N_C

There is no 20x1.35 listing on the wheel size chart for my new computer. But there is a forumla for multiplying the wheel diameter in mm by 3.14. That is supposed to get me the 4 digit number I need for the right setting. But what about the width? Is that a factor, or is there no need to worry about it?

Thank you.

John

Trek Rider

John, have you heard of the "roll out method"?

You place a tape measure on the ground and with you on the bike, you "roll" the tire one complete revolution. Take the measurement you got in inches and multiply it by 25.4. That will give you the tire circumfrence in mm's which is what your computer is looking for.

N_C

TR, thanks, I have used the roll out method before, but I don't think it'll work with this computer. It is the Topeak Panoram Wireless. It does not have the option to use the roll out method in the instructions. Which is why I did not use it.

Every computer I had in the past was a wired one so I'm guessing that is why the roll out method was shown as part of the instructions for those. I can go ahead and try to use it here & see what happens even though it is not listed as a method to use.

What is listed is the wheel chart & the formula that says WS=mm x 3.14. What that means is wheel diameter multiplied by 25.4 then multiplied by 3.14. But it says nothing about the width. Which is listed in the wheel chart for other wheel sizes, not including mine though. That is why I asked in the first place.

MikeR

All you need is the circumference of your wheel. The width does not matter.

That 3.14 is the value of pi. The math formula is Circumference = pi times Diameter. Since all you need is the circumference the “Roll out method” will also work fine. In fact it is a little closer to reality if you roll the bike while you are sitting on it, because it takes into account the slightly smaller circumference due to the tire being depressed by your weight. With the roll out method you have to measure in metric or convert to metric.

DieselDan

Most, if not all computers, the wheel calibration is in millimeters or centimeters. If it's four digits, it's mm. Three digits, it's cm.

Retro Grouch 

The rolling circumference is all that your computer needs because all the computer does is to count the wheel revolutions and convert that to MPH. The roll out method takes away all of the guesswork and indirect measurement errors so it's almost always more accurate.

The reason why the generic tire size instructions are width specific is because, if when everything else is equal, a wider tire will usually be a little taller.

dobber

The "roll out method" = "WS" = "Wheel Diameter x 3.14" = (they are all methods of determining the circumference, the roll out method being the most accurate)

The chart shows the various width because they pertain to different values in the internal database. A 700 x 28 is a different circumference then a 700 x 23. The fact that you directly measure the circumference eliminates the need to take width into consideration.