Speedo Accuracy

I understand the diameter will change significantly under load, but if you are measuring the actual circumference of the inflated wheel&tire, esp on a slick, that is your circumference input, and you don't need to measure diameter and calculate circumference.
 
You're almost there.
When you sit on the bike, you would agree that the tyre compresses?
Therefore, under that compression, the diameter of the wheel is smaller - yeah?
Right - the (true) circumference is the diameter multiplied by pi - correct?
Measure the radius under load, (axle centre to ground) and multiply by 2pi.
Or the exact same thing is to sit on the bike and roll forward one turn with your full weight on the bike.
If you still can't get your head around it, do the tests that I suggested, with a couple of different tyre pressures and you'll see what I mean.
I could spend another hour typing trying to explain this or you could spend 10 minutes testing.
... Steve
 
I've got you beat by a mile!

The tape measure sits ON the tread of the tire, all the way around. I don't care $ to doughnuts what the diameter is. I measure from point A all the way around the center tread of the tire to point A. This is the same as the cumbersome method of "rolling out" the tire distance, but faster and more reliable. The bike is upside down. The tape measure measures the tire, like you'd measure your belt. Hint: you don't need the chart that came with the kit.

If you know these measurements you can write them on your computer or a card in your saddle bag, and switch the computer to either measure when you trade tires for winter, unless you are using the meter for total miles ridden ever on an engine. I'd rather use an hour meter if I can find one that runs on 6v.

I hope you don't have to figure out your diameter to get your pants belt size. :)

You're almost there.
When you sit on the bike, you would agree that the tyre compresses?
Therefore, under that compression, the diameter of the wheel is smaller - yeah?
Right - the (true) circumference is the diameter multiplied by pi - correct?
Measure the radius under load, (axle centre to ground) and multiply by 2pi.
Or the exact same thing is to sit on the bike and roll forward one turn with your full weight on the bike.
If you still can't get your head around it, do the tests that I suggested, with a couple of different tyre pressures and you'll see what I mean.
I could spend another hour typing trying to explain this or you could spend 10 minutes testing.
... Steve
 
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Some people just don't want to listen/learn im afraid AussieSteve ...the whole tire compression and the math has seemed to baffle the poor chap :-S

KiM

Hello Kim - yeah. It only would have taken a couple of minutes to verify what I said. Enough for me. (After all, he's got me beat by a mile.)

... Steve
 
Correct. Directly measuring the circumference of the tire is definitely inaccurate. With any tire. Even if your tire pressure is extremely high, the error will be measurable - the wheel will move less than your direct measurement of circumference would suggest, and, the lower the tire pressure, the greater the error.

The technique is dead simple. Mark the floor, place the tire atop the mark, with the valve stem exactly on it. With your weight on the bike, roll the bike forward for one tire complete rotation, and mark the floor again. Then, directly measure the distance between the two marks. Dead simple. Not cumbersome, and the most accuracy. No calculation required.

happycheapskate - The calculation was just to show you how much the tire deflection under load actually impacts the accuracy of trying to measure circumference without being under load. Just a quarter-inch tire deflection under load represents nearly 2 percent error using your method. (the bike will be going about 2% slower than your speedo indicates)
 
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Just a quarter-inch tire deflection under load represents nearly 2 percent error using your method. (the bike will be going about 2% slower than your speedo indicates)

Yep - during my tests I measured 5cm, (2"), difference in circumference between an unladen wheel and loaded wheel with low pressure. (Wheel circumference 205cm - 2.4% error)

By the way, I only weigh 50 kg, (110lbs). A higher rider weight will result in a higher error.

Pretty good estimate, loquin.

...Steve
 
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I had the sexy wiminez at the store take mine! she put the start of the tape on my belly button and wrapped it around my waist all the way back to my belly button
Unfortunately she didnt take into consideration my need for breathing,
when i exhale my pants fall down :D:D:D

KiM
 
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