Pedal Assistance Sensor Disk for Hollowtech Bottom Brackets

thingiverse

SAFETY NOTE! This item uses rare earth magnets which are scary strong and should NEVER, EVER be given to children or left where they can play with them. If a child ingests more than one of these, they can end up pinching the gut and this can lead to DEATH. The magnets MUST be firmly glued in the assembly so they do not fall out. An extra thin layer of epoxy painted over the magnets and sensor ring is a GOOD IDEA. SAFETY NOTE! This is a Pedal Assistance Sensor disk and mounting for E-bike kits that need to be fitted to bikes with a Hollowtech(tm) bottom bracket. Most of the PAS sensors supplied with these kits are designed for square taper axle BB's. The Hollowtech BB simply has no clearance between pedal and BB to fit these sensor disks. The solution is to fit the sensor disk to the left hand crank arm instead of the axle with a custom bracket. The sensor disk is also a little larger in diameter to suit bikes where it's harder to fit the Hall sensor on the bottom tube or seat tube and get enough clearance. It's made in two parts - the sensor ring and the bracket which is epoxied (using epoxy putty) to the inside of the crank after you've established the correct position (I used Blu-tac for this!) A cable tie can be added later for some extra security. The bracket seen here was made to fit Shimano Deore M1500 cranks, but will probably fit a few others. It shouldn't be difficult to remix the bracket to suit your own cranks. You'll also need 12 Rare Earth magnets, 5mm in diameter and 4mm thick which you can get from e-bay. These are glued into the ring with superglue or epoxy. NOTE: you have to make sure that they are all fitted in the correct sense (same pole pointing toward sensor). The best way to establish this is to take the PAS sensor ring that came with the kit, determine which way around it was made to fit, and put the new magnets in the new sensor ring pointing in the same direction. Remember, similar poles repel, opposites attract, so another magnet should react to both sensors in the same way. On ebay try searching for "5mm x 4mm rare earth magnet" The two parts are attached with a countersunk screw so there's no interference with the sensor. The rib on the disk are designed to be secured more permanently to the crank in three additional positions using silicone compound if you wish.