I just installed a small, basic wireless cycle computer on my commuter bike today. It has a sensor attached to the fork that detects a small magnet attached to one of the spokes. The computer has a setting for various wheel sizes, which got me to thinking: there's no specific instruction for where on the spoke the magnet, and therefore the sensor, are supposed to go. I imagine different makes of bike are going to have the sensor and magnet at varying distances from the middle of the wheel; so does the wheel size really make any difference, as far as the computer is concerned? And if the magnet could be pretty much anywhere along the length of the spoke, how can the computer have any hope of accurately measuring speed and distance? Are these things just a waste of money? I mean, yes, they're a waste in the sense that they're unnecessary for most purposes other than idle curiosity, but do they even work???

The wheel size is configured so that the circumference is a known quantity, wihch lets the computer know how far you travelled in each revolution of the wheel and how long it takes per revolution.

It doesn't matter where the magnet is so long as it passes the sensor once per revolution.

You can put the magnet really far down on the spoke, really high, wherever, provided it passes the sensor. A whole spoke passes the fork at the same time. As long as your tyre size is good then it accurate.

And yes I think this thread is a joke aswell.

Stop thinking about your cyclometer.... use the dam thing (go ride)

Chris

You guys are mean to Bragi.

If it just lets you choose the wheel size then it won't be hugely accurate because different tyres will change that. It you manually measure the circumference and put that in they're generally pretty good.

They're useful from the point of knowing whether your breaking the speed limit or not. Also great for bragging rights.

This should help you to calibrate your cyclecomputer as accurately as possible.

https://www.sheldonbrown.com/cyclecom...libration.html

It's a joke. All someone needs to do is read the instructions that comes with the computer and they explain how it works and wheel sizes

The computer should have a way of inputting the circumference of the wheel, usually in millimeters. The instructions for the computer will show how to calibrate it; even the cheapest $5 department-store computers I've bought had these instructions.

I actually rode a 20 mile course with a GPS on my handlebar and found that my computer was about 2% off, so I subtracted 2% from the setting in the computer. Do not do this if you're going over a course with lots of curves; the GPS will probably underestimate the distance a bit by cutting corners between samples.

I hope you're joking about where the magnet is making a difference. It's counting revolutions, the center of the wheel doesn't turn any less times per mile than the outside.

That said, you're better off with the magnet nearer the center of the wheel; if you put it near the outside you have two bad effects:

1) the magnet is moving past the sensor very fast at high speeds and the computer may start missing revolutions.
2) the magnet's weight can unbalance the wheel more at the outside than near the center.

Don't assume what your wheel/tire circumference is, measure it!! I did this and my distance is right on when compared to using mapmyride.com and my car.

Once I get over 20 mph, my cheapo Cateye only outputs mph in increments of half miles per hour (20 mph, 20.5 mph, 21 mph, 21.5 mph, etc.). Is there any good reason why this should be?

If you REALLY wanna get it down to gnat's ass accuracy, measure the rollout of the wheel the sensor is going on while you're on the bike. Easier to do with two people.

Exactly. Inflate, mark, roll, mark, measure, input, go ride.

Shouldn't you take the average of several trials? Perhaps even on different surfaces? I mean, if you want to get your trip mileage accurate to an eighth of an inch or less, these things are important

Make sure the track is accurate first. Because we're trying to be accurate.

Oh yeah, I forgot. This should be done within a controlled, dust-free, vacuum. With lasers. Don't ever forget the lasers.

Getting back to magnet placement...

Depending on how heavy the magnet is and how "in tune" you are with your bike, placing the magnet closer to the rim may cause an annoying off-balance shake, especially at higher speeds.

This is from experience: Planet Bike magnets are really heavy, and you can feel the vibration from having it mounted close to the rim when you get over 20mph. I keep mine as close to the hub as possible, typically mounted between the spoke crossover and the hub.

It's okay, I deserve it. For some reason, the whole rpm x circumference thing didn't even occur to me. I'm not usually this stupid; no, really.

This thread inspired me to double check my own computer. I had it set to 2198mm (marathon winters 700x35c), and ended up resetting it to 2216mm (around 1% difference). Not huge, but I'm definitely going to look down at my computer and smile when I see my cruising speed is up .1mph!

But you have to ride in the exact line that equals 440 yards (usually one foot from the inside edge). But if it's a 400 meter track, you have to change your computer to km!

You ride around at 10.1 mph?

Sure. It's telling you that it has precision that's good enough to about 1-2%. At 10mph it can give you tenths, but can't pull that off at higher speeds. Basically, it's being honest about its precision rather than giving you an extra digit that isn't real.

I mean, they could give you 10 digits, but it wouldn't mean much.

Not knowing how your particular model works, it's harder to say why it specifically is having such problems. Generally, computers that have integration averaging should get a bit more accurate at moderate speeds, since it has more wheel revolutions to divide the error over.

It seems that any time I measure my wheel circumference, as soon as I forget exactly what that number was the battery dies, or I somehow hit the exact button sequence to erase or change it. So there I am, not knowing exactly how fast I am going, or exactly how far.

Strange, how this so far has not affected my ability to get anywhere.

The GPS also does not account for extra distance when traveling over hills. When you have at least four satellites available, most GPS units will give you the altitude, but it does not calculate that into your speed or distance.
This is why even GPS units that are designed for cycling, incorporate a wheel magnet/sensor to measure speed and distance.

In this cross-section of hilly vs. flat rides, the GPS only knows how far you went and how long it took. It doesn't take into account the hills in between.

I think that your cycling computer is actually correct and it is the GPS that is off by 2%.

Probably a valid theory, but I misspoke, I ADDED 2% to the comp distance. The GPS actually indicated a farther distance than the bike comp did, so if this did come into play, I should have added even more.

My bike comp reads to within less than a tenth what any of our three cars do for the same route, so I'm OK with it anyway.

I think riding a known distance might be more accurate than measuring rollout; tires compress and squirm a bit on pavement under load. It's probably not a lot, less than a mm or two per rev, so probably not worth worrying about.

Dang! You made digital images to answer the question, then uploaded them to your ISP's free storage area, got the URL and linked them here. The scary part is that I have actually done that before too. And I thought I was the only one who got that bored.