Choosing the right wheel size in my cycle computer

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.

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.

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

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.

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.

If you can enter a custom size you can measure your wheel using:

1. Measure you diameter (typically in centimeters).
2. Diameter * 3.1415 = custom size to enter.

Another method would be to mark the ground and tire with chalk where the tire hits, roll the bike forward and mark where it hits again. Then measure between the two marks.

Based on my experience, the rollout method (measuring the distance traveled during one wheel revolution while sitting on the bike) is a good starting point, but will not yield a setting that agrees with mapping software such as Ride With GPS. For me, what worked best was to use Ride With GPS, https://ridewithgps.com, to map out an exact one-mile route, using the satellite image to designate specific land marks such as the edge of a driveway as start and finish points. After riding the one-mile route, using my rollout measurement setting, I divided the actual distance (1 mile) by the computer's distance reading and multiplied the result by my current circumference setting to get the corrected circumference setting. I then entered the corrected number into the computer and rode the route again to check. For example, my rollout measurement was 2105 mm. After initially riding the one-mile route using this setting, my computer read .97 mi. Doing the math: 1/.97 = 1.0309278. Then 1.0309278 X 2105 = 2170. I reset the circumference to 2170 mm in the computer, rode the route again, and the computer showed a distance of 1.00 mile.

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 diameter of the wheel is 622 mm. The circumference of the wheel is 1954 mm. Thus, if you ride on wheels, you have to enter 1954 mm on the cycle computer.

However, most cyclists choose not to ride on wheels but rather ride on tires. This is when things become complicated. Most cyclists measure the weight of the products and get very angry if the manufacturer stated the weight of some component incorrectly in marketing. Most cyclists do not measure the width of a tyre, and even if they did, the width of an inflated tyre depends among other things on rim width. So if you buy a 28mm tire, and measure its width as 25mm, the manufacturer can claim that the stated 28mm width was on some other rim size than the one you're using.

Because of this, if you buy a 28mm tire you can actually get a 25mm tire, as the manufacturer has an incentive to sell lightweight products, and the easiest way to sell lightweight tires is to sell a 25mm tire as a 28mm tire.

Not only this, but the tire's rolling diameter and circumference are affected by the weight placed on the bike, whether the cycle computer uses front or rear wheel for measurement as the weight distribution is uneven (most e-bikes use rear wheel, most separate cycle computers use front wheel) and the inflation pressure.

A just-inflated 28mm front tire (7 bar) has about 0.3% larger rolling circumference than a tire inflated two weeks ago (5 bar) ¹.

The good news is that if you have some map service that is able to measure paths very accurately, and if you have a cycle computer with 0.01 km resolution, you can enter some guessed value as the rolling circumference and then ride a 30-50 km path. Use the map service to determine the precise length of the 30-50 km path. Then ride the path, taking no shortcuts and not stopping on any nearby shops. In principle, the longer the path the better, but on longer routes you need to stop somewhere to drink unless you carry a drinking bottle, and on even longer routes you need to stop somewhere to eat.

The true rolling circumference is true_path_length / cyclometer_trip_distance * guessed_value.

So, for example I rode a 34.92 km path where the cyclometer gave 35.28 km. The guessed rolling circumference was 2149 mm. Calculating 34.92 / 35.28 * 2149 gives 2127 mm as the true rolling circumference of these tires (Continental Grand Prix 5000 tube-type, 28mm stated width, 622mm bead seat diameter, on 622x19C rims, inflated to 7 bar and reinflated after two weeks when at 5 bar, at the measurement time pressure was approximately 6 bar, combined weight of bike and rider is 130 kg, this was the front wheel).

If you want to be very precise, you need to ride the path halfway between your regular tire inflation interval. That way, on recently inflated tires the error is in one direction, and on tires you have to inflate very soon the error is equal but in the other direction.

The largest problem of this is to find a good map service. Google Maps is not good in areas where bicyclists use bike paths and car drivers use roads. I use a regional service that gives the path length with 0.1 km precision and also shows markers between 0.2 km. I look at the path, and carefully measure with a ruler the distance between the last 0.2 km marker and the endpoint. I also carefully measure with a ruler the distance between two 0.2 km markers. Then with some math I can get the true length of the path to 0.01 km precision even though the service has only 0.1 km precision in its output.

If you do this procedure carefully, including doing the measurement halfway in tire inflation interval, the good news is that you can be accurate to within about ±0.2% as the error is in one direction with recently inflated tires and in the other direction with tires needing reinflation in a short time.

(1): I measured this on my old cycle computer that had 0.1 km resolution. It was based on the same route where a just inflated tire gave 34.3 km and two weeks ago inflated tire gave 34.4 km. Thus, the measurement is very imprecise due to the very low resolution. Also, the old cycle computer was an analog wireless one prone to misreceive counts, whereas the new cycle computer is a digital wireless one not prone to such misreception. I should re-do the measurement on my new cycle computer that has 0.01 km resolution.