Thursday, February 20, 2020

Deep Dive; Down The Rabbit Hole Of Tire Nomenclature

In the late 19th Century, it was all about diameter.
Okay! Today you had better be prepared to get your "nerd' on, because we're taking a deep dive into why tires have the designations that they do. Some of what I am going to share is knowledge I picked up along my journey as a bicycle mechanic. But I am going to credit a few folks right up front here. Thank you to Wes Williams, Clayton Wangbichler of WTB, and the inimitable Sheldon Brown (RIP) and his website.

I'm going to breeze through a LOT of technicalities here and focus only on a few salient points, so please bear this in mind. 

Wheels. That was what it all was about back in the late 19th Century. Wheels determined how fast or slow you could go, and this was figured out based upon a formula which required the measurement of the overall diameter of said wheel. Keep in mind that "tires' were nothing more than a steel strip over wood, or maybe a wired on rubber tube later on.  Pneumatic tires came around right around the turn of the century, but this wouldn't get sorted out until the 20th Century as tires you and I might recognize today. The point is, tires added little, if anything, to overall diameter.

Along with this tire/wheel thing, we had other factors which eventually lead to the "Safety Bike". Now wheels were not driven "directly", but by means of a crankset, chain, rear cog, and the rear wheel. This complicated things for the early wheelman, as it made figuring out your wheel development, or how far your wheel traveled with one revolution, much more of a chore. However; rear wheel diameter was still important to know so one could figure this out. Pnuematic tires also complicated things because they added to the overall wheel diameter. Now throw in bigger, or smaller volume of tires. Generally speaking, wider tires are going to add more diameter to a fixed diameter of wheel/rim. This radically changed gear development, so something had to be done to tame this cacophony of innovation and confusing tire/wheel sizes. This didn't happen for several years, thanks to two World Wars, innovations in automobiles and airplanes, and the ups and downs of economies. But eventually, it happened.

We pretty much have the French to thank for our predicament today. They helped develop a system of codification for wheels. Because the bicycle wheel was a worldwide phenomenon, we ended up with a mixed bag of Imperial and Metric measures for wheels. This really makes things hard, but we will stick to the metric side of things here.

The French made some wheels standardized around the 650mm diameter, but keep in mind, that's overall diameter. They way it was supposed to work was that if you had a skinny, low volume tire, you'd need a larger diameter rim to make the tire/rim combination equal 650mm. This was designated as 650A. Medium sized tires, with larger volume, required a smaller diameter rim, and thus the 650B designation. The largest volume tires went on even smaller rims, and these were designated 650C.

650B really doesn't tell us what size tires we are looking at anymore like it used to.
Copy and paste for 700 sized wheels. They were supposed to be 700mm overall diameter and rims varied in diameter depending upon tire volume. You now know that the "c" in 700c meant- originally- that a pretty good sized tire was supposed to be on there. But none of this applies in 2020. Obviously. That "c" in 700c is pretty much meaningless. You know this because a 29"er tire and a 700 X 44mm tire can both go on the same rim, but both result in vastly different diameters.

And of course, figuring out wheel/gear development is an arcane practice which few engage in anymore anyway. We keep saying "700c" because we need to call it something and traditions die hard. But now you know a little bit of that story. Now I want to address another letter "c".

That being the "c" people sometimes drop behind the millimeter measurement of a tire's width. Example: 700 X 44c. What is that? If we are measuring in millimeters, is the "c" for centimeters? Obviously that cannot be the case, although a 44cm wide tire would be rad on snow! No, here's what the deal is, as I have come to find out. Thanks to WTB's Clayton Wangbichler for the following.

Rims weren't always how we know them today. There were hookless rims, (Schwinn was famous for these), and there were a type of hooked bead rim called "crochet style". The "c" behind the tire designation for width would often have an upper case "C" to designate that the tire was designed for the crochet style rim bead. So, "44C" would be an example of how many companies would have designated such a thing. Eventually this important designation became meaningless as all rims became more or less standardized, and the "C" became "c", because.........tradition and marketing. It doesn't tell us anything.

In fact, Clayton from WTB told me that WTB has dropped the "c" on 700 and the "c" or "mm" from the width too. (Note the image above) So, there ya go. I've always wondered what the heck the 'c" was for on the width measurement, and now I know it doesn't really mean anything. It's just a mannerism, a vestige of days gone by. Not that you or anyone else should follow suit with WTB and stop using that, but, as Clayton told me, "We've all offended", and I think what he means is that we all were just doing what we had been seeing and not really asking "why?'. I think, given the history, it maybe will bring about a clearer way for talking about tires in the future. Not that we've gotten away from some other confusing bits. Like mixing metric and Imperial measurements!

For a bit of a visual presentation on the "A", "B", "C" French thing, here is a link to a GCN video which does a great job of explaining it.

Once again, a big thanks to all of you folks for reading.


Iowagriz said...

Good post Mark - so logical once you hear it. I had no idea what the C meant, knew of the B in 650b, never heard of the A - or the Crochet....

all very interesting

Michael Lemberger said...

I had always thought that the "c" in 700c referred to the 622mm rim size. Obviously it doesn't serve the original purpose of designating a specific tire/rim size, but does at least offer a standardized way to refer to a particular rim diameter.

R. Freeman said...

I know it's a typo, but the overall diameter of 650 sized wheels with tires mounted should be 650mm. 584mm is the bsd of 650b.

Guitar Ted said...

@R Freeman- Correct, and corrected. Thank you!

Skidmark said...

It seems it really should be 700_ something. I like 700c.