20" wheels "factor" for bike computer
 

In setting up my new Bike Friday I found an old but perfect condition wired bike computer in my parts box that I put on the BF. BF Mothership suggested a 20" wheel factor that resulted in a problem. Cadence works great, but speed and odometer are way overestimating . They also gave me a reference resource that is too damn complicated. I just don't want to do the calculations and measurements when riding time is so limited. So what I'm looking for is the number those of yall with 20"x1.75x wheels have used that produces close-enough results: OK is OK.

Explain yourself more clearly. Does your bike have 20 inch wheels? Is the computer set up for 26 inch wheels? Do you just want to know the difference in circumference? Or is it something else?

Here you go; can be helpful if you ever change tire size and/or diameter:

https://www.cateyeamerica.com/tire-size-calculator/

I always measure my circumference directly and use that number instead of what some chart says. Also, you need to keep in mind that there are 3 different sizes for 20" tires: ISO 406, 419, and 451 bead seat diameters.

Good luck.

Alo:

Does your bike have 20" wheels?: BF Mothership suggestd a 20" wheel factor--because I have 20" wheels.
Is the computer set up for 26" wheels? Since it was old but pefect condition It wasn't set up at all, but of course I could program it for various sizes ... but not down to 20".
Do [I} want to know the difference in circumference?: No, I want to know "the wheel factor ... number" that gets programmed into the computer that translates wheel revolutions into speed/distance, that's appropriate for 20" wheels. Maybe that's just the circumference, I don't know.
Or is it something else? YES. I want to try to avoid the measuring and math. Surely someone with 20" wheels can tell me what number they programmed into their computer ... and that should get me close.

But if not, I'll dig through Juan's and Moe's resources and get out my tape measure.

I roll the bike for one wheel revolution, measure that distance, enter into computer. I do not find it to be that difficult. Start with the valve stem at the lowest point, then roll for one revolution to get the valve to teh lowest point, not that difficult. It took me more time to write this post than to actually do it.

The Cateye Tire Size Calculator is a great tool. If you are looking for Wheel Size Diameter in mm and do not want to be bothered with entering data into the Cateye tool, I have used these successfully:

20 x 1.50 = 1490
20 x 1.75 = 1502
20 x 1.95 = 1575

These are approximations. Tires from different manufacturers will vary slightly and you may have to adjust accordingly.

Geezy Rider: THANKS!!!
 

Do you know how rare it is for people to actually answer the question asked? I'll try these. Then later when I have more time actually do the calculations.

You're not going to get accurate speed readings unless you actually roll the bike and measure the travel distance. And "factor" doesn't mean anything unless you know exactly what it is. It's not complicated math. It just wants to know your wheel diameter. It uses the magnet on your spoke to count each revolution. The measured travel distance formula is revolution x diameter x 3.14. the speed just takes the distance traveled and divides it by time.

Also the effective diameter decreases when you sit on the bike because the tire sinks under load and decreases the radius. So I sit on the bike and roll it 3 revolutions, take the distance traveled and divide by 3. My 2.1" wide tire is measured at an effective circumference of 1580mm at 3 bar pressure. If I don't sit on it, it's somewhere around 1590mm. Also if I decrease the pressure to 2.6 bar, I get something around 1550mm. This changes the speed and distance reading by up to 2%.

Probably not quite as rare as to be asked a question not so full of un-mentioned assumptions that it can be reliably answered. Since I live in Poland it helps to mention that I live in Poland otherwise I get answer that presumably work in Montana or Scotland. ;)