Road Cycling - Validating Bike computer settings with GPS

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DogBoy
07-05-04, 08:22 PM
Has anyone else done this, and does it sound like my result is screwy?

My cateye astrale 8 suggested that I set my computer at 2136 for my 700X28 tires. I noticed though, that the readout seemed to suggest I was going much faster than I really was. To test this, I took my Garmin GPSmap76S off my boat, and put it in my jersey for a quick ride (~7 miles). I converted the distance indicated by my bike computer (7.4 miles) to mm (1,609,344mm/mile) and divided by my bike setting 2,136 to compute the number of revolutions: 5,575

I then converted the distance indicated by the GPS (7.31 miles) to mm and divided by the number of revolutions to get my new bike setting: 2110.

So, aside from knowing I could have just done the tire-mark on the ground thing, does it seem strange that the mfg suggestion would be that far off?

Trek Rider
07-05-04, 08:30 PM
No tire manufacturer uses the exact diameter tire across the board. There are variations between manufacturers, model tires, and probably even from tire molds. The figure you were given to use is a guesstimate of wha your diameter actually is. The roll-out method is more accurate, or you could always go on a 50-60 mile ride and use a mapping program to get your distance and calculate what figure you should use (if you're really anal retentive about it).
CarlJStoneham
07-05-04, 10:55 PM
The manufacturer just uses 2*pi*r (or pi*diameter). They base their estimate on the typical radius of a wheel at a given size. Not very accurate. All you have to do is sit on the bike and the tire deforms slightly, resulting in a smaller circumference, hence the smaller number. As you ride for a week, you'll actually get smaller as you inevitably lose some air. Pump it back up, larger circumference. As it is, the maufacturer wasn't that far off. Only 1%. In fact, your GPS is likely not accurate uless you have the newer kind. Mapping programs aren't entirely reliable either (as soon as you get on local roads, you get in trouble :) ) To be perfectly honest, I'd suggest taking the average of the two. You'll never get a truly accurate mileage unless you change your cyclo every ride. Personally, use the higher number so you think you've gone farther, faster :D