What Does The “C” In Bicycle Tire Sizing Really Mean? (illustrated explanation)

Bike tire sizing is often a source of confusion sometimes even for experienced riders simply because there are too many sizes, numbers, symbols, and letters involved.

One of the characters that often confuses people is the letter “c” in a tire’s description. E.g., 700x25c.

The letter “C” in a tire’s parameters refers to an archaic French standard for bike tire sizing which relied on letters ranging from “A” to “D” to indicate a tire’s width and subsequently the circumference of the corresponding rim.

“A” equaled a thin tire and required a larger rim whereas “D” designated a wide tire and a smaller rim. The goal of this maneuver was to keep the outer circumference of the wheel the same regardless of the tire width.

Eventually, the industry stopped playing with rim sizes, and “C” became the standard for many adult bicycles.

image source: https://pixabay.com/bg/users/denimix-322016/

Wider Tire = Potentially Bigger and Taller

Wider tires “ballon up” to greater proportions and often have a larger outer diameter than the thinner ones designed for the same rim size.

My hardtail demonstrates this effect. The rear tire is Schwalbe Big Ben which is around 2″ wide. The front is VeeTireGalaxy measured at about 2.2″.

Originally, I rode with two Big Bens, but eventually, the rear wore off. Since I didn’t want to buy new tires and already had the VeeTire, I put it at the front and replaced the worn rear Big Ben with the original front one.

The result? The front end of the bike rose a little even though the rims are the same. The difference came from the wider, bigger tire and its tread.

Precisely because of this phenomenon and the desire to keep the outer tire diameter the same, in the past, wider tires were paired with smaller rims.

The image above illustrates two wheels that have the same outer diameter even though the rim and tire sizes are different.

The one on the right (700C) uses a wider tire and a smaller rim whereas the thinner one (700A) has a larger rim and a skinnier tire.

Having said that, a wider tire does not always guarantee a larger diameter.

Narrow tires could be made to fit on rims originally designed for wider tires.

FAQ: Why did people insist on preserving the wheel diameter the same?

The main purpose of keeping the overall diameter of the tire the same was to allow cyclists to quickly switch between different wheels depending on the terrain.

The unified wheel diameter ensured that the frame and the fork would accept the wheels without creating clearance problems.

FAQ: Why did that system end?

Two reasons come to mind:

  1. Rim manufacturers had a hard time with the demand.
  2. Changing the wheel required frequent readjustment of the brakes’ calipers because the friction surface would shift its position according to the rim’s size.

What Does “700C” Mean When Translated Into Modern Bike Terminology?

700c is an old French term indicating the approximate outer diameter of the wheel in millimeters. However, that standard is dated, and many of the tires marketed as 700 do not have a 700mm or 70cm diameter.

Therefore, 700 is only an expression of the rough external diameter of the tire. The real number varies according to the width of the tire, its thickness, and the tread pattern.

The inaccuracy of the old system and the great number of overlapping tire sizes is why the ISO 5775 system was created. Its purpose is to simplify tire sizing and eliminate needless confusion.

Hence why ISO 5775 focuses on the so-called bead seat diameter of the rim rather than the outer diameter of the tire.

What is the bead seat diameter?

To understand what the bead seat diameter is, one first has to learn what’s a bead seat.

The bead seat is the part of the rim to which the edge (bead) of the tire attaches. (consult the image above)

The bead seat diameter is the distance between two opposing bead seats of the rim.

According to the ISO 5775 standard, the 700c size corresponds to a tire requiring a rim with a 622mm bead seat diameter (BSD).

Road bikes, hybrids, cruisers, and mountain bikes with 29-inch wheels fit into this category.

Other Possible Explanations Behind the Letter “C”

While there are strong indications that the letter “C” in tire sizing is an archaism, some people disagree and present two different explanations.

Option A: “C= Clincher Tires

According to some, the letter “C”, indicates that the tire in question is a clincher type.

“Clinchers” are the most common bicycle tires. They work with an inner tube and connect to the rim with a lip (bead).

What is the likelihood of this option?

Technically, there may be some truth to it, but some tubular tires (non-clincher type) also have the letter “C” in their listing. Hence why this option isn’t very likely to be correct.

Option B: “C = Crochet Rim

The ISO 5775 standard for rim listing is as follows:

Rim Inner Diameter x Rim’s Inner Width + A letter code for the rim type.

E.g., 622x19C

The first value, 622, is the bead seat diameter in millimetres.

The second value, 19, is the inner rim width.

The letter “C” indicates that the rim is the crochet type – the most common rim these days.

(Note that those parameters describe the rim rather than the tire.)

Therefore, if the letter “C” in rim descriptions signifies the rim type, it may indicate the same in a tire listing (e.g., 700x25c).

What is the likelihood of this option?

I guess it’s possible, but the type of rim required by the tire is often listed separately on the tire. E.g., For Crotchet Type of Rims Only.

My Big Ben rear tire comes with this specification.

Having said that, both options are logical enough to be taken into consideration when searching for the origin of the letter “C” in tire sizing.

Frequently Asked Questions

What does 700x25c mean?

This is a classic road tire sizing relying on the old French standard.

700 indicates the outer diameter of the tire. However, it’s a nominal diameter a.k.a. “in name only”. The true diameter of the tire will vary.

Hence why 700c is more of an indication that a tire uses rims with a 622mm bead seat diameter rather than an explicit measurement.

Meanwhile, 25 indicates the outer width of the tire.

And the letter “C” could be seen either as a relic from the past as already stated or as an indication of the tire type (clincher).

Since the letter “C” sometimes finds itself in the description of tubular tires (a.k.a. non-clincher), I’d go with the relic explanation.

What does 25-622 mean?

This an ISO tire listing. 622 indicates the bead seat diameter of the rim in millimeters whereas 25 shows the outer width of the rim.

What does 28 x 1.00 mean?

This is tire sizing done in inches. 28″ is the outer diameter of the tire in inches. 1″ is the outer width of the tire in inches.

All three types of listings above could be found on the same tire as they practically indicate the same size in three different ways.

How do I find out what inner tube I need?

The parameters that matter the most when looking for an inner tube are:

  • Tire type (MTB, road..etc.)
  • Tire diameter (700C…etc.) or bead seat diameter (622mm…etc.)
  • Tire width
  • Valve type – Schrader (car valve) or Presta (French).

Note: usually, road tires rely on the old French system (e.g., 700x25c) whereas MTB tires use inches (e.g., 27.5×2.3)

Once you know all of that, look for a tire that satisfies the conditions.

For example, if your road bike tire is listed as 700Cx25, you will need an inner tube that matches that size and width.

Inner tubes cover a range of widths. E.g, 700×25-32. The width of your tire should fall within the listed range.

The valve type is also important and depends on the rim. Schrader valves are wider. Subsequently, the rims designed for them have bigger valve entrances. It’s not advisable to combine a Presta valve tube with a rim intended for a Schrader valve.

The Presta valve is slimmer and will move when installed on a Schrader rim. The slight movements may tear the inner tube near the valve

Using a Schrader valve with a rim designed for Presta isn’t a proper practice either because you will have to drill a bigger entrance hole for the Schrader valve to pass. The new diameter of the valve entrance will reduce the integrity of the rim and increase the chance of bending it.

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