traycerb

Will a magnetic-bar-style resistance unit work on aluminum rollers? Short answer is no, as aluminum is not magnetic, but I'm seeing conflicting experiences.

Presently I have some Nashbar Reduced Radius rollers https://www.nashbar.com/profile.cfm?sku=16868, but finally getting around to add some resistance. Tacx speedmatic and minoura's mag unit work, but are all belt-driven. I'm wanting a more low-maintenance solution like sportscrafters unit https://www.branfordbike.com/trainer/train8.html#item3a.

Their unit works b/c the sportscrafters drums they are designed for are alloy ie, have some ferrous material in them. downside of these is they are not realtime adjustable, but this is suitable for my needs.

I tested the Nashbar drums w/ a magnet, they don't seem to be alloy at all, but someone here said they got it to work:
https://www.bikeforums.net/showpost.p...8&postcount=55

I can't fathom how this would work though. The drum's axle is steel, but obviously fixed. Maybe the magnets are strong enough to affect the bearings??

OTOH, this post makes more sense, in that it doesn't work:
https://www.bikeforums.net/showpost.p...7&postcount=18

Anyway, I could order an individual alloy drum from Branford for $80, but there's the potential for size discrepancy, fit, etc not so safe for the drum to be flying off. May end up with the minoura unit anyway or just do the low-tech "towel below a roller" solution

cyccommute 

It works by magnetic induction. The metal doesn't need to be magnetic. Aluminum moving through a magnetic field will resist the movement by Farraday's law. Here's a quick illustration for how it works and here's a much more detailed explanation. Get past the very involved math and get to the explanation for the generator.

You can do a home experiment to see how it works with a strong magnet (a rare earth magnet from Radio Shack) and an aluminum tube just slightly larger than the magnet. Drop the magnet down the tube and it will slowly fall to the ground...much slower than it would if you just dropped it.

The magnet is creating a flow of current in the aluminum that is resisting the movement of the magnet through the tube and slowing it down. The same principle applies to the rollers. Put them in a magnetic field and try to spin them and they'll resist the movement. Move them faster and they'll resist more. Very simple and very clever.

Edit: Look at this one too. I may have to go make me one!

Edit again: Linked to the wrong video. Corrected and it's really cool.

traycerb

well i apparently retained nothing from my fields and waves course. see, this is why i left the physical sciences for the biological ones. less math too. sportscrafters can thank physics for my purchase (and cyccommute). good god but faraday does rock.

krazyderek

I thought about this, and saw some of the cool demonstration video's on youtube but when i held the saris mag unit it place and spun the rear drum i didn't get any increased resistance, however i could have dismissed this to early.

I'll do a full boar 20 min riding experiment today, one with my tacx mag unit, one with no mag unit (control) and one with the saris unit adjusted per the manufacturer's instructions on each level of resistance to see if it is compatible with nashbar rollers. I'll plead guilty, i simply thought regardless of Farraday's law, the drum walls were just to thin to be affected, and i thought for sure, a manufacturer would alter their rollers slightly so that their mag unit would only work with their rollers. But i forgot about the "faster it goes more resistance"-part, so i'll try and report back by tonight.

Yay! experiment!

krazyderek

Well i was indeed wrong. The Saris/Cycleops/sports crafters fixed mag unit does work on all Alu rollers.

Although it doesn't provide as much resistance as the TACX unit in any given gear, the resistance does increase fairly predictably through the gears. I did some testing and will post up the numbers later when i get home. But, let's just say i had no trouble getting over 300w in the middle chain ring on the highest setting of the fixed mag unit. It does however lack the convenience of a remote unit which is nice for intervals, less clumbsy than changing gears. The TACX remote also had a fine tuning dial aswell which allowed me to compensate for drift as the tires and belts warmed up and worked in during the workout. It's also very usefull for the graded MAP test allowing me to hit exact 25w increments which isn't always the case when switching gears with the fixed mag unit.

Last but certainly not least, one must consider the noise. With my more powerful Tacx resistance unit i can easily do lv2 and 3 workouts in my granny gear resulting in lower wheel speed and less noise. Mind you with the right tires rollers are still considerably quieter then trainers. While in the highest resistance for the mixed mag unit (saris /cycle ops/ sportscrafters) i had to shift up to the middle ring to get to hit lv3 wattages, thus resulting in louder workouts. Also in order to hit my interval wattage of 350-500 i'll have to be in the big chain ring resulting massive wheel speed, and some rattling/vibrations that may be coming from the rollers (might need replacing) or coming from the fixed mag unit it's self as it's just a spring loaded arm that could easily rattle when a small amount of vibrations hit it.

Anyways, i'll post a pic and data later if you want.
Thanks for the reminder cyccommute.

traycerb

Yah, no getting around the absence of a remote, but good to know it works.

Still, wonder if it's possible to increase the baseline resistance by adding some rare earth magnets https://www.rare-earth-magnets.com/Se...goryID-30.html. Haven't looked at the math, but I assume the resistance is proportional in some fashion (linearly? nonlinearly?) to the strength of the B (magnetic) field. Of course, none of this solves the fact that you simply won't have the degree of ctrl you have with the tacx (increments, fine tuning, etc.)

The whole apparatus could be a diy project, but probably more trouble than it's worth. still easier than your diy free-motion set up though (my next upgrade...)