esrevernitlepS

Does anyone know the exact circumference, in mm, of a 700 x 23C wheel? I need it for calibrating a computer I got. Using the numbers the instructions had, I was guessing it was somewhere between 2110 and 2120, but that's assuming their numbers were right.

Any help would be greatly appreciated.

Steev

It varies from tire to tire, but I measured what-ever Hutchison that came on my bike at 2095mm last night.

lebowitz

The manual for my Cateye Mity8 says 2100.

esrevernitlepS

Thanks.

HillRider

A 700c rim is 622 mm across the "bead seat". Bicycle tires are nearly circular in cross section so the nominal width is also the height. For a 700c wheel with a 23 mm tire, the diameter would be very close to 622+(2*23) = 668 mm. Then the circumference would be 668*Pi (3.14159....) = 2098.6 mm. So that's where the recommended 2100 came from.

I use 2090 mm (or 209 cm for my Cat Eye computers) since most "23 mm" tires are actually slightly smaller in width/height than their sidewall markings.

Little Darwin

1) Mark a spot on the ground and on the tire. Line them up. Roll the bike tire one revolution, mark the floor... measure between the two marks on the floor.

or

2) Measure diameter, outside to outside... multiply by pi.

The difference between 2100 and 2130 (10 more and less than your estimated range) is about 1%, so unless you need more accuracy, just pick something.

If you need ultimate accuracy, you should measure your specific tire with the air pressure you use and weight on it (I guess water bottles half full would be a good average)... do it on a surface similar to what you will be riding on... perhaps because of centrifugal force, you could ride at your average speed for even more accuracy...

I am just being silly of course, because I find myself wondering if the value in my computer is accurate or not... Even though my mileage or speed doesn't really matter except for my own information and self-improvement. I hope you come up with a number you can feel comfortable with. BTW - You may want to adjust it down occasionally for tire wear.

Sheldon Brown

That's only half a percent difference! How much accuracy do you think you need?

See: https://sheldonbrown.com/cyclecomputer-calibration

Sheldon "Numbers" Brown

Al1943

My numbers agree with Hillrider. I use 2090 for a Michelin Pro Race 700 X 23 on a Cateye rear wheel sensor computer with 118 psi air pressure. A measured circumferance does not take into account air pressure and load on the bike.

Al

esrevernitlepS

According to your calibrator, I was more like 26mm off (2118 before, 2092 now). Still, not much more than 1%, but it's nice to have things down to a science.

zonatandem

Now you also need to take into account the correct amount of air in the tire . . .

mactheknife68

For the ultra anal customers I have here's what we do (tire is inflated to rider's desired riding pressure)...

1. Mark floor at start point for rollout. Turn wheel so valve is over mark.

2. Weight bike while rolling three revolutions. Mark this second point.

3. Take measurement between marks (I measure in inches)-divide by 3.

4. Multiply by 25.4 for calibration number

Whatever's clever, yeah?

Little Darwin

Good point, if the level of accuracy is important, and you have the room, then the more revolutions measured the more likely a proper setting. (assuming no errors are made).

The rest of my post is just silliness:

Perhaps with a properly calibrated surface and a high speed video camera the proper calibration should be performed by the specific rider riding the specific equipment and tire pressure over 100 revolutions at cruising speed and then measuring to the nearest millimeter and dividing by 100...

And don't forget to round the result down to the next full millimeter as I don't want anyone posting speeds exagerated by specifying a larger circumference tire than they actually ride. I would hate for anyone to claim an average speed of 23.98712342314 mph when they really averaged only 23.98712342276 mph

HillRider

As long as we are being ultra-precise about this, using the valve core as an index isn't accurate enough as the distance from it to the mark on the floor makes aligning them too variable. So, make a THIN mark on the tire sidewall and use this to align with the floor mark. Much better precision!

thomson

I'll add one more to the ultra-precise argument (although I do not feel any of this accuracy is needed). You need to ensure the line you measure is the same line your three revolutions took. In other words, your bike must travel a stratight line if you are going to use a straight masuring tape to measure it. Fix the front wheel prior to the rollout.

Old Hammer Boy

It's gettin pretty anal around here!

roadfix

If using the rollout method, how do you know your tape measure is accurate?

Al1943

Still greater accuracy. One of my training areas is flat section line roads with survey pins in the centers of the intersections. The USGS topographic map confirms that these sections are full square miles with the intersections exactly one mile apart. If I want to go to the trouble I can ride a few miles and calibrate my computers to a fractional part of 1/100 of a mile (fraction of 5.28 feet or less than 1 %). A slight wobble in the front of the bike makes no difference because the sensor is in the back.
I rarely go to this much trouble.

Al

HillRider

Kidding aside, years ago I set my Cat Eye at 209 with the tires I had on the bike at the time and had the oportunity to ride a TAC-certified 10K (6.2137 miles) race course. The bike registered 6.20 miles over that distance. Plenty close enough.

Psimet2001 

Sig Figs people....Sig Figs.... Whats the resolution of the timing circuit in the computer??


Besides if you don't travel in an EXACTLY straight path during the rollout measurement then you might as well just chuck the computer, throw your hands up in the air, and resolve yourself to never knowing your ETA to the next brownie stop on the ride...

operator

Once you get in the neighbourhood, it's not so much the accuracy as the consistency. If you later find out that your measurements resulted in over/under comparing to some other more accurate method then you can fudge it.

If you keep changing the number in the computer, you'll have no idea at all.

Al1943

10K is actually 6.245 miles.

Al

cooker

Once you've programmed your computer you can double check it against the real world. Measure a straight stretch of road on www.gmap-pedometer.com, then ride it and see if your computer gives the right value.

HillRider

No, check your source. Three sources I have say 1K = .62137M so 10K = 6.2137 miles

sakarias

The rolling circumference of a weighted wheel is slightly smaller than an unweighted wheel.

Hmmm. What about recalibrating for tire wear? What if your tire is 5 psi low in pressure? What if you lose or gain weight?

One can never be too obsessive.

Well, maybe one CAN be too obsessive. As Sheldon Brown asked, just how accurate do you think you need to be?

Mike

Al1943

Hummm.... must be that carpet I've been smoking, sorry.