Road Bike Racing - For those who train with a power meter

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EvilGoodGuy
05-21-07, 09:58 AM
Is there a difference between watts on a compu trainer and watts on a power tapp?

For example, will a given effort on a compu trainer be the same as a given effort on the road with a power tapp? I've heard people get higher wattage readings on the road vs. compu trainer.

SirScott
05-21-07, 10:05 AM
There is often a difference, yes. Some trainers such as the Tacx Fortius or imagic allow for a "calibration factor" that will adjust the wattage readings to more closely match another meter.
Snicklefritz
05-21-07, 10:09 AM
I did a test on a computrainer last fall and also had my powertap collecting data at the same time. The powertap numbers were a bit higher.

jrennie
05-21-07, 10:16 AM
If you figure the more contact points the more loss to friction then:
ergomo off the BB would be highest
srm off the crank would be second
PT off the chain third
and Computrainer off the tire last.

The difference between in drivetrain loss is less than 5% so the numbers should be "close enough" a greater variable is some people have a much harder time putting in effort indoors than out and vice versa.
Snicklefritz
05-21-07, 10:38 AM
If you figure the more contact points the more loss to friction then:
ergomo off the BB would be highest
srm off the crank would be second
PT off the chain third
and Computrainer off the tire last.

The difference between in drivetrain loss is less than 5% so the numbers should be "close enough" a greater variable is some people have a much harder time putting in effort indoors than out and vice versa.


+1 I've heard more than a few coaches talk about how some people produce much better numbers on the road then they do in the lab. I know that I do.

Regarding PR, it's getting the info from the hub, not the chain. The chain tension method would be associated with the Polar stuff.
jrennie
05-21-07, 03:27 PM
Loss is through the chain and sprockets not the hub.
brianappleby
05-21-07, 05:51 PM
what snicklefritz is saying is that since the power reading comes from the hub, it's after the chain and sprokets have taken any energy out of the system.

to the OP. I've found that indoor conditions, even with a big fan, make my power output considerably lower than when riding outside. This is using the same power meter for both situations.