I found several fans and mag units that use belts to drive them by one of the rear rollers. All of the add-on units would require more moving parts, belts that might not fit just right and moving parts that would eventually wear out with one exception. I came real close to purchasing a magnetic resistance unit like this one, but couldn't be sure that it would fit my trainer.
The unit uses magnets to provide resistance even though the aluminum isn't magnetic! The magnets don't come in contact with the roller, so how do they provide resistance?
Turns out that the unit is a simple Eddy Current Brake. Click here to read more about Eddy Current Brakes.
The magnets themselves have a magnetic field which when placed very close (a few mm) to the moving aluminum roller (a conductor) create an eddy current in the roller.
A little help from Lenz Law explains how the moving alu roller's eddy currents interact with the magnetic field in such a way to oppose the movement of roller and provide a smooth "braking" effect. You can see in the video how moving magnets can induce an emf in the aluminum by induction in this short video. Now, rather than move the magnet, we're moving the conductor (aluminum roller) and it will produce the same effect. The braking effect increases as the speed of the roller increases.
What this means is that you can use gearing to adjust the speed of the roller and increase resistance, or just simply increase your cadence.
The same principle is used in mag-lev trains and roller coasters for braking.
You can clearly see the neodymium magnets here used as an eddy current brake to provide resistance in this flywheel exercise bike. You'll find similar set-ups in consumer elliptical and exercise bikes.
So... I took some rare earth neodymium magnets and hot-glued them to a piece of scrap wood cut 1-2mm longer than the inside dimension of the roller frame. Wedge it between the rails of the roller frame and adjust to desired distance / resistance. Here's some shots of the simple setup.
Use a scrap piece of 2x4 which I hot glued the nedoymium magnets to. I actually used a paddle bit to counter sink them a little bit. Didn't want to make them flush in case I had to move them for some reason.
I actually alternately reversed the polarity of the magnets to see if this might add a bit more resistance, but I haven't spent the time to switch them back to see the effect. I do know that it works this way.
I cut the piece just a mm or two long to make a snug but not too tight fit. This way I can remove it easily or adjust the distance between magnets and rollers. The beauty of this is that you can cut the wood to length to fit any trainer. Here you can see I've just begun to wedge it into place.
Adjust to desired distance. This looks like a mm or two to me.
Set in place ready to roll!
A quick spin of the roller with and without the magnets demonstrates the effect. Spin the roller faster and you see the aforementioned increase in braking effect.
No resistance
With resistance unit (eddy current brake)
With resistance unit (eddy current brake)
The important thing is that it works... very well in my opinion. You can vary the resistance easily by moving to a larger or smaller ring. Move the magnets closer to the roller for even more resistance or remove them all together for an easy spin.
The only drawback which I have yet to really measure or deal with is heat production in the aluminum roller due to it's electrical resistance as a conductor. You are basically transforming kinetic energy into thermal energy.
Some dyno's used to test automobiles and motorcycles need coolers to remove the heat produced during testing.
How hot does it get? I don't know. I'm planning a few high-resistance workouts in the next week or two to see how it performs. I'll report back. Update: just finished 45 min Spinervals 1.0 workout and it got warm, but not even remotely "hot." We'll see how it goes for an 1.5+ hr. ride later.
http://www.launc.tased.edu.au/online/sciences/physics/Lenz%27s.html
http://en.wikipedia.org/wiki/Eddy_current_brake
http://www.physics.ubc.ca/~outreach/phys420/p420_96/bruce/copper.html
http://alex-cycle.blogspot.com/2008_02_01_archive.html
http://valorathleticsinc.com/store/images/D/AC-2_Magnetic_Resistance.jpg
http://en.wikipedia.org/wiki/Eddy_current
http://www.overstock.com/Sports-Toys/Valor-Fitness-AC-2-Spin-Bike/3228573/product.html