Quiet, cheap, effective DIY magnetic resistance for metal rollers
08-08-10, 03:52 PM
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Quiet, cheap, effective DIY magnetic resistance for metal rollers
(i'm not a roadie, but posting this here since most posts on rollers are here)
i needed a way to get in some rides during a chicago winter, and i settled on rollers. rollers were better for me than trainers, because the demand to keep constantly focused fends off my boredom. Picked a set of Nashbar Reduced Radius Rollers.
i put some slicks on an ancient no-name mtb, and after the usual adjustment period, it worked great. i then ran into the common complaint regarding rollers: lack of resistance. i was able to get to highest gears without difficulty, but with the concomitant vibration and noise.
did some searching on resistance that would be suitable for my needs. since i live in an apt, i needed quiet, so wind units were out. since, at best, i'm really a recreational cyclist i wasn't looking to spend a lot of money. i found a sportcrafters magnetic unit, which is not the typical magnetic unit. it's literally just a bar of magnets, put just behind one of the rollers.
cheap enough, but there was concern that it was a little weak in the resistance dept. so i decided to build my own, and, surprisingly, it worked great.
Here's a pic of the whole setup:
Your first unrelated concern: WTF is up the tape? And WTF is up with the colors? Well, I ran into another complication of rollers: they chew up tires. Not sure how other rollers are, but Nashbar's rollers have fine grooves which left a spray of rubber behind my rollers. (this is before I knew about trainer-specific wheels). So I put electrical tape (orange) on the rollers, which solved that problem. The tape was a little noisy (squeaky), but an additional layer of masking tape (blue) solved that problem. Every now and then I have to replace a row of tape, but it's surprisingly infrequent. The electrical tape makes a great base layer, because it comes right off. As an added bonus, though the Nashbar rollers are quiet at baseline, the grooves cause a low droning sound, which the tape totally eliminated. Regarding the garish colors, well, it's just what I had at the time, and I've yet to replace it. In the meantime, go Broncos.
A close up of the resistance unit alone:
Almost self-explanatory. There are four magnets wrapped with tape to a thin wooden board, which in turn is screwed into to metal brackets hung on the roller frame. The magnets are rare earth magnets, which are much more powerful than the ceramic ones on the sportscrafters model, to the point of being dangerous. They snap violently together, are nearly impossible to separate, and make working in proximity with metal tools nearly impossible. I bought 6 of them (roughly 2 inches x 1 inch x1 /4" each) from this guy: https://www.supermagnetman.net/ for a total of $40.
I experimented with the polarity of the magnets, but it doesn't make much of difference. (Though it is important to buy magnets with the field oriented correctly). The blue styrofoam was meant to keep the magnets apart, but isn't really necessary.
Magnetic resistance unit in place:
One flaw of my my design is that I need to disassemble the rollers to slide the resistance unit onto the frame.
Close up of magnets and roller:
The resistance falls off dramatically with distance, so it's important to keep the magnets as near to the roller as possible. You can see it's really only a few millimeters away.
As you start to cycle, Faraday's law kicks in, and the resistance starts in. If the magnetic bar isn't latched into place, the forces will just push the magnetic bar away. I had two extra magnets which I used to hold the metal brackets into place.
Magnet holding bracket in place:
So, what's the level of resistance? Absolutely crushing. Keep in mind I'm just a recreational cyclist, but whereas before I could max out, now I have to push to keep my cadence above 80 in even the lowest gears.
In summary:
Advantages
1. No maintainance
2. 100% quiet
3. Small, low profile, easy to store
4. Great resistance
5. It's Carl-Weathers-cheap
Disadvantages:
1. Resistance obviously not adjustable on the fly. That distance of a few millimeters gives me a good baseline level of resistance, and I manage the rest through my gearing during my session. As close as it already is to the roller, I couldn't really increase the baseline resistance without adding magnets (which I have room to).
2. Resistance doesn't feel the same as the road.
3. It's Carl-Weathers-cheap. Even if it works, not everyone will want a $40 ghetto-looking bar of magnets hanging off their rollers. You know who you are.
4. Disadvantages common to all rollers: can't push yourself to the point of passing out, better to be outdoors (aka "why are we having this discussion in the summer?" Answer: STFU), etc.
Next upgrade will be converting them to free motion.
i needed a way to get in some rides during a chicago winter, and i settled on rollers. rollers were better for me than trainers, because the demand to keep constantly focused fends off my boredom. Picked a set of Nashbar Reduced Radius Rollers.
i put some slicks on an ancient no-name mtb, and after the usual adjustment period, it worked great. i then ran into the common complaint regarding rollers: lack of resistance. i was able to get to highest gears without difficulty, but with the concomitant vibration and noise.
did some searching on resistance that would be suitable for my needs. since i live in an apt, i needed quiet, so wind units were out. since, at best, i'm really a recreational cyclist i wasn't looking to spend a lot of money. i found a sportcrafters magnetic unit, which is not the typical magnetic unit. it's literally just a bar of magnets, put just behind one of the rollers.
cheap enough, but there was concern that it was a little weak in the resistance dept. so i decided to build my own, and, surprisingly, it worked great.
Here's a pic of the whole setup:
Your first unrelated concern: WTF is up the tape? And WTF is up with the colors? Well, I ran into another complication of rollers: they chew up tires. Not sure how other rollers are, but Nashbar's rollers have fine grooves which left a spray of rubber behind my rollers. (this is before I knew about trainer-specific wheels). So I put electrical tape (orange) on the rollers, which solved that problem. The tape was a little noisy (squeaky), but an additional layer of masking tape (blue) solved that problem. Every now and then I have to replace a row of tape, but it's surprisingly infrequent. The electrical tape makes a great base layer, because it comes right off. As an added bonus, though the Nashbar rollers are quiet at baseline, the grooves cause a low droning sound, which the tape totally eliminated. Regarding the garish colors, well, it's just what I had at the time, and I've yet to replace it. In the meantime, go Broncos.
A close up of the resistance unit alone:
Almost self-explanatory. There are four magnets wrapped with tape to a thin wooden board, which in turn is screwed into to metal brackets hung on the roller frame. The magnets are rare earth magnets, which are much more powerful than the ceramic ones on the sportscrafters model, to the point of being dangerous. They snap violently together, are nearly impossible to separate, and make working in proximity with metal tools nearly impossible. I bought 6 of them (roughly 2 inches x 1 inch x1 /4" each) from this guy: https://www.supermagnetman.net/ for a total of $40.
I experimented with the polarity of the magnets, but it doesn't make much of difference. (Though it is important to buy magnets with the field oriented correctly). The blue styrofoam was meant to keep the magnets apart, but isn't really necessary.
Magnetic resistance unit in place:
One flaw of my my design is that I need to disassemble the rollers to slide the resistance unit onto the frame.
Close up of magnets and roller:
The resistance falls off dramatically with distance, so it's important to keep the magnets as near to the roller as possible. You can see it's really only a few millimeters away.
As you start to cycle, Faraday's law kicks in, and the resistance starts in. If the magnetic bar isn't latched into place, the forces will just push the magnetic bar away. I had two extra magnets which I used to hold the metal brackets into place.
Magnet holding bracket in place:
So, what's the level of resistance? Absolutely crushing. Keep in mind I'm just a recreational cyclist, but whereas before I could max out, now I have to push to keep my cadence above 80 in even the lowest gears.
In summary:
Advantages
1. No maintainance
2. 100% quiet
3. Small, low profile, easy to store
4. Great resistance
5. It's Carl-Weathers-cheap
Disadvantages:
1. Resistance obviously not adjustable on the fly. That distance of a few millimeters gives me a good baseline level of resistance, and I manage the rest through my gearing during my session. As close as it already is to the roller, I couldn't really increase the baseline resistance without adding magnets (which I have room to).
2. Resistance doesn't feel the same as the road.
3. It's Carl-Weathers-cheap. Even if it works, not everyone will want a $40 ghetto-looking bar of magnets hanging off their rollers. You know who you are.
4. Disadvantages common to all rollers: can't push yourself to the point of passing out, better to be outdoors (aka "why are we having this discussion in the summer?" Answer: STFU), etc.
Next upgrade will be converting them to free motion.
08-08-10, 05:38 PM
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I'd be concerned w/ heat dissipation, but at the very least, that's a really cool physics experiment.
200W is pretty standard cruising power. Short (5 min) intervals could be as high as 500W (not me, but it's possible). That's a lot of power to be converted into heat on one Al drum without any real heat sink other than the drum itself.
200W is pretty standard cruising power. Short (5 min) intervals could be as high as 500W (not me, but it's possible). That's a lot of power to be converted into heat on one Al drum without any real heat sink other than the drum itself.
08-09-10, 12:26 PM
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That's very clever. Folks do something similar by magging fishing reels (revolving spool type). FYI, I believe it's Lentz's law that's at work here. Also, you can use a steel bar instead of wood. The magnets will stick and mounting them on steel makes them effectively stronger.
10-15-13, 04:23 PM
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This is brilliant and has just saved me wasting time and money hunting down a resistance unit for my cheapo Minoura rollers.
I was curious when I read this last Sunday so I went straight onto ebay and picked up a couple of 'super magnets' ("6kg pull") from ebay, and then sawed some wood to length. The magnets just arrived and I wanted to do a quick test; I taped the magnets to the wood and wedged the thing in place on the roller. Then I had a go.
It works! Feels a bit like a gradient of perhaps 5%. Going to experiment with more magnets.
I was curious when I read this last Sunday so I went straight onto ebay and picked up a couple of 'super magnets' ("6kg pull") from ebay, and then sawed some wood to length. The magnets just arrived and I wanted to do a quick test; I taped the magnets to the wood and wedged the thing in place on the roller. Then I had a go.
It works! Feels a bit like a gradient of perhaps 5%. Going to experiment with more magnets.
10-15-13, 05:08 PM
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Yeah, I did this too but skipped the whole wooden piece. I just stuck the magnet to the frame so that it sits very close to the drum and it instantly added enough resistance at the top end. Easy.
10-15-13, 05:27 PM
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10-16-13, 08:40 AM
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I bought this:
https://www.amazon.com/Applied-Magnet...tr_256374011_4
And did this:
And, yes, even with that small portion of the magnet actually near the AL roller, I get plenty of resistance. Super easy and compact. Thank you Faraday!
https://www.amazon.com/Applied-Magnet...tr_256374011_4
And did this:
And, yes, even with that small portion of the magnet actually near the AL roller, I get plenty of resistance. Super easy and compact. Thank you Faraday!
10-16-13, 08:43 PM
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I bought this:
https://www.amazon.com/Applied-Magnet...tr_256374011_4
And did this:
And, yes, even with that small portion of the magnet actually near the AL roller, I get plenty of resistance. Super easy and compact. Thank you Faraday!
https://www.amazon.com/Applied-Magnet...tr_256374011_4
And did this:
And, yes, even with that small portion of the magnet actually near the AL roller, I get plenty of resistance. Super easy and compact. Thank you Faraday!
10-16-13, 09:28 PM
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What you are doing is inducing eddy currents in the aluminium. As someone says above, Lenz's Law is the governing principle so the effect increases with conductivity, relative velocity and field strength.
The first two are given. To maximise field strength it's best to steer the field as much as possible using iron or steel about the thickness of the magnet. The field loses strength the further it has to travel through air (or aluminium).
If you want to keep the side by side orientation, put the magnets a bit closer together and mount them in pairs on steel with pole orientations reversed. In effect you want to make a big horseshoe magnet: N -> S -> Iron -> N -> S. and have the roller in the field from the last S to the first N.
Even better would be to mount them on opposite sides of the roller and complete the circuit with a big C shaped piece of steel, but that's probably a bbit ambitious.
The first two are given. To maximise field strength it's best to steer the field as much as possible using iron or steel about the thickness of the magnet. The field loses strength the further it has to travel through air (or aluminium).
If you want to keep the side by side orientation, put the magnets a bit closer together and mount them in pairs on steel with pole orientations reversed. In effect you want to make a big horseshoe magnet: N -> S -> Iron -> N -> S. and have the roller in the field from the last S to the first N.
Even better would be to mount them on opposite sides of the roller and complete the circuit with a big C shaped piece of steel, but that's probably a bbit ambitious.
10-16-13, 10:02 PM
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No magnet inside of the roller. As the AL passes by the magnet, an eddy current is induced in the AL. blah, blah blah...look up Faraday’s Law (Lenz’s Law describes the direction of the induced current.)...resistance. (Im on fall break and frankly can’t be bothered to teach...) But it works great and is super simple.
10-18-14, 06:18 AM
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I definitely want to add resistance to my cycleops rollers and this sounds like a great idea. I just dont get how the magnets add resistance to aluminum. I grabbed a couple of pretty strong magnets I found in the garage and held them close to the back roller as I spun it but did not notice any change in resistance. How does this work with aluminum?
Thanks
Brent
Thanks
Brent
10-18-14, 06:25 AM
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I definitely want to add resistance to my cycleops rollers and this sounds like a great idea. I just dont get how the magnets add resistance to aluminum. I grabbed a couple of pretty strong magnets I found in the garage and held them close to the back roller as I spun it but did not notice any change in resistance. How does this work with aluminum?
Thanks
Brent
Thanks
Brent
There is a cool demo you can do to convince yourself. Take two narrow diameter but same length tubes for which you can just barely drop your magnet through. One tube is Al, the other is nonmetal, say plastic. Hold the tubes vertically and watch the difference in times for each to traverse the length of the tubes. You don't need a stopwatch, the effect is large enough to notice.
*Ever look at a triple beam balance? You might notice that the side opposite the pan has a metal fin that runs between two (usually hidden) magnets. These magnets act to damp out oscillations that occur when you put something on the pan itself. It is the same effect.*
10-18-14, 06:49 AM
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A changing magnetic field will induce a current in the Al which acts in such a way as to reduce the change in magnetic flux through the Al. Geez, I am on fall break again and this question comes up. The faster the rollers are spinning, the bigger the effect, so you might not have noticed it at slow rotational speeds. You do need the magnet close to to the drum though.
There is a cool demo you can do to convince yourself. Take two narrow diameter but same length tubes for which you can just barely drop your magnet through. One tube is Al, the other is nonmetal, say plastic. Hold the tubes vertically and watch the difference in times for each to traverse the length of the tubes. You don't need a stopwatch, the effect is large enough to notice.
*Ever look at a triple beam balance? You might notice that the side opposite the pan has a metal fin that runs between two (usually hidden) magnets. These magnets act to damp out oscillations that occur when you put something on the pan itself. It is the same effect.*
There is a cool demo you can do to convince yourself. Take two narrow diameter but same length tubes for which you can just barely drop your magnet through. One tube is Al, the other is nonmetal, say plastic. Hold the tubes vertically and watch the difference in times for each to traverse the length of the tubes. You don't need a stopwatch, the effect is large enough to notice.
*Ever look at a triple beam balance? You might notice that the side opposite the pan has a metal fin that runs between two (usually hidden) magnets. These magnets act to damp out oscillations that occur when you put something on the pan itself. It is the same effect.*
04-15-15, 09:12 PM
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I would like to add that I did something similar after finding this thread and the results are exactly what I wanted!
I ran into the same problem where I started doing intervals but due to the lack of resistance, I couldn't get my HR as high as I need to.
Following the hold-strong-magnets-next-to-rollers principle, I went to Home Depot and got a metal angle bar. This allowed the magnets to stick to the bar without the need for tape or glue. I then cut this to size with my dremel and used a pair of clamps to hold it in place. I used about 14 3/4" x 1/16" magnets which I ordered off of Amazon.
I can definitely feel the resistance on the bike while holding the magnets a few mm away from the rollers. Not as much as a trainer but enough for me to get my HR up. The roller with the magnets does get warm due to those physics laws but it's still cool enough to touch and probably far cooler than a hot pavement in the middle of summer.
I ran into the same problem where I started doing intervals but due to the lack of resistance, I couldn't get my HR as high as I need to.
Following the hold-strong-magnets-next-to-rollers principle, I went to Home Depot and got a metal angle bar. This allowed the magnets to stick to the bar without the need for tape or glue. I then cut this to size with my dremel and used a pair of clamps to hold it in place. I used about 14 3/4" x 1/16" magnets which I ordered off of Amazon.
I can definitely feel the resistance on the bike while holding the magnets a few mm away from the rollers. Not as much as a trainer but enough for me to get my HR up. The roller with the magnets does get warm due to those physics laws but it's still cool enough to touch and probably far cooler than a hot pavement in the middle of summer.
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07-20-20, 09:14 AM
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I can definitely feel the resistance on the bike while holding the magnets a few mm away from the rollers. Not as much as a trainer but enough for me to get my HR up. The roller with the magnets does get warm due to those physics laws but it's still cool enough to touch and probably far cooler than a hot pavement in the middle of summer.
07-20-20, 07:15 PM
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Sorry for the zombie resurrection, did the heat ever become a problem? I like your solution with angle iron and clamps. Seems like it would make adding or subtracting magnets fairly easy, but I didn't want to lead to heating things up too much to warp the roller or damage the tires.
07-21-20, 05:00 AM
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Thanks, that's good to know. Actually, at the beginning of lockdown, I went on a nice, long, solo ride well away from people (well, it was supposed to be nice and long anyway), but I got a bad enough cut in my tire that required me to boot it up and abort the ride. Got a brand new, tire that was nicer than my others (because it was on sale and why not treat myself) but the only outdoor rides I've done since have been little cruises with the family. I know I'm wasting the nice tire on my rollers, but I don't feel like changing it out for one of the crappy ones I have laying around and I only have one wheelset currently built up with a powertap hub (I like the rollers for Zwift).
