Bicycle Mechanics - Mounting magnet and sensor thinger for computer

Word,
I just bought a new computer for my bicycle. It is a Speedzone sport. I understand how to put it on the handlebars and program it, but where on the fork do I place the sensor? In the instructions it doesn't say how far up or down, just a distance of 1-2mm between them. Thanks :)

Anywhere it fits really...... I do prefer to put mine as close to the center of the wheel as possible, as I think you get a more accurate reading. (magnet going slower so sensor picks it up better)

If it's a hard wired sensor, it really doesn't matter that much where on the fork it is.

Now if the thing is wireless, you'll want to keep it closer to the top of the fork. Some sensors have a fairly short transmit range.

Either way, just make sure the gap from the magnet to the sesnor isn't too large.

Anywhere it fits really...... I do prefer to put mine as close to the center of the wheel as possible, as I think you get a more accurate reading. (magnet going slower so sensor picks it up better)
You could also argue that further away from the centre, there's a longer period between activations, resulting in greater accuracy, less power used, longer permitted switch debounce/recovery time etc etc.

It really doesn't matter too much what with it only being activated once per revolution no matter where you put it, and modern silicon 'hall' switches are pretty efficient at detecting and resetting (I'm assuming it's not based on the older-tech magnetic reed switch)

Put it half-way out. A compromise. :) Or let your choice be governed simply by where it looks best, or causes the least obstruction, or simply fits most easily.

You could also argue that further away from the centre, there's a longer period between activations, resulting in greater accuracy, less power used, longer permitted switch debounce/recovery time etc etc.

It really doesn't matter too much what with it only being activated once per revolution no matter where you put it, and modern silicon 'hall' switches are pretty efficient at detecting and resetting (I'm assuming it's not based on the older-tech magnetic reed switch)

Put it half-way out. A compromise. :) Or let your choice be governed simply by where it looks best, or causes the least obstruction, or simply fits most easily.

Further away does not mean longer period between activations. It doesn't matter where you put it, if your wheel is doing 60rpm, the inside will have a slower rotational velocity than the outside of the wheel but it still still pass the sensor once per second.
The only difference being the magnet passes the sensor faster when it's on the outside. I think some of the old problems were that at high speeds, the magnet was passing the sensor too fast and the element inside had too much inertia and it wouldn't register but I haven't seen any modern computers with that problem. My cateye mity 8 sensor makes this "click" sound whenever the magnet passes near it, makes me think there's actually something inside that actuates whenever the magnet passes.
Just mount it where it's comfortable for you. Just in case, I'd zip-tie the wire to the part of the fork that faces backwards, not on the side, to avoid having it snag on anything.

The distance between the magnet and the sensor needs to be no greater than 2mm. So what you need to do is place the magent on the wheel where you can move it up and down the spokes, and find out where the sensor will go to obtain that distance. As far as cosmetics wrapping the cable around the front brake cable and behind the fork just to the inside once you get the sensor where it needs to be is a good idea.

You could also argue that further away from the centre, there's a longer period between activations,

Think about that for a few seconds... ;)

A wheel spinning at say, 60 rpm... every point on the wheel returns to its starting point every second...

<edit> Whoops I see someone already covered that point above.

Anyway, I like to set my magnet fairly far down the spoke. I've noticed skipped readings on Cateye computers when the magnet is closer to the rim. Lately I've been mounting it about 1" from the hub.

However my Polar wireless sensor forces an arrangement about 4" from the rim and that sensor hasn't skipped on me yet (up to 65 km/h at least).

Think about that for a few seconds... ;)
:lol: No; I know what everyone's getting at - I'm well aware that no matter where on the wheel it takes the same time to complete a single rotation (or does so many rpm etc.) :)

I know I should have put it better in the first place, instead of now; use "inactive/inactive states" rather than "between activations" etc. but anyway:

The difference is the magnet is not a single infinismal point - it actually has a fixed length.
Thus towards the centre hub, magnet / field / triggering area / whateveryouwanttocallit takes up a larger percentage of the total circular length, and further away (as the magnet / activator's size doesn't change), this percentage decreases. Therefore (assuming constant rotational velocity) it's 'active' for a shorter proportionn of the time, and 'inactive' for a longer proportion.

Example (totally fictional; designed for ease of analysis rather than reflection of reality);
magnet size = 2cm
circumference close to hub = 20cm
circumference out at the rim = 100cm
therefore:
proportions @ hub = (2/20 active) (18/20 inactive) = (10%) (90%)
proportions @ rim = (2/200active) (198/200inactive)= (1%) (99%)

Now what that means;
too close to the hub, it's active for a very long period of time. It'll use more power, it might be suspectible to jitters should you knock something and the mag/sensor vibrates within that 10%, it might not have long enough to switch off (reed-types use a return spring in the sensor which has to settle, hall switches have a decay curve)
too far away, and as has been pointed out - you might have the opposite problem: insufficient time to activate, esp. in reed switches.

I believe F1_Fan's experiences quite well illustrate this; cateyes have used reed switches (I've broken a couple) and the clicking will be this moving from one contact to the other (should barely be perceptible as it's incredibly fine metal in a vacuum tube) I don't know about the polar - for all I know it's just got a more sensitive reed switch, but with basic semiconductor prices these days, 'hall' switches can actually be cheaper!

Think that covered everything. :D Put it half-way. :)

Think that covered everything. :D Put it half-way. :)

Perfect.

:beer:

Ya, the Polar is a solid state unit. No clicky. Thus it's handy (when setting up or troubleshooting) that it has a LED that blinks for the first 50 revolutions.

Anyone have suggestions for mounting the sensors on bikes with a front suspension? The mounts supplied in the box with my friends' computer don't seem to be large enough to fit around his shock.

Are we gonna have to mickey mouse something with a zip tie?

theoretically, the closer to the hub the magnet is the less of an impact on the balance of the wheel, i think... although it probably donesn't make that much difference.

Word,
I just bought a new computer for my bicycle. It is a Speedzone sport. I understand how to put it on the handlebars and program it, but where on the fork do I place the sensor? In the instructions it doesn't say how far up or down, just a distance of 1-2mm between them. Thanks :)

i put mine at a distance on the fork so that the cable didn't have too much slack. i have one wrap up top around the handel bars, then it's tied once around the frame and once around the fork. nice and tight the whole way up. comes out to be about mid-way on the fork. also, for symetry (and wheel balance) i put the sensor on the opposite side of the tire valve.

Well I feel like a moron. I understand now, I put it halfway and oppisite the valve. I kept thinking, "The farther away from the middle, the longer it will take to get around, then the reading will be off." Golly, It's all one peice and stays together, duh. Thanks everyone for your help.
Ted

Well I feel like a moron. I understand now, I put it halfway and oppisite the valve.

I've got an opinion on this one too... go figure :D

Many rims are made with an insert in the seam area. It's put there just before the rim is joined.

Placing the magnet on the valve side may balance the wheel better. I remember (back in the day as it were) a company made weights that fit over the valve stem for such balancing purposes (unneccessary, IMO).

Hold the axle in your hand and see what part of the wheel ends up at the bottom