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I never thought I'd be on the receiving end of a catastrophic failure, but I am and I am left with some questions. While reading on Wikipedia I noticed it says a con of magnetic trainers is that they are prone to breaking.

During my training this morning, my trainer failed. And it failed hard. The flywheel shattered/exploded with a sound like a gunshot. A piece of it went through the wall behind my bike, while another shot through the front of my computer (playing a great training video of course). No injuries, but I can't believe this happened! I was in highest gear during a short sprint, so the wheel was moving at a pretty good clip, but I would have thought a trainer could handle that!

The trainer is a Minoura Mag Trainer, not sure on the exact model, but it's a little older.

That said, here are my questions:

  • In what ways do magnetic trainers commonly fail?
  • Was there anything I was doing wrong that caused the failure?
  • Are newer trainers designed better and/or can they accommodate a greater speed?
  • Would another type of trainer (fluid, wind, etc) failed like this?
  • Can I get a replacement flywheel?

Here's some pics of the carnage:

enter image description here

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Doesn't do anything to answer your question, but I think folks would like to know the brand of this trainer. –  Daniel R Hicks Mar 27 '14 at 14:48
From the pictures, it appears that this was "just" the flywheel that failed, and not the magnetic resistance unit. The flywheel is presumably made of diecast steel or some such and would not be subject to any unusual failure modes. Failure would probably be due to "fatigue", damage to the flywheel (a hard blow to the side of it, eg), or some combination of the two. It's a bit discouraging that the unit was not designed to fail in a "self-contained" fashion, either with a robust cover, or a metal strap around the wheel. –  Daniel R Hicks Mar 27 '14 at 14:56
I assumed they were machined aluminum and not cast. Either way, I'd expect stress cracks at the mounting hole and not in the center of the disk. –  Ritch Melton Mar 27 '14 at 15:14
Whatever they are (I've never looked at a trainer that closely) the flywheel should not have failed in that fashion, even if damaged. The flying pieces could easily have caused significant injury to the rider. I'd make a stink with the manufacturer/vendor. –  Daniel R Hicks Mar 27 '14 at 15:17
@DanielRHicks I added the brand. I've also emailed Minoura's support department. We'll see what happens. –  Aaron Mar 27 '14 at 15:32

1 Answer 1

up vote 5 down vote accepted

Let's start with some quick maths. I'm guessing the roller has a diameter of 40mm based on the picture. Since it's in contact with the tyre, whatever speed your speedo reads is how fast that roller is going. But it's 40mm in diameter. So, at 72kph / 45mph we have:

Speed = 72kph = 20m/s
Diameter = 40mm = 0.04m
Circumference = π d = 0.125m
Revs per second = 20 / 0.125 = 160 Hz (9500 rpm)

The magnetic rotor is bigger, perhaps 100mm diameter, and we want the centripetal acceleration:

a =  v² / r
  = 20² / 0.05 = 800 m/s² (about 80G!)

Note that that scales with the square of the speed. So at 108kph / 70mph you're going 50% faster but the force on the outside of the rotor is 2.25 times bigger (about 180G).

1. In what ways do magnetic trainers commonly fail?

The one you care about is overspeed. Either explosion due to too much centripetal acceleration, or an imbalance causing the shaft or mount to break. It looks as though your one failed that way, either through the trainer being knocked at some point or being slightly out of balance when it was made. I've also seen bearings fail through overheating, although that shouldn't happen now because bearings are so much better.

2. Was there anything I was doing wrong that caused the failure? 

Possibly going too fast, possibly going too far. If the flywheel was slightly out of balance it would probably fail eventually regardless of how slowly you went, but if you look at the centripetal acceleration, since that depends on the square of the velocity so does the "wear" caused by being out of balance.

3.  Are newer trainers designed better and/or can they accommodate a greater speed? 

Possibly, and I would hope so. It's been a few years since I broke one, but I've never been a world class cyclist and I have broken two different trainers simply by putting too much power into them (one admittedly at about 1300W). The problem is, to some extent, intractable. People want cheap, light, quiet trainers, but those trainers have to somehow get rid of 500W or more of power for as long as you care to sit on them. Doing that in a light, quiet, cheap container is hard.

4. Would another type of trainer (fluid, wind, etc) failed like this?

Probably. If you bought one with a bigger fan or other dump zone, no. But that would mean the old school "bike wheel with fan blades riveted on" that people hate so much. One alternative that would work is the "pedal powered generator" setup that the green kids are so fond of building, because they usually have bigger flywheels that consequently rotate more slowly (and as I keep saying, the problems go up with the square of the speed, so twice the roller diameter means 1/4 the force). But they're big, heavy and expensive compared to a trainer, and you have to build your own if you want that power output (commercial ones seem to top out at about 200W - I keep looking because I'd like one rated to 500W).

5. Can I get a replacement flywheel?

That's up to the manufacturer.

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It should be noted that 9500 rpm is not excessive for ordinary rotating equipment, so designing to handle stress at that speed should not be difficult. But also note the lower the resistance the higher the speed that can be achieved, so running a unit at very low resistance is apt to put more stress on it than running at a more realistic resistance. (And I'll repeat my statement that the machines should be designed so that any failure of a likely-to-fail component would be "contained" and not threaten injury to rider, bystanders, or nearby facilities.) –  Daniel R Hicks Mar 27 '14 at 23:52
@DanielRHicks I agree with the not failing and containment requirements. My point is that in practice they're not met :) Note that 10krpm bearings are a little more expensive than normal ones, and 20krpm ones more so. See "cheap light and quiet"... –  Mσᶎ Mar 28 '14 at 0:59
Of course, no one who isn't a TdF rider can realistically maintain 45 mph on the road for any extended period. If one is doing that speed on the trainer it's because they have the resistance set too low. (And one wonders why the CPSC hasn't raised a stink about trainers if they catastrophically fail so often.) –  Daniel R Hicks Mar 28 '14 at 1:19
It would be better to simply put a centrifugal governor on them. And, like the UCI, ban fairings and recumbents. –  Mσᶎ Mar 28 '14 at 1:47
Nah, those balls swinging around would be more dangerous than the current setup. (Realistically, a governor is just one more thing to fail. Though I can see maybe a gizmo that made an annoying noise if you go too fast.) –  Daniel R Hicks Mar 28 '14 at 1:50

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