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Stupid tire width question(9 posts)

Stupid tire width questionfiltersweep
Aug 31, 2002 8:34 PM
OK- if I am using tires that are 23mm wide and switch to 20s, does it also change the total radius (meaning the tire "height" also changes) ? My understanding is that it does, as evidenced by setting up computer callibration, but some guy I was speaking with (on a rather tangential philosophical issue) swears there is no difference... and I cannot find any info about this other than info regarding tire WIDTH.
Yes it does........Dave Hickey
Sep 1, 2002 4:25 AM
Look at your computer manual. My Specialized has different settings for 700 x 20,700 x 23, 700 x 25, etc...... It's not a big amount, but if you want to be accurate, you need to change your settings.
Exactly what I thought- nmfiltersweep
Sep 1, 2002 6:19 AM
Another problem is..DINOSAUR
Sep 1, 2002 7:31 AM
Manufacturers use different methods to measure their tires. A 700x23 with one brand might be equivalent to a 700x25 in another. The only way to get an accurate reading for your computer is to do a tire rollout.

Sheldon Brown has an article related to tire sizes on his website, but for the life of me I can't find it.
Another problem is..filtersweep
Sep 1, 2002 11:16 AM
I read Sheldon's article, but it never discussed tire depth- only width.

RE: different "methods" of measuring- manufacturers sometimes deliberately mislabel tires to make them appear lighter (ie. calling a smaller 21 or 22 a 23).
Another problem is..DINOSAUR
Sep 2, 2002 8:13 AM
I think the article I read (do you have the link?) talks about the size difference depending on the country of the manufacturer, (Japan , France, Germany etc).

I'm always switching off on brands of tires as I usually buy what is on sale. I use 700x23's and depending on the brand I notice that I get different readouts for the same distance. Not enough to make a big difference, but it could over a years time. Which makes me inclined to finally get of my lazy butt and do a tire roll-out and set my computer (been putting this off way too long)...
I found the link..DINOSAUR
Sep 2, 2002 9:24 AM
But the information I was seeking is not contained in the article, so now I'm wondering where I read it (?)...
re: Stupid tire width questionxcandrew
Sep 1, 2002 12:20 PM
The unsupported sections (beyond the rim) of tire casings have circular cross sections. An actual 23 mm tire would thus have a radius 1.5 mm greater than an actual 20 mm tire assuming the same tread depth.
Offer to place a tangential philosophical BET.Spoke Wrench
Sep 3, 2002 7:54 AM
Think of your tire as a tube running around your rim. The wider it is, the taller it is likely to be. That's not a perfect real world relationship, but it's close. Certainly in extreme cases, say, 700c X 23mm vs. 700c X 38mm, you can visually see a clear difference in height as well as width.