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How to add magnets to magnetic resistance exercise bike to increase resistance?

Hi, and thanks for reading my question.     

We have just bought an exercise bike, it uses the pedalling to power the magnetic resistance of the bike. Here is the problem: at the highest setting, the resistance is still not very much. 

We think that maybe we can increase the resistance by putting a strong permanent magnet near the flywheel, (we would cut a hole in the plastic cover to put the magnet near the flywheel).

Can someone explain how to figure out how strong a magnet would need to be? How much "pull" would probably have a significant effect? What type (neodymium?), and what shape would work best (disk, rod, etc...?).

We are willing to put about $50 into a magnet.  

If this doesn't work I think I will remove the cover and just use a "friction wheel" sitting on top of the flywheel. really ugly, but it should work.

Thanks for any help you guys can give!

I do not yet have a good picture in my mind of the mechanism specific to your exercise bike. My powers of clairvoyance are not yet developed enough to see your exercise bike sitting in a room in your house, and moreover to see underneath its opaque plastic cover, for to see what is happening with its flywheel and its magnets.

Perhaps it looks just like the one in this picture,
https://en.wikipedia.org/wiki/File:Magnetic_resist...
from the Wikipedia article on "stationary bicycle"
https://en.wikipedia.org/wiki/Stationary_bicycle

Or maybe it looks like something else I have not seen before?
https://www.google.com/search?hl=en&tbm=isch&q=mag...

If there is a way to make the flywheel turn faster, e.g. by changing the ratio of the sizes of the pulleys, or sprockets, driving it, I would expect that to cause the eddy currents to consume more power, and give you more resistance for the same pedalling speed.

By the way, typically the magnetic field surrounding a permanent magnet does not penetrate very far into the space surrounding the magnet. For this reason, I expect this eddy current braking magic, requires the magnet and flywheel be very physically close, e.g. distances of just a few millimeters, or less.

For example, looking at the picture from the mechanism from the Wiki article I linked to above, it looks like that bike has magnets made to neatly fit inside a ring-shaped channel at the rim of the flywheel.

What I'm saying is, depending on the geometry of the mechanism for your bike, it may be challenging to find magnets that will be a good "fit" to the surfaces on your flywheel where you want to make eddy currents happen.

I mean, you want to get the magnets very close to, but not touching, the moving flywheel, so they don't get ground into metal dust from rubbing against it.

I am guessing the size and strength of the magnets needed (since you ask about this) will be comparable to the magnets it came with. Sort of a naive guess, would be to say that twice as many magnets, or magnets twice as strong as the existing ones, would give you twice the resistance. Although, it might actually be a square-law thing, meaning twice the magnets gives you four times the resistance.

Actually, that is sort of hinted at, in the equation, on this page,
https://en.wikipedia.org/wiki/Eddy_current#Power_d...
under the heading "power dissipation of eddy currents", looking at this equation, P is proportional to B-squared. It is also proportional to f-squared, suggesting that speeding things up, will increase P, which I think was my original suggestion; i.e. change the ratio of the pulleys or sprockets to make the flywheel move faster for the same pedalling speed.

Urrgghh004 (author)  Jack A Lopez1 year ago
Hi,

Thanks for your answer. And, Yes, I think my bike is a lot like the picture in your link. I really don't want to completely disassemble the
"flywheel" assembly. I was hoping that I could just cut a hole in the top cover and use a mounted magnet with fine screw adjustments to adjust the distance between magnet and flywheel for optimum results.

So there is this crescent moon shaped piece, that holds the permanent magnets. Also there is some other mechanism which moves the crescent moon shaped magnet holder, closer to the flywheel when more mechanical resistance is desired, and farther away from the flywheel when less mechanical resistance is desired.

Moreover I am guessing outer rim of the flywheel itself, the part that moves fastest, is covered with a layer of good electrical conductor, like copper or aluminum.

That guess is based in part on a YouTube video I found recently of someone showing us the insides of a piece of exercise equipment, with a mechanism that looks very similar to the picture in the Wikipedia article on "Stationary bicycle". This mechanism has a crescent moon shaped magnet holder, mounted close to a flywheel. Interestingly the outer rim of the flywheel is covered with a layer of copper. This copper rim around the edge of the flywheel can be seen clearly in this video.

The flywheel in the picture from the Wikipedia article ?might? also have a copper rim, but it is hard to see it clearly because of the angle at which that picture was taken.

Anyway, the reason I was looking for YouTube videos on the topic of eddy currents and Lenz's Law, is because I believe these videos might help give you sort of a feel for how this magnetic damping, magnetic braking, works in practice.

For example, if you're going to add your own magnet, it has to be firmly held in place somehow, because the flywheel will try to drag it with it, and you can kind of "see" how that works from watching some of these videos.

Also I am wondering if your bike is truly set at its highest resistance setting.

That is to say, I am wondering if perhaps you arrived at this conclusion simply by looking at the bike from the perspective of its "dashboard"; i.e you turned a knob all the way clockwise, or you pushed some buttons and watched some little lights, or bars, on the display change, "boop-boop-boop -- okay, guess that's got it turned up all the way..."

I mean, I think a true measure of whether or not the resistance is turned up all the way, is to look at the gap between that crescent moon shaped thing and the flywheel. If these are so close they're almost touching each other, then that means the resistance is turned up all the way.

Anyway, I think I promised you some video links. The first two are better with sound turned off.

The first does not need sound because the narration is almost worthless. In fact the first 3 minutes is just blah-blah-blah-blah, and also so is everything after about 3min+30sec.

The second video is very good collection of eddy current demos, but there is no voice narration. The sound is just some generic techno music. I mean, leave it unmuted if you like generic dub-step techno.

The last one has good narration, plus amazingly, a chunk of solid copper the size of an apple pie, and a NdFeB magnet as big as bar of soap.





Dang! I forgot to link to the Wiki article on eddy current braking, here,

https://en.wikipedia.org/wiki/Eddy_current_brake

The pictures can give you an idea of the sort of typical geometry these eddy current brakes have.

Also I forgot to mention, a permanent magnet electric motor can be used as a eddy current brake; i.e the shaft gets harder to turn if you put a short-circuit, small electrical resistance, across its inputs, compared to those inputs being open-circuit.