Wheel Rotational Inertia Testing - Part 2

Before I start, I'd like to point out that the pendulum method is pretty simple, but its not the most concrete way to get inertia. Also, slight reading errors could have creeped in while measuring r0 (distance from the ceiling to center of wheels), and there could have been errors with starting or stopping the watch (which is why we took averages of two times), and there could have been errors with the wheels not swinging in one plane. I believe the 100 swings smoothed that out and our estimates should be pretty close to the real thing, unless something is drastically wrong with the results.

REAR WHEELS and RESULTS




Name : Compagnolo Proton Training Wheel, 24 spokes
Tire : Conti Ultra Sport Trainer Specific (Yellow)
Cassette : Compagnolo Centaur 10 speed
Weight = 3.58 lbs = 1623.860 grams

Time for 100 swings = 204 seconds.
Time for 1 swing = 2.04 sec
ro = 98.5 cm = 0.985 m
Ic = 0.0779976 kg m^2



Name : HED Jet 50 mm, 24 spokes
Tire : Conti Ultra Sport Road
Cassette : SRAM PG 970 9 speed
Weight = 3.39 lbs = 1537.678 grams

Time for 100 swings = 202 seconds.
Time for 1 swing = 2.02 sec
ro = 98cm = 0.98 m
Ic = 0.0506224 kg m^2


At this point, I thought let me switch to my race tires and see how the results are affected.

Name : HED Jet 50 mm, 24 spokes
Tire : Michelin Pro Race 2 Blue
Cassette : SRAM PG 970 9 speed
Weight = 3.19 lbs = 1446.959 grams

Time for 100 swings = 202 seconds.
Time for 1 swing = 2.02 sec
ro = 95.3cm = 0.953 m
Ic = 0.0835551 kg m^2

Hmm, that's a little strange. As I changed the tire to something lighter, the inertia increases from .0506224 to 0.0835551, which is an increase of roughly 39%. The next time we do this test, I think I'll retest this wheel again. I'm taking this with a pinch of salt, but again how off could we be?




Name : FSA RD-600 30mm, 24 spokes
Tire : Michelin Pro Race 2 Lite
Cassette : Compagnolo Centaur/Chorus (not sure) 10 speed
Weight = 3.19 lbs = 1446.959 grams

Time for 100 swings = 204 seconds.
Time for 1 swing = 2.04 sec
ro = 99cm = 0.99 m
Ic = 0.062691 kg m^2



Name : Arraya SuperHard CTL - 370, 32 spokes (I haven't even heard of this brand, it was on my commuter Bianchi)
Tire : Hutchinson Carbon Comp
Cassette : Shimano Ultegra 7 speed
Weight = 3.42 lbs = 1551.285 grams

Time for 100 swings = 206 seconds.
Time for 1 swing = 2.06 sec
ro = 100.4cm = 1.004m
Ic = 0.0780815 kg m^2




We can sort of compare these with what some of the values for wheels analytic cycling had on their website. I mean, our values are not so bad are they?

Rotational Inertia and Mass for Various Wheels

Wheel	Details	Ic (kg m^2)	Mass (gm)
Wire Spoke	Rear, Std Rim, 700, track, 36 spokes, w/o tire, w/ axle, nuts	0.0528	1177
Wire Spoke	Front, Std Rim, 700, 32 spokes, w/ tire, tube, rim strip, axle, skewer	0.0885	1264
Wire Spoke	Rear, Std Rim, 700, 32 spokes, w/12-21 cassette, tire, tube, rim strip, axle, skewer	0.0967	1804
Specialized tri-spoke	Front, 700, w/ tire, tube, axle, skewer	0.0904	1346
Specialized tri-spoke	Rear, 700, w/ 12-21 cassette, tire, tube, axle, skewer	0.1032	1771
Specialized tri-spoke	Front, 650, w/ tire, tube, axle, skewer	0.0683	1207
Mavic	Front, Std Rim, 650, 28 Bladed Spokes, w/ tire, tube, rim strip, axle, skewer	0.0632	1179
MTB	Front, 32 Spokes, w/ tire, tube, rim strip, axle, skewer	0.1504	1847