I searched for this, so my apologies if it has been covered.
Is there a formula to equate wattage to actual on-road mph? This would be done under “optimal” conditions ie: flat, no wind, good road surface.
I realize that grade/weight of rider and bike and wind all play into the equation. Also, it is my understanding that the relationship between power and speed is non-linear, as resistance increases in a non-linear fashion as speed increases…is that correct?
At any rate, is there a formula/equation for this? I’ve read a lot on the forum about power meters and I must say, I find it interesting.
There is–but there’s no need to make your head hurt w/calculations. Just go to Analyticcycling.com and plug in the numbers on the “speed given watts” section.
A couple of notes: the slope is set to 3%, so you’ll have to change it to zero.
And, of course, the biggy: Cda. The preset, IIRC, is .05, which is pretty high. Figuring out your actual Cda is the tricky part of this equation, though with a power meter and some free time it can be done with sufficient accuracy. There are a lot of good tips on biketechreview.com for figuring that part out.
Thank you for all your help. At the risk of sounding ignorant…OK, ignorance confirmed, what is Cda and IIRC? I’ve seen these terms used a lot on the forum and am clueless.
, of course, the biggy: Cda. The preset, IIRC, is .05, which is pretty high.
Good luck!
Clarification: The default CdA is actually 0.25, which is far lower than most triathletes. The ac.com webtool breaks it into its two parts: Cd = 0.5; and A = 0.5. The product equals 0.25.
It’s easier to use the tool by just setting one or the other to 1.0 and making the other one change between 0.28 and 0.35, which is where most triathletes are.
A few years ago, a guy from kurt Kinetic sent me an Excel spreadsheet that showed the wattage needed for each mph using the Kinetic trainer. While this was for the trainer, it is supposed to be equivalent to real world conditions. Here is what he wrote:
The Kinetic Trainer is calibrated for a 165 pound rider with a 23 pound bike, .7 frontal area, at sea level riding on rough asphalt road up a 1% grade.
(I found the Excel file…here are the inputs:
0.7 Frontal Area
0.5 Drag Coeffficient (NOTE: Cda of 0.35)
1.226 Air Density
85 Weight rider w bike (kg)
0.004 Rolling Resistance
0.01 Slope
m/sec Speed
100 Cadance
170 Crank Length
70 Pedal range)
Most of our information comes from Tom Compton who runs the Analytic Cycling web site. http://www.analyticcycling.com/ Tom is perhaps the most knowledgeable guy on the planet regarding cycling speed and power output requirements.
Check out the attached MS Excel spread sheet which details the speed to power relationship while riding the trainer. It is based upon the formula listed below.
Use the following formula to calculate your power output in watts while riding the Kinetic Trainer. If you add your average speed to this equation, you can calculate your average power. To record your average speed, it is necessary to connect a bike computer to your rear wheel.
= Average Power in Watts
Using 17.2 MPH average speed, your power out put is as follows:
= Average Power in Watts
90.210904 + = Average Power in Watts
90.210904 + = Average Power in Watts
90.210904 + 97.5333358848 = Average Power in Watts
187.7442398848 = Average Power in Watts
Here are some of the results from the file:
Speed (MPH) Power (Watts)
5 28.6
6 35.6
7 43.3
8 51.8
9 61.2
10 71.6
11 83.2
12 96.1
13 110.3
14 126.0
15 143.4
16 162.4
17 183.3
18 206.2
19 231.1
20 258.2
21 287.7
22 319.5
23 353.8
24 390.8
25 430.6
26 473.3
27 518.9
28 567.6
29 619.6
30 674.9
CdA is the product of two things: Cd is the coefficient of drag. It measures how hard a time the wind has in parting, splitting around your body and going back off the back. A is the frontal area, expressed in square meters. It measures how big you are, looking from the front and from the perspective of the wind. The product of the two determines how much force the wind pushes upon you as you ride along. How big times how slippery…CdA. The lower your CdA, the faster you go at a given amount of pedaling power.