When I was a kid, the bicycle odometers we could get were mounted so that as the wheel turned, either a magnet passed by a sensor and advanced the odometer, or a pin turned a little gear which was attached to a cable. I could really use something that works on the principal of the magnetic sort, though for something unrelated to biking. Ideally, it would let me program in a couple or 3 different circumferences, then calculate distance based on whichever factor I selected at the start of the measurement, but if it simply told me the number of rotations, I'd be fine with that.

I found something on eBay called magnetic punch counters which would give me that raw number, but they're all in Asia, and I'd really like to be using it much sooner than it would take to make that travel. I you know of something that might suit, and is available in the states, I'd be much obliged to hear from you.

Cateye bicycle computers all work on the spoke-mounted-magnet passing a fork-mounted sensor. When programming the unit, you can enter two sets of tire circumference, and switch between them. They have wired and wireless models, providing current speed, min/max speed, total distance, and two "trip"-distance odometers. Available at all price points. I own the Cateye "Strada" wireless model, and couldn't be happier (unless I had a spare $55 for the much larger screen of their "Padrone" model).

Get yourself a magnet, a reed switch, a pocket calculator and some wire. Wire the reed switch in parallel to the "=" key on the calculator. Set up the magnet and reed switch so the reed switch closes once per revolution. Program the calculator for a repeat add by pressing the 1, +, + keys. The calculator will now increment once per revolution, starting at 2. You can also enter the wheel circumference you want followed by ++. The calculator will now read out in distance traveled.

I wish I could say this was my idea. It appeared in the LAW Bulletin nearly 40 years ago.

Have you checked out the survey's options, roll on the ground measuring wheels? It's not hard to figure out what a complete revolution equates to for distance traveled forward and then simply count the rotations and doo the math. Andy.

most electronic cyclometers use a mag sensor on a wheel to count revolutions. And the number for conversion factor you enter from the chart is really the estimate of the wheel circumference in mm. Many have directions for measuring the wheel circumference to get a more accurate measurement.

also many meters have a bike1/bike 2 option that lets you use the same meter for 2 bikes, with different wheels sizse

so depending on the application and distance from measuring point to the meter a cyclometer might work for you

Unless they changed, sigmasport computers allow you to key in any wheel circumference from 0001mm to 9999mm. So , you could, for example punch in 1000mm, or 1 meter. Then every km would equal 1,000 revolutions, or 0.1km would equal 100revs. I don't know the speed you're considering, but with a round number wheel the speedometer would read in rpms (multiplied)

If you want greater resolution you could enter a wheel of only 100mm, or even 10mm. However you might run into limitations on the speedometer.

Otherwise, (just about) any bicycle computer sensor and magnet could be pulse counter, and you'd need some basic circuitry to read it out.

Or buy 2-3 CHEAP odometers and program each "head" for your purpose.
Swap heads as needed.

Thank you all; I'm very grateful. I've already done a little scouting with the information you've passed along.

I got one of these from Harbor Freight with a coupon, and I had great hopes for it. In fact, it does work, but it creates too much drag to do what I need. That drag is what eventually brought me to mind of my old bicycle odometers, and their magnet / sensor.

You need a Hall effect sensor, microprocessor, TFT display, solar panel, charge controller, and circuitry to interface all that stuff together. Oh, and an electrical engineer to put it all together. Too bad nothing off the shelf like that exists.

+1
I've been using one on my inline skates, and it has worked really well.

I tried using a Polar unit with wrist display first, but it wouldn't accept the wheel diameter.

I bought a CatEye Velo 7, largely based on Amazon's reviews. It doesn't have the option to enter two circumferences, but it's not expensive, and a 2nd head can be used as someone suggested. But it's limited to circumferences between 100cm and 299 cm. I need 76cm, so I'm trying to figure how I can use a 152cm setting, but my brain putzes before solving it.

As my user name suggests, we're a family of dyers. We dye wool yarn. Some of what we sell is put up in 20 yard hanks which will soon be made by wrapping it around a rotating drum whose circumference is 30". A second size partial skein is sold in 100 yard hanks with a circumference of 48". Those are made on a stepped portion of the same drum. The drum is long enough so that I can wind as many as 8 of the smaller ones at a time. The drum will be manually cranked using puleys & a timing belt with a mechanical advantage of 1:4.5, and I'm looking for a device to tell me when I'm done.

My bad. I didn't make my requirements very clear. Speed functions aren't really important. I'm looking to measure very short distances. You mention the idea of greater resolution, and I'm wondering if that would help me achieve a meaningful number the display of whatever unit I ultimately decide. I can almost apply what you're saying here, but there's something I'm not getting. My math was never great, and has only gone downhill over the years. How do I go about calculating the figure I need to enter so the display is one that I can interpret at a glance? The metric equivalents (762mm and 1220mm) of the imperial circumference measurements (30" and 48") I'm after is just fine if that's easier.

Read over this article-
Very Easy Cadence Meter For Your Bike <$12

You can see how the MATH is used to do something different. Count crank RPM instead of speed. The distance function is moot.
You can work it the other way to measure distance, where the speed is moot.
Just play with the numbers.
You may have to use several magnets to cut it down to usable "units", but just use the "Trip Meter" function and reset it for each use. (or keep the current odometer reading in your head and know when the next "mile" is logged.

That's an interesting article. Thanks for posting it, and for writing again. It'll take me a few days, but I'm on my way to take it through step 7 as that much makes sense already. The article was written in 2008, and that particular model of bicycle computer isn't still available except at a premium price, I'm going to start with the Velo 7 already in my possession, at least for the time being. My drum has 4 'spokes', so I'll start by using 4 magnets.

Okay, I'll give it a shot. Either way, I'll be back to let you know something. Thanks again.

I think I might have found something; look interesting to anyone else? They're wireless, 0mm to 9999mm circumference setting for $10 - Prime. The website has minimal additional information. It's news page is reporting Chinese New Year 2013. If the company has a specialty, it's in bluetooth headsets, but they seem to have no particular direction elsewise; they do have a couple of other bicycle products, and just one other bicycle computer that I saw which has managed only a mediocre performance in Amazon's reviews. I don't think I'll make any long term investments in the company, but I think I'll take 2 of these off their hands.

Hi, Bill;

I returned the Velo 7, and purchased 2 of the $10 wireless models with 0mm to 9999mm circumference settings. The directions are simpler to follow, but the buttons basically have the same functions. I have a sort of stepped drum that will be used to measure out two lengths of yarn. The smaller side has a circumference of 753mm. The larger, C = 1296mm. The drum has 4 evenly spaced spokes. I have the two magnets that came with these two units attached to opposite spokes. Additional magnets could be added to the other two spokes, or total magnets could be reduced to 1 without difficulty. A single installed sensor reads their every pass, and both computers receive the signal from the it; they are installed 26cm & 31cm from the sensor.

The smaller drum will need to measure sample hanks that are 21 yards / 19.202 meters
The larger drum will need to measure sample hanks that are 78 yards / 71.323 meters

I've read the "How it works" section at the bottom of the Instructables page to which you referred me many times, but I'm not math-wise enough to figure out how to form the equation. If I've given you enough information, and you have the time, could you show me how to think this through so I can do it when I need to replace these units someday?

Large Drum 71.323/1.296 = 54.96 Revolutions.
Small Drum 19.202/.753 = 25.52 Revolutions.

I'd use multiple magnets.
It may be easier to just experiment.
Plug in a number and see what you get for results.
If for example, your meter only reads 35% of your measured amount, divide the calibration number used by .35 and try again.
You may find you have to use more magnets on the "rim" of the drum to get enough resolution/accuracy.

The multiple magnet technique has been used by modern riders of "Penny-Farthing" (High wheel) bikes since their huge front wheels have a circumference exceeding what almost all cyclometers will allow. For example a bike with a 50" front wheel has a circumference of 399 mm. It can be equipped with two magnets 180 degrees apart and a calibration of 199.5 mm used.

Now why riders of a bike design from the 1880's would want an electronic speedometer/odometer is a question for another discussion

Not a particularly critical number, it appears from my tinkering. Doesn't seem to be much, if any, monitoring of if apparent speed changes are physically possible or not.
Two pulses = two revolutions as far as the bike computer is concerned.

Sure but if you don't put them 180 degrees apart the wheel will be unbalanced!

To add a bit more-
You could plug in the actual circumference of the wheel.
However, the odometers aren't really designed for such "short" distances.
On the smaller drum, you might just multiply the circumference by 10, and use 7.53.
Use a "similar" technique on the large drum.

The trouble with ANY digital display is the last digit is +/- 1 plus the actual accuracy of the display.
IF you can cause the display to read 10X for example, the last digit becomes +/- 1/10.

It might be simpler to use calibration values that would result in a specific "display" value such as 1.000km when your "desired" amount is reached.

One problem with only 1-2 magnets, your meter will read 1 or 1/2 revolution per click.
That means 1.296 or .648 meter resolution on the large drum.
Something like 5-10 magnets would allow much finer increments.

Another thing to consider is the time it takes to reset the "trip function" on the meter.
On the small drum, it might be faster to put on a paint spot and count 25 revolutions visually.

I don't know if your drum is hand cranked or motorized??

Thanks, all; have 100 magnets on order to play with this. Am reading for the time being. This is a lot more to think about.

For now, it's hand cranked. It uses timing pulleys and belt with a 1:5 ratio, so that makes counting rotations easier than it might otherwise be. Your point, Bill, about counting revolutions is well taken, and a good one. That may be the more efficient way to go. Will write again when I've played with magnets a while.

Nono, you're missing the big picture. Bike wheels are by nature unbalanced. By adding more items, you have a better chance of getting them balanced.

Have you ever balanced a prop shaft?
You find the heavy spot, then you put two jubilee clips around the shaft, 90 deg off the heavy spot and 180 deg apart. Then you turn the jubilee clips equal amounts towards each other and away from the heavy spot until the shaft is balanced.
On a bike, not much call for the jubilee clips, but we can use double magnets.

We should all be using double magnets to be able to benefit from the zen-like tranquility and the AMAZING improvements in ride quality, speed and stamina that comes from riding perfectly balanced wheels.
And as you return from your ride, expect your spouse to have gotten more beautiful, your friends more entertaining, your dog more well behaved and your kids grades to improve overnight.
(Oh, wait, did I overdo it now?)