Power to Speed Ratio

1

Hey friends,
I know there are countless factors that influence this, but very broadly speaking, are there any rules of thumb as to what speed range one can expect at a given power and weight (on a flat road/calm day)?

For instance, at roughly 79 kg (174 lbs), riding solo on a flat road with low wind, I hold 20 mph at 240 watts (which is tempo/low sweet spot range for me). Does that seem in the ballpark of where it should be? Wondering if my wheels/tires/aero situation is significantly holding me back, or if Iā€™m within reason. Thanks in advance!

1 Like
2

Enter your known variables and you see various info based on that.

image
image363Ɨ582 44.4 KB

This one is a comparison tool for different riders or conditions for the same rider.
Metric Units: Bike Calculator
English Units: Bike Calculator

3 Likes
3

The other one commonly used is Best Bike Split.

It is far more advanced but requires payment for some uses.

4

A bit dated but still good information: https://analyticcycling.com/ You can change weight, CDA (aero) and Crr - rolling resistance.

1 Like
5

Thanks for the resources. Very helpful.

1 Like
6

Total drag is comprised of many different contributions, but the biggest two are wind resistance and, if you are on an incline, gravity.

The power needed to overcome wind resistance grows as the third power of your speed, and is proportional to you CDA (your coefficient of drag times the frontal area).

Gravity is independent of speed, and you need to invest power over time (= energy) to lift yourself and your bike from your initial to your final height. When you go downhill, you get to convert that potential, stored energy into kinetic energy. Since the amount of potential energy is proportional to your total mass, heavier riders will get more energy than lighter riders, which gives them an advantage downhill.

So the total power to overcome drag is a function of speed. At very low speeds when you go uphill, wind resistance is very small. Conversely, at very high speeds on the flats, large increases in power only give you a moderate increase in speed. For example, a 50 % increase in power only gives you 14.4 % more speed, provided that all other things are equal.