Monthly Archives: December 2013

New testing platform – partial success

Since my last post, I’ve wound & fitted some new electromagnets, and made some test runs on the trainer..

Magnets

These are very similar to the previous builds, made from 1/2″ mild steel rod. I have drilled and tapped them at one end for attaching to a bracket. I’ve also added some nylon disks, cut from 40mm rod, just to hold the wire in place a little better.

mag1

This time I used 28 swg magnet wire. Unfortunately I ran out of patience when trying to count the turns by hand, so in the end just used the lathe to spool the wire on quickly. Each magnet took just under half a 250 meter roll, and has a total resistance of approx 20 ohms.

Attachment

The bracket was made from a section of rolled steel, bent into a U shape using a vice. This was then drilled to accept the magnets, and for screwing to the trainer base. The dimensions were then gently tweaked with a lump hammer, until I got a 4mm gap between the installed magnets (leaving 0.5mm each side of the aluminium disk).

mag2

Testing

As always, some good news and some bad news.

I must admit I was a bit anxious before the first test, seeing as how the previous testing on the Tacx platform had been such a damp squib. However, the good news was that this was unfounded – as I was achieving several hundred watts of resistance from these initial runs – result!

The bad news was twofold. Firstly the bracket has too much flex, and as I would increase the current, the attraction between the two magnets was pulling them closer together,  and into the brake disk (as you can see from the scoring on the disk in the last picture). This became the limiting factor for the maximum braking force I could dial in.

The second issue was vibration, courtesy of my junk flywheel. This was the limiting factor for the maximum speed I could ride. Although it was sort of bearable for quick test runs, no way I could actually train on this setup as it is.

But nevertheless, am treating this as mostly a success. To make further progress, I’m going to have to machine some parts with much better precision. Watch this space ;)