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Basic Road Tire Question(10 posts)

Basic Road Tire Questiondweiss
Aug 2, 2001 7:01 AM
What is the difference between a 700 x 23 and a 700 x 20 tire. Is the second dimension the width? Will they both fit on the same rim? And finally, why would you want one versus the other?

Thanks
Yup...and...MrCelloBoy
Aug 2, 2001 7:17 AM
the manufacturers measurements are ALL OVER THE BOARD as far as accuracy. Depends on the particular rim which width tires will fit which rims, and there's a bit of safety factor figured in.
See Sheldon Browns website at Harriscyclery.com regarding both issues.
Generally narrower tires for racing and wider for touring. Tandem teams rarely want to go below 28mm for anything but all-out racing.
Most folks I know ride 23mm.
wider is bettermr_spin
Aug 2, 2001 7:22 AM
Or so the saying goes. It's all a matter of opinion.

The thinner you go, the less rubber you need, so you give up a comfortable ride. But you go faster.

The wider you go, the more speed you give up because you need more rubber. That means greater rolling resistance and higher weight. Better handling, too, because the contact patch (the tire surface meeting the road) is bigger.

It's a tradeoff, but for most people, size 23 seems to be the size that gives the most comfort and the lowest effect on speed.

If you race, you'll probably want to race on size 20, because comfort is less important than speed. The exception is Paris-Roubaix, where you'll want at least size 25, and probably wider, to survive the pave!
Believe it or not, but..Steeve
Aug 2, 2001 7:52 AM
all other things being equal, a wider tire (such as 23mm vs 20mm) has LESS rolling resistance.
The reasoning is because of the shape of the tire's contact patch against the road.
A narrower tire will have a narrower but longer contact patch. The longer this contact patch, the more tire deformation that takes place, the more rolling resistance.
A wider tire will have a wider but shorter contact patch. Shorter patch=less deformation=less resistance.

Conversely however, a narrower tire has proportionately less aerodymanic drag that a wider one.

I presently I have 20mm front and 23mm rear.
wrong-oak
Aug 2, 2001 8:06 AM
I read the same article (or saw the same program, can't remember) that you did, but the problem with it is: "all things being equal..." All things are not equal, one tire a larger contact patch. "all things being equal" says that the contact patches have the same area, but one is wide and one is long. if this is true, then sure the wider one produces less heat & therefore less resistance. If you have two michelin axial pro tires a 700x21 and a 700x23, and they are both inflated to 125psi, the contact patch on the 700x23 is just as long as the patch on the 700x21, but the 700x23 is wider and therefore has a wider contact patch. And NO, it doesn't matter that both of them have the same weight rider on them. The article or program we saw was very misleading in this respect and I don't blame anyone for missing the catch. But a wider tire still has more rolling resistance than a skinney one and it is less aerodynamic. wider is slower.
Question...Whatever
Aug 2, 2001 9:37 AM
I think the article was in Bicycling Magazine a few months back...

IN THEORY (so I have qualified my starting point), wouldn't the contact patches be the same (square inches of rubber on the road)? I weigh 220 lbs, the tires are pumped up to 110 lbs/sq. in. Don't I need 1 sq. in. of rubber on the front and 1 on the back to hold me up? If I deflate the tires some, the contact patch has to be smaller....? Pump them harder, the patch is smaller...?

If so, then both tires (at the same pressure) have to put the same amount of rubber on the road...that is where the larger tire gets it width-wise, where as the narrow tire has a longer patch.

Comments, corrections? Any Michelin or Conti engineers out there?
Question...Cliff Oates
Aug 2, 2001 10:09 AM
I believe the source in question is an analysis that Jobst Brandt did for Avocet quite a while ago (10 years?). Here's a link to the rec.bicycles FAQ article on rolling resistance: http://draco.acs.uci.edu/rbfaq/FAQ/78.html
yup, good answser, you are correctclub
Aug 2, 2001 11:03 AM
also do a search for "hysteresis" for more on why fatter tires roll easier than skinny ones. On a perfectly glass smooth velodrome, skinny wins. Everywhere else, fat rules.
Yep, they'll fit on the same rim!jtolleson
Aug 2, 2001 7:27 AM
the second number:Rusty McNasty
Aug 2, 2001 7:45 AM
That second number is a profile height (the distance from the bead to the tread), which is CLOSE, but not the same, as the width. That can vary a lot, depending on construction.(FWIW, the second number on a tubular IS a width measurement, and is usually quite accurate.) If the tire says 700x20, you can usually count on the width actually being about 18mm.