SRM Amateur vs PowerTap Pro

Powertap if you're trying to save some money.

SRM if you're going for accuracy. Although Powertap is coming out with a version that might be more accurate.

nrhorwitz said:

Powertap if you're trying to save some money.

SRM if you're going for accuracy. Although Powertap is coming out with a version that might be more accurate.

The Power Tap is as accurate as the SRM Pro and more accurate in general than the SRM Amateur. There was a recent paper in Med and Sci in Sports and Exercise comparing the two systems. The issue with the SRM Amateur is that

1) it doesn't always come from the factory correctly calibrated (this is an issue with the SRM Pro to)
2) Amateur's slope can drift a lot and you may need to do regular calibration checks using a known certified mass at 4 points around the crank on both left and right crank to calculate an average slope
3) on the Amateur the left and right cranks (when applying a known force separately to each side) may well provide quite large differences which effects the accuracy

Looking at the above paper in MSSE, from memory (and this is my experience too) the Amateurs can be up to 10% off in terms of accuracy (i think the range in the paper was 2 - 10+%). With the PT in that paper the accuracy for the units was 2 to (something like) 2.9% (don't have the paper to hand).

Big advantage with the SRM is the ability to use any wheels. Big advantage with the PT is price and the ability to easily swap wheels between bikes and no calibration is required.

Disadvantage with the PT is in very heavy rain, the wiring kit can get waterlogged and stop working. Disadvantage with the SRM is price

ric

ric_stern/RST said:

1) it doesn't always come from the factory correctly calibrated (this is an issue with the SRM Pro to)
2) Amateur's slope can drift a lot and you may need to do regular calibration checks using a known certified mass at 4 points around the crank on both left and right crank to calculate an average slope
3) on the Amateur the left and right cranks (when applying a known force separately to each side) may well provide quite large differences which effects the accuracy

Not from a published study, but from several individuals who performed tests on their personal SRM-am:

4) A potential for a non-linear torque response from the strain gauges. e.g. you might calibrate it to read right at 3)lbs, but it might over or under report at different force levels. In practice what this means is that your accuracy would vary with cadence.

IMNSHO, the SRM am is clearly inferior to the PT pro or am in terms of accuracy. Not to mention the poor temperature compensation on all SRMs.

beerco said:

Not from a published study, but from several individuals who performed tests on their personal SRM-am:

4) A potential for a non-linear torque response from the strain gauges. e.g. you might calibrate it to read right at 3)lbs, but it might over or under report at different force levels. In practice what this means is that your accuracy would vary with cadence.

IMNSHO, the SRM am is clearly inferior to the PT pro or am in terms of accuracy. Not to mention the poor temperature compensation on all SRMs.

i agree with your summing up. and yes, while eating breakfast this morning 😄 i forgot to mention part 4 -- so, i recommend 4 different points around the cranks, both cranks and several different masses. it's such a freakin hassle.

ric