VeloReality Forum

This might be a stupid newbie question but here it is anyway.

I have a Wahoo Kickr Snap and I rode a nice and easy Lago d’Iseo route with about averages of 27km/h and 130w (watts reported by Kickr and saved to Polar unit). I decided to have a little hill experience in the end to raise my hr and rode a hill from West Flanders video. With the Lago d’Iseo I used 53 and 19, but for the hill I had to go to 39/27 just to get up the hill. My speed average was around 11km/h and the power was about the same as in the easy route. However the resistance was in completely different level and my heart level went high just as I wanted.

What puzzles me is the power output. If I used about the same cadenza, why the power output is the same for the easy resistance and the hard resistance? I understand the the speed was obviously much slower, but still the resistance was very hard when driving "up" the hill. If the final power output is about the same, shouldn't the resistance feel similar in both cases?

lemppari wrote:This might be a stupid newbie question but here it is anyway.

I have a Wahoo Kickr Snap and I rode a nice and easy Lago d’Iseo route with about averages of 27km/h and 130w (watts reported by Kickr and saved to Polar unit). I decided to have a little hill experience in the end to raise my hr and rode a hill from West Flanders video. With the Lago d’Iseo I used 53 and 19, but for the hill I had to go to 39/27 just to get up the hill. My speed average was around 11km/h and the power was about the same as in the easy route. However the resistance was in completely different level and my heart level went high just as I wanted.

What puzzles me is the power output. If I used about the same cadenza, why the power output is the same for the easy resistance and the hard resistance? I understand the the speed was obviously much slower, but still the resistance was very hard when driving "up" the hill. If the final power output is about the same, shouldn't the resistance feel similar in both cases?

Power=Force x Velocity

As you rode at the same power and your velocity had decreased from 27kph down to 11 kph you obviously had to apply more force: 27/11= 2.45 times more force. You did reduce it a bit by changing gears: (53/19)/(39/27)=1.93. Not enough to completely compensate so in the end you had to apply 2.45/1.93=1.27 times more force hence you felt more resistance.

Thanks! Of course... It's so easy to mix power with just the force. It's still a little confusing, because just looking at the power output, you can't tell how the training felt. You could be working very hard but still the power output is same as with the easy training.

lemppari wrote:Thanks! Of course... It's so easy to mix power with just the force. It's still a little confusing, because just looking at the power output, you can't tell how the training felt. You could be working very hard but still the power output is same as with the easy training.

Watts & heart rate are the numbers that say how hard it actually is & how hard it is for your body's circulation. If your training to become fitter then your goal will be to produce more watts for a given heart rate. What happens on steep hills though is torque increases as your cadence drops below a level thats efficient, so your adding strain to your muscles which is a bit like switching from a running workout to a weight training one..if you dont like doing weights then it will hurt but you may not actually be working your heart & lungs any harder if your heart rate remains the same...the solution is to either get used to it or use lower gears

Yes, those two (power and heart rate) should be combined to get a full picture. First reaction after working my a** off was a suprise since the power was the same as in the easy part.

BTW. I probably need to convert my West Flanders videos from 1080 to 720 since my garage computer doesn't have enough power to run them smoothly. Will there be any problems getting them recognized as licensed after converting?

lemppari wrote:BTW. I probably need to convert my West Flanders videos from 1080 to 720 since my garage computer doesn't have enough power to run them smoothly. Will there be any problems getting them recognized as licensed after converting?

your file must be renamed as following:

Let's say your original file was:
0013_BE-WestFlanders_1.1080.wmv

The 720p version must be renamed as
0013_BE-WestFlanders_1.720.wmv

Also do not keep the those in the same folder.

admin wrote:

lemppari wrote:This might be a stupid newbie question but here it is anyway.

I have a Wahoo Kickr Snap and I rode a nice and easy Lago d’Iseo route with about averages of 27km/h and 130w (watts reported by Kickr and saved to Polar unit). I decided to have a little hill experience in the end to raise my hr and rode a hill from West Flanders video. With the Lago d’Iseo I used 53 and 19, but for the hill I had to go to 39/27 just to get up the hill. My speed average was around 11km/h and the power was about the same as in the easy route. However the resistance was in completely different level and my heart level went high just as I wanted.

What puzzles me is the power output. If I used about the same cadenza, why the power output is the same for the easy resistance and the hard resistance? I understand the the speed was obviously much slower, but still the resistance was very hard when driving "up" the hill. If the final power output is about the same, shouldn't the resistance feel similar in both cases?

Power=Force x Velocity

As you rode at the same power and your velocity had decreased from 27kph down to 11 kph you obviously had to apply more force: 27/11= 2.45 times more force. You did reduce it a bit by changing gears: (53/19)/(39/27)=1.93. Not enough to completely compensate so in the end you had to apply 2.45/1.93=1.27 times more force hence you felt more resistance.

I am not sure I buy this... quite yet. The velocity in 'power=force x velocity' is the angular velocity of the pedals and not the velocity of the bike down the road, in the context a question about the force against the pedals, or the resistance at the pedals. I think the original point or question still stands - for a given power output and cadence ( = same angular velocity), the resistance should also be constant. No?

-Pascal

I am not sure I buy this... quite yet.

I am not sure I am trying to sell it to you. I am giving reasonable and correct explanation based on reported conditions. Read the whole thing more carefully. You should also account for "about the same" parts of original post. No real data.

And finally software tells weight of cyclist, road grade etc. etc to "Smart" trainer and it is the trainer that decides on what level of resistance it will apply at each particular moment based on input conditions. So if you wish you can open discussion with Wahoo Fitness and see what they say.

Using the original figures for speeds and gears ratios shows the cadence on the flat is 1.27 x the cadence uphill.
Assuming typical road bike 700-23C wheels, the actual cadences come out as 76.9 and 60.5 which is a ratio of 1.27.
But you get the same ratio whatever the wheel size.

Admin's calculation of 1.27 is the same thing and demonstrates a much lower cadence was used uphill.

Power = Force x Velocity = force at rear wheel x bicycle speed down the road.
This can be shown to be the same as (average) force at pedals x cadence. (ignoring pedal radius which is a constant for a given bike).

So cadence lower by a factor of 1.27 means force at pedals is up by a factor of 1.27 to maintain the same power.
That's why it felt harder uphill for the same power.
Admin's argument was correct.
The confusion comes about by assuming cadences were the same in each case when they are not.