NaBlade

So I got me a 2nd bike computer, a double-wireless Sigma for speed and cadence. Took it out for the first time this afternoon on the shortest loop I regularly do. Now my older bike computer (it's about 9 years old), Google Map Pedometer and TopoRoute all agree that this loop is about 15.7 miles. My new Sigma displayed 16.5 miles when I got home... *scratches head*

According to Sigma, there's 3 ways to measure wheel size: (1) Use Sigma's given value for my bike's 700cx23 tires, or (2) measure the diameter and multiply by 3.1416, or (3) measure the actual wheel's rollout on the floor. I used method #1 today, but I think I'll use method #3 tomorrow, which may be more accurate. My old computer on my 1st bike is too old, I've forgotten what I used; and it won't tell me what the current value is, I can only reset it to a new value.

Which method did you guys use on your computers? And do you think your getting accurate readings?

LIUser22

i use a cateye astrale 8 and it's accurate with gmaps. I also used the proconfig for 700x23. I would use method 3 as well if i were you, or borrow a garmin and measure it and compare.

toucci

I always suck it up and do the roll out. It's amazing how much variance there is among tires.

Yoshi

Well keep in mind that gmap and top route are all going to use straight lines, while your bike is not. That could definitely account for the 5% difference.

h2o_polo_boi

https://sheldonbrown.com/cyclecompute...n.html#rollout

i have the same computer as you. Sounds like the Sigma 1606L DTS. I used method #3 and it is almost dead-on with my Garmin GPS (doesn't deviate more than 1/4 mile in 50 or so miles). I also verified it at my school track for 1.0 miles. Pump your front wheel up to the correct pressure. Sit on your bike, roll in a straight line, and measure one full rotation in millimeters and enter it into the computer. you're odo will now be more accurate than a car's. also write the number down in case something happens to your computer.

Radiorick

Using a Continental Ultrasport on my front wheel, my roll out was 212cm.

Rick

cccorlew

I did roll out. I was careful to roll straight. I used a seam on the driveway to make sure. I made fine pen marks on my wheel and drew on teh driveway. I wasn't on the bike, which would have made it better. But it agrees with my other computers and GPS.

One BFer had a cool idea. Put toothpaste on the ground, ride over it at speend, carefully going straight. Measure. If you get 3 marls, measue the total and divide by 3.

NaBlade

Thanks guys. I'll measure rollout tomorrow, and do it while on the bike. The toothpaste idea is pretty good! I can just see my wife roll her eyes...