Boost 148 (rear)
Boost 148 is the most widespread mountain bike rear hub standard. It's 148 mm wide, 6 mm more than 142 (3 mm per side), and uses a 12 mm thru-axle. That extra width spreads the hub flanges, opening the spoke angle for a stiffer wheel with room for wide tires. It also shifts the chainline from 49 to 52 mm. It's not a 142 «with a spacer»: the whole hub positions the cassette 3 mm further out, and it requires a Boost frame and chainring.
If your MTB is from 2016 onward, it's almost certainly Boost. It's the width that made big 29-inch wheels stiff.
What it is and what changes
Boost doesn't change the cassette or chain: it changes where the whole hub sits in the frame. By spreading the flanges 3 mm per side, the spokes form a wider triangle and the wheel flexes less. Since the cassette ends up 3 mm further out, the chainring must move too (52 mm chainline), usually with a 3 mm chainring offset. That's why a Boost frame needs a Boost crank/ring.
Key data
What it fits
It fits frames designed as Boost 148 and needs a chainring with Boost chainline (52 mm, 3 mm offset). It won't fit 142 or 135 frames. A 142 hub can be adapted to a Boost frame with a spacer kit and rotor repositioning, but a native 148 hub can't be «shrunk» to 142.
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Frequently asked questions
Is Boost 148 compatible with a 142 frame?
Not directly. They're different widths; the frame must be 148 mm. You can only adapt a 142 hub to a Boost frame with a conversion kit, not the reverse.
Is a Boost cassette different?
No. The cassette mounts the same; what changes is that the whole hub positions it 3 mm further out. That's why the chainline shifts to 52 mm.
Do I need special cranks for Boost?
You need a 52 mm chainline, usually via a 3 mm offset chainring. Many cranks solve it with the ring, not the spindle.