bh85

Does anyone know what’s WTB Byway 650x47 tire circumference? I need it for my bike gps computer and I wasn’t able to find it anywhere online. Thanks for help!

Bill Kapaun

Grab a string and wrap around.
Unwrap and measure it.

dsbrantjr

Make a rough estimate and enter that. Take a ride on a known-length course, note the indicated distance, divide the known by the indicated then adjust the estimated circumference by that factor. If you do the math properly you should have the exact circumference.

Jeff Wills

Calculate or measure for yourself:
https://www.sheldonbrown.com/cycleco...ation.html#iso
https://www.sheldonbrown.com/cycleco...n.html#rollout

hokiefyd 

Place the valve stem at the very bottom of the wheel, as if it's intersecting the ground, and mark that spot. Roll one complete revolution, and mark that spot, and measure. That's the circumference. Enter that to start. Then ride a known distance and check your computer.

If your computer is reading less distance than actually traveled, then your entered circumference is larger than the tire actually is, creating fewer recorded revolutions than actually occurred. Adjust the entered circumference down by the % difference. If your route is 10 miles long and you recorded only 9 miles, then adjust your entered circumference 10% down and ride the route again.

If your computer is reading more distance than actually traveled, then your entered circumference is smaller than the tire actually is; adjust your entered circumference up by the % difference and re-check.

HillRider

What hokiefyd described is called a "rollout measurement" and is most accurately done by putting your weight on the bike and scribing matching chalk marks on the tire and on the road at the very bottom of the tire. Then roll the weighted bike forward at least one but better two or three revolutions in a straight line and make another chalk mark on the road when the tire mark is again straight down. Measure the distance between the road markings and that is your rollout distance. Use it to calculate the tire circumference and enter it into your cycle computer. Done carefully, this should be a quite accurate value.

A rough approximation can be had by assuming a circular tire cross section. A 650b rim is 584 mm BSD and if the tires are really 47 mm then the wheel diameter is 584+(2x47)= 678 mm and the circumference is Pi x 678 = 2130 mm or 213 cm.

FlMTNdude

BikeCalc.com - How to calculate Bicycle Wheel Size

That should tell you pretty much everything. I know that some manufacturers run a mm small or large, but even then I don’t bank they will actually be smaller.

dsbrantjr

The rollout method will be most accurate if the wheel is loaded similarly to the way it will be ridden, this is easier with a helper or a curb to help to stay upright while seated.

Andrew R Stewart 

Since rim width and tire pressure change the real life circumference any calculated number won't be the actual one. But sometimes "good enough" is good enough.

On our tandem we have two different Cat Eye computers. One's calibration number is three digits the other is four. Added to that is that the tires have a "3c" (I would say 3mm but we all know that molded/hot patched tire widths are not real numbers, they are labels) difference in size. My stoker and I never agree on how far we go Not that it matters as we get there at the same time. Andy (who doesn't run GPS but thought that it created the distance traveled from the satellites)

Bill Kapaun

People actually think the tire gets smaller when they sit on the bike?
I think of a tire like a rubber band that not under tension.
It doesn't matter what shape it's in. It's still the same length.

bh85

Thanks for help

hokiefyd 

What does decrease with weight (on the wheel) is rolling radius. It's true that the tire's material doesn't get "shorter", but it does deform into a flat contact patch. A straight line is shorter than the distance around an arc, so some of the circumference is "lost" when measuring length traveled. We think of that rollout measurement as circumference, but it's really the distance traveled by the hub/bike with one revolution of the wheel/tire system, and more deformation (and a shorter rolling radius) can impact this some.

In any event, that's why I recommend checking any rollout measurement against a known route, and making % adjustments. In theory, after two or three rides, you should have the computer pretty much bang-on.

davidad

This is the way to get a reasonably accurate measurement.