Tech Question re Ergo-Brain and Wheel Circumference

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Anonymous's picture
Anonymous

What an attention-grabbing subject line! Woo-hoo! But if you clicked through, perhaps you can explain to me how to calculate wheel circumference for the purpose of calibrating my Campy Ergo-Brain computer. I have been using it for years, all the while suspecting that my odometer was centimeters off (and they do add up!). It appears that the default setting is 2096, which i have always used, having standard 700cm wheels, but perhaps i have to factor in the tire diameter too. . . .?

I don't know. You tell me. Please.

Cat

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
how to calculate wheel circumference

Have the front wheel with the valve in the 6 o'clock position. Put a chalk mark on the floor at such position. Roll the bike directly forward once until the valve hole is @ 6 o'clock. Mark this position. Measure the distance between the two marks (and round down to account for your weight while on the bike - if you want to ever more precise). Having a second set of hands/eyes helps with this.

Anonymous's picture
Cat (not verified)

Peter - as a Jersey City resident, you make a big assumption that i have enough floorspace in my home to do this. Oh, and room for a friend too!

Can't i just look it up online?

thanks ;)

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
apt bldg hallway or sidewalk is suffice?

as for a friend, well if by chance if i'm passing by your neighborhood, i'd surely help.

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
more stewardly advice

If rolling your bike one wheel roation is too much trouble, then here is another idea. Remove the front wheel and wrap a piece of thread around the center of the tire. If you have no thread, then use an old cello string. As you wrap, use few pieces of tape to hold it in place.

Once wrapped around, cut and measure. From someone who used to do bike maintenace in an 8 square ft kitchen, I bet even Martha Stewart could handle either task in her cell block. ;-)

Anonymous's picture
Evan Marks (not verified)
Ride a lap of Central Park

Your total distance should be 6.05 to 6.1 miles.

If your cyclocomputer reading is too high, decrease the wheel diameter. Too low - increase it.

Anonymous's picture
Cat (not verified)
thanks, Evan and Peter! (nm)
Anonymous's picture
wheel man (not verified)
Numbers you need

You can measure, etc, etc.. but if you don't wrap the yarn well, if you don't roll a straight line, it will not be very exact.
But there are published charts depending on wheel diameter and size.

700 X 28 (28-622) 2136
700 X 25 (25-622) 2105
700 X 23 (23-622) 2097
700 X 20 (20-622) 2086

Yes, your weight and tire pressure will change these slightly. But, unless you always pump your tires to the exact same pressure and ride at the same temp, you will get some variance no matter how exact your measurement.
Sounds like your computer defaults to a 700x23. If you have wide 700 tires, you could be off by 2% just using the factory setting.

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
enumeracy, credibility & experience

"... but if you don't wrap the yarn well, if you don't roll a straight line, it will not be very exact...

""very exact""??? Either it is exact or it is not exact. Regardless, it doesn't have to be exact. Rather it just needs to be precise. From my personal experience doing such is quite acceptably precise. Notably, it also accounts for the tire's profile which ""wheel man""'s *un-cited* ""published"" chart does not.

For instance, are those values for slicks, cyclecross tires or something in between? Some tire manufacturers like Continental have more rubber on the tire ""tread"" while others place more on the sidewall like Michelin. Likewise, one tire manufacturer's 700 x 23 really fits more like another's 700 x 25, i.e. Avocett versus Vittoria. This is all hair splitting nonsense, really.

Using the measuring methods I mentioned one could expect the ""measuring error"" to be well within the 8 - 11 mm s, too (the difference between a couple tire sizes you list). Point is the measuring error is insignificant and such method works quite well."

Anonymous's picture
Peter Storey (not verified)
"Don't forget ""futility"""


Frankly, any of these methods works well enough to achieve an acceptable level of approximation, at which point Evan's method takes over. Ride a known distance and adjust accordingly.

Once you've calibrated it to the nearest .01 mile, you can come to terms with the fact that very few cue sheets (including mine) seem to be measured to any consistent level of accuracy.

Peter Storey

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
Agreed

"Perhaps you overlooked my comments of ""..This is all hair splitting nonsense, really..

I would use Evan's method to validate the setting is correct."

Anonymous's picture
wheel man (not verified)
Sorry for trying to help

Just thought a non-technical person might just want a number to plug into their computer so that they could go out and ride without buying yarn.
And I guess I could have cited:
http://www.campagnolo.com/pdf/ErgoBrain%20uso%20release%201_0.pdf
or
http://www.sheldonbrown.com/cyclecomputer_calibration.html

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
trying to help

"With the Sheldon Brown link you cited, it also includes the following:

The values on the chart will generally give a value accurate to within one or two percent, which is more than good enough for the vast majority of cyclists, and is much more accurate than most automobile odometers. If you require more accuracy, you can do a ""roll-out"" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution.

""Sorry for trying to help"" - really? While doing so at the expense of dismissing other's help. Hmm.
"

Anonymous's picture
Evan Marks (not verified)
"""...effective tire size is affected by tread thickness..."""

Soooo, if your cyclocomputer has the pickup on the rear wheel you'll need to adjust the circumference as the tire wears down.

LOL.

Anonymous's picture
don montalvo (not verified)
don't forget to adjust your setting if...

...the barometric pressure goes down. :)

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
rear wheel pickups - right said fred

i hate 'em. it makes it much harder to preserve your club ride's average speed when walking with the bike through the subway.


and yes...

~~~ JUST KIDDING!!! ~~~

cycling trips