Performance Wheel Test - A Look at Wheel Weight, Density, and Stiffness

Units of rotational inertia are impossible for most to understand and even harder to relate to, so we opted for a real-world test where each wheel and tire was mounted on the same car and spun up to 30 mph on the dyno. From there, we measured the amount of time that it took for each wheel to slow down to 5 mph (since data at lower speeds becomes more and more unreliable). The idea is that the wheel with less weight and rotational inertia will come to a stop faster than a wheel with more rotation inertial. To be consistent, each tire was inflated to the same cold pressures and given the same amount of rolls on the dyno under identical conditions.

Physical properties aside, the biggest question was whether these vastly different wheels would result in noticeably different lap times on our Figure 8 test track. The Figure 8 is basically two 200-foot-diameter circles spaced 500 feet apart from center to center. This configuration allows us to test how a car accelerates, brakes and performs in average lateral grip and transitional handling.

With apples-to-apples real-world testing in mind, we took out a mildly tuned Mitsubishi EVO 9 on JIC coilovers as our test car. Each wheel was pressurized to the same cold tire pressure and the car was topped off with fuel before each run so that weight did not change. Each brand-new set of wheels and tires were sent out for two scrub-down laps before starting its 10 continuous recorded laps. Through our vehicle telemetry data and driver input, we would be able to see if one set of wheels allowed the car to consistently accelerate or brake faster and have more stable steering transitions in addition to overall lap times.

The one test we didn’t get to pursue—and possibly the most important—concerns wheel strength. We had originally designed a very precise test on a hydraulic press to squish every wheel and measure how much each wheel would bend for a known amount of force applied, but that would have pegged the nerd meter. So we opted to get a rental car and have our test driver Andy Hope crash into some curbs for our testing/entertainment. Apparently, our legal department had some issues with that, so we unfortunately came up empty on real strength testing.

LAP TIMES

As expected, our real-world rotational inertia test did show a difference between lighter and heavier wheels. Wheels with less rotational inertia (i.e. easier to accelerate and decelerate) took less time to decelerate from 30 mph to 5 mph on the dyno. But more importantly, the time each wheel it took depended more on the tire temperature and how it changed rolling resistance. As the tires heated up after just one cold run, the change in rolling resistance was more than the difference between all the wheels.

As expected, the fully forged monoblock Volk RE30 was the lightest and densest among the wheels. At 17.4 lbs, it’s 3.5 lbs lighter than the heaviest wheel and more than 25 percent more dense than the least dense wheel while taking up the least volume. The two-piece Semi-Solid Forged SSR comes in just 1-lb heavier, even though its density is similar to that of the other conventional cast wheels. In terms of physical properties, each wheel reflects its price point almost proportionally.

This makes sense because most of the mass is in the tire. At 25.4 lbs, each tire weighs more than the wheel itself, and because it’s at the outer diameter of the rolling stock, its effect on rotational inertia will have a much larger effect than minor differences in rotational inertia within each wheel.

By Jay Chen

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