How does gravity affect Power reading

[random101]

G'day all,

I have a question regarding how power readings are affected on inclines/declines?

I'm using a power2max and my assumption is that I always read higher when climbing or sprinting uphill? (20min power is about 70watts higher when climbing). Max power has only been achieved uphill.

Is there any scientific text that explains why this happens?

I can assume gravity has an effect helping create more resistance helping you torque the pedals better (very different to wind resistance) when climbing so a higher FTP?

Having said that - I have had some good power readings in slight descents...

Or is all this my imagination...

[bigfatty]

When you’re time trialing, once you’re up to speed you’ve got a lot of energy in the system and as the pedals go around they’re merely topping up the energy required to sustain a fast pace. In a TT (high speed, flat road, high kinetic energy), the duration that your muscles have to fire is very small. You’re basically firing the muscle for a very short period of time every pedal stroke, but very quickly.

When you’re pedaling up a climb (low kinetic energy, traveling slowly, gravity holding you back), your legs impart force on the pedals for a much longer duration throughout the stroke, even though your cadence might the same as when TT’ing.

In short, your motor patterns are significantly different between time trialing and climbing.

http://cyclingtips.com.au/2013/09/climb ... -affected/

[random101]

Excellent thats the sort of article I was after.

Where does sprinting fall into that with the KE? as it's a 5-20sec max effort?

[bigfatty]

I know it feels like it is a much longer pedal stroke when sprinting but from everything I have read, it certainly states the Sprinting = Fast Twitch Effort. . ie; "pedals go around they’re merely topping up the energy required to sustain a fast pace". Though, I am not an authority on this.

an sprint uphill, a la Purito, well, I think this is stiil fast twitch. Again, I don't know for sure.

or maybe I got this all mixed up??? It is a very good question and I hope someone else chimes in.

[Rick]

I have noticed the same phenomenon for a long time.
I still don't understand exactly why, except to say that the load and momentum conditions are different. Some people seem to be able to put out similar power levels on flats and climbs, but I can't.
Sometimes I think that maybe it is entirely mental: On a climb, if you relax for an instant your speed immediately and dramatically falls. On the flats, I can back off a little without really noticing a change in speed, so I just keep gradually backing off a little so it doesn't hurt so bad. ????

[random101]

For me it's a significant difference, then again I have a bit to go to climb like an A grader and a bit to go to sprint and TT like a pro so... oh well.

But still curious to the physics behind it all