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Old 02-12-12, 02:22 AM   #1
Rdrcr
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Bike computer sensor question.

Does it make a difference if the sensor is put towards center of wheel or should it be placed more towards perimeter? I ask cause i've read that it makes no difference but I just installed one on my bike and went for a ride and it is telling me my max speed it 53 mph. I know my legs can't go that fast.

Last edited by Rdrcr; 02-12-12 at 02:27 AM.
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Old 02-12-12, 02:50 AM   #2
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I just installed two of these one on my bike and one on my wifes. First things first are you sure its reading mph and not kph. Second is it possible that the computer isn't set to your tire specifications. The tire size on my cyclocomputer was not even close to what my actual tire size. The placement doesnt matter you can put in in the close to the hub or right next to the rim and it will read the same.
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Old 02-12-12, 03:04 AM   #3
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Sheldon Brown states that it's better to place it nearer to the hub. When going fast on the bike and having the sensor toward the outside, it is possible that the sensor could be going fast enough to simply skip over the magnet without feeling it. Also it definitely sounds like a tire size is off.
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Old 02-12-12, 03:09 AM   #4
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Yes its listed in mph. I have the correct tire size put into the computer. My bike uses 26*1.95 and based on the instructions i put in info for 26*2.0. I doubt a .05 of a difference is affecting the computer that much.

Last edited by Rdrcr; 02-12-12 at 03:15 AM.
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Old 02-12-12, 07:37 AM   #5
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I don't know what model you have, but I recently installed a Cateye CC-RD300W [Strada Wireless]. The instructions say to...

* Install the sensor as close to the upper
part of the front fork as possible.


That would in turn require the magnet to be placed toward the outer wheel diameter to coincide with the sensor pickup "zone".

I assume this upper sensor positioning is to minimize the transmission distance to the computer. The higher placement also affords a safer environment from road generated water and dirt, I would think. In my case, the larger upper fork diameter offered a better sensor mounting area as well.

The only issue I had was having to swivel / move the sensor closer to the magnet's path (not just straight back in alignment), to get a proper reading.

Not sure what the speed issue is with yours, but I'd reset the computer (per instructions provided) and reinitialize from scratch.

If that doesn't do it, refer to these possible problem areas...

Interference may occur, resulting in incorrect data, if the computer is:
• Near a TV, PC, radio, motor, or in a car or train.
• Close to a railroad crossing, railway tracks, TV stations and/or radar base.
• Using with other wireless devices in close proximity.


If THAT doesn't fix the problem, I'd say return the unit, as the only other thing I can think of is if you inadvertently installed more than one magnet on the wheel.

Last edited by Maxturbo; 02-12-12 at 07:46 AM.
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Old 02-12-12, 08:20 AM   #6
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Quote:
Originally Posted by Rdrcr View Post
Does it make a difference if the sensor is put towards center of wheel or should it be placed more towards perimeter? I ask cause i've read that it makes no difference but I just installed one on my bike and went for a ride and it is telling me my max speed it 53 mph. I know my legs can't go that fast.
The speed at the perimeter is faster than at the hub. Sometimes, the speed farther from the hub causes problems but it will cause loss of counts of wheel rotation. Also, for wireless computers, the distance between the sensor and the display unit can be too large (and this will cause loss of counts).

If the position of the sensor is a problem, your speed will be lower than expected. Note that the magnet has to be close to the sensor and the shape of your fork may only allow a particular location.

If your speed is higher than expected, that means you are getting extra fake wheel rotation counts.

Wacky high speeds is usually the result of interference with an analog wireless speedometer.

Digital wireless speedometers might not always report the speed (due to interference) but they won't report fake speeds. That is, with digital wireless computers should never over report your speed.

Last edited by njkayaker; 02-12-12 at 08:27 AM.
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Old 02-12-12, 08:47 AM   #7
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rdrcr: It may not be the position of your magnet but its orientation; here's an article from Sheldon Brown which explains a problem similar to what you are seeing. http://sheldonbrown.com/cyclecomputer-magnet.html
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Old 02-12-12, 09:11 AM   #8
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Rdrcr, I had the same ultra fast readings from my wireless at first. I remounted the sensor and magnet closer to the hub to close the gap between the magnet and sensor and it settled down, for the most part.

I don't think it makes a difference where the magnet is mounted. The magnet will pass the sensor in the same time interval whether near the hub or near the rim. The magnet is going faster when near the rim, but still attached to the spoke.

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Old 02-12-12, 09:28 AM   #9
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Originally Posted by njkayaker View Post
Digital wireless speedometers might not always report the speed (due to interference) but they won't report fake speeds. That is, with digital wireless computers should never over report your speed.
Mine sometimes says I'm going 58 mph while sitting at red lights, always when the magnet is not even close to the sensor.
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Old 02-12-12, 09:38 AM   #10
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Refer to...

Quote:
Interference may occur, resulting in incorrect data, if the computer is:
• Near a TV, PC, radio, motor, or in a car or train.
• Close to a railroad crossing, railway tracks, TV stations and/or radar base.
• Using with other wireless devices in close proximity.
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Old 02-12-12, 10:29 AM   #11
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Is your question about placement of the sensor/sending unit or the magnet? Sounds like both. Mine are approx. midway on three bikes. The sensor and magnet need to pass very close to each other on each turn of the wheel.
Most gross errors in speed readouts are related to programming errors.
What kind of computer? Wired or wireless? Brand/model?
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Old 02-12-12, 08:15 PM   #12
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Originally Posted by bradtx View Post
I don't think it makes a difference where the magnet is mounted. The magnet will pass the sensor in the same time interval whether near the hub or near the rim. The magnet is going faster when near the rim, but still attached to the spoke.
Brad
Right. So long as the magnet triggers the sensor each and every time it makes a pass you should get an accurate speed reading, provided you have programmed the wheel+tire size correctly.

I've mounted the sensors on the fork near the hub, in the middle, and near the brake (depending on what arrangement is easiest to fit) without accuracy problems. For the wireless sensors you would think that closer to the readout head (the "receiver') would be better, everything else being equal, but I haven't found that to be a real issue. One might also think that the position of the readout (on the same side of the bars as the fork sensor) would be an issue, but again it doesn't seem to be.
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Old 02-12-12, 08:24 PM   #13
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What if the wheel is out of true? At the rim the wobble is more pronounced and may cause jitter in the magnetic switch which could read as false higher speeds. Closer to the hub the magnet won't move as much laterally. Also it's easier to get the detector close to the magnet near the hub. I can't think of anything else that might make a difference.
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Old 02-12-12, 08:44 PM   #14
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Quote:
Originally Posted by Rdrcr View Post
Does it make a difference if the sensor is put towards center of wheel or should it be placed more towards perimeter? I ask cause i've read that it makes no difference but I just installed one on my bike and went for a ride and it is telling me my max speed it 53 mph. I know my legs can't go that fast.
Just to check, did you enter the correct wheel diameter or circumference factor into the computer? The computer counts rotations by way of the magnet, then converts that to circumference, and rolling distance by virtue of the conversion factor you enter.

Many computers use a short cut to calculate miles or klicks. Rather than the extra step to convert, they simply use compensated circumference value to differentiate between the two. If you enter the "kilometer" diameter instead of the one for miles it'll read 1.6 times too high.
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