700c / 650c

Excuse my ignorance
but what do these numbers actually refer to ?.I understand its the wheel size I've often wondered but never bothered to find out exactly.Is 700c about 27"[i think so] so how do these 2 numbers relate to each other

Thanks in advance for clearing this up

Sheldon Brown's site has a nice article on this topic: http://www.sheldonbrown.com/tyre-sizing.html

Probably best to get a nice mug of tea/coffee ready before settling down to read it

44 views and no answer other than to see what sheldon says[said]

Ach knew i should have asked in the MTB section

The number is the nominal diameter of the inflated tyre. 700c would be roughly 700mm, 650c would be 650mm. The letter refers to the height of the tyre itself. So, for example, a 650c and a 650a may have roughly the same inflated diameter, but the c would have a fatter tyre.

This may appear puzzling because if you were to go out and find a bike fitted with 650a tyres they would probably be fatter than any 650c equipped bike you could find. So what's going on? Well once upon a time you could only get one size of 650c tyre, then some manufacturers started making different sized tyres to fit the 650c rim. And the same applies to the 700c rim. Futhermore they started making skinnier rims so that the original 'c' sized tyres wouldn't even fit.

26" mountainbike sized tyres are even worse. Hardly a one of them is 26" diameter. You can get anything from those monstrous fat Surly Endomorphs which are about 29" diameter to skinny 1" road tyres which are something like 24" diameter. They are all called 26" tyres which is frankly daft.

All this is why ERTRO (latterly ISO) tyre sizing came into being.

Nobody answered because you would need to go back into history quite a ways to understand.

Let it be said that wheel sizes are the direct result of various national standards that developed over time and because they stem from different systems, they are based upon different naming conventions that are in no way inter-related.

Because of the complete and utter confusion that reigned up until the 70's, a universal naming convention ETRTO was arrived at and is now marked n all tyres. The ETRTO convention defines how a tyre is to be measured and is always the same.

Your standard 27" x 1 1/4" tyre of yore is a 630 ETRTO dimension, the standard tubular size, which has been alternatively called 28", 700c, 27" and perhaps other names too according to distinct naming conventions that have held stead over the years, measures 622 ETRTO. Likewise the modern 29'er tyres for MTB are also 622 ETRTO.

As a general rule, most tyres having the same ETRTO diameter will fit on the same rims, as long as the difference in potential width is not overly great and the rims is set out to hold the particular tyre design on (i.e. some tyres require a "hook" bead)

BTW: there were 650A, 650B and 650C tyre sizes that are in no way interchangeable. The idea with the three "sizes" was to end up with a similar outer diameter of the tyre and not related to a rim size at all. The idea being that if you used a wheel with 650A, 650B or 650C, you would end up with a similar diameter outer tyre size as long as you didn't need to fit a rim brake.

Probably best to get a nice mug of tea/coffee ready before settling down to read it

Click to expand...

I now understand this part of the reply

Thanks for the answer,makes sense now


Kind of

To slightly simplify from my time trial experience:-

700c is the standard wheel size on road bikes.
650c (sometimes roughly called '26') is the normal front wheel size on time trial bikes.
520c (sometimes roughly called'24') was a very small front time trial wheel, fashionable for a short while in the mid-1980s but being cursed with never getting a really light tire made for it.

Citoyen du monde":2eayzrdf said:

Your standard 27" x 1 1/4" tyre of yore is a 630 ETRTO dimension, the standard tubular size, which has been alternatively called 28", 700c, 27" and perhaps other names too according to distinct naming conventions that have held stead over the years, measures 622 ETRTO. Likewise the modern 29'er tyres for MTB are also 622 ETRTO.

Click to expand...

You can't really say that 700c is the standard size for tubulars, since sprint rims can't be measured to the same standard as clincher rims. You measure a clincher rim at the bead seat, a sprint rim doesn't have bead seats. So while they are a very similar size they can't actually be said to be the same.

The fact that tubulars come out a very similar size to 622s is probably a coincidence. The first tubulars in that size apparently predated such skinny tyres being fitted to 622 rims. The 622 clincher rim was probably adopted as a sporting standard in mainland europe because with skinny tyres fitted to them came out a similar size, so they could be fitted to the same frames.

Steve Kish":2k0xt20n said:

To slightly simplify from my time trial experience:-

700c is the standard wheel size on road bikes.
650c (sometimes roughly called '26') is the normal front wheel size on time trial bikes.
520c (sometimes roughly called'24') was a very small front time trial wheel, fashionable for a short while in the mid-1980s but being cursed with never getting a really light tire made for it.

Click to expand...

OK as far as it goes, but remember that low pro time trial bikes are to some extent peculiar to the UK.

I tend to think of the 700c size as being equivalent to the old British 27 x 1 1/4" whereas 650 is the equivalent of the old British 26 x 1 1/4". As such 650c tends to be used on smaller frame sizes. Actually many small frames are a geometric compromise where they insist on using 700c wheels.

Also the 650c size is popular with some triathlon riders and triallers who have to comply with UCI rules. The UCI rules state that both wheels have to be the same size, so if you want a smaller front wheel to get a low riding position you're stuck with a smaller rear wheel.