I have a new bike computer and I can’t calibrate the computer. I have 650 wheels but the book doesn’t list the sizes that way. Is there another way to measure 650 wheels?
Same way you’d measure any wheel - with a rollout.
Start with the stem at the lowest point and mark the ground. Walk the bike (or wheel - but its easier with the whole bike) forward (or backwards) one revolution in a straight line. Mark the ground again. Measure the distance in mm, and that’s the number your computer is looking for.
Note that this measurement is affected by tyre selection and inflation - so make sure you air up first.
the bead diameter of a 650c wheel is 571mm. the diameter of the wheel, with a 20mm tire (compressed tire height) is:
diam = pi*(571+(2*20))
or 1920 mm.
Don’t use the calculation. it will be close, but a few mm’s off is a HUGE problem after a few hundred thousand revolutions.
Do the rollout test as described above. It only takes a few minutes.
I can never do the rollout test that precisely (within a mm while seated on the bike?)
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the bead diameter of a 650c wheel is 571mm. the diameter of the wheel, with a 20mm tire (compressed tire height) is:
diam = pi*(571+(2*20))
or 1920 mm.
Nice in theory. Not accurate in practice. Not all 20mm tires will roll out the same … not even when they’re the same brand/model. I roll out every new tire I mount on the sensor wheel of my bike. I repeat the roll-out three times and use the average figure. The variability can be several mms and enough to make a tenth or two difference in distance/average mph over a decent-length ride.
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You would use the actual measurement (compressed) of the tire to ground. It really does work.
I can never do the rollout test that precisely (within a mm while seated on the bike?)
Just lean as much weight as you can on the saddle as you walk beside the bike. That’s a sufficient approximation.
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Unless you are measuring for a certified course, does a tenth or two over a long ride matter?
Maybe I am just lazy, but I have never bothered to actually do a roll out test in my near 20 years of using cyclocomputers. Heck, the early Avocets only reported speed to the nearest 0.5 mph anyway.
Unless you are measuring for a certified course, does a tenth or two over a long ride matter?
In distance, not really. In average speed, heck yes!
Depends what you’re trying to get out of your rides, I suppose. I’m doing a lot of time trial training. I’m not typically going to lop 1/2 mph off my average speed in any one ride or two. Improvements are going to be in small increments. On training rides, I test myself all the time over intervals of various distances. I like to know if I did a tenth or two mph faster one day vs. a couple of days before. It’s not the only way I test, but it’s one of them.
As an aside, I love it when some clown boasts to me about how fast they did some ride, then I come to find out they don’t know how to calibrate a computer.
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