What's wrong ? powertap numbers too high !

Discussion in 'Power Training' started by abuck, Aug 15, 2010.

  1. abuck

    abuck New Member

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    Let me introduce first, This is my first season that I'm training hard and power meating is pretty new to me. I owns a powertap SL+ and a computrainer.

    I've done a 15K Time trial and my average power shows up as 332 watts which I really don't beleive this is right. I weight 165 LBS and I have a trek madone 5.1 with no aero bars. My time at the 15KM was 22.57mins and average speed of 38.8km/h (24.1mph). I think if I would push really 332 watts, my time would have been better isn't ?

    I am using Garmin Edge 705 and before the race, I did a calibration on it. What's weird is if I use my computrainer, the numbers seems to be pretty close to my powertap. I am doing some intervals with my computrainer at 350watts and after a minute, I am completely dead...

    What could be wrong with my powertap ?

    here's my info stats if it can help...

     
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  2. Alex Simmons

    Alex Simmons Member

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    There are many things that can influence the speed - power relationship.
    Your meter may well be reading incorrectly, however I would suggest considering the other possibilities:
    - your aerodynamics are not flash on the road bike
    - environmental conditions, especially wind
    - tyres and road surface
    - course gradients
     
  3. abuck

    abuck New Member

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    Thanks for you reply, yes you are right, these factors might impact the speed. However, i am just starting bicking this year, I don't think it's possible that I'm pushing 330watts ! Also, if I compare to my computrainer, I can only last maybe 2 minutes (3 at the max, max, max) at 350 watts. I could never last 330 watts in 23 minutes ! and if you look at my Peak 10min it shows as 350 watts. I could NEVER handle 330wats for 10 minutes in my computrainer !
     
  4. daveryanwyoming

    daveryanwyoming Well-Known Member

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    A few thoughts:

    - Computrainers aren't always that accurate. I ride them all winter and have seen big deltas between the CT readings and readings from torque tested PT and lately SRM power meters and that's after a substantial warmup and coast down calibration of the CT. Sometimes the CT (mine and those at commercial coaching facilities where I do indoor work in the winter) vary by as much as 30 watts above or below what my power meters record for a Threshold effort.

    - Many cyclists can't sustain the same power indoors on the CT as they can outdoors on the road. I've never seen a 20 or 30 minute indoor interval that came closer than 7% to my regular weekly outdoor L4 efforts. At the beginning of each winter my indoor numbers are usually 10% to 12% below my outdoor numbers but the gap closes with regular indoor training. Add extra race day motivation and outdoor numbers can be even higher than usual.

    - Did you remember to zero the torque on your PT before the ride? Your data 'suggests' that you did as you recorded samples of zero crank torque for the various listed durations but it's hard to be certain unless you zeroed the torque before the ride.

    - Have you done a static torque test on your PT wheel? That's the best way to rule out measurement errors or at least to understand the magnitude of those errors. You can't field calibrate a PT like you can an SRM but you can at least check it statically to see if the torque tube is way out of calibration. At the very least you can do the PT self test but that's not always definitive as it just measures the unloaded bias point of the PT strain gauge which doesn't always ensure that the strain gauge is accurate under load.

    - You averaged 10.77 m/s for your time trial. A quick look at analytic cycling and their speed for power calculator shows that for a total kit weight of 85 kg, at sea level on a less than perfect surface (or with something like higher Crr tires with butyl tubes) and a CdA of .35 (which isn't unheard of for a road bike in the drops especially if you haven't been fitted with emphasis on aerodynamics) you'd expect to average 10.78 m/s and that doesn't include the start, any cornering or a turnaround and neglects wind. IOW, given you were on a road bike and even assuming you stayed in the drops the whole time your speed isn't too surprising. Sure with a good road position you might get your drops CdA down below .30 but .32-.33 seems pretty common for cyclists that have worked on getting aero. A run of the mill bike shop fit for comfort probably won't get you anywhere near .30.

    FWIW a pair of clip on bars can bring your CdA down in a hurry and for a well fitted full time trial rig CdAs in the .22-.25 region are fairly common. IOW, that same 330 watts(assuming the PT was accurate) with a pair of fast rolling tires/tubes and an aero bar setup that gets your CdA down to a modest .25 would have increased your average speed to roughly 12.25 m/s or a time of 20.4 minutes if you neglect the start, cornering, turnaround and wind.

    Good luck,
    -Dave
     
  5. InPursuit

    InPursuit New Member

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    Is your powertap zero'd out?
     
  6. abuck

    abuck New Member

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    First, thansk for you info Dave, this is REALLY appreciated. Here's my feedback:

    Zero the torque means calibration, right ? Yes, I have make sure that I have done calibration before racing.

    No I haven't done it, I will do it after work tonight, I will let you know my results.

    Again, you are right, considering that my PT show me good results.

    Another interesting facts, which I don't trust PT numbers is I went for a V02max/lactate test and here's my results:
    [​IMG]

    based on those numbers, it's really not possible that I can do 330watts for 23 minutes.

    Anyway, I will do the static torque test tonight and I will post my finds :)

    thanks !
     
  7. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Well, zeroing the torque isn't technically a calibration (but Saris might call it that, I can't remember) it sets the zero point sort of like adjusting the dial on an older bathroom scale so that the needle points to zero before you step on it. But that doesn't mean the device is actually accurate when loaded and real calibration would involve checking a few points (possibly including the unloaded point and one or more in the operating range of interest) and adjusting the meter so that it was within some accuracy range. You can do this with an SRM and I believe you can do it with a Quarg but you can't actually do that with a PT, you can test the accuracy across a range of interest but if it's not reporting accurately you can't make any field adjustments to bring it into line.

    But that's mostly semantics, always zero the torque before you ride, especially if there are big temperature variations like taking the bike from a cool house outside on a hot day or vice versa in the winter.

    For the most repeatable results use a fixed weight of 20 kg or more during torque testing. You can use your body weight assuming you can balance with nearly all your weight on one pedal while making sure that pedal is horizontal and the rear wheel is braked. Some folks have good luck with this quick 'stomp test' method my results haven't been as consistent and repeatable, I guess my balance could use work.

    But since you suspect your PM is reading high by quite a bit you can probably learn what you need from a body weight stomp test since errors in that test will result in producing less torque than expected if you can't get all the weight balanced on a pedal or the crank isn't very close to horizontal. IOW, if you get a higher than expected torque reading on your PT CPU while doing a stomp test and you're sure about your weight and gear ratios then the meter is reading high as you suspect as stomping errors lead to low values not overly high values.
    Yes, it does seem unlikely that you sustained 330 watts for more than 20 minutes based on those results assuming it was a recent test and your fitness hasn't improved since then.

    Good luck,
    -Dave
     
  8. swampy1970

    swampy1970 Well-Known Member

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    But those numbers are not out of the question. I used to time trial at a level that would leave me with ~6mmol of blood lactate - verified indoors on an ergometer and outdoors on a track and at a point where the lactate curve was way past the classic deflection point. If you tack on a few percent to those power number from his test (for the indoor/outdoor factor) then you're entering the relms of plausable power numbers...

    ... Then again he might pop his clogs at 2mmol. ;)

    Static test the PT and then go have some 'fun' and see what the numbers look like.
     
  9. abuck

    abuck New Member

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    Ok, This is very weird.

    Here's my results:

    I have a 170mm crank lenght. The gear ratio used was 25/30 teeth.

    Based on the formula:
    (I used 50lbs to test)
    50 * 170 * 1/25.4 * 25/30 = 278.87 theorical. At the beginning, it was showing me some numbers around 300watts, 310watts but after like 10 try, I was receiving a number of 268 which is 10 watts less than expected.

    I try with my weight (165lbs)
    50 * 170 * 1/25.4 * 25/30 = 920 theorical. It was only reporting 838 but I was pressing the brake with one hand which could lead to this big step. But even everything, it seems that my powertap shows me highter number which doesn<t make any sence to me since I am complaining that my pt give me high numbers.

    Something that I want to add is that if I use my computrainer with my powertap, when the resistance is constant, my pt seems to read values ok.

    What could be wrong ?
     
  10. daveryanwyoming

    daveryanwyoming Well-Known Member

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    I'd tend to trust the tests with a fixed weight, especially since the body weight test came in low which can happen if your cranks aren't pretty close to horizontal (hard to be certain when you're standing directly above them) or you're putting some weight on the bars or wall to balance yourself.

    So in the early fixed weight tests your PT CPU was showing roughly 300 pound inches of torque (those aren't watts when the word watts is flashing) where your applied torque was roughly 279 or about 7% high but then after some testing your results were 4% low.

    I'd test again in a mid range gear ratio as I'm guessing you didn't ride your TT in a 25x30 combo just to try to keep things the same and I'd go through the steps carefully making sure you actually saw the peak CPU torque. One good approach is to hang the weight off of your pedal and slowly back up the bike watching to see when you get the highest torque readings.

    One thing to check is the axle lock nuts on the PT hub. Loose locknuts can give you inconsistent readings so it's worth a check. BTW, did you do the self test described in the manual? What value did you get for unloaded torque? If it's way out then that's another clue but if it's near the nominal 512 value you could still have problems under load.

    Good luck,
    -Dave


     
  11. abuck

    abuck New Member

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    Dave, I will do the test with severals gear ratio (especially, the one I've done with my time trial !). Also, will test when the bike moving slowly, this is actually a very good idea.

    And yes, I've done the same procedure with the manual. The unloaded torque was at 0 watts (the one on top). Middle one was at 669 and the bottom one too. This means that the calibration (or zero the torque if you will) was right, I assume !

    I know that the 512 is just a starting number but does that means that if I get 669, the strain gauge is very used or maybe tired ? Does that means anything ?

    I will do some other static test (different gear ratio, and I have different pounds too), I will post results !
     
  12. daveryanwyoming

    daveryanwyoming Well-Known Member

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    If you're getting a reading of 669 for the unloaded torque test then your strain gauge has some issues. IIRC, Saris tells you the test should be 512 +/- a small range but 669 is way out of whack.

    I look at the self test as a necessary but not sufficient condition, IOW, if you fail the self test (and your hub failed based on what you posted) then the hub probably has issues. But if you pass the self test it doesn't necessarily mean the strain gauge is accurate under load.

    I'd repeat the self test and if you still see a value much over 540 I'd call Saris. The good news is they respond to out of spec self test readings but tend to dismiss user torque test results but if you're still seeing 600+ for the self test you'll get their attention.

    Good luck,
    -Dave
     
  13. Alex Simmons

    Alex Simmons Member

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    Just to reaffirm what Dave has said, when in torque (or test) mode (the watts symbol is flashing), the top line is not reporting power (watts), it is instead reporting torque, in inch-pounds.

    If self test mode is that far out of spec (512 +/- 20 in-lbs) and your static mass tests are showing readings ranging more than +/- 2% out from actual, that unit is toast and needs to be repaired or replaced.

    One of the problems can be the torque zero drifting a lot during such tests. It needs to be zeroed before each static mass test. If it doesn't return to zero or near zero, then the test torque numbers will be wrong (one of the problems is it doesn't show negative numbers*). If it keeps drifting a lot, it's another sign it's probably toast (provided you have fresh batteries).

    * which is why I like to do the static mass calibration checks using the test mode as you see both the zero point and the weighted torque value - which highlights any issues with zero torque drift when you unweight the crank.

    When in torque display mode, all you see is the difference between weighted and the zero point, but you may not readily detect a drift in the torque zero.
     
  14. SteveI

    SteveI New Member

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    Just wanted to say, this is really really important with the PT. Tightening them with your fingers is not enough. You need to buy the right size cone spanner to be able to properly tighten them. I was getting quite a lot of variation in my PT readings before I bought the cone spanner to be able to properly tighten the nuts up.
     
  15. JibberJim

    JibberJim Member

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    An entirely believable time on the watts, I did 325watt 10 mile TT in 24:52 on a flat DC course on a fast day in full road set up on one of my bikes. You're well within a typical range of aerodynamics/rolling resistence there, all that means is how easy it is for you to improve and get that faster.
     
  16. smaryka

    smaryka Member

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    What about the new type of hub that doesn't come with a lock nut? They just slide on and off.
     
  17. SteveI

    SteveI New Member

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    I've never owned or seen one of those, but I'd have thought that if they just slide on and off, they won't be able to have the same problem. My hypothesis for why loose nuts caused a problem was that a tight clamping force could then exert a bending force on the axle as the nuts were fixed relative to the axle. If they just slide, this won't be able to happen. Perhaps that is why they changed the design.
     
  18. swampy1970

    swampy1970 Well-Known Member

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    You need a cone spanner and a torque wrench. The torque specs are given in the instruction manual.
     
  19. Pureshot78

    Pureshot78 New Member

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    I own two PT SL (wired) hubs and one gets 510 and the other 560 in test mode.
    The one that displays 510 i use on my trainer and the 560 is in a race wheel.

    When I switch wheels I go to test mode, option 6, then hold the select button until the numbers match.

    I haven't noticed any significant difference in my power for rides between the two wheels, but that doesn't mean there is none.

    Would you recommend that I try to send the 560 back to Saris?
     
  20. daveryanwyoming

    daveryanwyoming Well-Known Member

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    If it's still under warranty I would send it back as it fails their self test and getting a warranty service now will also extend your warranty another year.

    If it's out of warranty I'd start by carefully performing a static torque test with accurate weights on both wheesets to see how they match and how accurate the strain gauges are under load. If they're close enough for your purposes then you don't need service, if the 560 wheel is way out then you should get it repaired even if it costs you a bit. 560 isn't ridiculously high for the unloaded self test but it's still out of spec according to Saris so it may or may not indicate a bigger problem.

    -Dave
     
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