Hi Riders!

I’ve got a technical question about computation of power in Zwift.

My naive understanding is that power P (measured in watts) is the product of force F (measured in newtons?) and velocity V (measured in revolution per minute (rpm)): P=F*V. I think we have all experienced the relationship between these different quantities as Zwift would increase the level of resistance when your cadence does down *

I would like to know whether it is fine to simply compute Force as Power divided by Velocity (eg if i do 230 watts for 1 minute at 85 rpm, then F=230/85=2.706 - Then i think i would need to apply some other physics formula to convert it into newtons or something else relevant).

I am interested in this topic because i would like to know whether the force i apply on the pedals when i do strength workouts is similar to the one i apply when i do FTP at self selected cadence (approx 85 rpm) - For example i i do 200 watts at 60 rpm => Force = 3.5 vs 250 at 85 rpm => Force = 2.94. So would it be fine to say that my force dropped by 16%? (100(1-2.94/3.5)=16%)

Any geeky advices about these fascinating numbers are more than welcome!

Ride on and see you in watopia

Hi @Nicolas_Krucien

Welcome to the forum.

the formula is p = T * 2*π * rpm / 60

p = Rotational Power

T = Torque (Nm Newton meter) (Force * distance)

RPM = Revelations per minute.

So if you have power and want Torque then T= p / (2*π * rpm / 60)

But it get more complicated if your measure power at the ear wheel, then you have to take the gear ratio in to account.

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Hi @Gerrie_Delport_ODZ

First of all, many thanks for taking the time to answer my question - much appreciated!

I wanted to play a little bit with this fancy formula but unfortunately not much to do given that it boils down to T = P/V.

I’ve seen somewhere else that Torque (T) is the product of Force (F) and Distance (D). Would you know how to compute it? Basically i would like to know how much force i generate when i do legs strenghtening workouts at low cadence (say 210w at 60 rpm) compared to FTP effort at comfortable cadence (say 250w at 85 rpm).

May naive understanding is that at low cadence (< 65 rpm) the effort is primarily muscular (we quickly accumulate lactate in the muscles but heart rate remains relatively low) and at high cadence (> 80 rpm) effort tends to rely more on the cardiovascular system such that heart rate will quickly increase.

I am asking this question because sometimes it feels that FTP efforts at preferred rpm (in my case around 85) feel more difficult (ie require more pressure on the pedals) than strength efforts - But when i look at the maths this is definitely not true.

Thanks again for your help!

Yes T (N/m) = F (newtons) x Distance (meter)

If you measure power at the pedal then distance is your crank length in meters.

In classical mechanics its the (vector) cross product of the position vector and the force vector r X F (note r X F = - F X r) where r and F are vectors. The magnitude would be |r| |F| Sin(t) where t is the angle between the vectors.

BTW some crank based power meters include torque (magnitude) in their output stream as its determined by the strain gauges.