Inaccuracy of Pro 300PT indoorbike with powertap due heating up flywheel

Discussion in 'Power Training' started by PaulMD, Mar 7, 2007.

  1. PaulMD

    PaulMD New Member

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    I always thought that my Cycleops Pro 300PT with a powertap hub in the flywheel was pretty accurate. I have done two 60 minute timetrials and the last 30 minutes always felt a lot easier. That made me think because maybe the powertap was inaccurate during the hour due the fact that the flywheel because pretty hot after 30 minutes of riding > 250 watts.

    Two days ago I zero torqued before a training. After the training the flywheel was hot and my torque was 10 inch-lbs. After a couple of hours when the flywheel was at room tempature the torque was 0 inch-lbs. Yesterday before my training the torque was still 0 inch-lbs but after the training it was again 9 inch-lbs.

    - My question how much does this change in torque affect my average power? Is it off by more than 10 watts?
    - And is this a known problem?
     
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  2. rmur17

    rmur17 New Member

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    I don't have a 300PT but do plenty of ergo workouts on my Computrainer ... and I have encountered 'ergo drift' on there for long L4 intervals.

    It sounds like the 300PT could have a problem there. What was your average torque during the main interval and what was the average wheelspeed? From that you (we) can figure out how much the 10in-lb drift represented. If it was present for a significant portion of the workout, it would definitely result in over-reported power. Or if you have a .csv dump from the workout, you could subtract 10 in-lb from the torque in question and recalculate power per record.

    PT_power = Torque (at rear hub) x Wheelspeed

    if you were using metric units Torque would be in N.m and Wheelspeed in rad/s. With English units there are linear conversion factors involved.

    Back to the drift, I noted 5W perhaps up to 10W drift down in my CT power over regular 30-40min intervals. I found that:

    1) Placing a small external cooling fan (6") directly (fore-aft) at the load-generator; and,
    2) Keeping the room temp <15C

    reduced to drift virtually to zero. Definitely less than 1%. If nothing else that may help.
     
  3. PaulMD

    PaulMD New Member

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    I kept the wheel speed and the torque/wattage the same by changing the brake settings. My average torque is around 62 inch lbs and the cyleops computer reads 41.5 kilometer/hour. Does this mean that a change of zero torque to 10 inch lbs affects my power reading by 10/62 (=16%)??? Auww!!
     
  4. rmur17

    rmur17 New Member

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    I could be wrong (and someone will point that out) but I think you're correct. Given a pretty constant wheelspeed for your interval and an offset of 10 on a reading of 62 in-lbs, I'd make actual power ~52/62 or 84% of reported.

    If confirmed, that's a massive drop. And unfortunately one that folks may not spot unless they go to the trouble of checking the offset AFTER the main workout (and before cooldown). Not good :(

    Can you try a cold room or direct fan cooling as I suggested in my 1st post?

    It's likely something to confirm with Saris/Cycle-ops to see if there have been other reports.
     
  5. wiredued

    wiredued New Member

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    You have me confused a little when you talk of computrainer drift you mean the difference between what the computrainer is calibrated to show regardless of temperature compared to the PT on your bike right? But in the case of the 300PT he changes the resistance to match his target watts on the PT as the pads get warmer and only goes by what the power tap is measuring. Sounds like a reliable way of working out if you are paying attention to the power measurement the PT isn't going to drift if resistance is maintained manually.:confused:

     
  6. rmur17

    rmur17 New Member

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    well I'm definitely comparing the ergo mode setting on the Computrainer handlebar controller to power reported by the PT on my bike. I don't always use the PT on there - but for all important tests I do. I found that, without the fan, actual (PT) power would drift down 5-10W over the course of a long interval (30-40 min typical for me).

    re the 300PT, I've never seen one and don't know if it has an ergometer mode or not. Seeing it's an integral unit, there's no way of attaching another PM for comparison - you have only the 300PT reported power display for feedback. And if due to drift, that reported power gets higher and higher over time compared to actual .. then that's an issue with the unit IMHO.
     
  7. beerco

    beerco New Member

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    Kind of dissapointing to hear this. Rick, this device is basically a spin bike i.e. a solid steel 50lb (ish) flywheel with a PT at the hub - no ergo mode. I could see how becuase the hub is integrated into the flywheel that heat from it would be passed directly to the hub causing problems. I've ridden my road PT in conditions that started out cold in the morning (50f) and ended up in the 70s 80s without any offset problems but the flywheel could have much bigger temp swings I guess.

    Definitely call saris and see what they have to say. I'm not in the market for another trainer but that's the one I would buy if I had the $$. Now I'm not so sure.
     
  8. rmur17

    rmur17 New Member

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    Andy,
    Does it have an mechanical or electrical brake? I'd be interested in hearing what Saris have to say as well.
     
  9. jetnjeff

    jetnjeff New Member

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    The 300 pt use a v-brake and big friction "Brake" Pads



     
  10. PaulMD

    PaulMD New Member

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    Today I have emailed cycleops. I will post their reaction here when I get one.

    Yesterday I got the flu.When the fever is gone I will test the difference of 10 inch-lbs myself. I will zero torque the powertap hub and then cycle for an hour so the flywheel is hot and will have a zero torque of 10 inch-lbs. Then I will cycle a minute with 100rpm and then zero torque the hub and do a one minute with 100rpm again with the same resistance. In cyclingpeaks I will compare the 2x 1 minute.
     
  11. rmur17

    rmur17 New Member

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    sounds like an easy test but I fear you will find what we've predicted. In any case, I would be nice to hear what Saris have to say.
     
  12. PaulMD

    PaulMD New Member

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    I really hope not!! I did 297 watts for a 60 minute timetrial after 7 weeks of training, when the pro 300pt is inaccurate that's the reason for my 4.3watt/kg with only 7 weeks of training. :mad::mad::mad: But I can't test it because I am still sick. Fifth day on a row with fever :mad::mad::mad:
     
  13. wiredued

    wiredued New Member

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    How hot is the hub getting can you keep your hand on it?

     
  14. PaulMD

    PaulMD New Member

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    My hand won't burn, but it is pretty hot. The exact tempature I don't know.
     
  15. wiredued

    wiredued New Member

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    I hope a solution is being worked on at Saris that bike seemed so bomb proof I wouldn't have expected that.

     
  16. rmur17

    rmur17 New Member

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    sorry to hear about the fever :( . Let us know when you're back in the saddle or have heard from Saris.
     
  17. wiredued

    wiredued New Member

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    I second that for what it's worth PMD your one hour TT reports convinced me to try it I just did my first one on the KK and I like it better than 20 min TTs. The pace was more comfortable I still had enough breath to tell my wife to leave me alone if I had to but I didn't. I did 19.7mph=249.86 watts FTP (fuzzy numbers but improving fuzzy numbers):)

     
  18. attybiker

    attybiker New Member

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    I have the Pro300 PT as well and have also noticed the problem. I think I have a way around it as I warm up for around 20 minutes at a fairly good pace and then zero the torque. If I zeroed before the warmup and then after warmup I see a value of about 7-10 on the computer module after warmup. Once the torque is zeroed after warmup the 1-2 hour ride is fine. I've even looked at zeroing during the 1-2 hour rides and found 0 every time in the torque setting.
     
  19. PaulMD

    PaulMD New Member

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    At the moment I still don't have a reply from Cycleops. I am going to send them a second email.

    I am very dissapointed that this is a design problem of the Pro 300PT. All Pro 300PT flywheels heat up and all hubs will be inaccurate after 30 minutes of use.

    Now all my logfiles are worth less, when I look at them I understand why my heartrate drops 10bmp between minute 30 and minute 60 :mad::mad: I thought that it came due a good warming up of my body. When I knew these problem of the Pro 300PT I wouldn't have bought one. I hope Cycleops has a good solution but I am afraid that they haven't. The only other solution is that I get my money back so I can buy a regulair spinningbike with an isis bottombracket so I can install an SRM pro on it.
     
  20. rmur17

    rmur17 New Member

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    yeah I hear your frustration there. Decreasing HR indoors would certainly confirm the power drop as I believe most folks would increase or at least plateau but not drop at constant power. I have hundreds of ergo workout records and doubt if there's a single one showing HR decrease over time.

    What about the suggestion by Attybiker? I use a CT and warmup at tempo power for ~ 15min before performing 2-3 rolling resistance checks and accepting the 'calibration' (more of an offset rather than calibration). Perhaps a similar routine would enable your 300PT to give you acceptable results.
     
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