Boost 148 moves the rear hub flanges 3mm outboard on each side in order to improve the spoke bracing angle and increase wheel stiffness (click to enlarge).
Editors Note: This article was written by Art's Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances and breakdown component design. The original post can be found here.
By now you have heard about the new Boost 148 hub standards. But what exactly are they and what do they mean to you?
I previously wrote an article that discussed axle standards. You can see that there are a dozens of different standards, but when discussing the latest Boost 148 and Boost 110 hubs it is more important to just look at a couple of the older mountain bike standards.
In the beginning mountain bikes generally had quick releases with 100mm spacing in the front and 135mm spacing in the rear. Within the last 10 years, QR's have given way to the thru axle. The thru axle offers a much stiffer and more secure platform for bigger hits and harder cornering. On most mountain bikes, we now see 15mm or 20mm thru axles in the front and 12mm thru axles in the rear.
With the increasing popularity of thru axles, we saw the introduction of the 142x12mm hub and axle standard. These new 142mm hubs replaced the 135mm standard that had been around for years. But there is one important thing to remember about this standard: The distance between the two dropouts on the frame is still 135 millimeters, with an extra 3.5 millimeters of inset on each side built into the frame dropout. This 3.5 millimeters is used to guide the hub into the frame and self-center the wheel, making thru axle installation easier.
By moving the chainring outwards, you open up the frame, allowing for better tire clearance and shorter chainstays (click to enlarge).
Most hub manufactures offer replaceable hub end caps to convert their hubs between 135mm and 142×12. So while frame designs changed, the hubs for these two standards still have the same spoke flange spacing and other dimensions.
Now that we have some background on hub spacing, lets talk about wheel size. Originally everything was designed and engineered in the mountain industry around 26" mountain bike wheels. However, we now have 27.5" and 29" wheels, but the hubs are still the same width. With larger diameter hoops, spokes become longer. These longer spokes mounted in the same width hub create smaller spoke bracing angles.
Companies are starting to release product that will interface with the new hub standard (click to enlarge).
The spoke bracing angle is the angle formed between the spoke and the vertical plane. Larger spoke angles are able to balance a larger component of the lateral forces exerted in the horizontal plane during loading. So the end result is that 27.5" and 29" wheels are weaker than 26" wheels with current hub standards.
Enter Boost 148 and Boost 110. These two hubs move hub flanges outward. By moving the hub flanges outwards, spoke angles increase, creating a more stable base. So Boost standards maintain the ratio of spoke length to bracing angle seen in 26" wheels resulting in, "26" stiffness from a 29" wheel."
Continue to page 2 to learn more about the benefits of Boost 148 and 110 »
Boost 148 adds 3mm of spacing on each side of the hub. But unlike the 3.5mm difference from 135mm to 142mm, Boost 148 sees an increase in flange spacing, not just axle endcap width.
Boost 110 uses a 15mm thru axle and moves hub flanges outboard by 5mm when compared to 100mm hub standards. This 5mm outboard movement affects the spoke bracing in the same way as the 3mm shift on 148mm rear hubs, increasing spoke angles and creating, "the stiffness of a 26" wheel" in the front as well.
Now here comes the tricky part. With these minor shifts in flange spacing, the brake rotor mount location and the freehub body spacing have to be taken into consideration. Everything is shifted slightly outwards. So newer forks with the Boost 110 standard have also been designed to account for a shift in rotor and caliper location. That's why Boost 110 is actually different than the 110x20mm thru axle standard that already exists. The 110x20mm hubs seen on bigger forks still have the same rotor mounting as a 100x15mm hub.
Rotor shift is simpler than freehub body shift, because now with Boost 148 the chainline has been altered by 3mm. With a 3mm increase in chainline, it is necessary to move the chainrings outboard by 3mm as well. Luckily this 3mm shift can be accomplished with a redesigned crank arm spider. The Q-factor of the crankset and the BB width remain the same. For these reasons we will now see the introduction of Boost 148 specific cranksets that are necessary to achieve proper chainline.
With the shift outwards, the frame is afforded more clearance, which then allows the frame designer more tire clearance and will maintain a short chainstay.
A perfect example of this change in crankset design is shown by Shimano's introduction of their B-series cranksets with the release of Deore XT M8000 11-speed mountain groups. Shimano released standard 1x, 2x, and 3x cranksets for their M8000 series, but also introduced M8000 Boost 148 compatible cranksets alongside them.
Boost 110 uses a 15mm thru axle and moves hub flanges outboard by 5mm when compared to 100mm hub standards (click to enlarge).
This is a good indication that while Boost 148 and 110 will be around, they will not immediately overtake the industry and force you to buy a new frame and new wheels.
So if you are purchasing a new bicycle, Boost standards are definitely something to consider. But if you already have a killer ride, then don't worry, you will still be able to source parts for your 100mm and 142mm hubs for the foreseeable future.