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Would flywheel temperature be proportional to effort?….(5 posts)

Would flywheel temperature be proportional to effort?….TFerguson
Dec 19, 2002 2:09 PM
Riding indoors gives you lots of (too much) time to think..

While riding a spinner bike, I was wondering if using an infrared temperature sensor pointed at the flywheel would give the relative effort between two riders. All of the other differences should be negligible except for the "brake pad" on the flywheel. All of that energy must be dissipated as heat, therefore the flywheel temp should be proportional to the "effort". Yes? No?

This knowledge has no value what so ever, but like I said, there is lots of time to think.

Man get a tv in front of your trainer :) (nm)PODIUMBOUNDdotCA
Dec 19, 2002 2:51 PM
I would doubt it is a linear relationship ...Humma Hah
Dec 19, 2002 3:17 PM
... rate of energy production will be a linear function of power input produced by the rider, but the temperature rise would probably not be. Heat would be lost at a rate that is a function of temperature difference to the environment, which would tend to make the temperature rise roll off. Much of the heat would go into the massive flywheel, causing it to slowly increase in temperature with time (and is the reason those machines tend to not maintain constant drag thru an exercise session).

The typical system uses a belt with spring tension on it as the brake element. Torque then tends to be pretty closely related to the spring's tension. If the torque is constant and known, power can be obtained from torque and RPM. Unfortunately, that spring is usually adjustable all over heck and back, with no way of indicating the actual tension on the belt, so most exercycles of that type give wildly erroneous power readings.

A real dynomometer would measure the force applied to the brake, against flywheel RPM, and calculate power that way (I recently built such a device for a job at work). A reasonably good load cell for the job could probably be mass-produced for $20 or less, and would be similar to the weight-measuring component of a bathroom scale.

The other classic way of doing it is to drive a generator and measure power developed by the generator. The "simulator" built for the Daedalus 88 human-powered aircraft project was of this type.
Just point the temperature sensor to your forehead. nmBruno S
Dec 19, 2002 3:33 PM
The short answer is . . .Kerry
Dec 19, 2002 4:33 PM
Yes. But it would not be linear. For the long answer, we have to dip into some partial differential equations and consider radiative, conductive, and convective heat transfer, along with the heat capacity of the flywheel and the varying film resistance due to changes in flywheel speed. This is why measuring flywheel temperature would not be a very good way to determine power output.