The aluminium stock I ordered for the motor mount hasn’t arrived yet, so I’ve not been able to get things moved into the enclosure. However, I did get the M2 hardware for bolting my pulley to the servo horn, so have been experimenting with attachment options for the cable using the original wooden mounting board.
The short version is that the pulley does impart a small bend into the cable when relaxed (see photo above), but this doesn’t seem to have any significant impact on the accuracy.
To test this I put a dial gauge on the sliding magnet in the resistance unit, and monitored the movement of the magnet while trying various pulley attachment options.
With 100 resistance steps over the 30mm of travel, each movement should have been around 0.3mm, this was most accurate with a direct cable attachment to the pulley, and a bit more elastic when attached with a short leader cord. What was most noticeable (regardless of the cable attachment method) was that changes of direction weren’t as positive as I’d have hoped for.
The first one or two motor steps in a new direction don’t have much affect on the position of the magnet, despite the expected movement at the pulley. I don’t really have a good explanation why – my first thought was friction in the cable, but then I’d expect it to be worse returning at a light spring tension, but the effect is pretty constant in either direction and throughout the range of travel & spring tension.
Perhaps there’s a certain amount of give within the cable housing? Perhaps some lube will help anyway, though I’ll wait until the trainer is off the carpet and back in the garage. ;) I may yet make a new (shorter) cable anyway, as the length of the standard one puts the motor a couple of feet ahead of the front wheel (as it’s lying on the floor and not coming up to the bars).
Anyway, at worst this ‘feature’ seems to throw the position out by around 3 steps, so is easily overcome by the closed loop trimming of the resistance which attempts to match average power to the requested load.
Next week will hopefully see me finally(!) getting this into the new enclosure.