Bicycle Mechanics - cyclocomputer

Ok,
I bought a cateye computer today, easy installation. However, the manual doesn't specify how far from the hub or the outside diameter of the wheel the sensor needs to be mounted. I could only assume this would make a difference in accuracy. Am I wrong. Help me please!
-j

The closer to the hub the better. Will pass slower and allow the computer to read more accurately. Check out Sheldonbrown.com for a very extensive article on installing a computer.

The magnet passes the sensor ONCE per wheel revolution, no matter how close to the hub or rim.
Put your effort into finding a location where the magnet passes close to the sensor. The taper of the fork and angle of the spokes is what limits you to location.

It will pass once, but not as fast.

I've got another question... is it possible for my front wheel to flex while sprinting, causing the magnet to hit the sensor? When riding in the saddle, it's fine, but when I get up to sprint, I hear a clicking once every revolution. And I'm HOPING that it's just the magnet hitting the sensor.

I've got another question... is it possible for my front wheel to flex while sprinting, causing the magnet to hit the sensor? When riding in the saddle, it's fine, but when I get up to sprint, I hear a clicking once every revolution. And I'm HOPING that it's just the magnet hitting the sensor.
Yes, it is possible if the clearance between the magnet and the pickup is very close and, particularly, if the magnet and pickup are mounted out near the rim of the wheel.

The usual clearance recommendation is about 1/8" (3 mm) and I usually mount the magnet 4" -5" out from the hub center or right outside the second spoke crossing on a 3X laced wheel.

Anybody know what's the general rule of leeway the companies typically build into these units for calibration? I have a Vetta RT255L with the sensor mounted about 3 inches from the hub. When I used my first roll-out measurement I was getting high numbers. I went back and measured the roll-out at the sensor (diff of about 30mm) and the numbers seem closer to reality - though it's hard to know without an MIT grad and state trooper handy.

I've got another question... is it possible for my front wheel to flex while sprinting, causing the magnet to hit the sensor? When riding in the saddle, it's fine, but when I get up to sprint, I hear a clicking once every revolution. And I'm HOPING that it's just the magnet hitting the sensor.

This is easy to rule out. Take the magnet off and get out of the saddle.

Anybody know what's the general rule of leeway the companies typically build into these units for calibration? I have a Vetta RT255L with the sensor mounted about 3 inches from the hub. When I used my first roll-out measurement I was getting high numbers. I went back and measured the roll-out at the sensor (diff of about 30mm) and the numbers seem closer to reality - though it's hard to know without an MIT grad and state trooper handy.

Not really understanding this. The magnet passes the sensor once per revolution no matter where it's located, and your rollout is based on the tire's circumference - which doesn't change. What are you measuring?

Not really understanding this. The magnet passes the sensor once per revolution no matter where it's located, and your rollout is based on the tire's circumference - which doesn't change. What are you measuring?

Instead of measuring using the valve stem as the start/end reference, I used the magnet the second time. This resulted in a difference of 30mm. Using the first measurement, the computer reported that a park trail that's marked as about .5 a mile as over a mile. Using the second measurement it was about .6 or .7 - still not perfect, but closer.

Use the tire size chart that is in your manual!

Hard to do with a tire size not listed in the manual...

Hard to do with a tire size not listed in the manual...

You don't need a manual. It's the same number for everyone.

http://www.sheldonbrown.com/cyclecomputer-calibration.html

Instead of measuring using the valve stem as the start/end reference, I used the magnet the second time. This resulted in a difference of 30mm. Using the first measurement, the computer reported that a park trail that's marked as about .5 a mile as over a mile. Using the second measurement it was about .6 or .7 - still not perfect, but closer.

It sounds like you have it set on KM, not MI. Tell us what size tires you're using and we'll tell you the circumference number to plug in.

Hard to do with a tire size not listed in the manual...
Interpolate