Help with Cadence Sensor Magnet
 

Hello all. I have a Trek Emonda that is outfitted with the Bontrager Duotrap speed and cadence sensor. I really like the sensor being tucked away into the frame but I am having one major problem. The slip on cadence magnet that slides on to the crank arm is stretched too far.

I have no problem getting it on but the rubber is stretched so much that it snaps in a week. After the first snap my LBS gave me a replacement, same kind, and it just did the same thing after another week. The magnet isn't contacting anything that would rip it off. I've been really watching for that.
It just seems that my Ultegra cranks are too wide for the rubber sensor magnet.

I tried placing a rare earth magnet on the inside of the crank, sticking it to the pedal's spindle, but it's too powerful and sets off both the cadence and speed parts of the sensor.

Can anyone recommend a solution or another magnet that anyone uses? The 10mm size was perfect distance but the rubber just want large enough for the crank. I'm looking for something that will be secure yet not something that will same the crank arm.

Thanks in advance.

There is nothing nice you can say about the magnet approach. I just got rid of all of my magnets and went to the Wahoo Fitness RPM. It's an accelerometer based cadence device that is not much bigger than the stretch band magnet you are looking at and you don't need the receiver side. Never needs to be aligned with the sensor. Works fantastically well.

J.

I have a friend who has an accelerometer based sensor as well. But since I already have the receiver and something has to go on the crank arm anyway, I may as well have it be a cheap magnet rather than a whole new sensor.

Does anyone know if they make better sleeves or maybe one that relies on zip ties?

Sure. I just finally got tired of having to align magnets everytime I bumped it which was pretty much everytime I took the bike off the top of the car.

Garmin has a zip tie one. $12. Lots of times your LBS will have a butt load of them laying around and just give you one.



J.

Since the duotrap is in the frame, there's no issue of bumping it. And where the cadence magnet sits, I can't contact it with my foot at all. I've never had to readjust anything. I just need to replace the inferior magnet sleeve.

Get a 3x12mm or 1/8"x 1/2" rare earth magnet and stick it to the back of the crank arm with double stick tape or put it under a piece of gorilla tape. With duotrap, you're stuck with where the magnet has to go. With sensors that mount on the stay, you can often let the magnet stick to the inside end of the pedal spindle and align the sensor appropriately.

That's what I'd do. With the Duotrap arrangement, you might as well just stick the magnet down with double sided tape. You can find magnets of any size and shape on Amazon.

J.

I was having trouble with the cadence sensor and had to try a few different magnets so until I found one that was powerful enough to trip the cadence sensor without tripping the speed sensor. I finally went with these guys from Home Depot. I used one magnet attached to the magnet on the crank and used a strip of black electrical tape just to make sure it stays in place, but I honestly don't think I need it. Since then everything's been working fine.
http://bikeforums.net/attachment.php...hmentid=493264

The problem I think you'll have with crank magnets that attach with zip ties is slippage. I had a Cateye Double Wireless and used the magnet that came with it. It mounted nicely, but after riding on rough roads, the vibration would cause it to slide down to the bottom of crank, no matter how tight the ties were. I now use a rare earth magnet on the pedal with my sensor mounted on the stay. The Duotraps are nice but don't give you much wiggle room for the magnet positions.

My suggestion would be to search the different manufacturers web sites and look in their computer accessories for a cadence magnet type that might work for you.

I would probably just use tape.

Reason #93 to move to accelerometer based sensors. No magnet alignment issues anymore.

J.

Since I use my phone with a RFLKT and BT sensors the only BT based accelerometer available is the Wahoo RPM but that still leaves a magnetic speed sensor that I have to have. Luckily, I've not had any alignment issues because my bike is transported on a fork mount so I don't get the sensor bumped like you would on a trunk or hitch mount. Right now, the only time I have to align my sensor to the magnets is when I change the sensor battery.

You must have better luck than I. I don't put my bikes in the car but they go on hitch racks either on the back or on top. Either the magnet gets turned on the spoke or the sensor gets bumped somehow and they are out of alignment. Then multiply by both my wife's bikes and mine. The way you find out it's a problem when it malfunctions when you start riding. Then you have to stop, re-align, spin the wheel and verify. It's a pain.

Either way, probably within the next year, two at the most, with the steep drop in price of the actual accelerometer part thanks to smartphone volumes, you are going to to see the magnetic sensors go away anyhow. A very good thing.

There are several sensors that are in final testing now. I'm surprised that Wahoo hasn't announced one, it's got to be minor reprogramming on the cadence sensor they already have. I'm betting one shows up in time to ride with this spring.

J.

The only time I've ever had magnet alignment issues is when I've kicked the s/c sensor with my heel. You're right, battery changes too. Transport my bike in the car a lot. It's just never been an issue.

I got rid of the sensor when I got a power meter, and use GPS for speed. At the end of the day if the ride distance is off by 50 feet per mile it won't affect anything.