Road Cycling - Ok i'm resetting my computer and need help with the roll out.

Ok the directions say look on the tire and it is a 700X23C

OK so I look at the conversion and it's 2096 no prob


but I mesure the roll out and the roll out is 2146. WTF. what do I use for the setting?

it's a cateye astrale. what do I use? I'm gonna go for the 2096 setting for now but that dont make any sense if the actual roll out is diff is the setting for the roll out with the rim with out the tire or does it take into account the deflection in the tire with weight?


O well Help.

A rollout, done properly, is more accurate than the chart. The chart cannot account for differences in tires, inflation, etc. Repeat the rollout several times to make sure it is accurate.

BTW...I always just use the chart. It is close enought to suit me.

Definitely go with the results of the roll out. But,

1. do it several times to be sure;

2. make sure your tires are inflated at the pressure you usually ride with;

3. make sure your weight is distributed on the bike just like in your typical riding position;

4. if you know a local marathon course where they still have lines painted on the street, you could verify your setting and correct as needed. Naturally, any other precisely measured distance will work, and the longer the distance, the more accurate your test will be. But it needs to be precise. A car odometer is not precise enough.

Cheers,
Jamie

What Jamie said.

Not every tire of the same nominal size is the same actual size. A properly done rollout, that simulates actual riding, with several results averaged, will always be the most accurate.

The tables in the manuals can only be approximations. I have two bikes with 700x32c tires with roll-out measurements that are very different from each other (Conti's and Avocets), and both are different from the tables in the manuals -- which are in turn different from each other!

RichC

My wife and I both have cateye mity 3's.
set up both, doing roll out etc.
2 weeks later we ride the same ride and I
clock .4 miles further.
go figure.

Marty

I use the 2096.
I've compared the actual distance with what the car odometer read (15 mile trip). Same distance.

What exactly is rollout? I am trying to help my son select gears for his BMX bike and am using the gear charts which calculate rollout, but do not explain what it is??? I assume it has something to do with revolutions of gear to crank. Thanks!

The rollout they are referring to here is, I believe, the distance covered by 1 revolution of the wheel.
Or, more simply, the circumference.

The rollout I am refering to is from these charts
http://www.nzbmx.homestead.com/files/bmx_gear_calculator_online.htm

Which my sons current rollout calculated to 55.125 (Something)???

I mark my tire with some of my son's chalk and then ride down the asphalt driveway. Then I measure the marks on the driveway. I assume that I can't get any better than that.

Originally posted by SKYWLKR
but I mesure the roll out and the roll out is 2146. WTF. what do I use for the setting?
What are you confused about?
The fact that the roll-out doesn't match the calculated value? Don't worry about it. They never do. If you went to the trouble to do a roll-out, use those numbers.
Dealing with deflection? Two options: sit on the bike while somebody pushes you to do the roll-out, or estimate it. Both are pretty sketchy... if you're on the bike while someone pushes you, your weight distribution is going to be different than when riding. And they probably won't push you in a very straight line. To estimate, I determine the radius from an unweighted roll-out (2146/(2*pi)~=341.5 in your case), subtract my best guess as to the deflection from the radius, e.g. 2mm, re-calculate the circumference (2*pi*339.5=2133), and enter that in your computer.

definitely go with the roll out.

it will differ depending on tread choice, inflation, etc.

everyone in the bike clubs does it this way.

Land2Sea:
Here, the circumference of the tire is being discussed. The calculator you linked to calculates gear inches which are(Chainring Teeth/Sprocket Teeth)*Wheel Diameter(in inches). It's a common, but not entirely accurate method to determine how "big" a gear is. The number you end up with is the diameter of the back wheel that would have the same rollout (how far the wheel moves with one complete revoulution of the cranks) if you use gears with a 1:1 ratio. I hope that's clear enough. If not, Sheldon Brown has an article about it here (http://www.sheldonbrown.com/gain.html)

mark the ground with the valve stem at the ground with a sharp bit of chalk, move the bike forward untill the the valve is @ the ground again & mark again, measure & thats the number you use . the measurements in the charts are an average as a guide for people who ar,nt that fussy.i think diff. makes &types of tyres would vary in their circ.