First few workouts with PowerTap SL

[Pureshot78]

I have been browsing the threads here for some time now but just recently found a great deal on a PowerTap SL and joined the world of those training with power. Up until that time I had been simulating training with power by doing all of my workouts indoors (was forced to until recently by the weather here in Madison) using speed / cadence on a trainer as a guide.

Although I didn't know what wattage i was producing, I did know that i could do X Cadence in X Gear which produced X MPH, repeatable, and consistent.

A few questions then are in order.
1) I was roughly estimating my FT power to be 280-290 watts based on the power chart put out by cycleops and the "wattage computer" that kurt kinetic has available. (My roommate and I own one of each trainer if you were wondering.) When I put on my new PowerTap SL wheel, built into a Mavic Open Pro, I have been unable to average more than 250 watts in a 20 minute interval. Interestingly enough, when done on the trainer, I cannot maintain the same Cadence / Gear / Speed combination that I used to be able to on my other wheel. The other wheel was a Bontrager Select, nothing special, so could the difference in weight of the PowerTap SL cause that much of a change?? (It even has the same tire on it that the previous wheel did.)

2) I've included my first .wko file from a decent outdoor ride in the madison area. The same question applies, my peak 20 min power was 248 watts, but I would expect this to be a little lower than what I could do in a 2 x 20 based on the other intervals in the workout.
A little background on me:
Age: 27
Height: 5' 11"
Weight: 188
Years cycling: 1.5

P.S. I can't figure out how to upload a .wko file to the forum... any help?

 

[frenchyge]

A few questions then are in order.
1) I was roughly estimating my FT power to be 280-290 watts based on the power chart put out by cycleops and the "wattage computer" that kurt kinetic has available. (My roommate and I own one of each trainer if you were wondering.) When I put on my new PowerTap SL wheel, built into a Mavic Open Pro, I have been unable to average more than 250 watts in a 20 minute interval. Interestingly enough, when done on the trainer, I cannot maintain the same Cadence / Gear / Speed combination that I used to be able to on my other wheel. The other wheel was a Bontrager Select, nothing special, so could the difference in weight of the PowerTap SL cause that much of a change?? (It even has the same tire on it that the previous wheel did.)

Weight will not be a factor on a stationary trainer, but the contact pressure between the tire and resistance unit will be, as will tire inflation. Make sure you haven't cranked down the resistance unit on your PT wheel more than you had with your previous wheel. Also, the resistance of stationary trainers will drop off slightly during the warmup period, as the fluid loses some of its viscosity. Make sure your trainer is adequately warmed up before trying to compare cadence/gear/speed values from before.

P.S. I can't figure out how to upload a .wko file to the forum... any help?

I say zip it.... zip it good!

 

[RapDaddyo]

What tires are on each wheel? Tires (rolling resistance) can have a big impact on watts required to ride at a given speed, in addition to the tire pressure and other issues that frenchy mentioned.

 

[Pureshot78]

What tires are on each wheel? Tires (rolling resistance) can have a big impact on watts required to ride at a given speed, in addition to the tire pressure and other issues that frenchy mentioned.

I have Michelin Race Pro2 tires on my PowerTap Wheel and Front wheel I use for racing. The Bontrager now has a Conti HomeTrainer tire.

 

[RapDaddyo]

I have Michelin Race Pro2 tires on my PowerTap Wheel and Front wheel I use for racing. The Bontrager now has a Conti HomeTrainer tire.

I don't know about the Conti, but the Michelin tire has a very low rolling resistance. Rolling resistance is unlikely to be the explanation. It's not surprising that true power (measured by the hub) is different from power as measured by some trainers or derived from a chart supplied by the trainer mfgr. What is surprising is that you can't maintain the same cadence and speed if the resistance of the trainer, tire pressure, etc. are all identical.

 

[Pureshot78]

I don't know about the Conti, but the Michelin tire has a very low rolling resistance. Rolling resistance is unlikely to be the explanation. It's not surprising that true power (measured by the hub) is different from power as measured by some trainers or derived from a chart supplied by the trainer mfgr. What is surprising is that you can't maintain the same cadence and speed if the resistance of the trainer, tire pressure, etc. are all identical.

Now to make things REALLY wierd, my roommate rode my bike at the gear, cadence, speed that he is used to doing 20 minute intervals on the cycleops trainer and my powertap displayed 225 watts. Then he moved the bike to the Kurt Kinetic where I usually do my work outs and did a 20 minute interval at 275 watts. What gives???

 

[shawndoggy]

What gives???

Methinks that the two trainers have different resistence curves.

And you are calebrating the PT, right?

 

[Pureshot78]

Methinks that the two trainers have different resistence curves.

And you are calebrating the PT, right?

The PT has been correctly zeroed. I know this isn't scientific, but for a fixed Perceived Effort, if two trainers have different resistance curves, shouldn't the only difference be the speed at which the wheel is turning? My brain is telling me the wattage displayed should be nearly the same, am i crazy?

 

[RapDaddyo]

The PT has been correctly zeroed. I know this isn't scientific, but for a fixed Perceived Effort, if two trainers have different resistance curves, shouldn't the only difference be the speed at which the wheel is turning? My brain is telling me the wattage displayed should be nearly the same, am i crazy?

I think if you have zeroed the torque of the PT (and if it is performing to spec), you should find no difference in the effort required to ride at a given power (e.g., 250w) on two trainers, assuming you have a gear/resistance combo for that power on that trainer. Speed may be different between two trainers (even two identical trainers) and speed may change over time as the unit heats up. But, 250w=250w. Rely on the PT watts and forget about synchronizing the resistance on the trainers.

 

[frenchyge]

I think if you have zeroed the torque of the PT (and if it is performing to spec), you should find no difference in the effort required to ride at a given power (e.g., 250w) on two trainers, assuming you have a gear/resistance combo for that power on that trainer. Speed may be different between two trainers (even two identical trainers) and speed may change over time as the unit heats up. But, 250w=250w. Rely on the PT watts and forget about synchronizing the resistance on the trainers.

Agree. There may be a slight difference in feel between the two trainers because of the differing flywheel inertias, but that probably wouldn't explain the differences in power that you're talking about. Also, wheel slippage can produce a lower wattage for a given PE, so check that. Bottom line, ride on whichever trainer you're able to generate the most power vs. duration, or just get accustomed to the trainer you use most, and happy training.