Accurate Wheel sizing Schwalbe Marathon Supremes 700x35

[Andiroo]

Hi All

Simple thing but frustrated. I want a very accurate wheel circumference for my Schwalbe Marathon Supremes 700x35 so i can accurately set up my bike computer. Now when i roll the wheel and measure a complete circumference i get a measurement of 85.5". But when i look either at the schwalbe site or other sites on how to measure wheel size i get a different number.

Does anyone know have a definitive sizing or is the rolling measurement the best way to go?

Thanks

Andy

[John Nelson]

The rollout is the most accurate way to set your odometer. Don't worry about what Schwalbe says. Pump the tires up to the pressure you will run out tour, and load the bike with the load you will be carrying. Sit on the bike while you roll it out. I like to do three full revolutions. Make sure you go dead straight--follow a line on the pavement. I put a piece of tape on the front tire and have an assistant very precisely mark the pavement with a sharp pencil. Then I measure the distance with a steel tape measure. Done precisely, it doesn't get any more accurate than that.

[Andiroo]

Hey John

Proof the smartest riders really do hang out here!! I never considered the three full turns of the wheel for greater accuracy but makes a lot of sense. I also measured unloaded. Come the weekend i will get loaded up and do what you suggest. It is weird however that their seemed to be a material difference between what i measured and the official (?) stats. I mean if i gonna sweat for all those miles i dont want to lose a few hundred feet do I.

Thanks

Andy

[hem]

A quick look at the Cateye size chart says 217. So 2.54 X 85.5 = 217.17 I would say you are really close with your roll out measurement.
I have the Schwalbe Marathon Supremes 40mm and used the Cateye size chart and seems close to what I get with my Edge 500.

[PeteJack]

Depends how much precision you need. I used to measure the circumference of the tire but I've stopped bothering. the numbers that come with the instructions for your computer are quite accurate. On club rides of 30 -40 miles my mileage is rarely more than 0.2 miles different from everyone else's, that's 0.7%, supposing they are right and I'm not. It's plenty accurate for finding a turn on an ACA map (Mind you, ACA maps are usually pretty good but sometimes those distances are way off, so don't stake your life on it no matter how accurate your computer is) On a 1000 mile trip you may be off by +/- 1 mile, does it really matter?

[happyriding]

It is weird however that their seemed to be a material difference between what i measured and the official (?) stats.

What official stats are you referring to?  The 622?  That is the *inside* diameter of the tire, i.e. the diameter of the rim that the tire will fit on.  That measurement is not the same thing as the outside diameter of the tire while on the rim.  In other words, the diameter of the rim alone is smaller than the diameter of the rim plus the tire.

In fact, a tire will have a different outside diameter depending on the width(not the same thing as the diameter) of the rim it is mounted on.   On a wider rim, the tire will spread out more, and on a narrower rim the tire will stick up more.  Rims of a given diameter come in different widths.  The wider the rim, the stronger it is--but the downside is that a wider rim is heavier.

[Pat Lamb]

I'll usually go up to the top of a ridge near here and coast down after I've changed tires, just to check on the accuracy.  There's a couple of miles of measured mileposts, conveniently placed so I can check while going about as straight as I can.  It's easy enough to go in and adjust the wheel diameter in the cycling computer after you know how far off it is; mine are usually within 1%.  Start low, end up high, as the tire wears down.

In another sense, it doesn't really matter, because you'll be checking against somebody else's measurements, and you don't know how accurate those are.  Some of the AC maps seem to be off half a mile between measurements, and after adding up a day's worth of random errors in either direction, you learn to calculate in your head from one turn to the next, then keep your head up and your eyes open.  Similar stuff happens with others' routes in other places; that's why some people will circle the parking lot at the end of a century (to make sure their computer is over 100 miles), and others will complain about how far over 100 miles they ended up riding.

(But it drives me nuts when somebody says the ride I was on tonight is 27 or 28 miles.  It's 25.5, to within 1%, because that's what MY measurement says!!)

[mu2flyer]

after measureing the tire using the rolling method, we have a local park that has mile and 1/2 mile markers on the trail. I'm able to dail it in with longer distances.

[bogiesan]

after measureing the tire using the rolling method, we have a local park that has mile and 1/2 mile markers on the trail. I'm able to dail it in with longer distances.

Those are not likely to be very accurate. Standard computer-setting procedure is to roll the wheel out carefully three or mor times and average the results.

david boise ID

[DaveB]

"Milke Markers" on trails or roads are unlikely to be very accurate, just a guide.  The roll out method is by far the most accurate as the nominal tire size given by manufacturers is often not exact either.  I have a set of 700-32 marked tires that measure an actual 26 mm installed on standard road rims and their roll out confirms the actuual measurement.  A GPS can give very accurate distance measurements and can be used to calibrate a cyclometer if you have access to one.

[PeteJack]

that's why some people will circle the parking lot at the end of a century

One day on the Southern Tier we rolled into Blythe and my computer read 99.7 miles. For about a microsecond I thought about riding round the parking lot to make it 100, then sense prevailed, I opted for shower and food. Come to think of it I didn't even ask my pal what his computer read.

[Pat Lamb]

"Milke Markers" on trails or roads are unlikely to be very accurate, just a guide.  The roll out method is by far the most accurate as the nominal tire size given by manufacturers is often not exact either.  I have a set of 700-32 marked tires that measure an actual 26 mm installed on standard road rims and their roll out confirms the actuual measurement.  A GPS can give very accurate distance measurements and can be used to calibrate a cyclometer if you have access to one.

Depends on the location.  Many main roads have surveyed mile markers (sometimes even measured with chains!).  I figure it doesn't much matter how much ground I cover wandering back and forth, but it does matter how many miles of road I travel.  When you're on a U.S. highway, for instance, and rolling down the hill, it's basically a super-sensitive roll-out test; it covers more ground, so errors are diluted, and you measure distance traveled with your weight on the bike, which also tends to change the measurement process.  I could never put much confidence in measuring to the millimeter the distance between the stem being vertical.

[Westinghouse]

The electronic meters for bikes must be a lot better than scaling mileage off maps and using map distances. I did the ST in the winter of 2009-2010. Starting about 200 miles south of Saint Augustine, FL, I got 2700 miles from adding map distances together over 54 days to San Diego. I don't know the exact mileage, but I know it is hundreds of miles farther than 2700.

[bktourer1]

Try this:  http://www.sheldonbrown.com/cyclecomputer-calibration.html#ccc
this might help

[DaveB]

that's why some people will circle the parking lot at the end of a century

One day on the Southern Tier we rolled into Blythe and my computer read 99.7 miles. For about a microsecond I thought about riding round the parking lot to make it 100, then sense prevailed, I opted for shower and food. Come to think of it I didn't even ask my pal what his computer read.

Man, not me.  I'd have certainly ridden the extra few blocks to get the cyclometer to read 100.1!!!