Rebuild a powertap wheel and calibration?

Discussion in 'Power Training' started by bgoetz, Mar 27, 2013.

  1. bgoetz

    bgoetz Member

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    Does anyone know if rebuilding a PT hub into a new wheelset can have an effect on the calibration. My thought is no, but I just built my wheelset into an entire new wheelset. We used the same lacing pattern as the old wheel (1x drive/2x non). I had it on the rollers last night and my power seemed low when compared with my speed. I did go to tubeless and these wheels are 100x smoother than my old wheels, so maybe the higher speeds/less effort is a reflection of lower rolling resistance??
     
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  2. daveryanwyoming

    daveryanwyoming Well-Known Member

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    The torque tube shouldn't really change accuracy or calibration with a wheel rebuild assuming the wheel is actual round, true and well tensioned (who knows what would happen with floppy loose spokes and really bad build but the wheel would probably collapse anyway). But it's easy enough to do a static torque test to get some peace of mind and to help you trust your numbers.

    FWIW, I recently did some comparisons of three wheelsets on my rollers out of curiousity. I warmed up each set and tested each wheelset twice with repeatable results and checked tire pressures carefully. Power was measured with the same calibrated SRM and I checked torque offset before and after each run and there was no drift. All wheels pumped to the same pressure, e-motion rollers with resistance turned off, same position on road bike:

    - HED Ardenne C2 wheels, Vittoria Pave CG Clinchers, Butyl tubes (everyday wheels): 25.3 mph, 214 watts
    - HED Ardenne C2 wheels, Vittoria Pave CG Clinchers, Latex tubes (rainy crit wheels): 25.4 mph, 182 watts
    - HED JET 4 C2 wheels, Vittoria Corsa CX clinchers, Latex tubes (primary race wheels): 25.3 mph, 164 watts

    The first time through I just didn't believe it could be that big a difference so I repeated the tests and got the same results within a couple of watts for the same speed. On another night I did some rolling resistance vs pressure testing on just the race wheelsets and again the deltas were consistent. Sure the JET4s probably spin up faster just because they're more aero but the comparison between two sets of Ardennes with the same tires at the same pressure but butyl vs latex is shocking. You can probably guess why I'm racing my JET 4s in pretty much every race :)

    I wore through my every day Pave's and swapped them to a set of Conti GP4000s tires so I may have to do another one of these sessions.

    Anyway, I'd statically torque test your PT just so you know but different tires and wheels and things like tubeless could easily make big differences in terms of power vs. speed. BTW that's one issue I have with folks quoting their trainerroad or other trainer based power numbers as if they're valid. The difference in just changing tires and tubes can be a lot more than many guess.

    -Dave
     
  3. danfoz

    danfoz Well-Known Member

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    that difference between latex and butyl (assuming all else being equal) is astounding!
     
  4. RapDaddyo

    RapDaddyo Active Member

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    This is my experience as well. Even the difference in rolling resistance of two different clincher tires can be shocking.
     
  5. danfoz

    danfoz Well-Known Member

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    In addition (and not very well publicized), Continental claim the rolling resistance is 18% lower on a 23mm wide rim vs. a conventional 19mm rim.

    the combo of a faster rolling tire, a faster rolling rim, and a faster rolling tube must be cumulatively quite substantial if the inner tube alone is saving approx. 30 watts.
     
  6. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Continental does claim that, but to date no one has been able to independently demonstrate the same thing. IOW, at the same tire pressure with the same tires and same tubes independent tests like Al Morrison's Crr tests and others using similar procedures haven't been able to reproduce Conti's results.

    But that's not necessarily apples to apples since we don't actually run the same pressure on wide rims as narrow rims. From what I've seen my C2 rims with 23c tires at 95-100 psi roll similarly to my 19mm rims at 120 psi but with a much more plush ride and improved cornering feel.

    My 'tests' are just comparisons and not rigorous Crr tests as I don't do things like control for ambient or tire temps nor tire rollout differences which shouldn't be an issue in the comparisons above but in general with different size tires needs to be accounted for. [edit, actually I just thought about this and since the Corsa CXs are 23c tires and the Pave's are 24c tires and I used the same SRM head unit without rolling out the different tires and re-entering the wheel circumference the Corsa CXs get a bit of an artificial boost in recorded speed or a corresponding bonus in fewer watts to achieve the same speed. Again these aren't rigorous tests but the JET 4/CX combo was probably optimistic by a few percent]

    I'd take the Conti claims on wide rim Crr with a grain of salt until someone can reproduce those results but I will say there's not much Crr penalty when running wide rim clinchers at relatively low pressures and the ride is sure nice.

    -Dave
     
  7. danfoz

    danfoz Well-Known Member

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    I can see the apples to apples thing considering the air volume/psi diffs between the rim widths. But yes they sure do feel plush.
     
  8. Bigpikle

    Bigpikle New Member

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    These are very eye opening numbers in your tests Dave - I was also surprised there was such an aero difference on the rollers given no actual forward movement, but I guess the tyres still move through the air.

    I have some latex tubes to try when I go to the mountains next week so it will be interesting to see if they yield any benefit in the real world. I have times on the the same bike and tyres for all climbs I'll be doing, so I can see if it anything changes. You have also reignited my desire for those Zipp 202 Firecrests as well /img/vbsmilies/smilies/biggrin.gif
     
  9. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Maybe more importantly the spokes move through the air and the deep rims expose less of those churning spokes and a smoother transition to the rim as it stirs up the air. Honestly I have no idea of the magnitude of wheel spinning and aerodynamics but have seen it referenced in some discussions and it makes sense that different wheels require different amounts of energy just to spin in a stationary setting with discs likely taking an awful lot less as it's all skin drag.

    -Dave

    P.S. I'd be interested to see what you learn up in the mountains but personally I never really notice much when I'm out on the road. Sure, some wheels feel faster but day to day changes in things like tailwinds or even air temps can have such a big influence on speed, even on a climb, that I've never really pinned down power differences for speed outside of rigorous aero testing. I'd be curious to see what you notice but it wasn't till I tested on the rollers and then re-tested several times that I got my head around how big those power differences can be.
     
  10. bgoetz

    bgoetz Member

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    Thanks for confirming my thoughts that it is likely the new tire/wheel combo and increased efficiency. I did actually stomp test it, but I am not sure how confident I am with results from that. I tend to see fairly significant variability from weight position and crank position when doing those tests. Just a slight variation in crank position seems to swing my results 2-3%. This PT in particular has always stomped on the low side.
     
  11. fluro2au

    fluro2au New Member

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    Amazing difference there David. Thanks for sharing Do you have any other links to wheels, tyres and tubes and their rolling resistance Paul
     
  12. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Wheels and aero properties will mostly have to come from manufacturer's data which always needs to be viewed with a critical eye but for tire/tube data here's the best compilation to date:

    http://www.scribd.com/doc/76918277/AFM-Tire-Testing-Rev9

    Just remember this is per wheel data from smooth rollers, real world Crr and power differences are likely to be a bit higher unless you do all your riding on smooth alloy rollers or very smooth surfaces like wood velodromes.

    -Dave
     
  13. danfoz

    danfoz Well-Known Member

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    I seem to remember the site also shows the results of tire pressure on Crr. Increasing tire pressure lowers Crr, but ime that holds true on only the smoothest of roads. I have personally found I can actually roll faster at a slightly lower pressure on poor roads. I guess that's similar to a downhill skier seeking to avoid air at all costs.

    (Btw I see it's not a free resource anymore /img/vbsmilies/smilies/hissyfit.gif. And whatever app they now have surfacing the data slows my computer to a crawl, maybe it's just a gatekeeper app and the old skool pdf's are avialable after login?)
     
  14. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Technically rolling resistance continues to drop with increased pressures but overall system linear losses start to increase once you reach the point that most of the energy of tire/tube deflection at the contact point isn't returned to the road and gets transmitted up through the frame and fork to vibrate the whole system. For all practical purposes that increase in system loss is equivalent to an increase in rolling resistance itself even if the cause is a bit different.

    Tom Anhalt wrote a good piece which includes some field test results demonstrating this effective break point where increased pressure actually starts to slow you down. The ironic thing is that's the point a lot of folks start to feel like their tires are running fast as they start to feel those micro vibrations from the road and feel their tires are 'buzzing'. So a good rule of thumb for different road surfaces is to pump your tires up a bit at a time and do some quick test riding until you get to the point where the tires begin to feel fast, then let out a bit of air so they actually are fast.

    Here's Tom's article: http://www.slowtwitch.com/Tech/What_s_in_a_tube__1034.html

    -Dave
     
  15. dhk2

    dhk2 Active Member

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    Dave, it would be interesting if you could repeat the results of your roller tests on the three rear combinations outside on real roads. If the savings for latex tubes is even 10 watts out of 200 watts, that would be an impressive performance gain. I don't race, but that kind of savings would make fast club rides more fun.

    Last season rode the Corsa CX tires in 25mm. No power meter here, but they certainly seemed to roll faster and ride a bit plusher than my regular GP4000s. They made the steering a bit "heavier" too, almost like the tires were underinflated. Ran them at 120 psi, even though the sidewall rating said "min 130, max 160". I wore through the rear in about 1500 miles, but kept the front on through the next GP4000 rear. It picked up a couple of cuts in the rubber, but these didn't penetrate the casing which proved to be tougher than I expected.
     
  16. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Yeah, it would be interesting to see how those numbers translate to outdoor riding. It wouldn't be too tough to compare power at the same speed up a sufficiently steep hill but on the flats it really would require a full regression testing session and those are a major PITA in terms of finding suitable traffic free venues and calm windless times to test. Not sure if I'm motivated enough to do those tests but if I ever do them I'll post the results.

    FWIW there's some good published comparisons of Crr determined by roller testing and Crr determined by outdoor regression testing and the two compare favorably with outdoor Crr being a bit higher. Here's a chart Andy Coggan produced making such a comparison:
    [​IMG]
    So basically rolling resistance expressed as Crr measured outdoors tracks very well and nearly one to one with Crr measured on rollers but with a constant offset.
    This chart was snipped from Andy and Hunters power blog here: http://www.trainingandracingwithapowermeter.com/search/label/physics so it's not my data (but at least I'm giving appropriate credit).

    I might still do those tests some time but I'd expect similar results to what's shown above. In wattage instead of Crr terms that would suggest higher power lost to rolling resistance outdoors but the differences between the tire and tube combos would likely be similar to what was measured indoors.

    -Dave
    P.S. If it's not already obvious, those power numbers quoted above in my post on roller testing are about 75% due to the rollers themselves. IOW, the rolling resistance of the tires is likely around 45 to 50 watts but the total power of 165-200 or so represents the rolling resistance (plus bearings and other drag forces including spinning up the spoked wheels that move through the air even though the bike doesn't move) plus the power it takes to spin the rollers. And even though I had the resistance unit set to it's off position it still takes some power to spin the inertial drum that is also used as part of the magnetic braking unit.
     
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