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I'm looking at the Surly Krampus and Karate Monkey. It looks like the difference is basically that the Krampus is longer and fits bigger tires. But the Surly website says the Karate Monkey can fit both 27.5 and 29 wheels, but the Krampus can only fit 29. Why would that be? With disk brakes, shouldn't it always be possible to fit SMALLER wheels? I'm confused.
 

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Sure, you can physically attach the smaller wheels, but it will drop the bottom bracket significantly if the bike isn't expressly meant for it. That means your cranks will be hitting obstacles frequently, and in extreme cases (cornering) the ground.

The problem you encounter: [tire size]x[width] is not indicative of the actual size of the wheel, and so, the height of your bottom bracket. A 29x2 is, indeed, about 29" in diameter...but a 29x3 (what the Krampus is built for) is almost 30.5" in diameter. A 27.5x2.25 is almost 3 inches smaller in diameter than a 29x3, not at all what the "size" would imply.

You would be dropping your bottom bracket over an inch, to be generous. Not a great idea, considering that even on most modern bikes, a 175 crank at full extension would wind up ~3-4" from the ground.

Consider: do you hate it when you pedal over a 3" root, and it kisses the bottom of your pedals? Imagine that happening, but with a root/rock half that size.
 

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Why would you ever put smaller wheels on a Krampus? That's the whole point of the bike!

In any case, you can put 29s on the Karate Monkey, but it won't fit 29+. The Krampus is for 29+.
There were a couple people running 29+ on the KM around here. IIRC they had to run the rear axle in the back half of the dropouts, but it seemed to work.

I would reference the Surly forum for more info.

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