RiverHills

This is probably a dumb question because it's late and I'm tired and probably missing the obvious, but...

My bike's computer is set only to the wheel size of my bike. That's the only input in this one (Sigma) and most others I have found online. So, the computer knows my wheel size, but how does it know the radius or distance above the center of the wheel where the sensor is positioned? Obviously, the magnet at the top of the spoke would travel a longer circumference and would pass the sensor less often than positioning it close to the hub. So if all I do is enter my wheel size, how does the thing work no matter where on the spoke I place the magnet and sensor? Shouldn't the number that is entered into the computer be the actual circumference that the magnet travels in one rotation?

The whole reason for my question, in case you were wondering, is that I noticed my computer is about 2-3% off on mileage, and I need to tweak it. I thought I could just adjust the input into the computer, but apparently all it asks for is wheel size.

operator

The magnet goes around the sensor once per revolution. Irrespective of where it's placed on the spoke. The distance travelled by your bike in this one revolution is the same, obviously no matter where the magnet/sensor is placed.

The only thing changes is the speed at which it cross the magnet. Obviously the magnet further away from the hub is travelling faster, which is irrelevant to the speed calcaulation and therefore distance because one rotation, is one rotation.

Exactly what are you comparing mileage to that tells you it's 2-3% off? How do you know what you're comparing to is accurate?

Computers usually take into account the wheel size and the tire size when using a code. Since tires are never really what's marked and inflation plays a role if you're looking for hyper accurate distance then a rollout is your only option.

There's no reason to be this anal about data accuracy anyways. The stock numbers for a particular wheel/tire size gets me exactly or extremely close to what google maps/cue sheets tell me anyways. It'll never be perfect.

JanMM

Doesn't matter where the magnet is; the computer isn't measuring how fast the magnet is going in a circle but rather how fast the bike is traveling down the road. Entering the wheel size data into the computer lets it know how far the bike is moving with each revolution of the wheel. The sensor inputs how many wheel revolutions there are per time period. The wheel size info you are entering might not be accurate. Try measuring exactly how far your wheel rolls with each turn and compare that with the number listed for your wheel size. You should be able to enter the actual distance.

LesterOfPuppets

The computer just uses RPM of your wheel and circumference of wheel to compute speed. RPM will remain the same whether you run the sensor near the hub or near the rim.

Some computers will allow you to enter the circumference, which you'd want to figure by laying a tape measure out on your driveway or sidewalk, with tires inflated to typical pressure, then sitting on your bike and measuring out a revolution using your valvestem as a reference point.

Wanderer

Do a simple roll out test, to measure the outside roll of the tire, loaded. Input that figure into your computer, and you should be right on! Simple.

p.s. It's OK to be anal about accuracy!

Al1943

It makes almost no difference where on the wheel you place the magnet. The magnet passes the sensor once for each wheel revolution. There may be an argument that it's better to have the magnet closer to the hub than the rim because near the hub the sensor will have more time to sense the magnet. But in real practice it doesn't seem to matter.
What is important to your calibration is the wheel circumference number you input. You haven't said why you think your computer is inaccurate. You should not compare your computer results with a car because it's more likely that the car odometer is inaccurate. If you are sure that your computer is 2.5% inaccurate then change the circumference number by 2.5%.
I set my computers by riding several miles of known distances and then adjusting as needed. I use surveyed section road intersections that are exactly one mile apart.

Al

Mondoman

Yes, but for distance/speed calculations all you care about is how far the *tire surface* travels.

Nope, that's your mistake. No matter where the magnet is located, it will only pass the sensor 1 time for each turn of the wheel.

The way the computer works is this: it keeps track of time and when the magnet passes the sensor. Distance is calculated simply as (number of times past sensor) x (tire circumference). Speed is calculated as (distance) / (time), averaging over a number of seconds (maybe 10?). That's why the speed display takes a while to "catch up" when you change speed.

You can tweak it by slightly adjusting the wheel size. Remember, the actual circumference we care about is the *tire* circumference, not the wheel circumference. Since tires vary somewhat in size and circumference, the small error is likely due to the entered wheel+tire circumference (or diameter, or whatever) being slightly off from you actual wheel+tire circumference. Why not measure your actual wheel+tire diameter, calculate the circumference from that, and see if using that value fixes the problem?

DannoXYZ 

Then you really need to do the roll-out with your exact weight on the saddle. The tyre compresses with weight and it's radius is smaller with weight than without.

bmorey

The roll-out method is supposed to be the way to go for getting the wheel diameter. I just can't get it right for my new bike. A known distance for me is home to the first traffic lights. By GPS, in the car, and three previous cyclocomputers on two different bikes it's 1.85km. New bike, re-used cyclocomputer (Sigma) and very carefully-measured rollout: 1.94 km. I've never had any trouble callibrating a cyclocomputer until now but this beats me.

Road Fan

Conventional GPS, without map matching or any static augmentation sites, can have typical errors of 100 m or larger. Car speedometers are often calibrated by design to read a few percent high, and the regulations allow the error to be higher than that. I don't think there are very many accurate ways for a normal person to measure distance of a few kilometers. But still, if you're sure of the accurate number, just tweak the setting on your cyclocomputer until it reads the way you think it should.

Rollout should be accurate, but you have to measure the rollout with the bike weighted, with correct tire pressure, and with accuracy. You're concerned about 2 or 3 percent, so in 2074 mm (circumference of a 660mm wheel) that's a difference of 5.2 cm or 2 inches.

I think accuracy will ultimately depend on tire pressure.

BTW, I'm talking about US speedometer regulations, sorry, not familiar with Oz!

Magnet position won't affect accuracy. If the magnet is farther out on the wheel it will make a stronger signal in the sensor due to passing the sensor faster, but the computer counts the time between successive passings.

RiverHills

I know the computer is off by a couple percent because on my last few club rides using GPS-generated cue sheets, my final distance traveled at the end has been slightly more than what the cue sheet says. So last night I did a ride which my computer said was 6.25 miles. I measured it with Google Earth and came up with 6.09 miles. Obviously on a long ride, this coud add up to a large error.

I appreciate all the responses but my original question remains unanswered. Follow this logic...

Let's say my wheel circumference, or the distance traveled in one wheel revolution is 7' (to use a nice whole number). Now let's say I am cruising at constant speed of 10 mph, which is exactly the same as 14.67 feet per second. But even though my wheel is rotating at constant speed, the magnet placed at the top of the spoke would be rotating much faster than the magnet placed at the bottom of the spoke near the hub. So if my magnet is at the top of the spoke near the rim, it's passing the sensor at a much higher angular speed. So, why doesn't the computer perceive this faster count as a faster bike velocity? If I stop the bike, move the magnet down to the bottom of the spoke, and then go back to my constant speed of 10 mph, now the magnet is passing the sensor much less often, but the computer should still say 10 mph. How can the computer say 10 mph both times, if the rate at which the magnet passes the sensor- or the rate at which the computer perceives the bike to have traveled 7'- is different?

I understand that the computer uses the wheel size to convert revolutions into distance traveled...

bidaci

Because the computer doesn't care how fast the magnet passes the sensor just how many times it is passed in a given period of time. In close to the hub or out near the tire. The magnet will pass the sensor once per wheel revolution.

RiverHills

Right, but if I speed up, it only knows my speed has increased because the wheel revolutions has increased. It cares how many revolutions are completed in a given amount of time. So depending where on the radius of the spoke I place the magnet, it's going to register more or less RPMs.

bidaci

No it won't. Please go look at you wheel. Put a piece of tape on the spoke near the hub and at the wheel. Spin the wheel and presto chango, both pieces will pass the fork at the same time. 10 times in a minute is 10 revolutions whether the magnet is going quickly by the sensor or slowly. we are not measuring the speed of the magnet but the revolutions of the wheel assembly

RiverHills

That's right, we're not measuring the speed of the magnet, but the number of times it makes a revolution per time.

If I am riding at 15 mph and one revolution is equal to 7' traveled, this means my wheel is spinning at a rate of 3.14 revolutions per second.

So the answer is...even though the magnet would have different angular velocity depending on where it is placed on the spoke, the number of times it passes the fork would be the same, despite the speed at which it passes.

Wanderer

The exact reason for my use of the word "loaded" in my statement.

Wanderer

The computer doesn't care how "fast" it passes, it only cares that it "does" pass.

That's why they both pass only once per revolution.

Got it?

Little Darwin

Have you considered that you may not ride a straight enough line to exactly match the theoretical straight line that Google Earth would use between points?

A bicycle never travels a perfectly straight line... not even the professionals.

In the real universe, there will be some deviation from optimal.

However, since exact accuracy is so important to you. Multiply the number you used in your computer by 6.25/6.09 and come up with a new number. Enter that new number in the computer.

Try again.

Repeat this process with multiple routes until you come up with a number that is accurate enough for you. And always ride with the same tire pressure.

Be sure to do this about once a month to adjust for tire wear, and any changes in cycling skill for holding a line. Also be sure to repeat any time you change tires.

EDIT: This of course assumes that the accuracy of your input on Google Earth wasn't off by 3%, and what would make you think that clicking on a 20" screen would provide as much accuracy as riding on a 6 mile road due to the increased scale? The granularity isn't there on your screen as compared to on the road.

Second Edit: What makes you think that the person making the cue sheet didn't have a faulty odometer?

RiverHills

A deviation of .16 miles is equal to an extra 844.8. Even drunk off my arse there's no way the minor "zig-zagging" that occurs while riding down the road accounts for 844.8 feet in 6 miles. Try zig-zagging the ruler tool on Google Earth and you will see that it makes almost no difference.

gmason

The real problem here is density.

operator

Well. It is sort of looking that way. We've already given the OP the answers but he just refuses to accept them.

RiverHills

Scroll up and read my Post #15. Density? This thread is a vacuum.

jsharr

So if your mileage is 2-3% off, take the number you input into your computer in the setup phase, increase or decrease it accordingly by 2-3% to match the GPS number you trust, and reinput that number. Now go ride again and compare to GPS. Keeping doing this until you are satisifed with your mileage numbers.

RiverHills

How accurate do most GPS units claim to be?

jsharr

Cannot see that it really matters. I thought that this thread was started because your computer did not agree with your GPS, so my solution was to adjust the circumference number you input into the computer to match your GPS. I seldom ride with a computer that is turned on and never ride with a GPS.