# Bicycle Mechanics - CycloComputer magnet/sensor placement?

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ablang
07-24-08, 12:22 AM
I assume that the goal is to place the magnet/sensor as close to the rim as possible so that it can correctly get your mileage (assuming one inputted the correct tire size of course) w/o it hitting the brakes?

Am I right?

jeff^d
07-24-08, 12:31 AM
Doesn't really matter. One revolution is one revolution, whether at the rim or near the hub. You are correct that inputting correct tire size is important.

FlatFender
07-24-08, 12:34 AM
Doesn't really matter. One revolution is one revolution, whether at the rim or near the hub. You are correct that inputting correct tire size is important.

x2

jsmithepa
07-24-08, 02:25 AM
Am I right?
Wrong.

The sensor is a dumb switch that closes a contact when the magnet passes through it, it conveys no intelligent information other than counts how many turns the wheel does in an interval.

Then the puter says, Avr Speed = count x magic number stored by yr calibration.

EGUNWT
07-24-08, 09:20 AM
Wouldn't it make more sense to put the sensor close to the hub than on the outside? The farther away from the hub you get, the more torque the moving sensor will have, and the more out-of-balance the wheel will be.

supton
07-24-08, 09:36 AM
Perhaps. But the sensor is usually such a small weight, and one is usually not going that fast. Put it where it works best and the wires reach.

soma5
07-24-08, 10:11 AM
The only valid argument for placing the magnet (and sensor) closer to the hub is that the linear speed of the magnet relative to the sensor is lower so that there is less chance that a revolution will be missed (not counted). There has been one actual instance in my experience where moving the magnet and sensor closer to the hub made a difference in the reliability of a cycling computer, so I do this now routinely. The magnet is so light in comparison to, say, a valve stem, that the actual placement probably won't make a detectable difference in how the wheel rolls.

-soma5

carpediemracing
07-24-08, 10:12 AM
Wouldn't it make more sense to put the sensor close to the hub than on the outside? The farther away from the hub you get, the more torque the moving sensor will have, and the more out-of-balance the wheel will be.

I agree with this, depending on the size of the magnet. Some of them are pretty clunky.

It's hard to notice it until you get on rollers - then everything is obvious. The other time you notice it is when you're descending really fast - 50+ mph.

Having said that my magnet is smack in the middle of the spoke.

When I was a bit more concerned about such things I used to get slim flexible magnets and tape them to the side of the aero rims (TriSpoke or Rev-X) to keep things aero and light.

cdr

Sekine
07-24-08, 10:39 AM
Put your magnet exactly opposite the valve stem, that would 'balance' the weight.

Al1943
07-24-08, 12:12 PM
Put your magnet exactly opposite the valve stem, that would 'balance' the weight.

Not necessarily. The heavy side of most rims is where the joint is, usually directly opposite the valve stem. spin the wheel slowly and see where it stops. On my road wheels I put the magnet close to the valve stem for better balance. On my deep profile aero rims the valve stem is 60mm and is the heavy side so the magnet does go on the opposite side.

Al

jsmithepa
07-24-08, 12:19 PM
My 10 years old Avocet magnet is a RING mounted on the hub, nothing unsightly. Don't tell me Avocet got this thing patented and nobody else can use it. :eek:

Guessing some ppl would mount the thing up on top to avoid running a long wire.

Pig_Chaser
07-24-08, 01:46 PM
There's allot of considerations when placing a magnet/sensor on a wheel. But the short answer is: Adjust it for best performance.

Thumpic
07-24-08, 03:14 PM
if you put 2 magnets on different spokes you'll go faster and farther..........

HillRider
07-24-08, 05:34 PM
if you put 2 magnets on different spokes you'll go faster and farther..........

Joking aside, I've seen this done. I saw a reproduction "Penny-farthing" (highwheel) bike with a huge front wheel that was too large for the cyclometer's calibration numbers. The owner had put two magnets on the wheel and set the calibration number for half of the real value.

KevinF
07-24-08, 06:18 PM
The only valid argument for placing the magnet (and sensor) closer to the hub is that the linear speed of the magnet relative to the sensor is lower so that there is less chance that a revolution will be missed (not counted).

I have seen it argued on these forums that it is better to put the magnet close to the rim in order to produce a higher "spike" on the sensor. i.e., they were claiming that with the sensor close to the hub, it could still faintly detect the magnet at 180 degrees away and thus the "low point" reading to the "high point" reading won't be as great, and thus the sensor might not count some revolutions.

Not saying I believe that theory... Just pointing out that it has been said. Personally, I subscribe to the theory that getting the magic tire circumference number correct is far more important then magnet placement.

jsmithepa
07-25-08, 12:19 AM
There's allot of considerations when placing a magnet/sensor on a wheel. But the short answer is: Adjust it for best performance.
My verdict is, this is all much ado fer nuthing.

Booger1
07-25-08, 05:35 PM
I know I've had computers that didn't like the sensor out to far.It went by the pickup too fast and I had to move inward.

Mondoman
07-25-08, 06:01 PM
I suspect that the sensor is just a simple magnetic reed switch -- and thus either completely on or completely off, with no intermediate states and no worries about spike "size" as in KF's post.

Mentor58
07-25-08, 09:28 PM
I have seen it argued on these forums that it is better to put the magnet close to the rim in order to produce a higher "spike" on the sensor. i.e., they were claiming that with the sensor close to the hub, it could still faintly detect the magnet at 180 degrees away and thus the "low point" reading to the "high point" reading won't be as great, and thus the sensor might not count some revolutions.

Not saying I believe that theory... Just pointing out that it has been said. Personally, I subscribe to the theory that getting the magic tire circumference number correct is far more important then magnet placement.
Which is proof that on this forum (like all forums) there are people who have WAY too much time to think about things, and then argue them with a passion and intensity not usually found outside of people who should be on anti-psychotic drugs. :)

I just looked at all my rides, on each one I've got the sensor mounted about 5-6 inches up from the bottom of the fork. No real reason I suppose, just where I thought to put it.

M58