General Discussion Triathlon Talk » Low Power Readings on Powertap Pro + Rss Feed  
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2014-10-06 10:12 AM

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Oakville
Subject: Low Power Readings on Powertap Pro +

I was at a tri swap meet over the weekend and they had a set of Reynolds clincher wheels with a Powertap Pro +.

It was being sold "as is, but was only $200 so I thought I'd take a chance.

The good news is that it was working, but the power numbers seem to be about 70 to 100 watts too low.

I've been training on Golden Cheetah for the past year using virtual power with my Fluid 2 trainer and expected that my actual power would be lower, but not by that much... maybe 20 to 40 watts.

The setup on the Fluid 2 is the same as what I've always been doing - 2 1/2 turns after the wheel touches the tire and then pump to 100 psi.

I've tried calibrating it with my Garmin 500, but still shows these low numbers.

Any suggestions of what could be wrong? 

I would try changing the battery but it didn't come with the cap removal tool.  Could it be as simple as a low battery?

 

 



2014-10-06 10:19 AM
in reply to: Scott71

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Champion
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Knoxville area
Subject: RE: Low Power Readings on Powertap Pro +
Probably not the battery, but maybe. You should be able to get the cover off with some good hand grips. Could be the calibration. Virtual power is just a guess though and can we wildly inaccurate... so that's the first thing that stands out to me.

2014-10-06 10:19 AM
in reply to: Scott71

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Extreme Veteran
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Subject: RE: Low Power Readings on Powertap Pro +
Ha, the good news is the powertap is probably fine. The fluid 2 trainer curve is very very wrong in both GC and TrainerRoad. What kind of power does the PT say you're putting out? I recoiled in shock when I made the switch a year ago.

Congrats on the bargain!
2014-10-06 10:19 AM
in reply to: Scott71


35
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Subject: RE: Low Power Readings on Powertap Pro +
It could be reading correctly and your power is just lower than you thought. I wouldn't start saying it didn't work by comparing to virtual power. What sorts of numbers is it actually putting out for you?

I'd go out for one of your normal routes at a normal pace and record the power but ignore it during the ride. Then when you get home you can look to see if the power "makes sense." Eg if you rode a 30 mile loop and averaged 22 mph but had an average power of 122 then I would say something was off. Alternatively if you averaged 15 mph with a power of 130 then I'd say that would be plausible (of course ignoring lots of other factors in these examples).

I also think you can send it in to cycleops and have G3 internals put in for a few hundred bucks if it really is not working properly.
2014-10-06 10:19 AM
in reply to: Scott71

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over a barrier
Subject: RE: Low Power Readings on Powertap Pro +
In my experience virtual power is not reliable for real measurement.

When you spin down on the power tap is it reading zero watts (after your display captures the zeros)?
2014-10-06 10:38 AM
in reply to: fisherman76

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Oakville
Subject: RE: Low Power Readings on Powertap Pro +

Originally posted by fisherman76 Ha, the good news is the powertap is probably fine. The fluid 2 trainer curve is very very wrong in both GC and TrainerRoad. What kind of power does the PT say you're putting out? I recoiled in shock when I made the switch a year ago. Congrats on the bargain!

I haven't run a comparison along the entire curve yet, but a few numbers jumped out at me.  My FTP is about 240 and so when I hold this pace on GC the comparable wattage on my Garmin and the PT is only about 140 watts.

I have also been targeting sweet spot training between 210 and 220 on GC.  The PT was showing about 115 watts at this pace.

I'm by no means a monster on the bike, but on a flat out and back 30 km course this year I averaged 37 km/hr or about 23 mph.

I assume that I was pushing more than 140 watts to average 23 mph?

FYI I'm 165 pounds, 6 feet tall and riding a Felt B16 with FLO 60/90.

 



2014-10-06 10:44 AM
in reply to: running2far

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Oakville
Subject: RE: Low Power Readings on Powertap Pro +

Originally posted by running2far In my experience virtual power is not reliable for real measurement. When you spin down on the power tap is it reading zero watts (after your display captures the zeros)?

I'll double check tonight, but I do recall that it was zeroing with no effort. 

I did check to see if power only started to register after I hit a certain wattage threshold on GC, but even with very slight effort power was showing on the Garmin.

At the end of the day, even if the number is low then I was wondering if I could just train with it anyway.  I just won't be able to brag about my FTP on this forum! 

2014-10-06 10:56 AM
in reply to: 0

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Elite
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Subject: RE: Low Power Readings on Powertap Pro +

37 km/hr on 140w would be impressive!  

When you do the calibration on your Garmin, what kind of number do you get back?  I suspect with the older PT you have that you just get a 4 or 5 digit number that is exactly the same every time?

If so, any chance you know someone with a LYC that will show you the actual calibration number (I think it's supposed to be somewhere around 510?)



Edited by axteraa 2014-10-06 10:57 AM
2014-10-06 10:59 AM
in reply to: axteraa

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Elite
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PEI, Canada
Subject: RE: Low Power Readings on Powertap Pro +

FWIW, I'm 6'1", 170 lbs and to avg 37 km/hr on a flat course I would likely have to be in the 210w range.

2014-10-06 11:16 AM
in reply to: 0

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Subject: RE: Low Power Readings on Powertap Pro +

Don't judge a PMs accuracy by comparing it to virtual power or numbers on the trainer. 

Instead, go find a 15-20 minute section of road and hammer it like a race or a 20 minute test.  See if your power numbers are more in line with what you expect.  Ride that same 30 km section that you did at 23 mph and see what type of power it takes to do it.  Or at least roughly...since you won't have closed roads and full race gear.

I know a lot of people can use a PM on the trainer, and still hit their desired power zones.  I can't.  The instant I put my bike on the trainer, I have a very difficult time hitting higher power levels given the same effort I would put out on the road.  I have a FTP of around 270 and a lot of times it's difficult for me to ride on the trainer over 200 watts for say a 2x20' set.



Edited by Jason N 2014-10-06 11:18 AM
2014-10-06 11:19 AM
in reply to: Scott71

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Champion
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Subject: RE: Low Power Readings on Powertap Pro +

Call customer service after you have changed battery and zeroed out the PT and still getting low numbers.  Saris customer service is awesome but I'd not tell them you bought it used and I would be sure to call when you are near your bike (ie not from work) as they will problem solve with you on the phone.

I agree with others your old est power numbers may be off but your speed does not match up any where near your power should be for those speeds.

 



2014-10-06 11:50 AM
in reply to: axteraa

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Oakville
Subject: RE: Low Power Readings on Powertap Pro +

Originally posted by axteraa

37 km/hr on 140w would be impressive!  

When you do the calibration on your Garmin, what kind of number do you get back?  I suspect with the older PT you have that you just get a 4 or 5 digit number that is exactly the same every time?

If so, any chance you know someone with a LYC that will show you the actual calibration number (I think it's supposed to be somewhere around 510?)

The calibration number was coming back with a negative number.  I think it was -8800. I tried calibrating it twice and it was the same number both times.

I also tried the torque calibration test from this website - cyclingpowerlab - but pretty sure I screwed it up.  The torque value showing for the PT was 25% higher than what the actual torque should have been.  

2014-10-06 12:14 PM
in reply to: Scott71

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Subject: RE: Low Power Readings on Powertap Pro +
Don't you want to pump the tires to 100psi and than tighten it to 2-1/2 turn? I would assume that you want regular pressure on your tires before putting it on the trainer. Is the power curve similar to this speed vs power curve?

https://docs.google.com/spreadsheets/d/15tqVuiMyviNTqC5tFFo5In6tB6hY...
2014-10-06 12:42 PM
in reply to: KathyG

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Subject: RE: Low Power Readings on Powertap Pro +
Originally posted by KathyG

Call customer service after you have changed battery and zeroed out the PT and still getting low numbers.  Saris customer service is awesome but I'd not tell them you bought it used and I would be sure to call when you are near your bike (ie not from work) as they will problem solve with you on the phone.

I agree with others your old est power numbers may be off but your speed does not match up any where near your power should be for those speeds.

 




Saris doesn't care if you bought it used or not, they will offer to replace for a drastically lowered cost. Best customer service in the business, best be honest.

Before you give up completely
1) focus on spinning VERY consistently, no dead spots or wiggling or hips in your movement. If you can get power up to 200W or higher that way...it's probably ok
2) Be sure your tire pressure and roller height are good as mentioned before
3) See if you can get it up into the 400+W range on a short effort.

If all else fails call Saris.
2014-10-07 5:06 AM
in reply to: Scott71

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Subject: RE: Low Power Readings on Powertap Pro +
Hey Scott,

Long time no chat.

I have a Pro+ and the number I get back is -10614 which I believe is an ID, not a calibration number

The way I can tell if my PM is off is to ride it and look at the file in the AeroLab tab in GC and see if the numbers makes sense.
The bigger the hills the better since aero is less of a factor

If you have some ride files, I can give you a hand.
2014-10-07 9:01 AM
in reply to: marcag

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Oakville
Subject: RE: Low Power Readings on Powertap Pro +

Originally posted by marcag Hey Scott, Long time no chat. I have a Pro+ and the number I get back is -10614 which I believe is an ID, not a calibration number The way I can tell if my PM is off is to ride it and look at the file in the AeroLab tab in GC and see if the numbers makes sense. The bigger the hills the better since aero is less of a factor If you have some ride files, I can give you a hand.

Thanks Marc. As suggested by Jason above, I'm going to try a real world ride before I make any calls to Powertap.

I did some searching on the internet and it turns out that I likely did the torque test properly but was entering the wrong value in the calculator.  I thought the torque number from the Garmin was in foot pounds but its actually in newton meters.

Based on the test I ran, the PT is about 7% below what the actual torque should be, which may explain part of the disparity.

Are you going to do a 2nd run of the Power Mentor Group this winter?  I definitely saw improvements on my bike splits this season and was about 10 to 15 seconds faster per km on the run as well!

I would definitely be a lurker as I need some motivation to suffer through the 5 and 20 minute tests!

 



2014-10-07 11:15 AM
in reply to: Scott71

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Subject: RE: Low Power Readings on Powertap Pro +

As Marc mentioned, if you know how to use aerolab, finding a nice steady hill with decent road surface would be a much more accurate way to determine if your PT is reading correctly.  I just didn't know if you knew how to use aerolab (I'm not good at it myself), but if Marc is willing to help you, then that is the way to go.  Lots of factors could impact your power on a flat road, but you should at least be in the ballpark.  Certainly not going 23 mph at 140 watts if the PT is working properly.

2014-10-07 6:03 PM
in reply to: Scott71

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Subject: RE: Low Power Readings on Powertap Pro +
Originally posted by Scott71
Are you going to do a 2nd run of the Power Mentor Group this winter?  I definitely saw improvements on my bike splits this season and was about 10 to 15 seconds faster per km on the run as well!

I would definitely be a lurker as I need some motivation to suffer through the 5 and 20 minute tests!

 




Probably not in the format of last year.
I would like to do a virtual training group with all 3 sports but not sure what the best format is. Maybe mentor group but possibly other formats.
I'll ping you if I do it

Let me know if you need help with the aerolab stuff.
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