Wheel size and bike computer

[makbike]

I have a Bacchetta Giro 20. I have the computer sensor mounted on the rear wheel so that I can track mileage when I use my trainer in the winter. I've looked at the tire and it is stamped 26 x 1.0. If I use this information I'm suppose to program the wheel value as 1973 mm. However, when I measure the wheel according to the directions which came with the computer (Sigma 1606L) I get 24 1/8". If I use this meaurement I should according to the chart program the computer with a value of 1924 mm. Which one is correct? I'm so confused!

Thanks!

[BikeDutchess]

Ideally you wouldn't use the charts at all but measure the actual circumference of the tire. Inflate the tire to your ideal riding pressure. Place on your driveway or sidewalk (wherever it's a level surface) with the valve at the bottom of the wheel, closest to the sidewalk surface. Mark the sidewalk (with chalk or something) at the valve, move bike in straight line until the valve is down again, mark the endpoint, and measure the distance between the 2 marks.

[BleeckerSt_Girl]

Ideally you wouldn't use the charts at all but measure the actual circumference of the tire. Inflate the tire to your ideal riding pressure. Place on your driveway or sidewalk (wherever it's a level surface) with the valve at the bottom of the wheel, closest to the sidewalk surface. Mark the sidewalk (with chalk or something) at the valve, move bike in straight line until the valve is down again, mark the endpoint, and measure the distance between the 2 marks.

You should be sitting on your bike when you do this (it makes the wheel circ. smaller and more accurate) and at a slight downhill. Get someone to help you and hold the bars and count the valve turns while you roll. Record at least 4 or 5 rotations, more if possible, not just one. Then carefully measure the total distance with a metal tape measure and divide by the # of rotations you did. That will get you in the ballpark, but it won't be so accurate for a 40 mile ride or more. It's not "real" riding conditions.

After you set it, you can fine tune your accuracy by driving your car on a favorite bike route say 10 miles or so round trip from home and record the odometer reading. Then ride your bike the same route. If your bike says 10.7 miles and your car said 10, then set your bike computer wheel circumference a tiny bit smaller. If the bike computer says 9.5 miles but your car said 10, then set your bike wheel circum. a little bit larger and try it again. Best to use a favorite bike route that you ride often anyway to use as a fine tuning test. You only have to drive it once with your car. Keep resetting the wheel size until they are in sync.

[VeloVT]

Hmmm... I'm **definitely** no expert on this, but I've read that bike computers are actually more accurate than car odometers, so I might be a little leery of using the car to "calibrate" the bike computer. Confirm, maybe...

[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.