Question on Wheel Circumference & Bike Computer

[lette]

Bought the Cateye Astrale 8 for my road bike and love it. Bought a second mount for my hybrid. Thinking that since both bikes had 700 tires, I woundn't have to adjust the computer. Went for my usual morning ride, and found the mileage shorter than usual.

Computer adjusted for road bike 700x23 tires @ 110psi (but riding hybrid) = 29.67 miles
Computer adjusted correctly for hybrid 700x38 tires @ 90 psi = 30.84 miles

How can psi (or does it) affect distance when wheel circumference is exactly the same. I'm assuming that the tire width does not affect it???

mystified, Lette

[DebW]

Tire width does affect the circumference. It's secondary to diameter, but significant at the 5% level. Think of the tire cross-section being a circle, so if the tire width is larger, the distance from rim to road must be larger too.

[BleeckerSt_Girl]

Yes, tire width affects the wheel diameter significantly. Your bike will be higher off the ground with a 700x32c tire than with a 700x28c tire. That means a bigger circumference, naturally.
Also psi pressure affects it. Two tires the same exact size but inflated to different levels will have diff. circumf. as well. You will notice it when you sit on the bike. And you'll be sitting on your bike while the computer is calculating your ride BASED on your wheel circumference.
Also, when you measure your bike's wheel circumference in order to tell your computer, you should be ON your bike. Measure the circumf. of the wheel that you are going to attach the sensor to (front wheel?). Get someone to hold you at the handlebars to keep you from falling, and roll slowly along straight on a very slight decline if possible. Try to measure like 6 whole rotations (start at the valve and mark the road, of course), so you can then divide by 6 and get a more accurate measurement. Any little difference adds up bigtime over the miles.
(Notice how you'll get a different number if you are not on the bike when you measure circumference.)

[lette]

Boy you girls are smart!
Thanks, Lette