Rotor magnets


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3:14 PM
Jul 13, 2020
I've noticed several new eMTBs putting the magnet used to trigger the wheel rotation sensor on the brake rotor. Unfortunately, not every rotor has holes in the correct place, and some, like the last generation Shimano Centerlock rotors, don't really have a way to mount a magnet at all.

It looks like the brand new Shimano RT-MT800 centerlock rotors have a hole in the rotor spoke, I guarantee that was a design decision based on the trend toward rotor magnets.

I wonder if the manufacturers have gotten together and come up with a standard for the distance the magnet should be from the centerpoint of the hub? It seems like a 6 bolt rotor will have to utilize the mounting bolt somehow. The sensor may be adjustable, but the couple I've seen have utilized braze on mounts or integrated into the droupout for a cleaner look.
The Levo uses a small magnet holder that is mounted with two of the disc rotor screws - simple and effective. When I switched to a wheelset using center-lock rotors I just placed a small Nd magnet on the steel rotor. It held fine for over a year, but I’ve since added a bit of epoxy around it to prevent slippage.
In post #2 above “exodus” was how my iPad autocorrected “epoxy”. Now fixed.
Wonder if supply link, of rotors re branded, at different price levels, I seriously doubt these manufacturers have Tea. instead running a million parts, "modernizing" as the trends evolve, just my thoughts only, the market is huge.

I agree on the industry standard on these bikes. Would help a lot.

My Scott Genius came with a "thingy" that attached to the Shimano centerlock hub/disc. Pretty clean design. But does have a drawback...

A bit more info:

After breaking a spoke on that rear wheel, the acoustic rider in me said, just swap rear wheels until you have time to fix it. Well that turned into a bit more than swapping wheels as the magnet/speed sensor comes into play.

After much tinkery, I decided that just ziptie-ing the "thingy" to the new wheel was good enough. We'll see how the hack works out. Have not had a proper ride on it yet.

Although I may go with the "speed sensor hack" for a longer term fix so I can just swap wheels with no worries. (that's where you put the speed sensor on your crank/chainstay)
I picked up some 27.5 wheels to try out on my 19 levo. They were an eBay deal and came with centerlock hubs. So I bought some magnets and just stuck one on the centerlock adapter ring. I'm using 6 bolt rotors. No adhesive or anything and it worked fine. I did end up setting it with a dab of hot glue though just to keep it from shifting around a little. I carry a couple extra magnets in my tire repair kit just in case I ever lose one.
ebike rotor magnet.jpg
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I'm just reading this out of curiosity. I don't ride an eBike, but yesterday I worked on my friend's Levo. My day gig involves complicated computer controlled motor controls, so it made sense to offer assistance.

The whole eBike thing intrigues me, so I did a little exploring while I worked on the bike. The speed pickup on the frame is just a little magnetic reed switch. There's nothing mysterious at all about it. It's quite basic, and all that it needs to operate is a magnet in close proximity. It's similar to the pickup for the bike computer that mounts on your fork and has a magnet on a spoke.

If you're swapping out wheels/rotors/etc. and need to put something together, you can get tiny niobium magnets off eBay. They're cheap. Grab a handful so you'll have something to play with after you get your bike running. Place the magnet across from the pickup, at about the same distance. JB Weld it into place. It will work just like the factory setup.