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02-06-2008, 12:12 AM
Art & Science: Fooling the Dyno
How tires - and other variables - affect dyno readings
writer: Andrew Trevitt
http://images.sportrider.com/features/122_04_03_s+fooling_the_dyno+electric_eye.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_01.html)
We've known for some time now that repeatable, consistent dyno readings depend on a numberof factors, and one anomaly that crops up now and again is tire selection and wear. Thescenario 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 hisbike leaves the shop with less horsepower than it went in with. Another is the racercompeting in a horsepower-limited class. He dynos his bike several times over the course ofa weekend and is well under the maximum amount allowed, but after the race and a celebratoryburnout, his bike is suddenly over the limit.
The common element in both cases is tires, and not that either bike makes more or lesshorsepower. The sportbike rider who replaces his bald tire with a heavier new bun losespower. The racer who strips off rubber with his burnout gains power. Why is this? To findout, we spent a day at our dyno with inquiring minds and a stack of tires.
The Theory
We've extensively covered dynamometers and how they work ("Dyno-Might!" Feb. 2001, alsoonline at www.sportrider.com/0406 (http://www.sportrider.com/0406)), and the important fact here is that the more commondynos are of the inertia type. Using a drum with a known Moment of Inertia (MoI), torque canbe calculated by measuring how fast a bike can accelerate the drum. Knowing torque and rpm,horsepower can then be calculated.
http://images.sportrider.com/features/122_04_01_s+fooling_the_dyno+horsepower_dyno.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_02.html)
If the weight (or, more specifically, the MoI) difference between two tires is significantrelative to the drum, that will change how quickly the drum accelerates--and hence how muchhorsepower is measured. To find out exactly how great this effect is, we ran our '04Kawasaki ZX-6R on our SuperFlow CycleDyn (www.superflow.com (http://www.superflow.com), 800/471-7701) with threedifferent tires. Figure 1 shows their weight and MoI differences.
While our SuperFlow CycleDyn is an eddy-current dynamometer, the entire system (consistingof the drum itself along with the eddy-current apparatus and a large drum-driven fan) has aknown inertia value and can be used as an inertia dyno. For our standard dyno testing, weuse 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'sMoI plays a big role.
We ran our standard test procedure, which involves performing successive dyno runs untilmaximum peak horsepower is reached--generally, as the engine heats up an optimum temperaturewill be reached and passed. We repeated these tests for each tire, with the results shown inFigure 2. The heavier tires showed a significant drop in horsepower compared to the lightertire. While the peak horsepower for the Bridgestone BT-001 was slightly more than theheavier Metzeler MEZ4, in general the graph was somewhat lower, reflecting the higher MoI ofthe much taller BT-001. The difference between the lighter and heavier tires is definitelyenough 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 ourtest 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, buta virtually identical curve at higher rpm.
http://images.sportrider.com/features/122_04_02_s+fooling_the_dyno+horsepower_torque_dyn o.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_03.html)
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 andtire. One nice aspect of the SuperFlow dyno is that the drum has a large diameter, whichbetter approximates a real road surface than a smaller drum, and allows for a large contactpatch. We check for wheelspin occasionally (see the picture on pg. 114), and even on ourturbocharged 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 differentpressures to determine if that affected the outcome. Using the BT-012 at 25, 35 and then 45psi, 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 bestto eliminate as many variables as possible in any form of testing. That means using the sametire model with a consistent amount of wear. Next time you're scratching your head over adyno run, check your back tire--you may find a couple of free horsepower. -SR
We tried three different tires on our 2004 Kawasaki ZX-6R for this dyno test. The well-wornstock Bridgestone BT-012 was the lightest, and the Metzeler MEZ4 was the heaviest. The190/55-17 Bridgestone BT-001, slightly lighter than the MEZ4, had the highest MoI becauseit's a much larger tire:
Tire/Weight (lbs.) including wheel/Moment of Inertia (lbs. square inch)
Bridgestone BT-012/28.50/2061
Metzeler MEZ4/31.92/2545
Bridgestone BT-001/31.80/2594
How tires - and other variables - affect dyno readings
writer: Andrew Trevitt
http://images.sportrider.com/features/122_04_03_s+fooling_the_dyno+electric_eye.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_01.html)
We've known for some time now that repeatable, consistent dyno readings depend on a numberof factors, and one anomaly that crops up now and again is tire selection and wear. Thescenario 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 hisbike leaves the shop with less horsepower than it went in with. Another is the racercompeting in a horsepower-limited class. He dynos his bike several times over the course ofa weekend and is well under the maximum amount allowed, but after the race and a celebratoryburnout, his bike is suddenly over the limit.
The common element in both cases is tires, and not that either bike makes more or lesshorsepower. The sportbike rider who replaces his bald tire with a heavier new bun losespower. The racer who strips off rubber with his burnout gains power. Why is this? To findout, we spent a day at our dyno with inquiring minds and a stack of tires.
The Theory
We've extensively covered dynamometers and how they work ("Dyno-Might!" Feb. 2001, alsoonline at www.sportrider.com/0406 (http://www.sportrider.com/0406)), and the important fact here is that the more commondynos are of the inertia type. Using a drum with a known Moment of Inertia (MoI), torque canbe calculated by measuring how fast a bike can accelerate the drum. Knowing torque and rpm,horsepower can then be calculated.
http://images.sportrider.com/features/122_04_01_s+fooling_the_dyno+horsepower_dyno.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_02.html)
If the weight (or, more specifically, the MoI) difference between two tires is significantrelative to the drum, that will change how quickly the drum accelerates--and hence how muchhorsepower is measured. To find out exactly how great this effect is, we ran our '04Kawasaki ZX-6R on our SuperFlow CycleDyn (www.superflow.com (http://www.superflow.com), 800/471-7701) with threedifferent tires. Figure 1 shows their weight and MoI differences.
While our SuperFlow CycleDyn is an eddy-current dynamometer, the entire system (consistingof the drum itself along with the eddy-current apparatus and a large drum-driven fan) has aknown inertia value and can be used as an inertia dyno. For our standard dyno testing, weuse 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'sMoI plays a big role.
We ran our standard test procedure, which involves performing successive dyno runs untilmaximum peak horsepower is reached--generally, as the engine heats up an optimum temperaturewill be reached and passed. We repeated these tests for each tire, with the results shown inFigure 2. The heavier tires showed a significant drop in horsepower compared to the lightertire. While the peak horsepower for the Bridgestone BT-001 was slightly more than theheavier Metzeler MEZ4, in general the graph was somewhat lower, reflecting the higher MoI ofthe much taller BT-001. The difference between the lighter and heavier tires is definitelyenough 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 ourtest 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, buta virtually identical curve at higher rpm.
http://images.sportrider.com/features/122_04_02_s+fooling_the_dyno+horsepower_torque_dyn o.jpg (http://www.sportrider.com/tech/146_0406_fooling_the_dyno/photo_03.html)
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 andtire. One nice aspect of the SuperFlow dyno is that the drum has a large diameter, whichbetter approximates a real road surface than a smaller drum, and allows for a large contactpatch. We check for wheelspin occasionally (see the picture on pg. 114), and even on ourturbocharged 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 differentpressures to determine if that affected the outcome. Using the BT-012 at 25, 35 and then 45psi, 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 bestto eliminate as many variables as possible in any form of testing. That means using the sametire model with a consistent amount of wear. Next time you're scratching your head over adyno run, check your back tire--you may find a couple of free horsepower. -SR
We tried three different tires on our 2004 Kawasaki ZX-6R for this dyno test. The well-wornstock Bridgestone BT-012 was the lightest, and the Metzeler MEZ4 was the heaviest. The190/55-17 Bridgestone BT-001, slightly lighter than the MEZ4, had the highest MoI becauseit's a much larger tire:
Tire/Weight (lbs.) including wheel/Moment of Inertia (lbs. square inch)
Bridgestone BT-012/28.50/2061
Metzeler MEZ4/31.92/2545
Bridgestone BT-001/31.80/2594