 # Speed question and power question

I have two unrelated questions on tracking performance.

Speed question, specifically as it relates to a Kickr Bike. When you begin a video and Bluetooth connects to the bike, you have two speed options to choose from. One is the speed the bike records and the other is the Wahoo calculated speed. For those who have a Kickr Bike, any feel for which might be a more accurate representation of what your effort would yield in speed out on the road?

Power question, specifically calculating power when riding out on the road. I don’t have a power meter so I don’t have anything that calculate power when I am out on the road. Is there a calculator out on the web or is there a formula for calculating your power? I have the usual info to work with; distance, time, total climb.

Can’t help with the Kickr bike but you can work out power on a climb. This GCN video should help.

As I understand it, the required power to move a bike can be expressed as:

P(v) = a * v^3 + (b + c) * v

v being the velocity in m/s (speed in km/h divided by 3.6)

a = 1/2 * Rho_air * Cd * Area

with:

• Rho_air being the specific gravity of air at 1.0kg/m^3
• Cd being the drag coefficient of the rider and bike (dependent on how you position yourself. I’ve assumed a Cd of 0.92 in my calcs)
• Area being the frontal area of the rider and bike in m^2 (i’ve assumed the area to be 0.5m^2)

b = Crr * Mass * g

With:

• Crr being the rolling resistance of the tyres (I’m using 0.005)
• Mass being the weight of the rider and bike in kg (I’m using 75+10 = 85kg)
• g being gravity at 9.81 m/s^2

Then for the uphill sections you need to add to the c term:

c = Mass * g * gradient

with:

• Mass and g the same as above

Applying the above, I come to:

• 168W to travel 30km/h, and
• 362W to travel 40km/h.

This correlates very closely with my Kickr in Level 0 mode.

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To connect what @TrapMeSuf said directly to the OP’s question, I think (but am not positive) that speed calculated by SYSTM is calculated from power ( using @TrapMeSuf’s equations). I think that speed from the bike is calculated by multiplying the RPM of the flywheel by an assumed wheel circumference (which you can probably change in the bike configuration—at least you can with a KICKR trainer).

I’ll test it out and report back.

I don’t have a Kickr bike but the gen rec is use SUF/SYSTM speed. It’s a more accurate representation taking into account the effort required and will result in more accurate mileage if indoor mileage matters to you.

As for power calculation. There’s this:
https://www.gribble.org/cycling/power_v_speed.html

Good luck figuring it out while riding   Fwiw, I’d suggest picking up a single sided PM for outdoor riding. You can get one from 4iiii, or Stages in the \$400 range, or a PowerPod for a bit cheaper.

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