I have a 26 inch MTB frame and I want to replace the fork.
How can I distinguish a 26 inch fork from a 28 inch one? Which are the measurement to know?
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Suspension (or rigid) forks are not designed just for a particular wheel size, instead they have a nominal crown to axle length. Frames are built for a specific length fork (or a range of fork lengths) that will give the proper head tube angle. This takes into account both wheel size and length of suspension travel.
Forks designed for rim brakes will of course have attachment points located correctly for a particular rim diameter. (Credit to RLH for reminding me of this in a comment.)
This gets a bit tricky with suspension forks, as obviously they are designed to change length and may extend beyond their neutral length without a rider on the bike.
Measure the crown to axle length of your current fork and look for a replacement based on that, length of travel (and rim brake boss location). AFAIK forks for 26" wheel bikes with short suspension travel (>80mm) are not available new (commenters please prove me wrong on this).
You need to measure your axle to crown length and match it up. bikecad.ca/taxonomy/term/52
However 26" is now pretty much an obsolete standard, so you may have difficulty sourcing a satisfactory replacement