Wheel size and bike computer

[makbike]

I'm curious as to why one should not use the wheel size charts to program their computers? Are the charts not accurate?

[BleeckerSt_Girl]

Liza, I have heard that car odometers are pretty accurate, with the odd exception. When I did my calibrating, we had 3 cars at the time (now we have 2), and they ALL read exactly the same mileage for a 10 mile trip, and this was also confirmed by an online map-it program I mapped it out with, so I felt all that was a pretty good thing to rely on. Since then I have ridden 60 and 70 mile bike routes and then mapped them out online and my bike computer confirms exactly what the online program says the trip was. Good enough for me at this point!
Makbike- I found the charts were way off. Others I know have said the same thing to me. It's best to use the charts to get in the ballpark, then adjust it in real life.
But of course people do it the way they want to.

[TsPoet]

I'm curious as to why one should not use the wheel size charts to program their computers? Are the charts not accurate?

Because they will depend on the size of the tire, some are deaper than others. Also on the wieght of the rider and the amount of air in the tire.
I have a Giro 20, I always do the roll out method - sit on the bike, line up the valve with a crack in the side walk and roll 1-3 circles, mark the spot and measure it. Then convert to cm from inches if, like me, you only have a tape measure that does inches, and divide by 3.
That said, though, my memory is that I have mine set at 1923, but I have a 26" Conti GP 3000 on my rear wheel. If you've got the Kenda's that come stock, it might be a little bigger.
The fact that it's a Giro 20 won't mean too much to most on this forum, what's important is that it has a 26" wheel (vs 650), what is that, a 571? For more info than you ever wanted on the topic, check out Sheldon Brown.
http://www.sheldonbrown.com/tire_sizing.html

[indysteel]

How timely. I noticed on Saturday that my bike computer is overstating my mileage and, by extension, my speed. I'm going to have to measure my wheels to get a more accurate reading. Grrrr.

[BleeckerSt_Girl]

How timely. I noticed on Saturday that my bike computer is overstating my mileage and, by extension, my speed. I'm going to have to measure my wheels to get a more accurate reading. Grrrr.

If you feel your car odometer is accurate and you have a bit of patience, then you don't really have to actually measure your wheels.
Just drive a favorite regular bike route you do regularly, and mark what your car odometer reads for it. Then ride it on your bike. If the bike odometer reads higher than what the car said it was, then start resetting your tire circumference setting slightly lower by one or two digits at a time. Likewise, if your bike computer reading is lower than your car reading, adjust your computer setting for a slightly bigger wheel circumf. Make these adjustments a little at a time and then keep checking them whenever you do your regular ride. Re-ride and reset until your bike computer matches your car reading.

[MomOnBike]

We use the roll-out method, and accept that the figure is always going to be slightly off. DH's computer always seems to read faster/farther than mine does, but {shrug} maybe that just means he weaves alot.

An alternative to driving a route with your car is to use highway mile markers - if they are on a route you do. Again, one mile probably tell you much, but over 2-3 or so miles, you'll get an idea of the accuracy of your number.

[stacie]

try mapmyrun.com You can even calculate zigzagging across the street if you'd like. I've always just done as LisaSH says and trusted it was close enough. s

[li10up]

Now I know how to increase the speed and distance of my rides without all that pesky training! Just reset my wheel circumference!!!