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Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

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Old 12-30-08, 07:25 AM   #1
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Bike Computer Setting

I'm sure this is an easy question, but my wireless computer (a Sigma DTS 1606 L) always seems to record a bit high (more miles when compared to other riders). I have the wheel size set according to the manual. Without relocation any sensors, should I increase or decrease the setting? TIA.
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Old 12-30-08, 07:27 AM   #2
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What is your tire size?
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Old 12-30-08, 07:48 AM   #3
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what is your tire size?
700 x 23c
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Old 12-30-08, 08:14 AM   #4
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I haven't calibrated a computer in a while but IIRC, the number you set is the number of mm's in one revolution of the wheel. To answer your question, decrease the number.

[edit] I should add, try rolling out 1 revolution, measure in mm's, and use that number.
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Old 12-30-08, 08:19 AM   #5
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Thanks.
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Old 12-30-08, 10:10 AM   #6
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Also, try to have weight in the saddle while you measure the actual rolling circumference. I understand it makes a difference.
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Old 12-30-08, 10:21 AM   #7
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Easy way to do this rollout measurement is to put a drop of liquid on the center of the tread of the tire that has the sensor on it, sit on the bike and roll it forward in a straight line. Now measure the center to center distance of two adjacent wet spots on ground and use that number to set your computer.
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Old 12-30-08, 10:54 AM   #8
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I've done something similar with tape. I put a peice of tape on the ground and wrap the wheel with a peice of tape and then roll out the wheel three to five revolutions. Mark where the tape is on the ground and measure it. Then divide that number by the number of revolutions. The more revolutions the more accurate it will be. It can be a little tricky but I try to actually sit on the bike when I roll it out. If you are riding with people that are probably lighter than you then it makes sense that you are getting higher readings if they are also using the factory setting. If you weigh more then you will more than likely decrease the wheel diameter more than they will when you sit on the bike. All the computer does is count the revolutions and multiply it by the number that you have input.
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Old 12-30-08, 11:36 AM   #9
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No matter how accurate you get it, you will still not match your friends. It is unlikely that you are all riding the exact same line down the street.
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Old 12-30-08, 11:38 AM   #10
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Weighted rollout, FTW. I calibrated my computer via a 3x averaged rollout and rode along a stretch of street which was marked for a running race, and my computer was spot-on to the measured mile-markers painted on the street.
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Old 12-30-08, 05:14 PM   #11
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IF ALL your friends show CONSISTENTLY higher readings than you, then yours is probably wrong.
I just ose a string to measure my tire circumference and it seems to work just fine. A rubber band is the same length no matter what shape it's laying in!

IF you are a certain percent higher, just divide your current setting by 1 + the percent.
Example 7% high, divide by 1.07 and use that number.
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Old 12-30-08, 10:07 PM   #12
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IF ALL your friends show CONSISTENTLY higher readings than you, then yours is probably wrong.
I just ose a string to measure my tire circumference and it seems to work just fine. A rubber band is the same length no matter what shape it's laying in!
Not if you're applying force to the rubber band and deforming it.
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Old 12-31-08, 12:09 AM   #13
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I had the same issue with a Sigma computer and I think the Sigma manual may be wrong. My Cateye manual says 2096 is the number for a 700x23... try that.
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Old 12-31-08, 12:51 AM   #14
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Ask your friends how they calibrated theirs. If they all did the rollout measurement, they're probably fairly close. But they may all be going off the number in the manual, and you could be the one that's right.
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Old 12-31-08, 12:55 AM   #15
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Not if you're applying force to the rubber band and deforming it.
How about on a Thursday?

Any error induced by weighting the bike is so small, it's just as good as attempting to accurately measure roll out

Which wat does the error go by weighting the bike? More or less? You might make a flat spot on the tire, but you also increase the tire pressure and stretch it larger.
How many miles of tire wear should you recalibrate?

It's ALL much ado about NOTHING!
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Old 12-31-08, 03:05 AM   #16
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It doesn't make sense to use the number in the manual because different brands of tires can vary quite a bit even though they are supposedly the same size. I've got Continental Ultra 28 tires on one bike, and they are only 25 mm wide at full inflation. I use the rollout method to get it more accurate.
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Old 12-31-08, 03:20 AM   #17
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Quote:
Originally Posted by Snapperhead View Post
700 x 23c
Set it for 2097 mm and you should be within 1% OF DEAD ON, +/-
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