Dave Lehnen <

[email protected]> wrote in message news:<m

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> Cletus Lee wrote:

> > In article <d5BUb.11726$jH6.10872@newsread1.news.atl.earthlink.net>,

[email protected]
> > says...

> >

> >> Do the roll out test. Mark the tire, go in a straight line for a complete mark to mark tire

> >> revolution, multiply by 25.4, and enter it as the constant into the computer. That does it for

> >> most.

> >

> >

> > Or you could just measure in Millimeters and skip the math.

> >

> >

> >> "gcdoss" <

[email protected]> wrote in message

news:[email protected]...

> >>

> >>> My front wheel is a 37 x 406 and I can find no wheel that size in the Sigma Sport BC 600 chart

> >>> or in the Cateye Astrale 8 chart. It seems to run long by about 1/10th mile on speedometer

> >>> when checked with GPS or the mile markers on the trail.Right now it's at ws 990. I have tried

> >>> several setting and can't seem to get it right. A lot of you have that size tire and some may

> >>> use the Sigma computer as well. What WS setting do you use? By the end of the year if I get to

> >>> ride as much as I hope to, it could be off by 500 miles. Thats like a trip from Prescott to

> >>> Bakersfield and not acceptable.

> >

> >

> > I am a little confused about the wheel size input for a Sigma Sport. The nominal wheel

> > circumference for a 37-406 would be ~1507mm. Most bike computers calibrate to a wheel

> > circumference in millimeters. If this is true with the Sigma Sport, you should be using a number

> > closer to 1507mm

> >

> This series of Sigma computer is unusual in that if you select miles rather than kilometers for

> distance, the wheel circumference must be adjusted by the miles/kilometer ratio, or about .62137.

> Most brands do the conversion math and you still enter the circumference in mm or cm, but not

> Sigma. If the 1507 number is good, 936 would be the right number for a Sigma, at least for the

> BC600 (not sure about the newer models).

>

> > As suggested, the roll-out measurement is more accurate than any chart in an instruction

> > booklet. I would suggest several revolutions and take an average for a better approximation of

> > wheel circumference.

> >

> > FWIW, Most GPS I have found to be off(short) by 1%-2% in any non linear route..

> >

> > I have records on several 406 tires. Perhaps if you let us know the tire make and model, someone

> > on the list can provide the wheel circumference that they use for your tire. Who knows, you

> > might even get a consensus.

> >

> Roll-out is better than someone else's number. Your tires should be at you usual pressure, you

> should have your full weight on the bike, and measure for more than one revolution for best

> accuracy. While a GPS might give slight corner-cutting error on very twisty routes, they should be

> very accurate on fairly straight routes. Most charts I've seen with cyclometers list too-high

> circumferences, making distance and speed read too high.

>

> Dave Lehnen

Thanks, Having the help of 2 friends , we did the roll out thing again and came up with same number,

1492.25 mm. Using Dave Lehnen's formula I end up with 927.24 mph. This sure sounds good and I will

test it out on my next ride. We have a measured straight stretch that's 2 miles long so it should be

a good test. I'm glad someone knew the Sigma Sport didn't do the conversion internaly as I thought I

had it set.

Jerry