# best spot for computer wheel magnet



## RanGer498 (Feb 17, 2012)

I see people place there magnets on there wheels in different areas
center ,middle and far out side and Im wondering what is the best place for the wheel magnet to be placed ?

thanks


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## tihsepa (Nov 27, 2008)

It really dosent matter. I run two or three magnets to be sure though.


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## AndreyT (Dec 1, 2011)

RanGer498 said:


> Im wondering what is the best place for the wheel magnet to be placed ?


When you place your sensor on the front fork, placing the magnet (and the sensor) closer to the wheel rim will reduce the distance between the sensor and the computer itself (the latter being mounted on the handlebars). For wireless computers that might mean better reception.

For the popular penny-farthing bicycle the wireless range of the computer (as well as the cable length of the wired computer) will act as a natural limit of how close to the center of the wheel you can place the sensor. The rare bicycles with 700C wheels allow you to place the sensor anywhere you wish.

Otherwise, there's no difference.


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## Scott in MD (Jun 24, 2008)

I put my magnet 180 degrees around from the valve stem, to balance the rotating weight of the wheel. 

Does anyone else do this?


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## Yamabushi (Sep 30, 2008)

Scott in MD said:


> I put my magnet 180 degrees around from the valve stem, to balance the rotating weight of the wheel.
> 
> Does anyone else do this?


I do the same! :thumbsup:


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## Scott in MD (Jun 24, 2008)

Long time ago i spun my front wheel On my stand a few times, and every time it landed on stem down so I'm pretty sure the stem is the heaviest point on a true wheel (even with a welded Al joint). Besides, Zinn does this too.


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## Yamabushi (Sep 30, 2008)

Scott in MD said:


> Long time ago i spun my front wheel On my stand a few times, and every time it landed on stem down so I'm pretty sure the stem is the heaviest point on a true wheel (even with a welded Al joint). Besides, Zinn does this too.


Yep, that's my logic as well. And as a bonus, it's more aesthetically pleasing.


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## Pitts Pilot (Dec 5, 2011)

tihsepa said:


> It really dosent matter. I run two or three magnets to be sure though.


...and average 60 mph! I like it.

I also go opposite the valve stem. I go as far toward the center of the wheel as I can, as it experiences less centrifugal force (I think) and passes the sensor more slowly.


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## Yamabushi (Sep 30, 2008)

Pitts Pilot said:


> I go as far toward the center of the wheel as I can, as it experiences less centrifugal force (I think) and passes the sensor more slowly.


I do that as well! Great minds and all that!


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## AndreyT (Dec 1, 2011)

Pitts Pilot said:


> I also go opposite the valve stem. I go as far toward the center of the wheel as I can, as it experiences less centrifugal force (I think) and passes the sensor more slowly.


It will indeed experience less centrifugal force (although the magnet mount is supposed to be able to handle even the largest one). 

As for "passing the sensor more slowly"... I'm not sure it is a good thing. For the sake of precision, the magnetic field from the magnet (as it approaches the sensor) should ramp up as quickly as possible. I.e. the "sharper" is the event produced by the magnet - the better.


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## AndreyT (Dec 1, 2011)

Scott in MD said:


> Long time ago i spun my front wheel On my stand a few times, and every time it landed on stem down so I'm pretty sure the stem is the heaviest point on a true wheel (even with a welded Al joint). Besides, Zinn does this too.


On an unloaded wheel in "laboratory" conditions it might make a difference. On a bike that is actually ridden on the actual road the influence created by that disbalance is absolutely inconsequential.

Needless to say, to couterbalance the valve stem it is not sufficient to just place the magnet somewhere on the opposite spoke. It is important to determine the exact distance from the center of the wheel at which the balance is achieved. (And then, when you change, say, the 8-Ball novelty valve cap to a Rocket Ship novelty valve cap, a rebalance is required).


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## Scott in MD (Jun 24, 2008)

I believe what I'm saying is true because I watched my wheel roll to a stop on the stand with stem down every time I spun it. I know that Zinn's book recommends this magnet and stem technique because I read it. And I believe that balancing on an unloaded stand is relevant because car wheels are balanced using exactly same "unloaded" principle: take them off the car, mount them on a tool, spin them up, find the heavy spot, put some weight across from it. (Or have you found some way to balance your car wheels while they are "loaded" on the axle and the road? Maybe you mathematically calculate the right spot for the balance weights by observing the shimmy of the steering wheel?). 

And in a world where guys shave their legs so they can race aerodymanically faster to the coffee shop, not much is inconsequential.


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## skepticman (Dec 25, 2005)

Scott in MD said:


> Or have you found some way to balance your car wheels while they are "loaded" on the axle and the road?


There is the Road Force Balancer.

Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix


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## Scott in MD (Jun 24, 2008)

skepticman said:


> There is the Road Force Balancer.
> 
> Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix


Thanks. I just STOLE one of these on eBae for less than sixteen grand. After I get it set up for my bike wheel I'll have that damn magnet centered right on the spot.


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## JCavilia (Sep 12, 2005)

AndreyT said:


> It will indeed experience less centrifugal force (although the magnet mount is supposed to be able to handle even the largest one).
> 
> As for "passing the sensor more slowly"... I'm not sure it is a good thing.* For the sake of precision, the magnetic field from the magnet (as it approaches the sensor) should ramp up as quickly as possible. I.e. the "sharper" is the event produced by the magnet - the better.*


Actually doesn't matter how fast it ramps up. The slope of the rise is irrelevant to the sensor because it "senses" the magnetic field in a very simple way - it's a reed switch, not a proximity sensor. A springy flexible steel contact is pulled on by the magnet, and when it's pulled far enough it contacts the other contact and closes the circuit. It springs back and breaks the circuit when the magnet goes away. So as long as the contact happens every pass, at about the same point in the revolution, you get good data.

If the field were weak enough (due to a combination of weak magnet and too much distance between magnet and sensor), too _fast_ a pass could in theory result in missed pulses if the reed didn't pull all the way closed before the magnet departed. I'm not sure that would ever happen in real life. Certainly not if you placed them close enough.

I agree with you that the balance thing is irrelevant. The amount of imbalance caused by the magnet would be undectable on the road by any rider, I'd bet.


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## Kerry Irons (Feb 25, 2002)

*Balance*



Scott in MD said:


> I put my magnet 180 degrees around from the valve stem, to balance the rotating weight of the wheel.
> 
> Does anyone else do this?


Unless of course you have wheels like mine (Velocity Aerohead rim) where even with a valve installed, the rim joint is heavier and so the stem ends up at the top of the wheel when you let it spin down. And of course as someone else mentioned, wheel balance on a modern road bicyle is meaningless. But other than that, you're absolutely right


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## martinrjensen (Sep 23, 2007)

a friend of mind did that with the magnet. Had it in as close to center as possible. Turns out that the computer did not reset itself fast enough to work properly and when he moved it out a bit it worked fine. Cateye I think.


Pitts Pilot said:


> ...and average 60 mph! I like it.
> 
> I also go opposite the valve stem. I go as far toward the center of the wheel as I can, as it experiences less centrifugal force (I think) and passes the sensor more slowly.


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## Nicole Hamilton (Sep 5, 2010)

JCavilia said:


> The slope of the rise is irrelevant to the sensor because it "senses" the magnetic field in a very simple way - it's a reed switch, not a proximity sensor.


The speed sensor in my old wired Sigma clicks when the magnet is under it, so it definitely could be a reed switch. But there's no click from the sensor for my Garmin 500; I suspect that's probably a Hall effect device.


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## flatlander_48 (Nov 16, 2005)

RanGer498 said:


> I see people place there magnets on there wheels in different areas
> center ,middle and far out side and Im wondering what is the best place for the wheel magnet to be placed ?
> 
> thanks


Read the manufacturer's instructions...


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## Al1943 (Jun 23, 2003)

Kerry Irons said:


> Unless of course you have wheels like mine (Velocity Aerohead rim) where even with a valve installed, the rim joint is heavier and so the stem ends up at the top of the wheel when you let it spin down. And of course as someone else mentioned, wheel balance on a modern road bicyle is meaningless. But other than that, you're absolutely right


I think it's more common for the rim joint to be the heavy side instead of the valve stem. Three out of four of may wheel sets are heavier at the rim joint, opposite the valve. I put the magnets on the valve stem side.


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## curiousmike (Apr 4, 2012)

Newb - just purchased and installed a Cateye wireless on my bike.
The instructions asked to put the magnet as far away from the hub as possible.

Question: Unless I'm reading the instructions wrong, they also said to put the receiver on the back side of the fork leg. If, for whatever reason the receive decides to catch a spoke, something is going to get smashed.
Looking on the "what was your first bike thread (with photos)", I've noticed most people mount the receiver on the *front* of the fork leg. That makes sense to me as it will go with the direction of the wheel... so if it ever slips inward, the spoke would thwack it back outwards.


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## picview (Feb 22, 2012)

JCavilia said:


> If the field were weak enough (due to a combination of weak magnet and too much distance between magnet and sensor), too _fast_ a pass could in theory result in missed pulses if the reed didn't pull all the way closed before the magnet departed. I'm not sure that would ever happen in real life. Certainly not if you placed them close enough.


I had a cheap wired cycling computer that read the wrong speed when I'd get it up to about 28-29 mph. It would miss a count or two and read ~11-12 mph when I was doing double that. Moved the sensor closer to the wheel hub (so the magnet would move slower) and the problem went away.

Furthermore, that little bit of magnet mass has a smaller affect on the wheel's moment of inertia when it's closer to the hub. So close to the hub is it for me!


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## gumbafish (Jan 11, 2011)

I guess I am the only one that didn't give this any thought, I have a combo cadence/speed sensor so my magnet placement had to conform to the placement that worked for cadence.


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