Aug 8, 2003 7:32 AM
|I know, "Use the roll-out method!" I've heard this said numerous times.
My question is, are you sitting on the bike when you do your roll-out? When sitting on you bike, the tires obviously compress to some small degree that varies with rider weight, air pressure, etc., thereby decreasing the diameter of your tire/wheel and hence decreasing the circumference. It seems to me that to be accurate, you would need to be sitting on the bike in your typical riding position with the front end weighted as it would be while on the road when you make your measurement. I know this is being picky, but do any of you do it this way?
I realize that the difference would be small, proabably somewhere around a 1-2%. Does this make sense?
I'm bored at work and analytically minded. Sorry.
|math is hard||mohair_chair|
Aug 8, 2003 7:48 AM
|The unit of measurement on most computers is a centimeter, so your measurement is going to be rounded to the nearest centimeter. Right off the bat you have lost accuracy. Worrying about a fractional increase or decrease isn't going to mean much unless it adds up to more than a centimeter over an entire revolution, and I doubt that it does.
If you want to be really anal about it, remember that as you ride, your tires will wear down, making the circumference of the wheel smaller, so you probably need to remeasure before every ride. In fact, you should probably take a measurement every 10 miles or so to be more precise, or sooner on particulary rough pavement.
|That is tiny...||biknben|
Aug 8, 2003 8:05 AM
|The tire is only compressed at the point of contact with the ground. The difference will be miniscule. I would guess a milimeter or less.
My computers accept measurements to the nearest milimeter. I have one (cateye cordless) that only allows 5 mm increments.
I doubt the difference between a weighted measurement and unweighted one will provide anything noticeable.
Just food for thought: Do you realize the circumference will reduce as the tire wears? Do you remeasure the circumference to maintain accuracy?
|I agree it is a pretty small differnece, but...||kermit|
Aug 8, 2003 9:28 AM
|if I do the math for by set-up there is a noticeable difference. Mind you, I'm riding a CX bike with 30c tires at about 85psi on the road. I will be springing for an extra set of road wheels and tires sometime in the future.
For my set-up, my tires easily compress 5mm. In essence, you're decreasing the radius by 5mm, therefore, the diameter is decreased by double that (or 10mm) and your circumference by 3.14x1cm. Comparing the results of weighted and unweighted roll-outs I get roughly 211cm vs 214cm. A pretty small difference (1.5%), but somewhat significant over 1000's of revolutions to us anal types.
When I get around to doing my first century I'll make sure I go an extra mile or two just in case my tires wear down significantly during the ride!
I suppose it only matters to us anal types, right MrDan! ;o) (no offense intended)
|No offense taken...||MrDan|
Aug 11, 2003 6:17 AM
|The differences can really add up if you're working off a cue sheet with lots 'o turns...
|definately do the roll-out...||MrDan|
Aug 8, 2003 8:15 AM
|with your weight on it and tires properly inflated. I'm 150lbs, and I'd estimate that the wheel compliance is more on the order of .1-.125 inches or 3mm give or take... I ride 700x25's about 90-95psi. This does add up, but I think the manufacturers canned calibrations (for my Sigma anyway) seem to take the "average" factors into account. Even though the tire wears, that would be about 1-1.5mm over the entire life of the tire - and fronts can go a long time... do the roll-out and do as many revolutions of the wheel as you can accurately measure and then divide to get the right number for the most accuracy.
|Any physics majors help here?||Spoiler|
Aug 8, 2003 11:31 AM
|I'd call in a physics dude on this one, but you may not be changing the diameter.
If you compress at the contact patch, the air volume has to go somewhere. That somewhere is distributed throughout the rest of the circumference, expanding, or buldging the tire elsewhere. Maybe it's expanding at the sidewalls instead of expanding at the tread, so the circumference is decreased. Or maybe, it expands elsewhere along the tread, so the circumference stays the same.
OR maybe the air molecules get smooshed closer together, decreased the circumference.
Tonight, I'll try a rollout with 60 psi, then I'll try a rollout with 110 psi.
Can I confuse the issue further? What happens if you try a rollout with the front tire at 60 psi and the rear tire at 110 psi?