power meter magnet attachment
 

Has anyone had any experience with a crank based power meter magnet (specifically quarq) continually breaking off and sticking into the chain? and what have you done to fix it to the frame so that it won't come off. I have used both the wire guide mount, which broke off after 3 rides, as well as the putty, which broke off during the first ride. I'm looking for something that won't damage the carbon frame if i ever want to remove it.

Hi,

Sounds like poor installation more than anything else.

Generally mount magnets as inboard as possible.

A crank based magnet does cadence, not power.

rgds, sreten.

sreten, that's what i thought too, but for some reason, when the magnet drops off, i no longer get power readings. i have a quarq elsa. and the manual actually states "The included magnet must be installed for the power meter to function." so i don't know what's going on there. but i definitely lose power readings when the magnet comes off.

Power is torque times angular velocity, you need to know the cadence to get the angular velocity. I assume the crank has some sort of strain gauge to allow for torque measurement...

from my limited knoweldge of the inner workings of the power meter, i assume that there is or are strain gauge(s). and most of my physics was left in the classroom after finals :) thanks for the refresher. that makes sense.

i just picked up my bike from the LBS after they installed the magnet for the third time. (1 original, 2 drops on rides). the mechanic suggested i try installing a chain guide, so that my chain won't drop on the inside. i insisted that that is not the problem. since both times that it came off, i was riding on flat roads, and not shifting, or at least not to my recollection; anyway, nothing that should under normal circumstances cause the chain to drop. and that i had not had any chain dropping issues until this.

but then i did remember the week before i got the power meter installed, i dropped the chain on a climb. but that was the only time since i bought the bike a year ago.

and i can see how if the chain drops, it might stick to the magnet. but i don't think that it should pull it out.

this time when they mounted the magnet, the mechanic said that he left a little bit more space.
i wonder if that's going to work.

Without a picture of the application it's tough to see the actual problem. But is sounds like a simple mechanical problem, of mounting the magnet without using a strong adhesive.

Possibly you can improvise a pocket, maybe by cutting up a blister package of the right size and shape, with extensions to either side of the magnet, Pocket the magnet, and use wire ties to hold the assembly to the chainstay. You might add a sheet of rubber (cut up a balloon) underneath for traction.

If the magnetic pull isn't strong, you might push the magnet into a short length of rubber or vinyl tubing, and use the cable ties o hold it on, closing both ends right up against the magnet.

i don't think they used an additional adhesive. they just used the putty that comes with the power meter. so when it dries, it hardens. the install looks like a pimple on the bike. a mound of putty, with the magnet at the top. i've heard of some people gluing the magnet to the frame.
i've actually had one recommendation to use 3M double sided tape.

the pull on the magnet is actually pretty strong. so i'm not afraid that i will ever lose the magnet b/c it will stick to something on the way down. that usually happens to be my chain.