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Old 05-02-06, 08:49 AM   #1
Dinstee
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Computer/wheel question

I have just gotten a Trek Incite 8I computer (wireless) and have a question regarding setting up the wheel size. The computer, as do just about all computers, has several preset wheel size configurations to choose from including a custom setting. I have a 26" 1.6 semi slick on my MTB but the closest preset is 26 1.5. The next size is 1.9. Is the difference between 1.5 & 1.6 too small for me to worry about or should look at using the custom wheel size setting? For now I am using the 1.5 setting.
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Old 05-02-06, 08:54 AM   #2
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A custom setting of 201 cm or 2010 mm (or 79.2" if the calibration numbers are in inches) should be about right is the tire is close to the stated size.
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Old 05-02-06, 08:57 AM   #3
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thanks. I see that 2010 mm is corresponding to the 26x 1.5- so i'm going to just leave it alone. (Or put my 26x1.9 back on)

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Old 05-03-06, 01:22 PM   #4
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Get a tapemeasure, prerfably a fabric one, and mesure around the circuforence of the actaul wheel in question.
- Or put the valve at the bottom of the wheel, mark the ground, roll it round and back to the valve at the bottom, mark the ground again. Then mesure that distance!

Also, random fact of the day.
- If you see the circufrence to 1667mm and set the computer to display Kh/h it will count tens of revs (Ie, 32.5Km/h = 325rpm)


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Old 05-03-06, 01:54 PM   #5
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Get a tapemeasure, prerfably a fabric one, and mesure around the circuforence of the actaul wheel in question.

Daniel
That will give you a number too large because it does not account for the loaded weight.

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Old 05-03-06, 07:27 PM   #6
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"Rollout" measured with the correct tire pressure and your weight on the bike is, theoretically, the most accurate way to determine the true wheel circumference but it has to be done properly or it's no better than the calculated value. You will be right within a fraction of 1% if you use the calculated value.
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Old 05-03-06, 08:59 PM   #7
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"Rollout" measured with the correct tire pressure and your weight on the bike is, theoretically, the most accurate way to determine the true wheel circumference but it has to be done properly or it's no better than the calculated value. You will be right within a fraction of 1% if you use the calculated value.
The discrepancy is more if the sensor works off of the rear wheel, less if off of the front wheel. I tend to forget that because I've always run rear wheel pickup computers.
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Old 05-04-06, 11:28 AM   #8
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I thought most computer instructions also included a method for measuring it yourself. Something like marking off 20' and measuring the distance for one full wheel rotation. It's supposed to be more accurate too, since tires can vary (especially mtb tires).
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Old 05-04-06, 11:39 AM   #9
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I thought most computer instructions also included a method for measuring it yourself. Something like marking off 20' and measuring the distance for one full wheel rotation. It's supposed to be more accurate too, since tires can vary (especially mtb tires).
You can mark of several rotations on the floor and take the average but this needs to be done with the tires fully inflated and the bike with the full weight of the rider and gear.

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Old 05-04-06, 11:57 AM   #10
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You can mark of several rotations on the floor and take the average but this needs to be done with the tires fully inflated and the bike with the full weight of the rider and gear.

Al
Sorry, forgot to mention that. I would do this with someone holding me, riding the bike backwards. Just like the rollout for junior gear restrictions.
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Old 05-04-06, 11:59 AM   #11
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If your computer measures in millimeters:

If you want to be relatively accurate and still impress people with how far you ride, or your speed as recorded on your computer, pick one size larger than your tire and/or round up to the next increment of 10.

If you want to be relatively accurate and still make sure that you don't say 100 miles unless you are absolutely sure you rode at least 100 miles, pick one size smaller than your tire and/or round down to the next increment of 10.

Either way, the two numbers (high and low) are likely to be within 2%... A difference between 2000 and 2020 millimeter circumference is 1%.

If the difference between 30.0 mph and 30.3 mph is critical to you, then you should do a roll-out test... otherwise, an estimate based on closest match in the computer's ducumentation should be fine for most people.
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