We've known for some time now that repeatable, consistent dyno readings depend on a number of factors, and one anomaly that crops up now and again is tire selection and wear. The scenario can play out in a number of ways, but here are the two we most commonly encounter: One is the sportbike rider who goes to his local shop for a tuneup and new tires, and his bike leaves the shop with less horsepower than it went in with. Another is the racer competing in a horsepower-limited class. He dynos his bike several times over the course of a weekend and is well under the maximum amount allowed, but after the race and a celebratory burnout, his bike is suddenly over the limit.

The common element in both cases is tires, and not that either bike makes more or less horsepower. The sportbike rider who replaces his bald tire with a heavier new bun loses power. The racer who strips off rubber with his burnout gains power. Why is this? To find out, we spent a day at our dyno with inquiring minds and a stack of tires.

Tire Weight (lbs.)
Including Wheel Moment of Inertia
(lb.in)
Bridgestone BT-012 28.50 2061
Metzeler MEZ4 31.92 2545
Bridgestone BT-001 31.80 2594

Horsepower and torque for the three different tires. The heavier tires showed a significant drop in peak horsepower. Note the differences between the MEZ4 and BT-001, which have contrasting weight and MoI numbers.
The theory
We've extensively covered dynamometers and how they work ("Dyno-Might!" Feb. 2001, also online at www.sportrider.com/0406), and the important fact here is that the more common dynos are of the inertia type. Using a drum with a known Moment of Inertia (MoI), torque can be calculated by measuring how fast a bike can accelerate the drum. Knowing torque and rpm, horsepower can then be calculated.

If the weight (or, more specifically, the MoI) difference between two tires is significant relative to the drum, that will change how quickly the drum accelerates--and hence how much horsepower is measured. To find out exactly how great this effect is, we ran our '04 Kawasaki ZX-6R on our SuperFlow CycleDyn (www.superflow.com, 800/471-7701) with three different tires. Figure 1 shows their weight and MoI differences.

To determine if gearing affects horsepower readings, we performed back-to-back runs in fourth and fifth gears. While at lower rpm a slight difference was noted, the graphs are very similar at higher revs.
While our SuperFlow CycleDyn is an eddy-current dynamometer, the entire system (consisting of the drum itself along with the eddy-current apparatus and a large drum-driven fan) has a known inertia value and can be used as an inertia dyno. For our standard dyno testing, we use a "controlled run," which places a small amount of eddy-current load on the bike. Still, the majority of the load is from the inertia of the system, and as we will see, the tire's MoI plays a big role.

We ran our standard test procedure, which involves performing successive dyno runs until maximum peak horsepower is reached--generally, as the engine heats up an optimum temperature will be reached and passed. We repeated these tests for each tire, with the results shown in Figure 2. The heavier tires showed a significant drop in horsepower compared to the lighter tire. While the peak horsepower for the Bridgestone BT-001 was slightly more than the heavier Metzeler MEZ4, in general the graph was somewhat lower, reflecting the higher MoI of the much taller BT-001. The difference between the lighter and heavier tires is definitely enough that tire selection is one dyno-testing variable that must be tightly controlled.

To find out if the change in gearing due to the taller tire plays a part, we repeated our test for the BT-001 using fifth gear instead of fourth (our usual method). The results, graphed in Figure 3, showed slightly more horsepower at lower rpm with the taller gear, but a virtually identical curve at higher rpm.

Another concern with different tires is slippage on the dynamometer's drum during a test. Our CycleDyn includes an apparatus that can check for any wheelspin between the drum and tire. One nice aspect of the SuperFlow dyno is that the drum has a large diameter, which better approximates a real road surface than a smaller drum, and allows for a large contact patch. We check for wheelspin occasionally (see the picture on pg. 114), and even on our turbocharged Hayabusa, which pumped out more than 200 horsepower ("Blow Your Own," Oct. 2002), we've never had a problem.

As a final test to satisfy another curiosity, we ran the same tire at three different pressures to determine if that affected the outcome. Using the BT-012 at 25, 35 and then 45 psi, we found virtually no difference in horsepower across the rev band.

While gearing and tire pressure may not play a big part in dyno readings, it's always best to eliminate as many variables as possible in any form of testing. That means using the same tire model with a consistent amount of wear. Next time you're scratching your head over a dyno run, check your back tire--you may find a couple of free horsepower.