## Friday, November 23, 2007

### 1Wheel 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

#### 1 comment:

1. On the 5th anniversary of this Post, I thought I'd offer you a comment :)
Your efforts inspired me to experiment with a cast magnesium Jongbloed 202 Modular Wheel Center (formula car part). Long story short, I find that this method's results are sensitive to pendulum length, and twice as sensitive to swing period. While I can easily improve my length measurement accuracy, I think accurate swing time measurements will require the use of a video camera and editor...
Thanks, and have a Happy Thanksgiving

Thank you. I read every single comment.