accurate computer reading?

how.u.doin7

Hi guys,

wondering if u can help. I have just bought a cateye computer and was wondering if anyone has an accurate measurement for the wheel distance in millimeters? I have measured myself but still think its a bit vague. My tire is 26 X 2.25 , I currently have reading on my computer at 2077mm. Just wondering what setting others have theres at ?

Cheers

xtremedash

Get it exact by putting a tape measure on the floor and roll the wheel along until 1 revolution has passed. Voila, your exact measurement for your bike. I used the valve as a reference point to get 1 revolution.

how.u.doin7

hi , many thanks for the reply,

yes i have done that already with the valve pointing to the floor, just wondering if there was a more accurate reading than this as its down to the mm and didnt think that when I just had the valve as a ref point was accurate enough?

xgeek

I would not worry that much about it. If you are 2mm out on a circumference of 2077mm that is just under 0.1% error (or 1 miles in a 1000 miles ridden). I bet my car is not as accurate as that.

xtreem

Plus, when you seat on your bike the tire is deforming and by that
you have smaller wheel circumference.

You can only set up computer corectly if you ride your bike on rims.

Electricpics

Xtreem wrote:

Plus, when you seat on your bike the tire is deforming and by that
you have smaller wheel circumference.

You can only set up computer corectly if you ride your bike on rims.

?? The tyre deforms locally but each revolution still has to travel over the full cirumference.

My Cateye computer has a list of decimal wheel sizes (47-559 etc) to select from so you don't need to measure the curcumference. It's accurate enough - just a couple of feet out on a measured mile.

supersonic

You can measure 10 revolutions outside, and divide by ten to be a little more accurate. Do it while seated on the bike.

how.u.doin7

Cheers for the reply guys that's great :-)

MarvintheAndroid

Quote : "?? The tyre deforms locally but each revolution still has to travel over the full cirumference."

No no no no no - the result of tyre deformation is that the portion of tyre in contact with the road is flat not curved, so at that point the "circumference" is the distance along a straight line not along an arc. Rider weight and tyre pressure will vary the amount of deformation.

Here's the method I use.

Apply some paint to one of the tyre knobbles near the valve - a car touch-up brush is ideal. Then, while it is still wet, sit on the bike as normal and gently roll forward in a straight line along a garage floor or similar so it leaves a paint mark on the floor. After one complete revolution you should get a second paint mark (you can have a helper refresh the paint halfway along if necessary).

Use a tape measure to check the distance between the leading edge of the first mark and the leading edge of the second. This is your effective circumference and it should be accurate to around 1mm.

If you have room, you can do 2 or more revolutions and divide the result for greater accuracy.

If you need convincing, do it again without any weight on the bike and see what happens !!!

Marv