SRM vs Computrainer wattage discrepancy?

[Max358]

I recently bought an SRM after using the Computrainer for several months, and the first ride I did with both gave about a 50w spread between the two measuring devices. I've read posts here that show a fairly close relationship when comparing devices, so I'm a little stumped on the big spread I see. Any tips? Many thanks -
MAX

 

[Woofer]

I recently bought an SRM after using the Computrainer for several months, and the first ride I did with both gave about a 50w spread between the two measuring devices. I've read posts here that show a fairly close relationship when comparing devices, so I'm a little stumped on the big spread I see. Any tips? Many thanks -
MAX

Calibrated SRM?
Zeroed SRM?
Calibrated and warmed up Computrainer?
Drivetrain losses taken into account?
The only people who claim the Computrainer is as accurate as an SRM are selling Computrainers or trying to rationalize their purchase of a Computrainer...

 

[PhilH]

Calibrated SRM?
The only people who claim the Computrainer is as accurate as an SRM are selling Computrainers or trying to rationalize their purchase of a Computrainer...

or have taken the time to check the calibration ( and if necessary calibrate) of their computrainer against their (in my case powertap) other powermenter.

right Rick

Phil

 

[rmur17]

or have taken the time to check the calibration ( and if necessary calibrate) of their computrainer against their (in my case powertap) other powermenter.

right Rick

Phil

yes ... it's all in a slight twist of the wrist. Good advice from Woofer except for the rant at the end

I've adjusted three CT's here and after over a year on mine have obtained super repeatablilty between my CT and PT (the others I've not rechecked). Error better than +/- 1% or better than either manufacturer claims when averaged across intervals/rides of at least 5-min in length.

For the OP, yes check your zeroing first, CT calibration (rrc) 2nd, SRM slope 3rd and report back. Drivetrain should be only 2% or so with the SRM reading higher of course.

FWIW I adjusted my SRM down a little to match my PT in a mid-gear/cadence check I ran. I hate having different devices giving different readings!

rick

 

[Max358]

Thanks all. I will report back after taking all the steps mentioned above. My SRM is reading about 50 lower than the CT, not good for the morale after thinking I'm making progress on the CT. Ha! Thanks again - MAX

 

[Max358]

Thanks all. I will report back after taking all the steps mentioned above. My SRM is reading about 50 lower than the CT, not good for the morale after thinking I'm making progress on the CT. Ha! Thanks again - MAX

 

[JIM WV]

yes ... it's all in a slight twist of the wrist. Good advice from Woofer except for the rant at the end

I've adjusted three CT's here and after over a year on mine have obtained super repeatablilty between my CT and PT (the others I've not rechecked). Error better than +/- 1% or better than either manufacturer claims when averaged across intervals/rides of at least 5-min in length.

For the OP, yes check your zeroing first, CT calibration (rrc) 2nd, SRM slope 3rd and report back. Drivetrain should be only 2% or so with the SRM reading higher of course.

FWIW I adjusted my SRM down a little to match my PT in a mid-gear/cadence check I ran. I hate having different devices giving different readings!

rick

Rmur17:

Got a question as I've got a discrepancy between my PT and CT, about 30 watts or so. How do you calibrate/adjust the CT? Are you referring to something other than the coast down calibration at the beginning of a workout? When you say adjustment, I get the feeling that you are mentioning something else. I've always done the coast down religiously but can't seem to get the two different pieces of equipment to be any closer in readout. Thanks

 

[rmur17]

Rmur17:

Got a question as I've got a discrepancy between my PT and CT, about 30 watts or so. How do you calibrate/adjust the CT? Are you referring to something other than the coast down calibration at the beginning of a workout? When you say adjustment, I get the feeling that you are mentioning something else. I've always done the coast down religiously but can't seem to get the two different pieces of equipment to be any closer in readout. Thanks

had a long reply written but my web connection dropped out. Yes I meant adjusting the slope of the CT's load curve such that the power will agree with your PM of choice. You have to be happy with that PM of course and ensure it's not a zeroing issue etc. I found the newish CT's were within 8% of my PT .... which really isn't bad considering the vagaries of a tire/roller interface..

rmur

 

[rmur17]

had a long reply written but my web connection dropped out. Yes I meant adjusting the slope of the CT's load curve such that the power will agree with your PM of choice. You have to be happy with that PM of course and ensure it's not a zeroing issue etc. I found the newish CT's were within 8% of my PT .... which really isn't bad considering the vagaries of a tire/roller interface..

rmur

Followup: here's a plot of my recent (Nov.05 - Feb.06) PT reported versus CT power over the past few months. Data was from either ergo intervals of 5+ min in length (pretty steady state) or rides up to 1:30 in 3D mode (steady-state to pretty variable L3 hilly rides).

I think they're plenty close enough over a reasonably wide average power range, wheelspeed etc.

So it is possible to obtain very accurate and consistent results between the two

rmur

 

[acoggan]

So it is possible to obtain very accurate and consistent results between the two

Racermate should hire you.

 

[PhilH]

I can attest to the accuracy once the two are calibrated. My CT originally read consistently 30w high across a large range of power. Early last year I followed these directions and after 15 minutes of patient tweaking and it now averages +- 3 watts in ergo mode (depends on how picky I am with my roll-down calibration).

Just to let you know my CT is from 1996 and I still use the nintendo system with it. Ninety percent of the time it is being run just using the handlebar controller. I run the powertap at the same time because this old system has no way to record data

Phil

 

[Max358]

So you adjusted your CT to match the PT? Why not adjust the PT to match the CT? Generally, which one should be calibrated/adjusted to match the other, i.e., which one controls? Does it matter? 30 watts is pretty significant, and it seems like one would like to know the "true" wattage he/she could produce.
MAX

 

[acoggan]

So you adjusted your CT to match the PT? Why not adjust the PT to match the CT? Generally, which one should be calibrated/adjusted to match the other, i.e., which one controls? Does it matter? 30 watts is pretty significant, and it seems like one would like to know the "true" wattage he/she could produce.
MAX

Unlike an SRM, you can't recalibrate a PowerTap yourself. However, you can check the accuracy of the calibration of the strain gages by doing a rather simple "stomp test" (i.e., apply a known torque to the pedal, and verify that the PowerTap reads as it shoulds). Many people have done this, and almost without exception they find that the factory calibration is within specifications (i.e., w/in +/-2%). Nonetheless, I still always recommend that people check the accuracy of their PowerTap when they first get it and periodically thereafter, just to be sure.

OTOH, unlike a strain gage based device such as an SRM or PowerTap, the only way of verifying the accuracy of a Computrainer is by comparing it against another device of known accuracy. Many people have done this as well, with varying results - that is, some people find that their Computrainer is quite accurate, whereas others find that it isn't. In general, it seems that newer Computrainers tend to be more accurate than older ones, but I am not aware of any formal evaluations of that impression, and I also don't know if the apparent difference relates to calibration drift in the field, or to improved quality control at the factory level as a result of increased consumer awareness/demand.

The answer to your question, then, is that if the Computrainer doesn't agree with another device of known accuracy, then the Computrainer needs to be recalibrated. This used to require sending the load generator back to the manufacturer, but after suffiicient attention was focussed on the issue Roger Moore of Racermate finally divulged the location of the potentiometer that must be "tweaked" to change the calibration, thus making it possible for people like Rick Murphy to do it themselves.

 

[JIM WV]

The answer to your question, then, is that if the Computrainer doesn't agree with another device of known accuracy, then the Computrainer needs to be recalibrated. This used to require sending the load generator back to the manufacturer, but after suffiicient attention was focussed on the issue Roger Moore of Racermate finally divulged the location of the potentiometer that must be "tweaked" to change the calibration, thus making it possible for people like Rick Murphy to do it themselves.

Andy, Rick, anyone! How do you tweak the "potentiometer"?! I've played around with the coast down numbers and just can't get my PT and CT to align. I am satisfied that my PT is accurate, I've done a stomp test and zeroed it.

 

[Max358]

Thanks, Andy. I bought a new CT in November 05 and a 300 hour old DA $RM about 3 weeks ago. I suspect the SRM is the one that is off since it's been ridden for some time and was on a different bike with different chainrings. I guess I was just hoping, for the amount of money I invested in these devices, that they would straight away be dead on each other. You gave a good description for the PT, but any other tips for the SRM besides the posts above?

 

[Woofer]

Thanks, Andy. I bought a new CT in November 05 and a 300 hour old DA $RM about 3 weeks ago. I suspect the SRM is the one that is off since it's been ridden for some time and was on a different bike with different chainrings. I guess I was just hoping, for the amount of money I invested in these devices, that they would straight away be dead on each other. You gave a good description for the PT, but any other tips for the SRM besides the posts above?

If you buy *any* powermeter, it's a small investment in time compared to the cost of original purchase to actually do a periodic calibration or check of the calibration and a zero before every ride where applicable. If you don't, you have no idea if the powermeter is anywhere close to accurate - the AIS did a study of many, many SRM's and found this to be an issue which could be mitigated by the steps listed above. You have to do the zero thing on the SRM while it *settles* into position after being moved between bikes or chainrings changed. If you don't want to be bothered to doing periodic checking (like tuning a car) get a Polar since it's not possible to calibrate or zero it other than changing the chain weight as it accumulates gunk.

 

[Woofer]

So you adjusted your CT to match the PT? Why not adjust the PT to match the CT? Generally, which one should be calibrated/adjusted to match the other, i.e., which one controls? Does it matter? 30 watts is pretty significant, and it seems like one would like to know the "true" wattage he/she could produce.
MAX

The PT CPU reads the zero offset from the hub so someone would have to make something that simulates the hub radio signal and use this to send the artificial offset to the CPU or possibly one might fiddle with the rear wheel while adjusting the zero offset. One might be able to do this with the SRM by playing with the crank arms when setting the zero offset but why mess with something that is accurate and will need to be accurate when off the CT as well. In both cases one would have to do the reverse calculation of what the torque offset correction should be to match the CT's inaccuracy.

 

[acoggan]

I guess I was just hoping, for the amount of money I invested in these devices, that they would straight away be dead on each other.

You'd think that would be the case, but as you've learned, often it is not. Maybe if we all keep pounding our fists on the table long enough and hard enough, though, the manufacturers will hear us.

You gave a good description for the PT, but any other tips for the SRM besides the posts above?

Since strain-gaged based devices are generally quite linear and since you can't change the calibration of a PowerTap yourself, I've never seen much reason to do a multi-point evaluation of one - to me, the odds of a PowerTap being correct at the particular torque you apply but not at another seem sufficiently low that I've always been satisfied with a single-point verification (esp. since Graber happily swapped out the one PowerTap that I tested which was off by 3%). With an SRM, though, I think it's worth doing a multi-point calibration, if for no other reason than it will give you the peace-of-mind of knowing that yours provides a truly linear response (some Amateur cranks have been found to be decidedly non-linear). Other than that, though, about the only recommendation I can make when following the instructions from SRM (which can be found in the online manual on their website) is to make sure that you load up the crank with enough weight to span the range of torques that you'd normally expect to encounter.

I should add that, in the past, I've been really anal and, e.g., checked the response of my SRMs at 4-8 positions (by removing and replacing the left crank on the 4- or 8-sided spindle), checked them with various chainlines in both rings, etc. However, while both of my Pro cranks exhibit distinct "hot" and "cold" spots as a function of position and/or chain angle, the torque actually applied to powermeter in use will always be highest when either the right or left pedal is forward. In that regard, then, it makes sense to favor using the slope that is determined when the crank is in this position.

 

[acoggan]

Andy, Rick, anyone! How do you tweak the "potentiometer"?! I've played around with the coast down numbers and just can't get my PT and CT to align. I am satisfied that my PT is accurate, I've done a stomp test and zeroed it.

Rick's going to have to help you out with that one, as other than being aware that it can be done, that's the limit of my knowledge. I do vaguely recall that the potentiometer screw can be found on a circuit board that is hiding behind a label on the load generator, but whether it's on the side, top, or what-have-you I don't remember. I also don't know how much to suggest that you initially turn the screw, although I do recall Rick saying that it was quite sensitive since it is only a 1-turn potentiometer.

(Note: I'm also interested in getting Rick's input, since I've got a friend with a 2 y old Computrainer that reads higher than his recently-calibrated SRM by ~15 W, when if anything you'd expect it to read ~5 W below due to frictional losses in the drivetrain.)

 

[rmur17]

Rick's going to have to help you out with that one, as other than being aware that it can be done, that's the limit of my knowledge. I do vaguely recall that the potentiometer screw can be found on a circuit board that is hiding behind a label on the load generator, but whether it's on the side, top, or what-have-you I don't remember. I also don't know how much to suggest that you initially turn the screw, although I do recall Rick saying that it was quite sensitive since it is only a 1-turn potentiometer.

(Note: I'm also interested in getting Rick's input, since I've got a friend with a 2 y old Computrainer that reads higher than his recently-calibrated SRM by ~15 W, when if anything you'd expect it to read ~5 W below due to frictional losses in the drivetrain.)

too busy today to write a proper description ... in general I recommend a 3-step process in which you first check the slope and offset for your CT and chosen PM. Then look at the data to ensure that the characteristics are linear and simply incorrect in slope. Finally, knowing the difference in slope you can be sure that correcting the slope will bring the two inline.

AC, did you get the 'characterization' step procedure I sent a few days ago? I'll take that and append a description of how to find, prepare the test, adjust the pot and re-test. It's sensitive but seemingly pretty robust once it's literally tweaked into position. I'm talking perhaps 1/8 or less of a turn here to correct perhaps a 5% difference. The only tools required are an Allen key to loosen the flywheel on the shaft and a very small flathead screwdriver with at least 1" of shaft.

Works like magic though when it's done ... just pay attention to your tires, decent warmup for the CT, a repeatable rrc # and you should get quite accurate CT power in all modes (well haven't tried pure spinscan so I should leave that one out).

talk later,
rmur