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-   -   Problem with Garmin 705 Cadence / Speed sensor (http://www.bikeforums.net/electronics-lighting-gadgets/584529-problem-garmin-705-cadence-speed-sensor.html)

PAlt 09-13-09 01:41 PM

Problem with Garmin 705 Cadence / Speed sensor
 
I am having problems getting the cadence sensor on My Garmin 705 to "pick up" the magnet consistently on the crank arm on one of my bikes. I'm currently set up on two different bikes, a Guru Carbonio Ti and a Litespeed Tuscany (1998 vintage).

The Guru works fine, it's the Litespeed that seem to have the issue. The sensor is new, and picks up the wheel magnet with no problem. It will sense the crank magnet sometimes, but not consistently enough to get a reading on the head unit.

The chainstays on the Litespeed are curved and I'm using a SRAM Rival crank, which is slightly machined out on the backside of the crankarm. In order to get the sensor to pick up the magnet at all, I have to angle the sensor out away from the wheel. This means that the magnet is VERY close to the sensor only at it's top. I've tried different magnets from the old parts bin, shimming the magnet (which still didn't push it out enough to matter), changed the placement of the sensor on the chainstay to further back, which helped to straighten the angle some, but apparently not enough.

Wondering if a stronger magnet might help, but I'm open to any suggestions...

PAlt 09-16-09 05:53 PM

Quote:

Originally Posted by PAlt (Post 9667266)
I am having problems getting the cadence sensor on My Garmin 705 to "pick up" the magnet consistently on the crank arm on one of my bikes. I'm currently set up on two different bikes, a Guru Carbonio Ti and a Litespeed Tuscany (1998 vintage).

The Guru works fine, it's the Litespeed that seem to have the issue. The sensor is new, and picks up the wheel magnet with no problem. It will sense the crank magnet sometimes, but not consistently enough to get a reading on the head unit.

The chainstays on the Litespeed are curved and I'm using a SRAM Rival crank, which is slightly machined out on the backside of the crankarm. In order to get the sensor to pick up the magnet at all, I have to angle the sensor out away from the wheel. This means that the magnet is VERY close to the sensor only at it's top. I've tried different magnets from the old parts bin, shimming the magnet (which still didn't push it out enough to matter), changed the placement of the sensor on the chainstay to further back, which helped to straighten the angle some, but apparently not enough.

Wondering if a stronger magnet might help, but I'm open to any suggestions...

Bueller??? NO ideas???

tippy 09-16-09 07:52 PM

Pictures of the non-working setup would be nice ...

tippy 09-16-09 08:00 PM

Does the magnet pass the sensor at the indicator mark on the sensor? Are you get the appropriate LED flash (can't remember if CAD is the red or green flash).
I've found that if it is set reasonable close and you see flashes, it should work.

PAlt 09-17-09 07:30 PM

Quote:

Originally Posted by tippy (Post 9689033)
Does the magnet pass the sensor at the indicator mark on the sensor? Are you get the appropriate LED flash (can't remember if CAD is the red or green flash).
I've found that if it is set reasonable close and you see flashes, it should work.

The diode will flash sometimes. As I indicated, the sensor sometimes picks up the magnet, other times not, but obviously not consistently enough to give me a cadence reading. Will attempt to post a few pictures this weekend. Thanks for responding!

shoerhino 09-17-09 07:39 PM

I've noticed this behavior as well. The magnet needs to pass very close to the sensor. I've never trued stronger magnets but I would suspect that it might work better.

bbrase 09-19-09 07:23 PM

Better (stronger) magnets are a probable solution. What I use are the 1/2"x1/16" Neodymium magnets from here: http://www.unitednuclear.com/magnets.htm . The 1/2" diameter fits almost perfectly on the inboard end of the pedal spindle when fully inserted into the crank arm. Even if your pedal spindles aren't long enough to go fully through the crank arm, these magnets are strong enough to lock in place there and not move. Stack a couple of the magnets on top of each other (or get the 1/8" thick ones) to increase the strength and close the gap to the sensor a bit. Of course this won't work if you have Ti pedals. On my 2003 Litespeed Tuscany I have nearly a 3/4" gap between the magnets and sensor and cadence works reliably. I have two of the 1/16" magnets stacked on this bike, although I suspect it would be reliable with only one. Also this is a much "cleaner" look than the stock magnet with the tie wrap around the crank arm. I've had this setup since Feb 2009 with no problems and no lost magnets.

If any of this isn't clear just ask, I'll post some pictures.

Shimagnolo 09-19-09 08:22 PM

Quote:

Originally Posted by bbrase (Post 9706530)
The 1/2" diameter fits almost perfectly on the inboard end of the pedal spindle when fully inserted into the crank arm. Even if your pedal spindles aren't long enough to go fully through the crank arm, these magnets are strong enough to lock in place there and not move. Stack a couple of the magnets on top of each other (or get the 1/8" thick ones) to increase the strength and close the gap to the sensor a bit.

Now *that* is a slick idea!
On my road bike, the crank is so far from the chainstay, that I had to carve a section from a 1"
wooden dowel rod to lift the magnet nearly 1/2" to get it close enough to the cadence sensor.
It looks kludgey.


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