The Street > The Bike Shop
Yakob/Merged article on tyre size
G:
Surprised nobody has made a thread about this yet; shows how dead this place is still I guess. I dont really do Facebook so CBA to comment on the article directly so making this thread instead.
http://themerged.com/the-inaccuracy-of-tyre-labelling-the-birth-of-the-21-wheel/
I was really pleased to see this on the Merged, it is a useful interesting article that people ought to be quite interested in. MTBers are going crazy with the wheel and tyre sizes these days (though not making the one I want to see which is 26+). And we are slightly going the same way.
However I would like to clarify a few things.
Tyre size IS almost entirely predictable. It isn't simple to state, but it does follow a logical predictable path. Shove the tyre on a given rim and it will place the beads in two known locations. Pump the tyre up and it will form the INSIDE of the tyre to a circle. Our rim size is based off a constant bead seat diameter and with a certain size tyre (1.95 as I remember) should give a 20" OD.
As rims get wider, the tyre settles down more into the rim so the tyre gets wider AND the OD of the wheel gets a little smaller (very little).
As rims get narrower, the tyre is lifted up out of the rim and the tyre forms a smaller circle and sits up higher making the OD larger (a very small amount).
As tyres get wider (on the same rim) they make a bigger circle which is both wider and higher (by about the same amount) so a genuinely 2.5" tyre would be 0.5" wider and 0.5" taller than a 2" wide one, so the wheel would jump from a 20" OD to a 21" OD.
At the pressures we run, a bigger tyre casing (of the same strength) will stretch a LOT more than a smaller one. On a 2" tyre at 90psi there will be about 15MPa of stress on the tyre casing, with a 2.5" tyre at the same pressure there will be nearly 19Mpa. 25% more stress gives approx 25% more strain and over a tyre that is 25% wider. So a 2.5" tyre will likely expand by 56% more than a 2" tyre due to pressure.
Unfortunately I dont know the details of the two 22" standards or what tyres are available, but it would be cool to make a little table if anyone has the info.
:)
G.
The Brawn:
Surly tread pattern should play a part in this too? A street tyre in a certain size would stretch less that a dirt orientated tyre of the same size?
tecnic1:
Rubber is just really flakey and inconsistant.
I worked as a mechanic at one of these high performance indoor karting centers while I was in college, and we went through a lot of tires. We also had a timing system that was reporting times to customers to the thousandth of a second, so we got tons of complaints about such and such kart is a dog, or such and such kart is really fast or whatever.
So one of the things I would do to to equalize karts was mess around with the rear tire diameters. Tires came in boxes of 12, and within a box there would be variation of up to an inch in inflated diameter (it was around 31-32 inches). I actually plotted four or five boxes at one point (as a bored engineering student, I tended to do shit like that) and it took two deviations to capture 75% of the tires.
I wrote that whole expirence off to buying cheap, rental kart tires, but once I graduated and got my current job, one of the first things I worked on was an inflatable EPDM seal for a 36 inch steam valve. This bitch was crazy. It was inspected and measured at numerous steps, held to insane tolerances, had a rejection rate that a non-government customer would never accept, I mean this seal was visually inspected three times by three different organizations before it was X-rayed. It would have been cheaper to press $100 bills into an O-ring to seal this valve.
One of the qualification tests I had to do was to inflate the unrestrained seal to some pressure lower than the working pressure and measure the inflated height. Inspite of the inspections and tolerances and rejections, two deviations to capture 75% of the seals. My sample size was a lot smaller (12 seals), but still, tons of variation.
Based on those expirences, I think it's difficult, even within rubber products manufactured from the same lot of materials to the same specifications cured on the same day in the same mold, to predict inflated dimensions of rubber things with the sort of accuracy we're used to.
Granted, there are some significant differences between bike tires, go-kart tires, and inflatable steam seals, e.g., the rubber compounds, thickness and so on. Additionally, just pulling numbers out of my ass, I would guess that if a mean bike tire diameter is 21 inches, two deviations would get you 20.875-20.125, which is likely not significant.
I'm not sure I have a relevent point here, but when that article came out, this variation in inflated diameters was what came to mind. It's not something you can fix, or even predict, it's just part of working with inflated rubber.
ediotism:
^^ thanks for the insight. it's a joy to read some decent posts on bikeguide again.
how come i've never seen you post before??
EDIT: i take that back. i have read a lot of your posts before, just didn't really register your name. were you on bikeguide before teh server wipe?
JFax:
I wished I had some intelligent thing to say in relation to this topic, but it is even beyond my bike-nerdiness.
I ride "yo olde faithful" as the article ends with. I've been riding 2.1" KHEs since they came out and similar tyres before that. I now ride G-sport birdcages, but relied on Dragonfly High-5s for a long time, which had increadibly small widths. I would say that the tyres behave bit differently, but I cant say I notice much, I think I went through more tyres back then though.
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