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I've just bought these rims off eBay:

http://www.ebay.co.uk/itm/Mercian-built-Wolber-rims-Shimano-hubs-vintage-27-inch-good-condition-/270921291817?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item3f142ad829#ht_500wt_1401

I've fitted a 27 1/4 tyre to the front, no problem.

But the rear only accepts a 28in Tyre, and the 27 is too big...

Why is this? And what size would this make the rim?

Many thanks

From what I've gathered here: http://www.ctc.org.uk/desktopdefault.aspx?tabid=3802

It would appear that some 28 tyres fit onto rims with smaller diameters.

Now, that raises a new question, why would Mercian sell a pair of wheels with different size rims? - Or wouldn't they?

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Could you tell us exactely what is written on the tires' sidewall? Dimensions, numbers, etc? 27 and 28 are "unnoficial" numbers which might mean different sizes, sometimes. –  heltonbiker Mar 10 '12 at 16:01
    
It says in one of the images that the wheel is a 27". Where are you getting this 28" number from? 28" wheels are relatively rare. –  user973810 Mar 10 '12 at 16:17
    
They're supposed to both be 27", I'm fitting continental tyres, both the same. –  MChandler Mar 10 '12 at 17:04
    
And the 28" is the tyre size according to the tyre. –  MChandler Mar 10 '12 at 17:04
2  
Because tire sizes are idiotic. –  Daniel R Hicks Mar 10 '12 at 19:53
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2 Answers 2

up vote 2 down vote accepted

European ISO 622mm (AKA 700c) tires are often labeled as 28" tires. Confusingly, the very rare 635mm tire is also labeled 28". This is unfortunate and leads to much confusion. The common 27" tires are ISO 630mm, which means that you will almost always find that tires labeled as 28" are actually smaller in diameter than those labeled as 27".

The sizing system for bicycle tires is a confusing mixture of "standards" from different companies and nations. The definitive Sheldon Brown tire sizing article has more detail than you'll likely ever need.

If your 27" (630mm) tire is slightly too big for your rim, it's likely that the rim is 622mm. It's probably worthwhile to visit your LBS; there are lots of standards and when it comes to tires you definitely want to make sure they are properly fitted. Having a tire come off the rim while riding is an unpleasant experience.

As commented above, the difference in sizes between the wheels is probably because the wheelset didn't originate as a pair and came from different sources.

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Seems that they're not a pair! Seller is giving me a partial refund, thanks for your help! –  MChandler Mar 11 '12 at 13:37
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With the tires removed the wheels should be the same height when stood next to each other.In the picture it is dfficult to tell because of the lean.If you are installing tires of different styles or manufacturers one may be more difficult than the other to mount but still be the correct size.Take the tire that would mount and see if it will fit on the other wheel.If they don't fit you may have a Dutch wheel which were 28"It has a bead seat diameter of 642mm also called 700,700a or 700b not to be confused with the modern standard 700c.this sitehttp://casanovasadventures.com/catalog/bike/p2010.has auseful page on wheel sizing.

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