MTB WHEEL SIZES – WHAT’S THE DIFFERENCE?

But what size are they really?
During the rise of 650b wheels, naysayers argued that even the term – and its ’27.5in’ alternative – didn’t make sense. They were right. But then, none of the other sizes do either.
The numbers in 650b (and 700c, for the record) are the millimeter diameter of the wheel including the tire, so unless your tires are exactly 66mm/78mm tall respectively, they’re wrong.
Meanwhile, the letters refer to the bead seat diameter (BSD) necessary to get that outer diameter. They’re leftovers from an archaic French system that kept outside diameters constant, a system that worked by fitting smaller or larger wheels whenever you changed tire width. You probably aren’t doing that…
In reality, the BSD of 29in wheels is 622mm, 27.5in are 584mm and 26in are 559mm. That’s 24.5in, 23in and 22in respectively! So never mind the jargon – today they’re all really just labels, not accurate measurements.

The middle way – the K-Force Light MTB 27.5 – is proving a very popular choice.

Making bigger wheels work
Taller wheels are basically longer levers, so they flex more. Reducing this without adding excessive weight, especially at the rim, is the tricky part.
Lengthening the axle and spreading the hub flanges (the bit the spokes lace to) is a great way to bring strength back. Just as you’re harder to shove over with your feet spread wide, a wider axle provides greater resistance to flex across the wheel’s radius.

Your frame and fork dictate the axle width. In the past, 135mm rears and 100mm fronts were near ubiquitous, though 142mm rear axles – which don’t spread the spokes any wider, but add locating pockets in the frame – supplanted them even before 26in died. Hollow 12/15/20mm screw-thru axles also add significant stiffness (and safety). Good wheels convert between the sizes with simple spacers.
Boost sizing, with 148mm/110mm axles, is slowly gaining ground, though it requires compatible frames and forks. 150mm-plus rears remain the preserve of downhill bikes – at this width, heel clearance becomes an issue.
Bigger rims must be stronger too, but that adds weight. Carbon fiber is ideal for both reducing mass and boosting stiffness. Extra width also brings greater tyre support and more aggressively square profiles for railing those corners, as well as inherent extra stiffness. For years rims were stuck at 19mm or even 17mm widths, leaving 2.35in tires pinched and prone to flopping over.

Cyclocross – the best of both worlds?
Road bike axles spent decades at 130mm rear and 100mm front, but the advent of disc brakes has forced a rise to 135mm for rear clearance – good news for CX riders, who’ll find 29er MTB hoops and dedicated CX wheels cross-compatible.

But while CX choice is enviable, what about mountain bikers? Should you go 27.5in or 29in? In truth, progress remains rapid and the waters are as muddy as our sport, but as a rule of thumb, the more aggressively you ride, the more you should look towards 27.5in.