SRM -vs- Powertap

[FORT TT]

I've just learned that you can get a hub, that will measure the amount of watts you produce. (Powertap)

It is to be mounted as rear hub, as you all probably know. But is it as accurate as an SRM crank? I am only thinking about the measuring of watts produced, not all the other cool things an SRM crank can do.

Am I way off, or will there inevitably be a loss of power through the chain compared to measuring directly at the crank?

[ctkrohn]

The Powertap claims accuracy to plus/minus 2%, I believe. Bicycle drivetrains are very efficient, so you lose only a minimal amount of power in the chain.

The Powertap is definitely a better option than an SRM in my opinion because it's so much cheaper and not any less accurate. You lose some flexibility, but if you want you can buy a race Powertap wheel and a training Powertap wheel for less than the price of an SRM Professional.

[MikeM]

I had the same thought about the accuracy of powertap with regard to drivetrain losses. My concern would be the change in power readings over time.

I understand that you have to calibrate the SRM, but I would have anticipated that there would be more drift as a result of chain wear (in the case of the powertap) then the strain guages drifting over time.

Can anyone comment who has used both of these systems extensively? What we need now is for some hotshot to develop a system based on the bending of the pedal axle...

Mike

[peterpen]

Accuracy is basically equivalent between the SRM and the Power Tap. The PT may read slightly lower than the SRM due to drivetrain losses, but that isn't terribly significant as it is fairly constant, and losses due to chain wear are within the margin of error. There were problems with early versions of the PT in re: moisture getting into the hub, but they seemed to be fixed with the PT SL version I have as I've used it in pouring rain repeatedly with no problems.
If you live somehwere very cold (train in temps below freezing) the PT SL may not be the best as the batteries it uses are not rated below 0 celsius. IMO, the chief drawback to the PT is if you like to run different wheels for different conditions/ courses.
I think buying a PT SL was the single best equipment purchase I've ever made - and probably the only one that has made a significant difference in how fast I go.

[dedaciai]

Fort TT,

If you can, wait until June or July of this year when Powertap releases the Powertap SL Wireless. It uses the same technology as a cordless phone to transmit data to the computer.

Landis has been using his since last year's Tour. I am not sure how accurate the SRM vs. Powertap, but for me, if it is within 2-3% should be good enough for training and racing.

I am actually waiting for the wireless version myself because I change between wheels and bikes a lot and it would be too much of a hassle to deal with wires or additional mounting kits.

My post did not answer your question, but at least it gives you something else to think about when considering your purchase.

[Skyleth]

all the other cool things an SRM crank can do.

what are the other cool things that an SRM can do besides telling you wattage?

here's a long discussion on Powertap v. SRM

I have a powertap simply becuase I can't afford a SRM, but it's a tradeoff, w/ the powertap you can use any crankset and bike you want, with a SRM you can use any wheelset you want but are stuck w/ a bike.

I went to a seminar by Andy Coggan one of the premier Power Based coaches out there, and he was talking about how he had the Power Tap and SRM on at the same time and could see the drivetrain loss, but both meters measured accurate to within a few watts of one another, not enough to lean one way or another. It's sort of like how people measure horsepower in a car, you can measure at the engine or at the wheel.

[STARNUT]

I don't the actual wattage is the most important thing. I think repeatability is. even if it is off by 5 watts, it should always be off by 5 watts. Accuracy vs Precision. Which one is more precise? At the Lim siminar he said that a clean drive train came make a 2 watt difference. He was also refering to powertap measurements a lot. I think this would imply that the powertap is very precise. Precise enough to see a 2 watt difference between clean and dirty drive trains. That dosen't mean that an SRM is not precise, just that he didn't use very much data from an SRM.

Starnut

[FORT TT]

Thanks for all of your answers.

I'm not thinking of buying either, they are cool, but I can't see the purpose of it.
Actually, I can, but there is a million things I would rather have for my bike than some machine to discuss with on my rides..

I don't quite see the main difference in usage compared to a cadence sensor, that also is a gimmick to evaluate the training.

It was just a matter of my brain going wild with this question and I knew that I could count on you guys!

[flying wombat]

Thanks for all of your answers.

I'm not thinking of buying either, they are cool, but I can't see the purpose of it.
Actually, I can, but there is a million things I would rather have for my bike than some machine to discuss with on my rides..

I don't quite see the main difference in usage compared to a cadence sensor, that also is a gimmick to evaluate the training.

It was just a matter of my brain going wild with this question and I knew that I could count on you guys!

There is a Huge difference between cadence sensor and power training. Power is an absolute measurement of what you are doing, having trouble hanging onto the group? Are they going faster today or are you tired? If I change my position on the TT bike how does if affect my power? Power can tell me that I am struggling to maintain my output, so its time to drop off and take it easy. There is a reason that a huge percentage of pro riders use powermeters to analyze their training.
If you arent racing seriously then there is not a lot of point to a powermeter, but I would rather have a powermeter than some lightweight gimmick piece on my bike that saves less than a trip to the portajohn. But to each their own.

[FORT TT]

Thanks for all of your answers.

I'm not thinking of buying either, they are cool, but I can't see the purpose of it.
Actually, I can, but there is a million things I would rather have for my bike than some machine to discuss with on my rides..

I don't quite see the main difference in usage compared to a cadence sensor, that also is a gimmick to evaluate the training.

It was just a matter of my brain going wild with this question and I knew that I could count on you guys!

There is a Huge difference between cadence sensor and power training. Power is an absolute measurement of what you are doing, having trouble hanging onto the group? Are they going faster today or are you tired? If I change my position on the TT bike how does if affect my power? Power can tell me that I am struggling to maintain my output, so its time to drop off and take it easy. There is a reason that a huge percentage of pro riders use powermeters to analyze their training.
If you arent racing seriously then there is not a lot of point to a powermeter, but I would rather have a powermeter than some lightweight gimmick piece on my bike that saves less than a trip to the portajohn. But to each their own.

Okay, I get your point. But as yu stated, if I don't ride for muvh more than the fun of it, it's a waste of dough...