Eddy Current Resistance Unit for Aluminum Bike Rollers

So I decided that in order to gain some fitness, not just control while spinning, I would need to add some resistance. Turns out, Performance no longer carries the resistance unit for these rollers. So I started searching around the 'net for some units I might be able to modify to fit my set.

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)

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

DIY Free Motion Rollers

I've been riding my indoor magnetic/fluid trainer since 2004. I bought it to maintain some cycling form and endurance through the winter here in New England. It has served me well, though some of the sensation of riding your bike on the road is lost.

So... I started looking at rollers... and it seems they're nothing new. Here's some history of bike rollers and their use in gold sprints racing.

I actually considered trying to make some, including taking apart an old treadmill for bearings and rollers. I also considered using PVC pipe and some wood to build a set like these. Decided that I didn't have the time/money to fool around with building a set that might not work, so I began looking for a used set.

One failed attempt at buying a used set off of Craigslist and a subsequent 20% sale online, I decided to pick up a set of Travel Trac Aluminum Rollers from Performance Bike for around $120 plus shipping.

I went for the less expensive aluminum rollers rather than the PVC set which came with a built-in, belt driven resistance unit. I figured I could add a resistance unit later if I wanted to (which I found out later they don't make anymore). I'd read in a few places that the aluminum rollers created a bit more noise than the PVC, but there was no worrying about UV damage to the rollers as I guess is a concern with the PV if used or left in the sunlight.

It took a few days for them to arrive, and my first spin took me sideways off the rollers... second attempt... not so bad... third attempt and I'm spinning freely.

The rollers are definitely provide a realistic sensation compared to the rear wheel stationary trainers.

In my search to build a set, I happened across a few posts/videos on do-it-yourself free motion rollers and thought I could easily modify my new rollers to do the same.   Took some roller blade wheels off of some of my son's old roller blades and mounted them via some custom brackets to the bottom of the roller frame.

Spent an hour fashioning a frame from some 2x4's and here's the result:

Frame fashioned from two 2x4's and some medium density fiberboard that I had laying around that came from my treadmill shipping box.  I actually need to shorten the frame some, as I wanted to keep it long to see how much movement I had before I cut it to the right length.  The frame is very lightweight and manageable.  I imagine I could possible put a couple of strategically placed hinges on it to make it even more portable.  Pretty simple the way it is though.

Glued and then screwed the 2x4 from the underside to keep it looking clean.

The Travel Trac Technique Alu rollers have removable "feet."  I used the machine screw that came with the roller to fasten some cut-to-length angled aluminum I was fortunate to get from my father.  The aluminum bracket had a taper to it so it took some countersinking and drilling to get the screw to set just right.

 

 Below you can see the roller assembly set on top of the wooden frame. 

I purchased my only hardware for the build at a discount hardware store for about $4.

Remove one of the hooks from each elastic bungie and clamp to frame.  I later determined that to get the right movement, you want the bungie to be attached so it is "just" taught - not stretched with too much tension.

Check back for some video of the rollers in action!

So now what do you do with a set of modified rollers?

Modify them some more... see how I made my own bike roller mag resistance unit for aluminum rollers.

Check out this video which I referred to quite a bit in designing mine.