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I have an exercise bike that works by a (I'm assuming solid iron) flywheel and a 'horseshoe' of magnets that are moved in and out to set the difficulty.

I have attached an image of the same design found on Google Images so you can see what I have:

view of exercise bike mechanism

What I want to do is make the bike harder to pedal. For this, I have bought a bunch of neodymium magnets online, after reading that they were the strongest magnets you can easily get.

I bought a bunch of circular ones and smaller flat ones. I've routed an 'arm' out of MDF to screw them onto - so far I've only done the circular ones but I would have gone on to try the flat ones as well. I've illustrated what I'm doing here:

demo of lever arm

So I'm using the MDF arm as a lever to move the magnets in and out in the same way as the horseshoe would. The idea is to test the concept before I go messing around with the real horseshoe.

And I'm glad I tested it because so far, nothing is working as I would intuitively expect. The magnets have no effect at all on the flywheel!

What's going on?

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  • $\begingroup$ Are you getting them close enough? $\endgroup$ – Solar Mike May 28 '19 at 9:22
  • $\begingroup$ Which question are you asking? How does it work? Why does it not work? $\endgroup$ – Solar Mike May 28 '19 at 10:46
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    $\begingroup$ Are you sure it's a magnet system and not a friction belt or air flow system? $\endgroup$ – ratchet freak May 29 '19 at 13:53
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    $\begingroup$ How are you arranging the polarity of each magnet? for best effect the magnetic field should alternate with each magnet. $\endgroup$ – ratchet freak May 29 '19 at 16:31
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    $\begingroup$ @ratchetfreak Okay - having made sure the magnets were alternating poles, I now feel a very slight effect. But these are very strong magnets and the effect is barely noticeable. What are the physics at play here to explain this? $\endgroup$ – Tim May 29 '19 at 17:39
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I've designed magnetic brakes like this and have had no issues with a setup similar to yours.

I think the system will work even without a special arrangement of magnets.

My guess as to why it's not working, is that the magnet bracket may be made from iron, and may be concentrating the flux on the old system, improving it's power compared to your new one.

I would gather more data by having someone spin the flywheel, then (carefully) holding a magnet near it. You should be able to feel the drag force and get an idea of what's happening.

The gap will need to be very small, is your new gap as small as the old one?

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if you fix/glue the magnets to the flywheel spaced equally apart and have a coil of wire wound around an iron core close to it you have in effect a generator.

Short circuiting the ends of the 2 coil wires will cause a resistance to the pedal effort and in effect a brake that is variable if you also have a variable resistance in the coil wires.

A 2mm gap between the magnets and the coil face will be sufficient to generate a reasonable amount of electricity which can be controlled by the variable resistance.

You "could" have a fair bit of heat generated in the coil of wire when the electricity is almost a dead short so I would advise having a load in the circuit in the form of a 240 volt 60 to 100 watt light bulb (by experiment) to safely burn off the power as a light source......no cooling fan needed. Ian.

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  • $\begingroup$ Thanks very much for this solution, Ian! For the sake of my (and others') understanding of the physics involved, are you able to explain why what I was doing wasn't working? $\endgroup$ – Tim Jun 1 '19 at 11:07
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    $\begingroup$ TBH if OP is sweating away on a trainer, a fan would be an excellent use of any generated power. It gets hot when you're not moving forward. $\endgroup$ – Criggie Jun 17 '20 at 5:04

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