% increases in power required to improve average TT speed

robkit

imagine a flat, windless TT course of 1O or 25 miles and that a rider needs to produce X constant watts to finish in a time equivalent to average speed 20mph.

obviously X is a function of frontal area, drag, rolling resistance, etc, but in relative terms does anyone have estimates of the factors of X required to increment for each successive 1mph, upto say 30mph?

i would find this very interesting, both to scope the level of curvature (ie getting from 24 to 25 requiring more power than getting from 20 to 21 due to the exponential increases in air resistance), and to figure out what sort of improvements are realistic from a given starting point.

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ric_stern/RST

analyticcycling.com will allow you to manipulate many variables. Try the Static Forces on Rider pages

ric

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robkit

(frontal area .5m2, weight 75kg, gradient 0, cranks 172.5, all other inputs @ default)
http://www.analyticcycling.com/ForcesPower_Page.html
mph met/sec watts increment factor increment time (mins) time (mm:ss) 1 mile
20 8.9408 136 0 1.00 0.00 30.00 30:0 3:0
21 9.38784 154 19 1.14 1.14 28.57 28:34 2:51
22 9.83488 175 20 1.29 1.13 27.27 27:16 2:43
23 10.28192 197 22 1.45 1.13 26.09 26:5 2:36
24 10.72896 221 24 1.63 1.12 25.00 25:0 2:30
25 11.176 247 26 1.82 1.12 24.00 24:0 2:24
26 11.62304 275 28 2.02 1.11 23.08 23:4 2:18
27 12.07008 305 30 2.25 1.11 22.22 22:13 2:13
28 12.51712 337 32 2.48 1.11 21.43 21:25 2:8
29 12.96416 372 35 2.74 1.10 20.69 20:41 2:4
30 13.4112 409 37 3.01 1.10 20.00 20:0 2:0

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velomanct

most riders will never get close to .5m2 frontal area, unless you're a small rider or have a Lance type budget and wind tunnel.

just for example, I am a tall rider at 6'2" and 172lbs. it would take between 500 and 550 watts to go 30mph, a lot more than 400.

in a real world example, on a fast rolling/flat course, it takes me an average power of 320watts to average 23mph. but that is without aero equipment.

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ric_stern/RST

I guess it may depend by what you mean by small rider, and you don't need a big budget... When i last rode a TT (seriously) i recorded an avg speed of ~ 42.5 km/hr on a standard road bike with no aero anything (28 spoke Open Pro rims) normal drop handlebars, raced in separate jersey and shorts. This required an avg power of 311 W (SRM Pro, that had been calibrated). This would give a *rough* ball-park estimate of CdA of ~ 0.27 m^2. I'm 1.75m and 69 kg.

Prior to Chris Boardman breaking the Hour Record for the first time, he conducted a series of trials on an indoor velodrome using different bike positions. One of these was a 'standard' Merckx type position. At 409 W avg, he travelled at ~49.5 km/hr, giving an estimated CdA of ~ 0.25 m^2. Boardman is (was) the same height and mass as myself.

Using your data of 172 lb and 23 mph and 320 W, we can roughly estimate that your CdA is 0.43 m^2. I would think (although i've never seen you) that it would be quite possible to make you more aerodynamic, such that you can travel faster for the same or less power. It would seem, initially at least, that you are 'giving away' power unnecessarily

Ric

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Smartt/RST

Keep in mind that the analytic cycling model is asking for the combination of bike and rider for the weight variable. So if you weigh 78kg, your total weight is likely to be around 86kg. I just mention this because the numbers posted by robkit would change dramatically in order to account for another 10-11kgs...as you allude to.

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velomanct

yes I have accounted for bike weight too.

perhaps the reason why i might be wasting power is because I try to keep my effort the same throughout the tempo session. but in a real TT i would ease up more on downhills and push the climbs more, which in turn would likely give me a slightly faster speed for the same average power.

don't forget that when I said it took me 320watts to average 23mph, that doesn't mean it takes 320watts to go 23mph on a flat road with no wind. i was riding a rolling course with wind and corners and so on. in perfect conditions on a flat road, it would likely take me 290-300watts to do 23mph

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"friendship, family, religion. These are the three demons you must slay if you wish to succeed in business!" -Mr. Burns
The faster you go, the fewer passing cars

Mansmind

not doubting your word, but that sounds high to me. It takes me ~ 245 watts to ride 23 mph...on a flat road, no head wind, etc.

bikeguy

Go to analyticcycling.com and plug in for speed, in the given speed, find power section. I'd guess riding 30 mph requires 3.3x as much power as riding at 20 mph. The air drag scales as ground velocity cubed, given that air velocity=ground velocity (no wind).

Another great program is the bike power calculator from machinehead software. Then you can calculate pretty much anything you want, like power at given speed, drag area and Crr etc. There's an annoying popup, you can suffer it if you don't have the cash, like I do, or pay up and get rid of it.

-Bikeguy

velomanct

what's your height and weight, and is that in a standard road position?

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The faster you go, the fewer passing cars

Mansmind

6', 184 lbs. right now, position on the hoods. I suppose I stretch out a little more than what I'm accustomed seeing other riders do, but I don't see that many other riders so I'm not sure.

I have quite a bit of upper body mass (previous weight training), I would expect my drag to be quite large, but perhaps not.

ric_stern/RST

probably takes me a few watts less, when riding around, which is in line with me being not as tall or as heavy as you

ric

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Mansmind

What really pisses me off is averaging 18mph and 270 watts for an hour, got to love those hills. Actually I think I'm starting to appreciate them, even enjoy them...but it would be nice to find a 40-50 mile ride somewhere just to get the speed up for a change.

velomanct

wow, I thought you would be much smaller. I suppose its hard to judge when there is no wind on a dead flat road, but I know it takes me at least 280watts to do 23mph. I have done the stomp test on my powertap and it is accurate too. torque zeroed of course. I have never be good at time trials, maybe my position is slow.

looking at todays ride I was doing 300watts at 23.8mph on a flat section, I dont think there was wind.

__________________
"friendship, family, religion. These are the three demons you must slay if you wish to succeed in business!" -Mr. Burns
The faster you go, the fewer passing cars