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I have bought the Campangolo Khamsin wheelset, and now I would like to set the right wheel size in my Trek Incist 8i cycle computer. Before I do the custom wheel setting, I would like to know what would be the best choice?

The wheel has ETRTO 622x15c printed on it.

The computer comes with the following wheel sizes defined:

700x20, 700x23, 700x25, 700x28, 700x32, 700x35, 700x38, 26x1.5, 26x1.5, 26x1.90, 26x1.95, 26x2.0, 26x2.1, 26x2.2.

So, if anyone here knows what would be the closest match I would be happy to know.

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Does the computer allows for manually entering a custom value? –  heltonbiker Sep 10 '12 at 20:01
    
Is that what's printed on the TIRE, or the RIM? You want to find the numbers printed on the TIRE. Then you need to understand that the numbers may be expressed several ways -- too many to enumerate here, but Sheldon Brown has a fairly good article on the issue. But 622 is the same as 700c, so if you really have a 15mm wide tire (more likely that's the rim size) you'd go with the narrowest 700 value available. Most likely that's the rim size, though, and you have a 700x23, x25, or x27. –  Daniel R Hicks Sep 10 '12 at 20:04
    
Note that, as a rough approximation, the 622 number is the diameter of the rim, and the diameter of the tire is the rim diameter plus twice the tire width. Multiply times pi (3.142) to get circumference (which is what many cyclometers use as the "raw" number). –  Daniel R Hicks Sep 10 '12 at 23:36
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6 Answers

up vote 14 down vote accepted

I have not had much luck with any of the preset settings (on my cateye computers) always using custom. Here is how I recommend to measure the circumference:

  1. Inflate your tire to desired psi
  2. Put a mark of chalk on the garage floor and bike tire
  3. Sit on my bike and roll forward one revolution until the bike tire mark comes back to the floor
  4. Mark the end position on the ground
  5. Measure the distance between the marks on the ground, convert to mm (for the Cateye) and set the custom tire circumference.

I have found differences between different tires (both 700 x 25) and even differences with the same tire (air pressure difference).

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+1, for taking an actual measurement. Although the tire sizing spec is a good starting point, you really have to just measure it one way or another to get good accuracy. –  Angelo Sep 10 '12 at 16:39
    
Depending on the tire, the size can vary quite a bit. Rolling out is the best method. Recheck if you change to different tire. –  Ken Hiatt Sep 10 '12 at 16:39
    
Rolling out is the only method that captures the intrinsic nature of the phenomenon of interest to be measured: the distance ridden by the bike per wheel revolution. If you have available space and tape length, you can measure two or three revolutions. Also, substitutes for chalk (including water, ink, etc.) can be used. +1 –  heltonbiker Sep 10 '12 at 20:00
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Best to roll several revolutions, if you can manage it. (Move the wife's car out of the garage temporarily.) –  Daniel R Hicks Sep 11 '12 at 3:07
    
it might make a difference whether you're sitting on the bike at the time, although it's probably negligible at high pressures.(what's a car doing in the garage?) –  JamesBradbury Dec 9 '12 at 8:09
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Look on your tire for the proper size. Different tire sizes change the overall diameter of the wheel and amount of distance traveled per revolution of your wheel.

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Most likely it will be 700xsomething listed on your tire. 622 is the standard wheel size referred to as 700c. I'm guessing due to the 15mm width that your tire is going to be something like 700x23/700x25/700x28. 29er tires fit on the same diameter rims, but with a larger rim width to accommodate a wider tire. –  Benzo Sep 10 '12 at 15:55
    
More details on tire sizing here: sheldonbrown.com/tire-sizing.html –  Benzo Sep 10 '12 at 15:57
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I'm assuming that when you say the "wheel" has a size of 622x15c listed on it, you mean the rim and not the tire. As others have mentioned, the tire size is what you need. Almost any 700c tire will fit on your rim, assuming that your frame has the clearance to accommodate it. Once you've got your tire on there, check the size listed on it and match it up with the sizes you've listed.

It's worth noting that any 700xwhatever tire will not necessarily be exactly the same size as other tires with the same size listed on them. The size is an estimate. If you want the most exact measurement possible, or if your tire size isn't listed, you can measure it yourself like so:

Put a chalk mark on the ground. Position your front wheel directly on top of the chalk mark with the valve as low as it will go, which means it will also be directly on top of the chalk mark. With the chalk in your hand, roll the bike along one full tire rotation until the valve comes to rest at it's lowest position. Put another chalk mark. Measure the distance in millimeters. Use that number instead of whatever the chart tells you. And, if you want the most precise measurement possible, sit on your bike while you roll it along and get a friend to do the marking. This wall account for the way your weight distorts the tire.

Incidentally, this measurement process is all explained in the manuals of most cycle computers, page 24 of your particular manual. http://www.trekbikes.com/pdf/owners_manuals/06_Incite_6i_8i.pdf

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I have a Cateye Double Wireless, and I was surprised at how much the tire circumference in the instructions varied from what I was measuring and observing.

I run 700x25C Gatorskins, and the Cateye instructions said to use a circumference of 2105mm, but I was measuring a circumference of 2155mm, when the tire was inflated on my wheel. That's more than a 2% difference. In actuality, it can be shown that the Cateye number (2105mm) is a lot closer to the true circumference (when a rider is on the bike, compressing the tire, and changing the effective radius of the wheel/tire and circumference of the tire) than 2155mm is. So if you can live with a small amount of inaccuracy, I would say that you should go with the number in the instructions that came with your computer. It's probably something they've considered more carefully than you might think.

I tried to explain this to a friend, and his reply was "your inner nerd is showing," which is probably true. But, I have degrees in math, and I don't care.

What I found, for my bike & my weight, was:

1) Sitting on the bike causes the tire to compress about 5mm. (My computer sensor reads off my back wheel, which carries most of my weight.) So when I measure the circumference of the tire, that must be taken into account. There is, effectively, a loaded and unloaded circumference. In my case, the unloaded circumference of a new 700x25C Gatorskin is about 2155mm. The loaded circumference is around 2124mm.

2) The tire wears over time. With 3200 miles on the rear tire, it's squared off pretty good, and the worn unloaded circumference is about 2140mm, whereas the worn loaded circumference is around 2109mm. These tires are typically good for 4000 miles, maybe 4500. Near the end of its life, the number given by Cateye turns out to be pretty close for this tire. But not all tires of a given size have the same circumference. Schwalbe Marathon Plus tires, for example, are taller (larger circumference) than Gatorskins.

3) Can I find a "set it and forget it" number? I could just use the figure supplied by Cateye, 2105mm, or I could interpolate and use (2124+2105)/2=2115 (rounded). Over the life of the tire, this would cause my speed and mileage to be overestimated when the tire is new and underestimated when the tire is worn, but it should average out by the time the tire is replaced, assuming the tire achieves an average lifespan.

2% is not a huge error, but over a year's mileage for me, it would cheat me out of a longish ride's mileage (somewhere between 40 and 50 miles), at least in my records.

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2% is actually pretty good especially when you compare it to car speedometers (consumer.org.nz/news/view/speedometer-accuracy). With cars there's a lot of room for error because there are many different tread patterns and thicknesses that can adversely affect the accuracy. –  Kibbee Sep 11 '12 at 12:56
    
+1 for inner geekiness. –  JamesBradbury Dec 9 '12 at 8:14
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622 is the ISO naming scheme for 700c wheels. Since the choices given to you don't list 700x15 I would think you'd have to enter a custom setting. I don't imagine any of the 26 inch measurements would apply to you either. Mountain bikes have a bead seat distance of 559 mm, meaning you would need a tire that is 622+15 - 559 = 78 mm, or about 3 inches on a 26 inch rim to come up to the same radius. This is based on the information I was able to obtain from the Sheldon Brown tire sizing page. This assumes that you have 15c tires on the bike, which would be quite narrow, but not unheard of. If you can supply the numbers from the side of the tire wall then we could provide you with a more specific answer.

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You will likely find out as I did, that it can be a bit of trial and error when setting the cycle computer to accurately measure mileage. The settings are usually set according to the size ( diameter and width ) of the tires. You can get fairly close to being accurate by using teh suggested settings in the directions for your computer. There are a few things you must take into consideration. First of all you must make sure your tires are inflated to the pressure normally used. Secondly, consider the position you must place the sending unit on the fork, and the magnet position on the spoke. The further towards the outer edge of the wheel the magnet can be placed, the more accurate the measurement will be with the computer initial setting by the instructions. Some forks won't allow you to set the receiving unit so the magnet is close to the rim of the wheel. Due to my fork design, my sensor is down probably two inches from the edge of the wheel, closer to the center of the wheel than the instructions recommend. I've found the easiest method is to travel a known measured distance, like a mile, then set your computer so that it will read the same mile over the known distance. Then be sure to write the information down so when it's time for a battery change, the measurements are easily reset in the computer.

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