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I first came across this when I was looking at the Surly LHT, but I started to see it everywhere. It seems to me they can just put on a longer fork or something, or is it more for the back wheel?

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I don't know why, but don't see it across the board, most bicycles I've looked at use 700c down to 50cm. I have three 700c bikes all around 52cm (each measures frame size a bit differently). –  Glenn Gervais Feb 16 '12 at 20:36
    
Is this question specifically about touring bikes? If so, we should tag appropriately. –  Neil Fein Feb 16 '12 at 23:28
    
It's more about bikes in general. Why would you need a bigger bike frame for bigger tires? –  Chris Belsole Feb 17 '12 at 1:46

2 Answers 2

up vote 6 down vote accepted

The real issue is in the top tube length. Basically, for shorter riders you need to move the handlebars closer to the seat. But then you have to deal with the wheel possibly colliding with the pedals ("toe strike"), changing the head angle or the fork's rake, which compromises the handling, and/or having a proportionally longer top tube than would otherwise be indicated by the seat tube length.

Rodriguez Cycles did an excellent multipart essay on choosing between 650c and 700c wheels for road bikes, and the illustrated Cliff notes they provide should prove illuminating.

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My Randonee frame size is in the high 40s, and it's a 700c bike. My toes clear the front wheel on a good day, and sometimes on a mediocre one. –  Neil Fein Feb 16 '12 at 23:28
    
Just looking at the 2009 Novara Randonee geometry, looks like the XS (47cm ST) has a ~53CM effective top tube, plus a 55mm rake fork. The medium is a 55cm ST and 56cm ETT - the reason for the relatively longer top tube on the XS is to push out that front wheel. The compromise there is that you end up more stretched-out than might necessarily be ideal. –  lantius Feb 16 '12 at 23:36
    
@lantius So Hypothetically if they angled the head tube forward a bit you could have a smaller bike with bigger wheels? I guess stress on the frame is why they would not do this since more of your weight would be on the head tube due to the increased angle. Kind of like a chopper bike. –  Chris Belsole Feb 17 '12 at 1:51
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Angling the head tube more upsets handling. Front end geometry is a delicate balance. –  Daniel R Hicks Feb 17 '12 at 2:01
    
I ran this past an ex framebuilder/designer I know and his answer was basically the same: toe overlap or other geometry issues with a short top tube and big wheels. –  freiheit Feb 17 '12 at 5:44

A bike frame does not have to be above 56cm at all. In fact, the 56cm is a measure of only one aspect of the bike's geometry.

It depends greatly on the bike. The Long Haul Trucker, for example, my only be sold in frames 56cm and above, but several other Surly bikes, such as the Cross-Check and the Pacer come in sizes as small as 42cm.

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The LHT is sold down to 42 cm. The desired geometry can't really be done with a 700c wheel. –  Batman Jan 13 at 5:58

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