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

Originally Posted by PaulMD

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?

Originally Posted by rmur17

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.

Originally Posted by PaulMD

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!!

Originally Posted by rmur17

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.

Originally Posted by wiredued

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.

Originally Posted by PaulMD

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.

Originally Posted by beerco

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.

Originally Posted by rmur17

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

Originally Posted by PaulMD

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.

Originally Posted by rmur17

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.

Originally Posted by PaulMD

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. But I can't test it because I am still sick. Fifth day on a row with fever

Originally Posted by wiredued

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

Originally Posted by PaulMD

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