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Wheel size confusion!!!!! PLEASE HELP

PimmsoclockPimmsoclock Posts: 5
edited September 2010 in Workshop
Ok so,
I've always had 700c wheels but recently a car hit my bike and bent the frame and forks so I've had to 'go shopping' for a new bike.

I was looking for a 80s frame which I have found and is my size (seat tube and top tube) but then the man told me it had 25 inch wheels, which confused me to no end. I thought 700c wheels were 26" and my pal thought they were 27". When I turn 622mm into inches its 24" ARGHHHHHH

Also I am oblivious to 25 inch wheels, i.e as far as I knew/know 25 inch wheels don't exist.

So what I'm asking do 25 inch wheels exist? Are they way smaller than 700c? Are they 700c? And finally can 25inch wheels(if they are) be replaced for 700c?


  • AidanRAidanR Posts: 1,142
    edited September 2010
    No wheel has ever been referred to as 25". As an older road bike, it'll be 27" (ISO 630mm). 26" is standard mountain bike (ISO 559mm) but may also refer to a host of other sizes, including 650c (ISO 571mm) seen on some smaller road and triathlon bikes. 28" is sometimes used to describe 700c wheels (ISO 622mm), but can also mean other sizes. Confusingly 29" is used to refer to larger mountain bike wheels, but these are in fact 700c too.

    The problem is that there are four different systems for tyre sizing - inches with fractions (28 x 1 1/2"), inches with decimals (29 x 1.5"), French sizing (700x37c) and ISO (37-622). All those examples refer to the same sized tyre. Go figure.

    One problem is that ISO refers to the diameter of the rim (bead seat diameter), whereas the inches and French systems refer roughly to the diameter of the tyre, hence the problem converting between them.
    Bike lover and part-time cyclist.
  • keef66keef66 Posts: 13,123
    Kids bikes often have 24 inch wheels, standard MTB wheels are 26 inch. Road bikes today are 700c which are bigger. Road bikes used to be 27 inch.

    never heard of a 25 inch wheel. Google Sheldon Brown; he's bound to have something encyclopaedic to say on the subject.

    Fitting 700c wheels to that frame will depend on whether the brakes have the adjustment to align the pad to the rim
  • keef66keef66 Posts: 13,123
    Must learn to think / type faster....
  • laelae Posts: 555

    These tables should help.

    There's no such thing as 25" wheels, not as far as I know anyway. But the actual diameter of the rims is different to the 'named' wheel size, annoyingly.

    If the seller has physically measured the rim diameter he might have come up with 25". As you say, 622 (i.e. 700c) is 24" so we can expect 25" to be slightly more than 622mm.

    630mm happens to be the size of 27" rims which were used on older road bikes, which are just a tad bigger than 700c and were still common in the early 80s. I'd say almost certainly that the bike has 27" rims. Get the seller to quote you the tyre size to check.

    You can swap old 27" rims for modern 700c rims if a) your brake pads can drop around 4mm and b) you have screw-on freewheel hubs in 700c wheels (widely available on ebay or from fixed-gear specialists) OR you spread the rear dropouts by 10mm to accept a modern, wider, freehub wheel.


    Just for reference, 700c wheels aren't 26" or 27", and there are FOUR different rim sizes known as 26"! Knowing the tyre size is the best way to check.
  • laelae Posts: 555
    keef66 wrote:
    Must learn to think / type faster....

    You're telling me! Three responses in the time it took to write mine!
  • Thanks for all the replies, I thought I was going mad!

    I'm guessing hes literally measuring the wheel with a tape measure then atleast now I can make sense of it! :D
  • keef66keef66 Posts: 13,123
    Suggests he's not too familiar with cycle technology.

    caveat emptor and all that......
  • A pretty good estimate of the overall diameter of a wheel including tyre is

    2 x IRD + BSD

    Where IRD and BSD are the two numbers describing your tyre size. (Internal Rim Diameter and Bead Set Diameter)

    28 - 622 for example would give you an overall wheel diameter of

    2 x 28 + 622 = 678mm = 26.69 inches.

    Some people might refer to such a wheel as a 26”, others 27”. So stick to the ISO/ETRTO numbers on the tyre.

    Incidentally, if you multiply the diameter by Pi you get the circumference which you can plug into your cycle computer.
    I may be a minority of one but that doesn't prevent me from being right.
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