Can I used 650B / 27.5" forks with 26" wheels?

Absolutely.  With good 26" forks getting hard to come by, using a new 27.5" fork makes perfect sense.

What's the difference?

If you compare a 26" and 27.5" fork of the same travel, the axle to crown (A2C) measurement of the 27.5" fork will be about half an inch (12.5mm) longer. This might slacken your head angle by around half a degree.  If you go with a slightly shorter travel fork, then the A2C will be about the same and you'll have no change in head angle.  For example, the 160mm 26" Rockshox Pike has exactly the same A2C as the 150mm 27.5/650B Pike (542mm).

What about offset?

Newer 27.5"/650B forks typically have a different fork offset to 26" forks.  Once again, taking the Rockshox Pike as an example, the 26" Pike has a 40mm offset and the 27.5"/650B Pike has a 42mm offset.  This increased offset gives a reduced mechanical trail on the Pike 27.5"/650B when using a 27.5"/650B wheel.  This reduced mechanical trail makes the fork feel as responsive as the 26" version with a 26" wheel.  Using a 26" wheel in the 27.5" Pike will give you slightly less trail than a 26" Pike which should make things feel a little more responsive when climbing and a little less stable when descending.  This will, however, the counteracted by the slightly slacker head angle you get with the longer A2C.

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