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Old 03-05-08, 12:47 PM   #1
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Watts/kg and "Opposing Forces"

I read an article that says the following:

"Power to weight ratio is important because it requires more power for a heavier rider to travel the same velocity than for a lighter rider. However, it is important to remember that gravitational resistance is just one of many opposing forces while riding and only a small percentage of the opposing force on a flat road."*

With the last sentence in mind I wondered: If I, a 69kg rider, maintain 1100 watts in a 20-second sprint (15.9 watts/kg) alongside an 88kg rider maintaining 1400 watts (also 15.9 watts/kg), who goes faster? To reduce the number of opposing forces we'll call this a drag race on a flat road with no wind and no wheelsucking/drafting. Two riders with different mass but generating identical power to weight ratios sprinting alongside one another. Who goes faster?


*(C) Colin Sandberg
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Old 03-05-08, 12:49 PM   #2
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Originally Posted by Davey View Post
I read an article that says the following:

"Power to weight ratio is important because it requires more power for a heavier rider to travel the same velocity than for a lighter rider. However, it is important to remember that gravitational resistance is just one of many opposing forces while riding and only a small percentage of the opposing force on a flat road."*

With the last sentence in mind I wondered: If I, a 69kg rider, maintain 1100 watts in a 20-second sprint (15.9 watts/kg) alongside an 88kg rider maintaining 1400 watts (also 15.9 watts/kg), who goes faster? To reduce the number of opposing forces we'll call this a drag race on a flat road with no wind and no wheelsucking/drafting. Two riders with different mass but generating identical power to weight ratios sprinting alongside one another. Who goes faster?


*(C) Colin Sandberg
The bigger dude.

His watts/CdA ratio will almost certainly be greater.
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Old 03-05-08, 12:58 PM   #3
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Big guy wins.


Here's a tangent:

How is the "A" in CdA related to weight? If you make the crazy assumption that volume (mass) is equal to HxWxD and area is equal to HxW, then you would say that the square root of a rider's area should be proportional to the cube root of his mass... or that the rider's area is proportional to his mass raised to the 2/3 power. The [huge] assumption here is that the ratios of height to width to depth are fairly constant among riders.

So maybe the metric for steady-state, flat, constant speed vs. drag power should be W/[kg^(2/3)].

Of course, a sprint is not steady-state; the riders have to accelerate against their masses.
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Old 03-05-08, 01:16 PM   #4
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What speed are you starting from? If starting from a dead stop, the difference will be smaller.
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Old 03-05-08, 01:19 PM   #5
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Originally Posted by Phantoj View Post

So maybe the metric for steady-state, flat, constant speed vs. drag power should be W/[kg^(2/3)].
R Stern uses W/Kg^0.67 when profiling athletes.

http://www.cyclecoach.com/pageID-dow..._MAP_zones.htm
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Old 03-05-08, 01:28 PM   #6
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There is a lot more to it than w/kg.... just sayin
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Old 03-05-08, 02:01 PM   #7
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Quote:
Originally Posted by Davey View Post
With the last sentence in mind I wondered: If I, a 69kg rider, maintain 1100 watts in a 20-second sprint (15.9 watts/kg) alongside an 88kg rider maintaining 1400 watts (also 15.9 watts/kg), who goes faster? To reduce the number of opposing forces we'll call this a drag race on a flat road with no wind and no wheelsucking/drafting. Two riders with different mass but generating identical power to weight ratios sprinting alongside one another. Who goes faster?
The answer is http://www.analyticcycling.com
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Old 03-05-08, 02:01 PM   #8
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Figure it this way - since frontal area is sort of similar for many riders, make it a minor point.

Major point is power, another is weight, since both vary greatly.

Pure wattage relates to flat type speed. Threshold relates to TT speed.

Watt/kg relates to climbing speed, minor is acceleration speed.

Max (sub 10 second) wattage relates to jump in a sprint.

20-30 second wattage relates to sustainable top speed in a sprint.

This is how I approach it. Seems to hold true at the most basic levels (i.e. 100-200 watt and/or 5 kg deltas). At fine tuning levels ("will that 4 watt savings with better tires make me faster in the sprint") then it's not right.

hope this helps,
cdr
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Old 03-05-08, 02:09 PM   #9
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Figure it this way - since frontal area is sort of similar for many riders, make it a minor point.
Comparing CdA values over a range of riders shows about the same variation as weight of bike plus rider.
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Old 03-05-08, 02:33 PM   #10
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Thank you all for reminding me why I went into television instead of physics...I now feel dumber than usual.
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Old 03-05-08, 02:39 PM   #11
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Comparing CdA values over a range of riders shows about the same variation as weight of bike plus rider.
In what riding position?
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Old 03-05-08, 03:52 PM   #12
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In what riding position?
All positions.
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Old 03-05-08, 04:32 PM   #13
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Interesting. Any references you have bookmarked, asgelle?
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Old 03-05-08, 04:33 PM   #14
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I read an article that says the following:

. Who goes faster?


*(C) Colin Sandberg

The one with the better positioning at the beginning of the sprint.
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Old 03-05-08, 05:03 PM   #15
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Interesting. Any references you have bookmarked, asgelle?
None specifically, biketechreview used to have a collection of data, but I can't find it on the new site. Reading around different places though, I've seen a low CdA of about 0.2 and a high of about 0.3 on aerobars so roughly a 50% difference. That's about the same span I'd expect to see in competitive or fast recreational riders.
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Old 03-05-08, 05:19 PM   #16
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The one with the better positioning at the beginning of the sprint.
Actually it's the one who doesn't loose his earpiece following the lead-out.
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Old 03-05-08, 05:35 PM   #17
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All positions.
Hard to buy that based on the huge difference in frontal area between someone sitting up on the hoods and someone in a full TT position...
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Old 03-05-08, 05:43 PM   #18
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Interesting. Any references you have bookmarked, asgelle?
from the saris prologue video:

Slipstream prologue pacing vid post #16

tom d - .212?

ryder h - .2298

just from those two measurements, there is about an 8% difference ~ 70kg rider and 75.9kg rider.

i am sure there are extremes on both ends that would drastically increase the percent differences.
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Old 03-05-08, 05:51 PM   #19
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Hard to buy that based on the huge difference in frontal area between someone sitting up on the hoods and someone in a full TT position...
the variation in time trial position is likely smaller than the variation of any other positions (i.e. the best and worst position of racers in drops will likely be greater than the estimated 50% differnece of time trial position). certainly if you compared sitting up right and time trialing the percent difference in cda would be far greater than body mass percent difference of any rider.
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Old 03-05-08, 06:25 PM   #20
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What I want to know is how the heck does a little guy like Levi (relatively low steady state watts compared to much larger riders like Cancellara) ride a faster relatively flat TT?
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Old 03-05-08, 06:30 PM   #21
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... Levi (relatively low steady state watts compared to much larger riders like Cancellara) ...
Do you have his power data?
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Old 03-05-08, 06:30 PM   #22
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What I want to know is how the heck does a little guy like Levi (relatively low steady state watts compared to much larger riders like Cancellara) ride a faster relatively flat TT?
Johan? He turns all his riders into stars, Tyler, Roberto, Floyd.....
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Old 03-06-08, 11:21 PM   #23
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What I want to know is how the heck does a little guy like Levi (relatively low steady state watts compared to much larger riders like Cancellara) ride a faster relatively flat TT?

Aero as anything.
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Old 03-06-08, 11:48 PM   #24
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What I want to know is how the heck does a little guy like Levi (relatively low steady state watts compared to much larger riders like Cancellara) ride a faster relatively flat TT?
Let's see him ride a faster TT in a one-day event. Levi is fast, but his exceptional talent is recovery after continuous hard stages every day.
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Old 03-07-08, 12:07 AM   #25
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What I want to know is how the heck does a little guy like Levi (relatively low steady state watts compared to much larger riders like Cancellara) ride a faster relatively flat TT?
I recall Cancellera talking about how he was really feeling the day before that stage and how that affected him. As for the TT itself I was under the impression it was a bit rollier if anything (weight coming in a bit more there I guess), you being a CA guy would know best, what was the course like?

Oh and as stated above, Levi seems to have the CdA of a small child
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