mounted to an axle and rolled down an incline of a known slope. The amount of time it takes for the wheel to roll a set distance from a stop is used (along with the wheel's weight, the slope angle and the axle diameter) to calculate the wheel's MoI. We did five tests, discarding the highest and lowest results and averaging the remaining three values for each wheel. The numbers listed include the rear wheel's cush drive and hardware (but not the sprocket) and a small axle used to conduct the test that has negligible influence.

The Ride
A few pounds make a huge difference
As part of our test, we took our GSX-R1000 to a Hypercycle (818/988-8860, www.hypercycle.com) track day at the Streets of Willow and mounted up the Blackstone Tek hoops halfway through the day. We immediately felt a big change in the effort required to turn the GSX-R-it flipped from side to side with less effort, and feedback from the front end was much better on turn-in. The change can best be described as being similar to hopping off a GSX-R1000 and onto a GSX-R750-it's that big a difference. One change definitely not in the GSX-R750 direction was the 1000's acceleration with the wheels installed-the big Suzuki gained speed more easily, and felt like it picked up a few ponies with the swap.

The lighter unsprung weight of the Blackstone Tek hoops also made a difference in the bike's suspension. The GSX-R tracked better over the Street's rough patches, and felt more compliant in general. With more time on the wheels, we'd want to make some suspension adjustments to account for the decreased unsprung weight.

On the downside, the normally solid Suzuki was noticeably less stable in fast transitions, and we encountered some brake problems with the wheels mounted. A slight pulsation under heavy braking from fast speeds with the stock wheels turned into a major chatter, and it felt like there was less feedback under braking.

Lap times during the test were inconclusive-a stiff wind came up once we had the Blackstone Tek wheels installed, and our man went a hair slower than with the stock rims.

Hoop It Up Extras
For additional images and information for the February 2004 wheel test, simply click here.