I just received a new set of rollers (emotion, mag resistance, etc.) and I'm wondering what are the issues with metering power. InsideRide provides a nice Excel spreadsheet for power estimation at each mag setting and, while it can't really account for tire/tube friction, it does try to adjust the power slope to account for weight. (Oh wait, it seems like I remember a discussion that said weight didn't have much of a play in frictional forces.)
As you'd expect, the power curves are linear. To get a decent estimate of power on the rollers, do I need to do anything other than selecting a trainer from the "Set Trainer..." list that has a similar power curve?
Eventually, I want try a PT wheel on the rollers and take my own measurements. Once I have accurate measurements in hand, how should I go about adding a custom curve to the "Set Trainer..." list? Is that possible?
I have a feeling I'm overlooking some fundamental issues; otherwise, rollers would be in iBike2 now. What are the difficulties?
Rollers
Re: Rollers
I don't use the trainer mode of iBike, so I can't address that... but if these are real rollers that aren't attached to the bike, you'll have a large and somewhat variable resistance just from the rolling resistance of the tires. That is why rollers are often used without external resistance. If the mag is well calibrated and the assumed tires are close to what you have, the supplied curve should be in the ballpark though.
Re: Rollers
Thanks rruff. Yes, these are rollers -- nothing attached to the bike. But unlike my old set of rollers, these have a built-in flywheel with a 4 position magnet for additional resistance. (I like the resistance of a trainer, but I'm not fond of having my bike locked into one.)
It's the "variable" part of your reply that I'm puzzling over. Is that all due to tires, tubes, and pressure? Does weight come into play? Is there any reason the power curve wouldn't be essentially linear?
Since I don't have a DFPM (waiting for Quarq to come out with the compact -- 1st Quarter 2009 -- D'oh), it seems like the easiest way to determine my custom curves is to throw on a PT wheel with my training tires/tubes at riding pressure, collect data points on each mag setting, and then determine slope and plot the lines for each resistance level.
Once I do that, can I import the new power "curves" into iBike2 for use in trainer mode?
It's the "variable" part of your reply that I'm puzzling over. Is that all due to tires, tubes, and pressure? Does weight come into play? Is there any reason the power curve wouldn't be essentially linear?
Since I don't have a DFPM (waiting for Quarq to come out with the compact -- 1st Quarter 2009 -- D'oh), it seems like the easiest way to determine my custom curves is to throw on a PT wheel with my training tires/tubes at riding pressure, collect data points on each mag setting, and then determine slope and plot the lines for each resistance level.
Once I do that, can I import the new power "curves" into iBike2 for use in trainer mode?
Re: Rollers
The variability is due to different tires, tubes, and temperatures. If you figure out curves for the tires and tubes you will be using, then it should be fine... assuming that that mag is consistent. I'd expect it to be close to linear... there is some wind drag, but this is small when you are stationary.
Re: Rollers
OK -- got it:
1) wind drag on the wheels is non-linear, but negligible;
2) frictional forces are a large component and are linear, but vary widely depending on tires, tubes, pressure, and temperature;
3) flywheel and mag resistance should be linear.
To nail down (2), hold tires, tubes, and pressure constant, spin tires up to temperature, use a DFPM to sample power output at various speeds, and plot the power curve. Seems easy enough.
Outstanding question: how does one add the new power curves to the trainer list in iBike2?
1) wind drag on the wheels is non-linear, but negligible;
2) frictional forces are a large component and are linear, but vary widely depending on tires, tubes, pressure, and temperature;
3) flywheel and mag resistance should be linear.
To nail down (2), hold tires, tubes, and pressure constant, spin tires up to temperature, use a DFPM to sample power output at various speeds, and plot the power curve. Seems easy enough.
Outstanding question: how does one add the new power curves to the trainer list in iBike2?