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agreement between two powertap hubs

Satish PunnaSatish Punna
in General Training Discussions

PT hubs state accuracy of 2%.  Anyone have real world experience regarding whether power numbers from two different powertap wheels (either on the same bike or different bikes) is practically comparable? Thanks!

Comments

  • russGrussG
    Posted By Satish Punna on 14 Nov 2012 07:00 AM

    PT hubs state accuracy of 2%.  Anyone have real world experience regarding whether power numbers from two different powertap wheels (either on the same bike or different bikes) is practically comparable? Thanks!



    No experience, but if I wanted to know then I would set up on the trainer and ride at a selected cadence and the same gear on each wheel for a long enough period to factor out any minor statistical variation, and compare my power results.

    A more sophisticated test that requires a speed-cadence sensor would be to configure TrainerRoad to use Virtual Power and to hook up your cycle-computer to read the hub power and run basically the same test.  Your comparison can then be adjusted for minor variations by scaling the virtual power.

     

  • John WithrowJohn Withrow
    I have a training wheelset and a race wheelset that both have Powertap hubs. I never did the "actual" testing that Russell suggests, but after lots and lots of riding and knowing what my FTP sets feel like, as far as I could tell, they were pretty much exactly the same. I always did my final FTP test before a big race on my Race Wheel set, and it always came in right where I would have expected it to. So after a bunch of those, I used the two interchangeably without any fear of variance between them.
  • Satish PunnaSatish Punna

    This is exactly what I wanted to know.  Thanks!  I have a training wheel with PT hub and am budgeting for a race wheel with PT hub next year, and I'm going through my usual process of overthinking things. 

  • Dino SartiDino Sarti
    I have been toying with a little test in my mind, but have not done it yet. I could put my Powertap wheel on my bike with the SRM. I could then pair one with my Garmin 910 and the other with my Garmin 500. Then, I could compare the results.... That would be interesting.... I just need to find the time to Nerd out on power....
  • Satish PunnaSatish Punna
    Posted By Dino Sarti on 14 Nov 2012 01:23 PM

    I have been toying with a little test in my mind, but have not done it yet. I could put my Powertap wheel on my bike with the SRM. I could then pair one with my Garmin 910 and the other with my Garmin 500. Then, I could compare the results.... That would be interesting.... I just need to find the time to Nerd out on power....



    I have seen some similar tests posted on the web.  Some say the power numbers reported by the PT hub seem to be lower than SRM or quarq, attributed to drivetrain loss. I was wondering how interchangeable the numbers would be between two PT wheels. 

  • Dino SartiDino Sarti
    I heard/read the same about drive train loss. I got the original question, but was doing a bit of a highjack... Sorry!
  • Satish PunnaSatish Punna
    Posted By Dino Sarti on 14 Nov 2012 05:03 PM

    I heard/read the same about drive train loss. I got the original question, but was doing a bit of a highjack... Sorry!



    No worries.  Having an SRM and powertap is a problem I wish I had. I think I'm going down the road toward two powertaps ( shhhh. Don't tell my wife ). 

  • William JenksWilliam Jenks
    There MUST be drive train loss. The second law says so. :-) Can't remember where I read it, and I can't vouch for the accuracy, but there's supposed to be a couple of watts loss in using badly crossed gears even (because of the bending of the chain)
  • Dino SartiDino Sarti
    Not to continue the highjack, but according to my reading and a conversation with Rich S. at Wheelbuilder, the conventional wisdom is that drive train loss between the crank and hub is about 5 watts worth of measurement. Which is no big deal, because it is supposed to be constant at all power outputs. Not a like a percentage that would increase the number of watts lost at higher outputs.
  • Satish PunnaSatish Punna

     I read the 5 w thing as well, so some people adjust their computers when switching back and forth between the two types of power meters.  I thought that minimizing drive train loss due to cross chaining was another benefit to having compact cranks since they allow most people to stay in the middle of their cassettes ( not sure where I read that).

  • Carlo_TangonanCarlo_Tangonan
    you can use the stomp test to figure out each powertap's torque reading. http://www.cyclepowermeters.com/powertap-garmin-calibration-check-76-c.asp
  • Dino SartiDino Sarti
    I love the name "stomp test". Sounds like a caveman science....
  • Satish PunnaSatish Punna
    Posted By Dino Sarti on 14 Nov 2012 10:22 PM

    Not to continue the highjack, but according to my reading and a conversation with Rich S. at Wheelbuilder, the conventional wisdom is that drive train loss between the crank and hub is about 5 watts worth of measurement. Which is no big deal, because it is supposed to be constant at all power outputs. Not a like a percentage that would increase the number of watts lost at higher outputs.

     

    Has anyone who does IM ever toyed with the idea of re-lubricating their drivetrains DURING a race?

  • Dino SartiDino Sarti

    Some guys try urine during a race, but that's another thread.

  • Satish PunnaSatish Punna

     LOL As soon as I posted this I KNEW someone was going to say that!

  • Dino SartiDino Sarti
    That joke wrote itself! Ba-dump-bump! I am here all week folks.... image
  • John WithrowJohn Withrow
    Oh... that was a joke... I am totally guilty of that lubrication technique mid-race.
  • Joe MatchetteJoe Matchette
    @John -- you gotta do the left-side lean....keeps it away from the drivetrain. image
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