I remember the earlier tires, some with contradictory printed labels together with molded-in ISO sizing. For instance, older 27X1-1/4" tires sometimes had tiny raised lettering that read "26-630", which reflected their actual 26mm width. All of the popular, early "27X1" and "700X25" tires measured much closer to 20mm wide, and some even had the molded lettering "20-630" and "20-622", respectively.
Today's 20 and 23mm tires are all within a mm of actual width, but larger tires often deviate as much as several mm to the small side. Curiously, one Michelin Dynamic 32mm tire I bought was actually 2-3mm LARGER than stated.
Tube sizes should reflect actual width, but this varies. I measure tubes in the flattened state, which often reflects the stated size, and find that they can stretch safely into tires whose width is 25% larger than that measurement. A (flatted) tube that measures 25% larger than the tire's inflated width also can be fitted without creases since there is added volume inside the rim cavity, between the beads. An oversized tube may allow one to ride a punctured tube for miles before making the repair, since there is a good bit of a self-sealing effect when the tube wall is not in tension. An undersized tube loses air almost instantly by comparison.
David Snyder Auburn, CA usa
Mark Stonich wrote:
> "Back in the Day" people believed that smaller=faster, and manufacturers
> realized they could brag about the light weight of their tires by making
> them much smaller than their nominal sizes. Lately there seems to be a
> trend toward more honest marking of sizes, at least with the Panasonic and
> Michelins I've installed lately.
>
> I've seen older Conti 27 x 1-1/4" tires that measured just over 25mm on
> Super Champ Mod 58s (Hardly a narrow rim). New 27 x 1-1/4" Panaracers
> measured 31.5mm on the same rim.
>
> I had a customer say she had to have "At least 32mm" tires on a bike I was
> building up for her, when all I could fit, with fenders, were 28s. Then I
> saw her older bike with "700-32" Specialized Touring tires that measured
> under 25mm on Matrix rims.
>
> However, now that tire sizes are starting to reflect reality, the
> relationship between nominal and actual inner tube sizes has become such
> that if you install them in a similarly labeled tire they get stretched
> thinner than a condom when inflated. I've been buying 700c x 35-43mm
> tubes for 28-32mm tires.
>
> At 12/1/2009 03:14 PM -0800, David Boston wrote:
>>List-
>> I was rooting through old stuff and found some Specialized Turbo
>> Infinity/SK4
>> tires marked 700x25. I was going to toss them, but decided to do a
>> trial mounting
>> and see how they would do. Great, as it turns out, but I noticed the
>> actual size of
>> these 700x25 tires to be smaller than the current day 700x23 tires I
>> removed, which
>> were Conti Grand Prix 3000s. Both measurements, across and off the rim,
>> were
>> smaller. Why is this? Have the numbers come to represent something
>> different?
>> They could be mis-sized, but I recall back in the day during the
>> introduction of
>> skinnier clincher tires that 25c would be the narrowest I would want to
>> ride on the
>> road, and today I'm riding 23c tires with no consideration. Well, I
>> guess because
>> they're bigger. So, have tires come to reflect the overall cultural
>> obesity situation
>> or is there some other "inflationary" cause for this? Anyone else
>> notice this in older
>> tires? Was this a gradual change or did I miss a tire sizing
>> revolution somewhere?
>>
>>Dave Boston
>>Tucson AZ USA
>
> Mark Stonich;
> BikeSmith Design & Fabrication
> 5349 Elliot Ave S. Minneapolis, Minnesota 55417 USA
> Ph. (612) 824-2372 http://bikesmithdesign.com
> http://mnhpva.org