Sorry for the slight detour in topic, but this is bike related and science related, so i thought there may be some interest.
My daughter is interested in sustainability/environmental issues. The project she picked this year is to capture the energy from a bike trainer and reuse it. We’ve talked her out of making this an undergrad electrical engineering project by keeping it mechanical. So her idea is to hook up a chain to a smart trainer and then use that to run a fan to cool the rider while they’re doing a workout.
The build is pretty simple, but the science questions can get deep; or at least middle school science fair deep. My questions to this group:
Any concerns about using a smart trainer in this way? Could we be damaging it?
Would there be increased tension in the contraption that offsets any benefit from the cooling?
If i used pedal power to compare, should the readings be close, or will the smart trainer give me what it thinks its doing vs the pedals giving me what its actually doing (smart trainer plus the contraption)?
My plan is to use a steady 100W custom workout on TR to test.
I’ve got a set of Kreitler rollers with the Headwind fan and can tell you that the addition of a fan is no minor resistance increase. Here’s a link to their wattage calculations based on the opening of the fan duct: Wattage Information — Kreitler Rollers
I know this isn’t at all what you asked, but it seemed relevant to me.
I would think as long as the fan stays connected to the trainer in some way, the spin down calibration should account for the increased resistance from the fan contraption? I would think the trainer would just assume you’ve put the worlds slowest tire on, or used superglue as chain lube.
As noted above though this should be pretty easily testable. With a 100 watt workout…you should feel a difference in effort with vs without the fan if the calibration is not doing its job…I would think.
Adding a fan would / could be a lot of additional resistance. Air Assault, and Airdyne bikes work in this way - the he fan is the only source of resistance, and if you’ve ever ridden one of these bikes, the resistance easily gets into the 100s of watts. I don’t know if a trainer spin down could account for this level of added resistance.
If she ends up going the electrical route, she may be able to get a few ideas from here:
As long as you minimize the additional bearing load between the trainer’s cassette and the connection to the fan, there should be minimal change to loading mechanical to the trainer.
This relates to your final question. You are essentially increasing the load on the rider with the additional resistance given from the fan. The type of fan and level of air you move will dictate the delta there. Fan selection and drive ratio will all play into the final results.
Your second idea is most correct. You are introducing a new load on the rider, that the trainer will not necessarily register. The pedal force & power data will be the “real” load on the rider, while I predict the trainer will likely be lower.
How much delta will result from the initial delta between trainer/pedals in the default state with no fan. Then the friction from the drive system and the actual wind resistance of the fan itself will add load to the rider that the trainer will largely not see.
One x-factor here that I can’t predict is the general action of the trainer and it’s flywheel with the addition of a new “drag” source. It will almost certainly reduce the spindown time from the increased resistance.
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