Cat-eye computer ?
How accurate are these computers? I am setting mine by the tire circumference but am leary as to how accurate it will be.
Extremely accurate, if you've calibrated them correctly!
Mine is off by less than 0.4% - I've measured. That's less than 2000 ft off after a century (100 miles).
Brand of computer shouldn't matter, at least not if they're simple enough in their design. The more complicated they get, the more things that can go wrong...
One thing to think about, though, is how precisely you are able to set the wheel size. Some models only allow you to set a certain, pre-set, wheel size, such as maybe "25-622" or "47-559". These models would be slightly less accurate, unless you happen to pick a perfect "fit" for your particular wheels. But they'd still be accurate to within maybe 1-2%, and that's not too bad.
It helps to measure the wheel circumference while you are actually on the bike, as different tire/rider combinations can create different results. This is only, of course, if you want to be super-accurate.
There is only 1 setting for each tire circumference in the directions. If you use their settings and they are off then you are screwed.
How do you do the roll out method to calibrate these computers
OK. As compulsivebikr says, this is if you want to be super-accurate.
Originally Posted by babysaph
You want to get the exact rolling circumference of your front tire, and enter the number on your computer. Each computer has a different means of entering the figure.
Find a nice smooth surface (like a garage floor or the sidewalk). It helps to draw a string along the surface so you can roll the bike straight, otherwise a wobbly path will give you an inaccurate reading. Just stretch it out and tape it down at both ends. You need about 10 feet of string for a single revolution of the wheel -- but you'll get the super-accurate results if you roll out two revolutions, so use 20 feet of string.
Make sure your front tire is pumped up to normal pressure (what you'd use on tour).
Put a piece of white tape on the outside of the tire at some point (say, at the valve stem). The piece of tape should touch the pavement but be visible as you straddle the bike. Put a vertical mark on the tape so you can precisely identify the spot where it touches the pavement. At that spot on the pavement (at one end of the string), put a mark. You want to hit it exactly where the tire-mark is vertical; otherwise it can be off a few mm and this will add up over a ride.
Get on the saddle. You'll want to "crab" down the pavement, along the string, with all your weight on the bike (this compresses the tire, which you need to do if you want to get an accurate measurement of rolling circumference). Go two revolutions (for better accuracy), and stop when the tire-mark is exactly vertical again. Mark this location.
Measure from start to end; divide by two if you've done two revolutions.
If you're super-anal-accurate, you could do this 3 or 4 times. Believe me, you'll get different results each time. Average 'em. Done.
Edit: Oh, one more thing! Write the number down on a small piece of tape, with indelible ink, and stick it to the back of your cycle computer. At some point on your tour, the batteries will die or the buttons will get jabbed and you'll have to enter it again. I can never remember the circumference setting more than a week or so....
Last edited by EmmCeeBee; 03-21-06 at 10:17 PM.
I set mine by measuring the radius (from hub centre to floor/ground), and subtracted an appropriate amount to compensate for the loaded condition (i.e. me being on the bike).
But there's a much easier way to do it, if your computer allows you to enter your own measurement.
First, set the wheel size as you normally would.
Ride a known distance, and check what the computer says (make this a relatively long distance - maybe 40-50 miles or so).
Let's say you set the wheel circumference to 2105 mm the first time. Your ride happens to be exactly 47.13 miles (according to some digital map/route planning system), and your computer shows 47.92 miles.
Now, divide 47.13 by 47.92, and you get 0.984. Next, multiply 2105 by 0.984, and you get 2071 mm. Enter this new circumference into the computer.
This is, of course, just another form of rollout test, but a much more accurate one, and a more FUN one! You get to ride too!