justin_le wrote:What we need is a magnet PAS ring that attaches inside the inner chainring, and is split in 2 semi-circular pieces so that you can insert it in place without even removing the crank arm.
But what does it mount to? Modern cranks don't have any exposed axle, so it would have to be the chainring.
Yes, I was thinking something that would pinch the inner chainring. So long as the pinching device was on an arm of sorts that could slide in and out to accommodate different chainring ID's then this would be fairly universal. The only annoyance is that it seems like there is equal distribution of 4 and 5 bolt chainrings, so you'd have to consider the spacings between these pinching arms to make it work on both.
Hang on why make it two pieces, just make it one piece with a cutout.
I thought of that too. I guess it depends on how tight of a resolution you want with the magnetic poles, since some of the splined spindles have large OD's and would require a decently wide cutout. Riding with the THUN sensor currently using 16 pulses per revolution, it really helps to facilitate an instant response from the PAS control that you can't get from say a 5 magnet ring.
Obvious would be to use the chain ring mounts, as they are standardised. But that would still probably require using longer chainring nut/bolt things.
Not to mention that even with the chainring mounts there are still quite a few "standards" http://sheldonbrown.com/cribsheet-bcd.html
Hang-on, most of those nuts/bolts are magnetic (at least the ones I have). How about just get some 6mm diameter rare earth magnet, stick them on the inside of the chain ring bolts, then mount the sensor. That could work. Wonder if the magnets would stay on?
Most likely they would. Another approach would be to use magnets on the chainring bolts as a method of holding the PAS ring itself in place, and then use a smaller diameter but higher resolution set of magnets for the PAS pickup. With this approach you could have your plastic PAS piece include several sets of mounting magnet holes to account for the different 4 and 5 bolt hole standards, and then the user would simply pop and glue a set of magnets into the appropriate holes for their crankarm.
Down tube size and shape vary drastically, I was thinking the seat tube would be better, ie. standardised in size and round most of the time. But some front derailuers brackets sit really low (like on my Gaint MTB dura ace) and would get in the way, so it may not be a universal solution. The sensor mount would need some degree of adjustment if an existing bracket is in the way. Maybe back to mounting it off the bottom bracket nut, not sure.
Hmm, if there is one thing that is largely standardized it's the length and diameter of a bottom bracket shell. So if you had a pickup that uses two zip ties or narrow hose clamps going around either end of the bottom bracket tube, I think that would do it. It also forces a consistent radial distance from the axis so that there won't be alignment issues if you have a double set of magnets for quadrature encoding. Just need make sure there is a channel and space for the cable routing that often passes under the BB device.