Kelinen

Hi all,

I have bought my first road bike, a giant defy 3. Now i want to know my wheel circumference for my speed calculator. My tires ares Vittoria Zaffiro 700x25c, but when i google that, i get different circumferences:
2125 mm
2110 mm
2105 mm
2108 mm

And the standard in the computer is 2124 mm.

Can you help me out?

Thanks.

FBinNY 

It's impossible without actually measuring, which I'll come back to. But are you aware that the difference high to low in the various estimates is less than 1% (0.8%).

In your shoes, I'd toss out the 2125 number and assume it's somewhere closer to the 3 lower numbers which vary high to low by less than 1/4% so I'd go with 2108, and probably be within 1/8% of the true number. If you averaged all four you'd still be at 2112 so pick something between 2108 and 2112 and accept that you can't be more accurate than that.

If you want to measure you can do a rollout. You'll need a plumb bob, (weight on a string), a tape measure, and a piece of chalk. Start with the valve exactly at the top or bottom, verified by the plumb lining up the axle and valve. Mark a line on the pavement. Roll the bike in a straight line exactly one revolution (check with plumb line) and mark a second line. Measure the rollout (circumference). If you have a friend to work with, it'll be more accurate of you sit on the bike to compress the tire during the rollout measurement.

Now that you think you have the computer calibrated to within 1/2% accuracy, here's the bad news. Front wheel computers will always read high. That's because the front wheel rolls farther than the back wheel, always tracking a wider arc on all curves. Since nobody rides a perfectly straight, and roads have curves anyway, this can add up. If you want ultra accuracy you'll mount the sensor on the rear wheel, which tracks a straighter line.

In short, don't go crazy, use 2108, or something slightly larger to compensate for the tracking error and be secure that you're accurate enough, within 50' over a mile.

Kelinen

Thanks, i'll go with 2108mm.

First i measured it like you said, but its impossible to be as accurate as mm, so i searched on google. I thought that there would be some specific numbers for those tires, but i didnt find them ;-)

I also did a quick calculation, and noticed that on 100km, the error couldnt be higher than 1km, so it's not that important as i thought it would be.

Thanks anyway :-)

Ex Pres 

700c = 622mm (rim bead seat diameter)
25 = 25mm + 25mm (cause it's on both sides of the rim)
add 'em up and you get 672mm, multiply by pi = 2111
But that's only in theory. Most tires are smaller than stated, so the manufacturers can make it look like their tires for any given size are lighter.

wphamilton

Just dab some oil or grease on the tire, roll forward and measure between the spots.

You can calculate an approximate number (the highest it could be) by (622 + 2 times 25) times 3.14 which is where the 2110 comes from. Depending on your weight and air pressure the tire mashes down so it isn't as big around, accounting for the smaller numbers. It won't always be exactly right no matter what you choose so I'd just pick one in the middle.

Edit: one more thing to complicate things. The Sheldon Brown site claims that some computers use an imprecise conversion factor which throws their calculation off by 0.5% I can't verify if this is true but .995 x 2124 (your computer's recommended setting) arrives at near the calculated circumference of 2110. So you just might be better off using 2124 or a few smaller.

10 Wheels

Look on the sidewall of the tire. That brand sometimes has the numbers listed there.
2105 be best

HillRider

Ex Pres' calculation is what the manufacturer's use for their calibration tables and runs pretty true to correct. The assumption is that a bike tire is pretty much circular in cross section so the height above the bead seat is the same as the width. For your 700x25 (assuming it really is a 25) the number he gave, 2011, is theoretically correct.

For my 700x23 tires, I use 2090 (theoretical is 2098) and my Cat-Eye agrees with my Garmin GPS to within 0.4 miles over 100 miles and the Garmin actually reads slightly higher.

mikepwagner

I wonder what "conversion" he's talking about. That would an enormous computer error.

Mike

wphamilton

It's the metric to miles per hour conversion using 1.6 vs the correct 1.609. Why any computer would do that, don't ask me.

On closer look I'm not buying into it. A computer using 1.6 would need a smaller tire setting, not a larger one, to make mph come out right. The similar numbers are a coincidence. So ... never mind.

Kelinen

So conclusion (yes, it's a 700x25):

theoretically, it's 2111 mm, but with the errors, 2108 is a good approach to reality?

fietsbob

sitting on the bike compresses the tire, changing the roll out, circumference ,
so pump up the tires to the proper pressure, and have an assistant,
to have a more accurate number, combine several wheel rotations,
and then divide the number of rotations of the total length..

measure.. the number is ... DIY..

Al1943

This is true and is more significant if the computer's sensor works on the rear wheel which caries a much larger share of the weight.

Retro Grouch 

Why do you want to know?

If you think that "good enough" is good enough, just program in whatever tire circumference number the computer set up instructions suggest. You might be off by a couple percentage points but that's - well - good enough. If, after finishing a ride my computer says 49.88 miles, I don't ride a lap of the parking lot because I think that "good enough" is good enough and nobody else cares anyway.

If you're the kind of person who has to have all his clocks indicate exactly the same time, you need to roll out your tire. I'd roll it out for 3 or 4 revolutions and divide to minimize your measurement error. You can minimize, but you can't completely eliminate measurement error. In the end, all you've really accomplished is to redefine "good enough".

HillRider

You just described me to a "T". I have indeed ridden a circle around the parking lot to get my cyclometer to read 100.1 miles when the century ended with it showing 99.8 miles! And yes, I hack my watches against the NIST atomic clock.

As to cyclomneter calibrations, i have indeed done the roll-out procedure and the Cat-Eye calibration table seems very close to what I came up with. Also, various makes and models of "700x23" tires differ slightly in their true dimensions so you would have to repeat the roll-out with every tire change. I don't. i do set the calibration number slightly under the theoretical value (2090 vs 2098) and assume my actual distance is at least as far as the reading.

seedsbelize 

I set mine by the rollout method. Then adjust it against the kilometer markers on the road. It may or may not be any more accurate, but it does agree with officialdom. How accurate a measurement are you going to get using chalk and reading mm? Even using a pencil?

HillRider

The only truly accurate way to calibrate any wheel based odometer (bike, car, whatever) these days is to run it against a GPS for a significant distance. I've got a Garmin Forerunner and have used it on my bike along with my Cat-Eyes and the GPS is the final arbiter of how far you've gone. If you are really concerned about your calibration, buy or borrow a GPS and use it as the standard.

FBinNY 

There's a difference between what's measured by a GPS based odometer and a wheel based odometer. The GPS unit will measure the net distance traveled, or the distance between 2 points, while a wheel mounted unit will measure the actual distance traveler along the route.

In the big picture the difference can be very tiny, or not at all, but they are different. Imagine, that you ride tight circles in a parking lot for a while. The GPS will say you haven;t moved, but the wheel odometer will record that you did. That's an extreme example, but for a more practical example, imagine your climbing a very steep hill and decide to slalom up it. The GPS unit will only record the shorter straight line up the hill.

HillRider

True and I've seen this discrepancy when riding in congested areas with a lot of turns. The GPS will report slightly less distance than the Cat-Eye, say 0.05 miles in 5 miles of city riding. Once I get out onto more open roads with fewer interruptions, the GPS actually reports a slightly longer distance than my Cat-Eye.

That's why I said you want to compare the GPS to your cyclometer "for a significant distance" and preferably on fairly open roads. Incidentally, modern GPS's are sensitive to quite short distances so some of your slalom wiggles will probably record too.

Looigi

The GPS calculates the straight line distance between points it logs. If it logs points frequently, the total distance over a winding track more accurate than if it logs points infrequently. The Garmin 705 can be set to use "smart" recording or to log points every second.

Retro Grouch 

Even better, install identical computers on the front and rear wheels of a tandem. The front one will always record a "skosh" more distance because the rear wheel cuts every corner by a little bit.

If we come up a bit short on a century ride I'd go by the captain's computer because - well - I'm the captain. Then I'll tell Mrs. Grouch that she can't claim credit for riding a full century because her computer proves that she didn't go that far.

FBinNY 

Absolutely right. Folks get worked up trying to get precision accuracy for what's inherently an inaccurate measurement. Worrying about a difference in rollout of 2005 vs 2010 is quibbling about a possible error of 0.25% when real world variables are far more than that.

Back when I used an odometer (don't bother any more) I entered a decent approximation of the wheel rollout. Then checked my readings against the mile markers along the road, and adjusted the figure slightly based on the comparison. After a while, I simply stopped caring and when the battery died, I scrapped the unit.

If I go on an extended tour in an unfamiliar area, I'll probably buy another simple unit (speed distance & time) so I can navigate using maps (if they still print them). Super accuracy doesn't exist or matter.

Doohickie

To measure your circumference, ride your bike through a small puddle and look at the wet spots that follow. Measure the distance between the wet spots with a tape measure and that's your circumference.

BJC

When you have two types of tire measurements, "Old-style" and ISO, listed on a bicycle tire, use ISO to calculate wheel circumference.

ERTRO measure notation: "tire width"-"rim diameter", since my bicycle tire size is listed as 26 x 1.75 American & 47-559 ISO, I will use 47-559 in the following formula:

Circumference = [(2 x tire width)+ rim diameter] x pi

Circumference = [(2 x 47) + 559] x 3.14159265359

Circumference = [94 + 559] x 3.14159265359

Circumference = 653 x 3.14159265359

Circumference = 2051.46000279 mm

Therefore, I would enter 2051 mm in my Bell F12 bike computer as the wheel value/circumference.

AnkleWork

Even after four years, empiricism still beats theory.

Besides, everyone knows pie are round and cornbread are square.

SquidPuppet

Four years.