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Road Cycling It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle. -- Ernest Hemingway

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Old 02-02-08, 02:13 PM   #1
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Rotating mass: what does it cost you?

Having a power meter is too much fun. I was wanted to quantify the impact of a heavy rim on an all-out sprint. This sprint had a 5s mean maximal power of 1527W.

The short story:
Average cost of a heavy rim (Velocity Deep-V) vs. a light rim (Zipp 280/303), during the entire sprint is 0.2% of my power, or 2.6W out of 1301W available.

The MAXIMUM cost was 0.25%, or 4.0W out of 1604W available.

This is just for the rim, not counting anything else on the wheel.

That's not much difference. At all. There are extremely rare circumstances where 0.2% would be the difference between winning and losing a sprint. Still, for the same price as a Deep-V, an Aerohead will chew up half of the difference, so you're down to 1.3W.

One interesting (?) point is that my max acceleration was 1.56 m/s (0.16 G )

The long story:
I took my recent 5s best sprint power file (1527W), and exported it as a .csv.

In Excel, I worked next to the sprint data to calculate angular acceleration of the wheels during the sprint. I calculated a moment of inertia for the rim (550g Deep V, assumed a thin ring concentrated at the bead seat diameter).

From this, I was able to calculate the angular acceleration for each line of data during the sprint. Then I calculated the torque required to generate that angular acceleration on a 311mm radius ring of 0.55kg.

Looking at the torque required, I compared that to the recorded torque at the hub, and came up with a %

The calculations actually show a greater effect since I assumed the weight was at the bead seat diameter, rather than somewhere in the middle. I also assumed the high weight for the Deep-V and the low weight for the Zipp.

Since the recorded speed is affected about one row below the torque hits, I did some offsetting for my equations.

The spreadsheet is available for review here (check around line 1000). Please let me know if you see any errors -- it's been a long time since physics 101...

If you've never seen a power file, it might be interesting to view anyway (you can graph Watts yourself).

Last edited by waterrockets; 02-02-08 at 02:19 PM.
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Old 02-02-08, 02:18 PM   #2
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Waterrockets = BF mythbuster

"Spin up" is nothing but marketing hype, thank you for providing some solid data which proves it.
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Old 02-02-08, 02:24 PM   #3
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But a stiffer bottom bracket transmits more power, right? Did you and UT dude ever get together to test that one out.

Edited to add that stupid winking smiley.
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Old 02-02-08, 02:38 PM   #4
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Waterrockets,

Very interesting! Thanks.

I didn't open the xls file, but does that file correlate the wattage and the type of terrain and speed (elevation, speed) ?
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Old 02-02-08, 02:42 PM   #5
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I think rotating mass costs you at least a couple of dollars a gram.

Oh, sorry... I didn't realize that this was one of those threads based on practical facts
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Old 02-02-08, 02:50 PM   #6
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so aero wins over weight on flats once again?
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Old 02-02-08, 02:51 PM   #7
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Waterrockets = BF mythbuster

"Spin up" is nothing but marketing hype, thank you for providing some solid data which proves it.
Lance beating out all of those other guys on much "better" bikes, wheels, and Campy is the mythbuster (would YOUR first choice in the TDF be Trek and Bontrager?)
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Old 02-02-08, 02:59 PM   #8
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(would YOUR first choice in the TDF be Trek and Bontrager?)
No but my choice would have nothing to do with performance.
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Old 02-02-08, 02:59 PM   #9
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Lance beating out all of those other guys on much "better" bikes, wheels, and Campy is the mythbuster (would YOUR first choice in the TDF be Trek and Bontrager?)
Why is campy the mythbuster? I don't get it, even if I read your sentence the way you want me to read it.

Actually, don't answer. Just go spend more money on bike schwag. The economy needs it.
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Old 02-02-08, 03:02 PM   #10
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so aero wins over weight on flats once again?
Dude, that's heresy
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Old 02-02-08, 03:58 PM   #11
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Can't find that thread on the wattage effects of various aero equipment in wind tunnel tests. We should compare those numbers to these weight wattages to make a comprehensive review. I think aero makes the bigger effect tho with these numbers.
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Old 02-02-08, 04:57 PM   #12
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Originally Posted by ericcox View Post
But a stiffer bottom bracket transmits more power, right? Did you and UT dude ever get together to test that one out.
We haven't yet -- you'll hear about it here first. He's still game, but we're both pretty busy now...

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Originally Posted by ericcox View Post
Edited to add that stupid winking smiley.
No smiley needed for that one

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Waterrockets,

Very interesting! Thanks.

I didn't open the xls file, but does that file correlate the wattage and the type of terrain and speed (elevation, speed) ?
No, this was just a power file. The sprint was a flat 200m, in fairly calm air. This evaluation was simply looking at how hard it is to rotationally accelerate heavy rims.

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so aero wins over weight on flats once again?
Why do you think I'm riding Deep-Vs?

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Can't find that thread on the wattage effects of various aero equipment in wind tunnel tests. We should compare those numbers to these weight wattages to make a comprehensive review. I think aero makes the bigger effect tho with these numbers.
Absolutely it does. This is making the case for heavy aero wheels like PlanetX deep carbons over the lighter ones
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Old 02-02-08, 05:48 PM   #13
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Originally Posted by waterrockets View Post
Having a power meter is too much fun. I was wanted to quantify the impact of a heavy rim on an all-out sprint. This sprint had a 5s mean maximal power of 1527W.

The short story:
Average cost of a heavy rim (Velocity Deep-V) vs. a light rim (Zipp 280/303), during the entire sprint is 0.2% of my power, or 2.6W out of 1301W available.

The MAXIMUM cost was 0.25%, or 4.0W out of 1604W available.

This is just for the rim, not counting anything else on the wheel.

That's not much difference. At all. There are extremely rare circumstances where 0.2% would be the difference between winning and losing a sprint. Still, for the same price as a Deep-V, an Aerohead will chew up half of the difference, so you're down to 1.3W.

One interesting (?) point is that my max acceleration was 1.56 m/s (0.16 G )

The long story:
I took my recent 5s best sprint power file (1527W), and exported it as a .csv.

In Excel, I worked next to the sprint data to calculate angular acceleration of the wheels during the sprint. I calculated a moment of inertia for the rim (550g Deep V, assumed a thin ring concentrated at the bead seat diameter).

From this, I was able to calculate the angular acceleration for each line of data during the sprint. Then I calculated the torque required to generate that angular acceleration on a 311mm radius ring of 0.55kg.

Looking at the torque required, I compared that to the recorded torque at the hub, and came up with a %

The calculations actually show a greater effect since I assumed the weight was at the bead seat diameter, rather than somewhere in the middle. I also assumed the high weight for the Deep-V and the low weight for the Zipp.

Since the recorded speed is affected about one row below the torque hits, I did some offsetting for my equations.

The spreadsheet is available for review here (check around line 1000). Please let me know if you see any errors -- it's been a long time since physics 101...

If you've never seen a power file, it might be interesting to view anyway (you can graph Watts yourself).
Hmm, that's pretty impressive, considering Cippo and Boonen usually developed around 1750-1800 watts at peak, albeit probably for around 15-20 seconds.
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Old 02-02-08, 10:26 PM   #14
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Hmm, that's pretty impressive, considering Cippo and Boonen usually developed around 1750-1800 watts at peak, albeit probably for around 15-20 seconds.
Well, they combine that with an FTP adequate for keeping up with a TdF stage. Then, they get to add 300W (!) to my 5s power... that's like adding a decent time-trialist to your sprint. They're much much faster. I am happy to be over 1500W though.
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Old 02-02-08, 10:31 PM   #15
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So, I have to say I'm a little surprised I'm not seeing any challenges here.

I can't wait until I get the frame stiffness test done then
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Old 02-03-08, 12:11 AM   #16
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So, I have to say I'm a little surprised I'm not seeing any challenges here.

I can't wait until I get the frame stiffness test done then
maybe becuase a lot of us can't afford that kind of equipment.......
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Old 02-03-08, 01:15 AM   #17
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One possible challenge would be in the experimental method. I haven't actually sat down to figure out how the power meter measure power, but being human means you innately introduce some significant variance in power generation. Considering the difference is only 0.2% between runs, unless you ran the exercise a large number of times with consistent data as demonstrated by statistical analysis, in the end, these numbers aren't much better than heresay. (Had a phys chem prof drill the importance of stats into us, sadly it stuck)

Then again, the same can be said for a lot of the other data we're basing our assumptions on in aero and such. And, who's got the time to do these things multiple times without a grant of sorts? One question though...did you do the runs one after another? and if so which one first?

Overall props for adding some numbers I'm starting to regret my recent rim buy now
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Old 02-03-08, 03:51 AM   #18
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does this mean spending money on new wheels won't have an effect on actual speed? like it won't make me go from a cruising of speed 20mph to 22mph?
and i should just concentrate on buying more food?
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Old 02-03-08, 04:05 AM   #19
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One possible challenge would be in the experimental method. I haven't actually sat down to figure out how the power meter measure power, but being human means you innately introduce some significant variance in power generation. Considering the difference is only 0.2% between runs, unless you ran the exercise a large number of times with consistent data as demonstrated by statistical analysis, in the end, these numbers aren't much better than heresay. (Had a phys chem prof drill the importance of stats into us, sadly it stuck)

Then again, the same can be said for a lot of the other data we're basing our assumptions on in aero and such. And, who's got the time to do these things multiple times without a grant of sorts? One question though...did you do the runs one after another? and if so which one first?

Overall props for adding some numbers I'm starting to regret my recent rim buy now
its late, but from what i can gather from his post he took one run and plugged in the numbers from the different rims from there. not nearly as many variables as the "im gonna put on a different set of wheels and try that same sprint again" method. i dont understand what he did, but it sounds like he knows what hes doing.
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Old 02-03-08, 08:24 AM   #20
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One possible challenge would be in the experimental method. I haven't actually sat down to figure out how the power meter measure power, but being human means you innately introduce some significant variance in power generation. Considering the difference is only 0.2% between runs, unless you ran the exercise a large number of times with consistent data as demonstrated by statistical analysis, in the end, these numbers aren't much better than heresay. (Had a phys chem prof drill the importance of stats into us, sadly it stuck)
That would be true if the intent were to determine how fast he accelerates in a sprint, but that's not the point. The purpose here is to see roughly how important rotational acceleration is compared to other forces acting on the bike/rider. Since "important" is not a highly precise measure, there is no need for high precision in the data. Face it, if he accelerated ~50% faster next time and ~50% slower the time after that, the conclusions wouldn't change.

Data is taken from a single sprint and analyzed. To me the questions are: 1) is the sprint typical of what most riders would see so that the results can be generalized, and it seems they are. 2) Is the method of analysis accurate so that the conclusions derived from the data are supported. For the most part that also seems to be true, especially since these results are pretty much in agreement with other similar analyses. The only open questions in my mind are the errors introduced due to the numerical differentiation of speed to get acceleration and from the relatively long 1.26 sec. sampling rate. But again it is doubtful these errors would change the conclusion that rotational inertia is insignificant.
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Old 02-03-08, 09:12 AM   #21
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Interesting, but Wikipedia has this pretty well (but not perfectly) covered in Bicycle Weight and Power

The general rule of thumb is that taking a gram off the wheels is equal to taking between 1.5 and 2 grams off the bike.

The article analyzes some example differences to come up with numbers ranging from 0.02 MPH (500 grams saved in a sprint) to 0.06MPH (2.2kg saving on a 7% road grade). A 0.04 MPH improvement is only about a meter a minute difference.

The article then devotes a few paragraphs as to speculations as to why such small calculated differences feel so much different to riders.
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Old 02-03-08, 09:18 AM   #22
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The general rule of thumb is that taking a gram off the wheels is equal to taking between 1.5 and 2 grams off the bike.
I don't see this anywhere in the article, and it is far from true.
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Old 02-03-08, 09:24 AM   #23
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I don't see this anywhere in the article, and it is far from true.
Try reading the section on "Kinetic Energy" again (about a 1/3 way into the article). It provides equations comparing kinetic energy against "rotating mass" to examine the energy impacts of rotating versus non-rotating mass, summing up with:

Quote:
In other words, a mass on the tire has twice the kinetic energy of a non-rotating mass on the bike. There is a kernel of truth in the old saying that "A pound off the wheels = 2 pounds off the frame."[3]

This all depends, of course, on how well a thin hoop approximates the bicycle wheel. In reality, all the mass cannot be at the radius. For comparison, the opposite extreme might be a disk wheel where the mass is distributed evenly throughout the interior. In this case and so the resulting total kinetic energy becomes . A pound off the disk wheels = only 1.5 pounds off the frame. Most real bicycle wheels will be somewhere between these two extremes.
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Old 02-03-08, 09:42 AM   #24
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Try reading the section on "Kinetic Energy" again (about a 1/3 way into the article). It provides equations comparing kinetic energy against "rotating mass" to examine the energy impacts of rotating versus non-rotating mass, summing up with:
Yup...its true...angular momentum which is function of moment of inertia. Common knowledge in fact. This whole thing is pretty nebulous however if comparing a good road wheel to an ultra light road wheel as Waterrockets proved in his experiment. There is also the dynamic of conservation of angular momentum or flywheel effect which is relevant in non sprinting circumstances. Momentum and fractionally heavy wheelsets is good to maintaining momentum in slight elevation changes and descending and side wind resiliency and why honestly, a few grams unless racing at the elite level means very little. An ounce of frame weight is even less relevant.
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Old 02-03-08, 09:57 AM   #25
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The ironic thing about the Wikipedia article is that where it says "There is a kernel of truth in the old saying that "A pound off the wheels = 2 pounds off the frame.[3]", the citation 3 leads to a page that debunks the importance of rotating mass. http://www.charles.whitaker.name/wheels.html

Heavier wheels are a tiny bit harder to accelerate, but hold their speed better when you ease up. Thus they tend to even out the surge and lag that occurs with every pedal stroke, or every variation in rider effort. On a flat course, that may actually improve your speed, or at least offers no disadvantage, as Ondrej Sosenko proved when he set the hour record in 2005 with deliberately heavy rims. http://www.cyclingnews.com/news/?id=...l05/jul19news6

In other words, the myth of the importance of rotating mass is totally impervious to facts.
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