# Calcuting forces on a slope (cycling) -online calculators wrong?

I'm trying to deduce the forces acting on a cyclist going up a hill and I need help. There's 2 questions:

Q.1

I've looked at online calculator already in existence:

http://www.analyticcycling.com/ForcesPower_Page.html
http://www.gribble.org/cycling/power_v_speed.html

What puzzles me about these calculators is when I caculate the parallel force I get numbers that are different to these calculator. but are the same as http://thecraftycanvas.com/library/...pers/incline-force-calculator-problem-solver/

e.g. for a 5º slope and a 78 kg rider+equipment: -9.81 * sin(rad5) * 78 = 66.6 N "thecraftycanvas.com" get the same result, yet Gribble and Analytic cycling get 38.2 N

Q.2

Following on from this, what I am trying to do is calculate what the force are acting on the rider by doing a calibration run down the hill, the other forces (air resistance (wind, pressure), friction) besides gravity being the difference between the force on the calibration run and a hypothetical run just with the force of gravity.

So that when the rider goes up hill in X amount of time we can calculate the how much work against gravity by itself and all the other forces rolled together (deduced from the calibration run).

Am I missing something fundamental?

A.T.
yet Gribble and Analytic cycling get 38.2 N
You confuse slope angle and percent grade.

tan(slope angle) * 100 = percent grade

Am I missing something fundamental?
Air drag is velocity dependent. You can let him roll downhill, and measure the acceleration at different velocities. From acceleration you get the net force. Substract gravity to get air drag + rolling resistance. Transmission loss is not included.

Last edited:
I did indeed A. T., thanks!

A.T.