# Magnetic Resisted Exercise Bike Theory Question

1. Mar 18, 2013

### JustRob

I am an electrical engineer disabled in a snow sking accident and am now a quadriplegic. I'd like to design an exercise arm bike and make it magnetically resisted because friction resisted is not conducive to minute changes in resistance.

I have a magnetic resisted trainer for a road bicycle where you take the front wheel off of a road bike, fasten the forks in the trainer frame and the back tire sets on a roller. At one end of this roller is a small aluminum disk roughly 1/8" thick and 4" in diameter and attached to the spinning roller. On one side of the spinning disk is a disk the same diameter with six 3/4" diameter magnets (appear to be ceramic) arranged around the perimeter of the 4" disk evenly spaced (as if every 10 minutes around a clock). This disk is fixed and cannot move.

On the other side of the spinning aluminum disk is an identical magnet disk with magnets configured the same way but allowed to rotate 30 degrees. The strength of the magnetic field is varied by rotating one magnet disk causing the magnets to more eclipse the corresponding magnet on the other side of the spinning aluminum disk.

I can't find good information explaining the theory behind this. I have read all of the wiki pages on eddy current brakes. I'd like to see an equation that would explain what is going on. I got 1" neodymium magnets and positioned them around a 5" disk with a 1/8" thick disk between but the resistance is minute. I suspect it is because the rotational velocity is no where near as fast as the road bike trainer. I have also been told that the thickness of the spinning disk between is too thin.

Any help or direction is GREATLY appreciated!!!

Thanks

2. Mar 21, 2013

### Staff: Mentor

Eddy currents require a variable magnetic field - I don't know the relative orientation of your magnets, but one setting will have a very inhomogeneous magnetic field, and the other configuration will have a more uniform field.
Probably. Alternative explanations would be a bad placement of the magnets (field too uniform), or weak magnets.
I would try to get a thickness similar to the size of the magnets, but that might depend on details of the implementation.

3. Mar 21, 2013

### JustRob

I ultimately would like to use either one or two electromagnets. With that said and the knowledge that it is a changing magnetic field let me ask you this.

If I change the configuration so that on one side of the aluminum wheel I'm spinning is an electromagnet facing the wheel with a 1/16" airgap and mounted toward the outer perimeter of the spinning wheel and opposing it on the other side of the spinning wheel an identical electromagnet mounted with the same air gap.

The electromagnet on one side would be powered so that the center is north pole and the intensity be sinusoidal from minimum to maximum magnetic field.

The opposing electromagnet would have a steady state south pole field where the intensity would vary the strength of the resistance.

Do you forsee that working?

4. Mar 22, 2013

### Staff: Mentor

Why do you want to vary both?
The setup will give a deceleration, which depends on the currents in the electromagnets, sure.

5. Mar 22, 2013

### JustRob

The electromagnet set up with the sinusoidal variation would provide the varying magnetic field to induce the eddy current and braking action. The steady state magnetic field I thought I'd use the magnitude to provide more or less resistance.

I guess what I'd could do is have one electromagnet set to constant full power with maximum magnetic field and simply vary the magnitude of the dc sine wave powering the opposing electromagnet.

By deceleration do you mean it will creating a resistance to someone spinning the shaft which would be attached to the spinning aluminum disc between the electromagnets?

6. Mar 22, 2013

### Staff: Mentor

No, not that. You need a spatial variation, this automatically translates to a time-dependent variation for the rotating disk.
Right

7. Mar 22, 2013

### JustRob

Sorry but I'm confused. No, not what? By spacial variation do you mean a distance from the spinning disc variation? If so doesn't a sinusoidal magnetic field strength simulate that? If it is more like having magnets pass by each other would just cycling power on and off to one electromagnet work?

Again confused, do you feel this will work to resist someone pedalling the handles?

8. Mar 22, 2013

### Staff: Mentor

No, not what you posted.

Here is an image:

A permanent magnetic field with a rotating disk. All the time, parts of the disk enter the magnetic field, and other parts leave it, leading to a time-dependent field for those regions, and therefore a braking force.
I think that is the whole point of the device?

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9. Mar 22, 2013

### JustRob

Just to be crystal clear:

That spinning disc is just all aluminum?

I appreciate all the help. Would a crude equation describing the resistance be:

F = ω * β * A

where;
F: force resisting the disc
ω: rotational velocity of the disk
β: magnetic field strength
A: magnetic field area

And again, I realize it is very crude.

10. Mar 23, 2013

### Staff: Mentor

Anything conducting will work, aluminium is conducting.

This equation cannot be right, as you can check via the units. In addition, it does not include the conductivity of aluminium.
Playing with the units, $\frac{1}{s^2} T^2 m^6 \sigma \mu_0$ with a conductivity $\sigma$ has the units of a force, but I am not sure how to construct the length to the 6th power and if that has anything to do with the force.

11. Apr 8, 2016

### Newsome

I have a magnetic resisted indoor trainer for a road bicycle where you take the front wheel off of a road bike, fasten the front forks in the trainer ... I can't find good information explaining the theory behind this.

12. Apr 8, 2016

### mp3car

Not sure this adds a whole lot, but perhaps it will add something... My elliptical machine uses a magnetic resistance setup, it's pretty simple. When you "pedal" the elliptical, it spins an iron flywheel, and there is a semi-circular (maybe about 6" long) permanent magnet that is positioned so that it's parallel lengthwise to the direction of rotation (picture placing a slightly curved hand close to a bicycle wheel, with your hand parallel to the bike frame (almost like your drinking water from your hand). There is a small motor that changes the distance between the magnet and the iron flywheel to change the resistance... a very simple design...