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what is the REAL size difference between 700c and 27"

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what is the REAL size difference between 700c and 27"

Old 07-19-07, 01:44 PM
  #1  
mcompton1973
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what is the REAL size difference between 700c and 27"

ok, I have searched, I read Sheldon's page...I still dont know the answer.

27" tire is (roughly) 27" Diameter.
700c tire is what? I have been told that it is 29" roughly.
and on Sheldons sight he says 29"...BUT...if that were true, then why is it that brakes on older bikes have to be changed or adjusted down? seems to me that means they are smaller....

I am sorry, and it really does not matter I guess....just bugging me that I dont know.
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Old 07-19-07, 01:52 PM
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Wheels' denominations are truly misleading.

An objective measure is that of the bead seat diameter.
27" wheels have BSD=630mm (24.8")
700c wheels have BSD=622mm (24.5"), so the former is 8mm larger in diameter.

The reason wheels are called 27", 26" or 29" is because they have approximately those diameters when some tire is seated. Of course it really depends on the tire, hence the confusion. A 700c rim with a big tire (29er) can end up bigger than a 27" rim with a small tire.

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Old 07-19-07, 02:09 PM
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Originally Posted by mcompton1973 View Post
ok, I have searched, I read Sheldon's page...I still dont know the answer.
It's on Sheldon's tire sizing page.
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Old 07-19-07, 02:10 PM
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If you look at this table for setting type D cyclocomputers, it tells of the radius of certain wheel/tire combinations... since type D cyclocomputers use radius to set wheel size.

http://sheldonbrown.com/cyclecomp_d.html

You will notice that the typical 27 inch tires (27 x 1 1/4) are between 700X32 and 700X35 in their over-all radius...

So any 700c tire that is 35 mm or wider will be taller than a 27" and anything 32 mm or narrower will be shorter...

By the way, 29" is the term for a massive 700c... which is why it is bigger than a 27"
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Old 07-19-07, 02:14 PM
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Originally Posted by mcompton1973 View Post
ok, I have searched, I read Sheldon's page...I still dont know the answer.

27" tire is (roughly) 27" Diameter.
700c tire is what? I have been told that it is 29" roughly.
and on Sheldons sight he says 29"...BUT...if that were true, then why is it that brakes on older bikes have to be changed or adjusted down? seems to me that means they are smaller....

I am sorry, and it really does not matter I guess....just bugging me that I dont know.
Ok, here's some reality. I measured the diameter of my tubular wheels (700c here in the USA) with 23 mm tires, diameter is 676 or so mm. It's not easy to see it exactly.

Tony Oliver, in his great book "Touring Bicycles," gives a table of diameters v. ISO/ERTRO specs. My number is not far from his. He also gives a diameter of around 700 mm for 27 x 1 1/4.

So the real difference is a a larger diameter by about 20 mm, more if you compare 19 mm track sew-ups (660mm) to the 27x1 1/4's. This is enough to significantly affect whether you can use fenders or not, or even if you can upsize rims and tires on some frames, such as (lol!) converting that Colnago into a tourer.

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