garethzbarker

My bike computer doesn't have my bike friday wheel on its preset list but I can manually set the mm. I thought I had it right but a side by side with gps shows I'm off. anyone know what the computer setting for a 451 wheel 1-1/8 tire is?
thanks

venturi95

Get on a long smooth level stretch of pavement, put the velve stem straight down, mark that spot, roll the wheel perfectly straight with a little load on the front tire for 3 revolutions or so, measure distance (in millimeters) of 3 (or what ever number of) revolutions. Divide by 3 (or # of revolutions) to find the number you need to put in your computer. I have only seen computers use millimetres for "roll out" entry, which will always be more accurate than a generic one size fits all number in a manual. A helper will make this task much easier.

Boudicca

Or take the lazy way out, and let Sheldon do the work for you.

https://sheldonbrown.com/cyclecomputer-print.html

garethzbarker

Unfortunately, there seems to be a difference between the 1-1/4 and 1-1/8 enough to make a difference noticeable on a 5km ride because it didn't work when I tried his number. Probably due to the fact that a smaller wheel needs to be a little more accurate and possible because it will dip a bit more under a load. Looks like I have to measure.

fietsbob

Certainly a discrepancy in circumference as a error, would add up more
in a small wheel that rotates more times, than a bigger wheel, to cover a Km

Dynocoaster 

If the tire is a 1 1/8 is it a 451?

garethzbarker

I'm stupid... why do I keep mixing those up. Yep it's a 451. And the setting on the cateye page lists it at 1545 in case anyone comes here looking. I'll test it out vs gps and report back.

rkokish

I have never been completely accurate using the roll-out method. GPS is not completely accurate and tires of the same size vary a bit by brand and model. I like to get close using a chart, then find a road or trail with mile markers and see how far off I am after 10 miles, adjust accordingly and repeat. One mile doesn't do it because that marker may also be off. Go 10 miles if you can. Once you have it set to your satisfaction, write down the number or you'll have to do it all over again when you replace the battery.

az2008

I like the idea of doing multiples to reduce the margin of error.

What I did in the past is dab some water on the tire and roll it one revolution. Measure the distance between the wet marks on the concrete.