rumrunn6

re: Bike Computer Wheel Circumference Chart(s)

Hmm ... scratching my head cuz the chart that came with my computer doesn't jive with Sheldon Brown's and also I don't understand why a 27 x 1" wheel tire combo would be different than a 27 1/4" wheel tire combo.

I tried measuring my wheels' circumferences but it is too awkward with a metal tape measure, so i'd like to use the chart. I've been riding different bikes with different wheel/tire combos and I'll be using yet another tomorrow, as well as changing a tire size on a current bike. I want to get this right and also want my personal performance comparisons to be accurate. Hope that makes sense.

OK, 1st issue chart differences:

My computer's chart:
27 x 1 = 2136
27 x 1 1/4 = 2155
700 x 32c = 2155
700 x 28c = 2136

Sheldon Brown's chart:
https://www.sheldonbrown.com/cyclecomp_f.html

27 x 1 = 2145
27 x 1 1/4 = 2161
700 x 32c = 2155
700 x 28c = 2136

So it looks like my settings for my 700 x 28 tires will be fine cuz both charts match. But I'm concerned about my 27 x 1" setting. Should I set 2145 or 2136?

I'm reluctant to use the rollout method cuz it's dark and rainy out and I don't have any chalk.

2nd issue - why would the circumference measurement be different between the 27 x 1 and the
27 x 1 1/4???

grinningfool

Put a piece of tape on the floor, and align your valve stem with it. Then roll your bike one wheel revolution, and put another piece of tape on the floor aligned with the stem again. Then measure the distance from one piece of tape to the other. That is your wheel circumfrence.

MudPie

Grinningfool's method is the best, and it could be done indoors on a hard surface. Instead of indexing off the valve stem, I will put a pen mark on the side of the tread as close to the floor as possible, and use that as an index. Also, I place masking tape on the floor and place distance marks on it using the tire mark as an index. Then measure with a steel tape measure. I know this sounds geeky, but I will do the same measurement multiple times (like 10) and then take the average value.

waldowales

There is .4% difference between the two figures. I'd be surprised it you could get that close on a rollout.

Retro Grouch 

If you're that particular about being exactly right, the roll out method is the only way to go. The tire size label on the side means nothing. The exact same tire installed on different rims might be different.

Personally, I don't care that much. I just use the numbers that come printed on the computer directions. If I can't find the right tire size, I interpolate. What are you using this data for? Sometimes good enough is good enough.

noglider 

If you're super-particular, you should do the rollout with you on the bike, because your weight will deflect the tire a bit. Have a friend do the marking and measuring.

Jeff Wills

And do the rollout for 10 revolutions of the wheel. Divide the result by 10 and Bob's your uncle.

I_bRAD

An easier way is to measure from the hub to the floor and mulitply by 2pi.

roberth33tiger

i agree with the 2 pi method for ease and accuracy. pi equals 3.1416. measure from floor to
axle center IN MILLIMETERS, multiply by 2 pi. round the answer to the amount of digits your
computer requires.

RonH

Use the chart that came with the computer.

demoncyclist

+1 on rollout if you must be dead nuts accurate. As for the circumference question- it is the same reason that the measurement is different for differing width 700c tires. The wider tire has a slightly larger circumference.

Metzinger

0.4% of 2155 is 8.6mm. I think I could measure a bit closer than that. With a pencil.

Results would be curious here.

Nogyro

The instructions that came with my Cateye said to do the rollout method with one twist, they said to be sitting on the bike. The tire squish will effect overall distance as well.

I_bRAD

By curious, do you mean correct? Otherwise I'm not sure what you're getting at.

Al1943

Because a 27 x 1 1/4 tire has a bigger circumference than a 27 x 1 ??

The roll out measurements I've seen have been very inaccurate. I think mostly because a roll out is such a small sample of a bike ride. If you are going to go with a roll out do what some others have suggested, 10 revolutions with the full weight of the rider on the bike, then divide by 10. It will make a difference.
When I want a more accurate calibration I ride a known surveyed mile from survey pin to survey pin then adjust the circumference setting. But I usually just go by the chart and round down slightly. They tend to be a little optimistic.

Al

bikinfool

There are going to be differences among tire sizes, manufacturers don't always measure the same way, so whatever tires Sheldon used might not have much to do with the ones your computer mfr used, etc. I'd think that weighted rollout with your particular setups for most accurate figures. I was never was that anal with my bike computers (and haven't used any for years). You'll still end up with differences from other ways of measuring (stated map distances, gps, your friends' computers, etc).

dvs cycles

It would only be dead on if we were using solid tires that don't deform and you measure with the bike exactly upright.
Roll out is the only way to go. I find the charts to be too high of a number. If you want to read faster speeds than you are actually riding use those.

I_bRAD

If you sit on the bike and have someone measure that's accurate enough for me. Hell, even without someone sitting on it. I'm happy if I know I went around 100kms on a ride. I don't need to know I went 101.3kms.

Metzinger

I meant varied.
DVS covered one reason.
In addition, you'd have to measure over 6 times more precisely to equal that of a roll-out. (2XPi)

Bianchigirll 

it sounds funny that you are concerned about the circumfrence difference between 27x 1, 27x1 1/4 a whole 1/4 difference. but dont seem alarmed that a 700x23 and 700x25 are different by 3MM same as a 700x28 is different from a 700x32 by 4MM. since there is roughly (sorry I am not exact enough here) 25MM to an inch that make the difference between a 700x28 and 700x32 about 4 or 5 MM OMG!!!

dobber

And on a 1 mile bike ride you'd be off 21 feet.

Theres a limit to the required accuracy in some cases. This is one of them.

coldfeet

+1 if you're worried about being totally upright, measure on both sides without moving and divide by 2.

the problem with the rollout method is you would have to keep a totally straight line.

And yes, I think this is a little bit of overkill, the chart with the computer is probably close enough.

tatfiend 

IMO the reason the charts tend to be off compared to rollout is that most tires are smaller than their marked size.

Using rollout or, better yet, a measured mile/kilometer should be the most accurate method if you want to be dead on but even most car odometers are off by more than the inaccuracy caused by using the chart with your bike computer.

Retro Grouch 

Even car odometers? Car odometers suck!

Car odometers have much more tire tread wear differential than a bicycle tire. They also are affected by every curve. Most certified running courses are measured using a calibrated bicycle odometer. They won't even consider anything that's measured using a car odometer.

CaptainCool

If you want to be dead on, calibrate your computer against a short course measured by a GPS unit or gmap-pedometer.