It’s rare that the bike industry sits still for long when it comes to the pursuit of performance gains. Boost is a perfect example. It’s a new front and rear hub spacing standard that results in increased wheel strength on all wheel sizes as well as added design flexibility. As tire widths continue to expand, and modern geometries with short chainstays for quick acceleration remain a goal, Boost fits the bill. Here’s the long and short of what Boost is all about, and why it’s likely to catch on.
Let’s all take a moment to reflect on how far mountain biking has come since the days of 7-speed freewheels and rear axles that snapped like chicken bones. Chainstay lengths used to be as long as an NBA forward’s arm, and 1.9" tires looked aggressive—yet neither got us anywhere very fast. It’s a good thing those days are behind us.
One thing that hasn’t changed, however, is that mountain bikers want to ride longer, farther, and faster. These simple desires drive innovation and lead to developing equipment that helps make this possible. Boost is one such innovation that allows riders to experience new, wider tire widths and wheel sizes without compromising modern trail geometry or durability.
Boost 110 Or Boost Front
“110” as in 110mm wide. This comes from the same reasoning that leads to 148mm in back—a stiffer, stronger wheel, etc. Boost employs a 15mm thru-axle, and the Over Locknut Dimension (OLD) is where the 110mm comes from.
We haven’t seen an end cap kit for converting 15 x 100mm hubs to 15 x 110mm, but it is theoretically possible. A non-disc side end cap would be 10mm longer, and you would have to re-dish or rebuild your wheel.
Boost 148 Or Boost Rear
“148” as in, you guessed it, 148mm wide. With a wider hub shell comes better bracing angles of the spokes, which results in a stiffer, stronger wheel. This is a big advantage for 29ers, as a correctly built wheel of this size on this new dimension yields a wheel of similar strength and stiffness to a standard 26" rim with 142mm hubs. Expect to see bikes in the future with 141mm OLD hubs for bikes with QR dropouts, similar to the 135mm hubs we see today.
Compatibility Of Cranks
A wider rear hub puts the cassette farther outboard, so in order to get the best shifting performance out of the derailleurs, the chainline (the line from the chainring to the center of the cassette) needs to be moved to accommodate that new position. Boost bikes compensate with adjustments to the chainring position.
The good news is that this may only mean a new chainring or crank spider for an existing crank. The crank arm Q factor (the distance between a rider’s feet when they’re on the pedals) stays the same. On SRAM 2x10 cranks, the chainline of the inner ring is 49mm, with the outer ring placed at approximately 53mm. For most Shimano 2x cranks, the inner chainring is at 48mm, and the outer ring is at approximately 52mm. With a 2x crank on a new frame, mount a 1x ring to the outer position.
Triple chainring cranks have a 50–51mm chainline in the middle ring, and may or may not provide enough clearance for 1x adaptation on a Boost rear end. Changing the spider on SRAM cranks to a Boost-compatible version, replacing the spider with a direct mount chainring with a +3mm chainline, or using new cranks are going to be the only options for a 2x or 3x system on a 148mm-spaced bike. Coincidentally, ideal chainline is around 47mm for 142 hubs and 50mm for 148. So if the frame/tire maintains the 6mm industry standard for clearance of chainring-to-frame and chain-to-tire in the lowest gear, a change may not be required. However, this has the potential to severely limit chainring choice and potentially be unsafe. (QBP does not recommend the use of any drivetrain component combination that does not meet manufacturer specifications.)