Change, it has been noted, does not occur in a vacuum. Change is a dynamic force, a reaction to shifting needs. Once change occurs, the scene is then set for more changes to be enacted. And so we evolve. A recent tectonic movement in the mountain bike world that is still rippling through the industry was the widespread shift from the relatively freshly adopted 142x12mm rear hub spacing to an even newer 148x12mm spacing. 6mm, not much, but it caught many people on both sides of the bike – industry and consumer alike – with their metaphorical pants down.
The shift to 148mm “boost” spacing was not, contrary to the opinions of conspiracy theorists, an industrywide effort to shove planned obsolescence down the throats of unsuspecting consumers and force them to buy new bikes every year. If you believe that, cruise over to our True Deals section and pick yourself up some deeply discounted 142x12 wheels. Instead, it was the result of an evolution in componentry that has been ongoing for, ohhhh, about a century. It is not the first time hub spacing has abruptly flared outward by a few millimeters, and we are willing to bet it won’t be the last. In order to get our heads around the “why” of this, let’s take a walk back through the long and varied history of the bicycle hub.
In the beginning
Ahh, the beginning. In the beginning, bikes had huge front wheels and tiny rear wheels. Riders pedaled the front wheel directly, crankarms attached to the front hub, and the diameter of the wheel determined the bike’s gearing. This is where the term “gear-inch” originated. In order to try and create some sort of bracing angle and subsequent strength for a 52”-or-so diameter wheel, the hub flanges were wide-set, resulting in front hub spacing of around 170-180mm. With the advent of the “safety” bicycle in the late 1880s, where the front and rear wheels were sized the same, wheel diameters came down to around half that of the old Penny-Farthings, and because there was now a centrally mounted crankset driving the rear wheel with all the subsequent chain alignment issues that would become part and parcel of the bicycle experience for the next 130 years, rear hub spacing narrowed way down. One cog mounted to a rear wheel, maybe a coaster brake for the luxury of deceleration by some form other than dragging a shoe on the ground, bolted into arrow straight chainstays – for the next few decades, and found even today on track bikes and BMXers, rear hub spacing was a modest 110mm.
In spite of the protests from the purists, by the early 20th century, riders were clamoring for multiple gears. Internal geared hubs showed up – Sturmey Archer, Pedersen, Sachs all hit the market between 1902 and 1904, and by 1909 there were 14 different companies offering internally geared, 3-speed hubs. These newfangled hubs took up a little more real estate than the simple hubs preceding them, and so rear triangles found themselves getting spread apart just a hair, to around 114mm. Meanwhile, looming on the technological horizon, casting a shadow that would define almost every single rear hub spacing dimension change to follow, was the rear derailleur. Henri Desgrange, the father of the Tour De France, saw the coming of multi-speed bicycles, and in 1902 uttered the famous words “Isn’t it better to triumph by the strength of your muscles than by the artifice of the derailleur? We are getting soft.” If his position had stuck, bike builders would have faced far fewer design challenges over the coming century.
The derailleur won out, showing up at Le Tour in 1937, and signaling the onset of a drivetrain arms race that started out quietly enough, but would turn red-hot by the end of the 20th century. From the late 1930s until the 1980s, a rear hub for a bicycle was basically defined by a thread-on 5-speed freewheel, a 9mm diameter axle, and 120mm spacing. Little jumps, wider and wider. But here we are, 120mm spacing already. And only 5 gears to account for.
The Arms Race
In the 1980s, a whole lot began happening concurrently. Six, then seven, then eight speed rear gear clusters arrived on the scene. Mountain bikes became increasingly popular. And the thread-on freewheel as we know it reached the end of its evolutionary line.
Six- and seven-speed freewheels saw chainstays nudge out a little wider, to 126mm. But the move to eight, then nine-speed rear clusters really threw a wrench in the works. Rear axles found on freewheel style hubs had to contend with over an inch of unsupported axle hanging out between the drive side hub bearing and the dropout. Strong road riders and cyclotourists had plenty of experience with bent rear axles, but the new crop of mountain bikers wreaked havoc on the spindly little things. So, by the early 1990s, there was a wholesale shift toward the much stronger axles found in cassette hubs (thanks to the hub bearings on the drive side moving all the way outboard to close to the axle tip), along with the advent of index shifting which made it much easier for riders to embrace the proliferation of gears sprouting out of rear wheels. 130mm became the norm for road spacing, and mountain bikes jumped eagerly to 135mm, and everyone was happy.
For a little while.
Thing was, the distance between the hub flanges was getting narrower as the number of gears increased. This led to less available spoke bracing angle, and less ability to symmetrically dish a rear wheel, which meant that overall wheel strength began to be compromised. Then disc brakes got added to the mix. Then people rightfully began to question the sanity of holding those wheels – and all the massive braking and torque forces operating on them – in place with a cam that clamped a 5mm piece of wire running through a hollow 9mm axle; an “innovation” that had genuine merit when it was thought first thought up in 1917.
Welcome to the 21st Century!
In recent years, the quick release axle, as pioneered by Tullio Campagnolo, has thankfully slipped out of the mix with regard to performance mountain bikes, and is even fading from favor on the road. MTB rear axles are now 12mm, sliding through machined dropouts that completely enclose the axle. Brake calipers and derailleurs are always aligned where they should be, and it is impossible to install a wheel incorrectly. This is change for the good. And, as gears have continued to proliferate to 10-speeds and beyond, rear hub spacing stepped outward again, to 142mm. More room for the gears, room for the brake rotor, better bracing angle and wheel dish for stronger wheels, everyone rejoice!
Meanwhile, modern metal forming technology, the onset of molded carbon fiber frames, and the advent of 1x drivetrains allowed designers to shorten chainstays while still maintaining reasonable crank offset. The fact that we can ride long travel full suspension 29ers with shorter chainstays than our 26” race hardtails of a decade ago is pretty damn amazing. Take a moment to think about all the layered technological evolutions that have allowed that reality to happen. It’s awesome - change upon change that allowed the next wave of evolution to occur.
And this is where we get to Boost spacing, another little 6mm jump from the new-new of just a few short years ago, and why it is a good thing, and not part of the global conspiracy to make you buy new stuff. There are now TWELVE gears attached to the back of today’s high end mountain bike! That’s two more gears than your dad’s racing ten speed without even needing a front derailleur. That’s eleven more gears than Henri Desgrange ever imagined would be found on a Tour De France racebike, and he never in his wildest dreams would have envisioned something like the Red Bull Rampage. There’s a super powerful disc brake. There’s a wheel, maybe a 29” wheel, that can be built light and strong thanks to wider hub flange spacing. There’s room to fit a fat tire in short chainstays without interfering with the chain. There’s a front wheel that is symmetrical and strong.
This is evolution. In technology, as in nature, a dynamic landscape where adaptation and advantage are shaping our experience. This evolution is incrementally making the bikes we ride better. There’s no way you could convince Jared Graves to compete on the EWS circuit with a decade old Stumpjumper, nor would you find Peter Sagan very stoked about racing Paris-Roubaix on a 1957 Atala. The move to Boost spacing incrementally improves the breed, just like the move to 142x12 from 135x10 improved the breed. It’s a little better than what was before, not necessarily a game changer, but an important rung along the evolutionary ladder. Each change as it happens seems like it offers a small advantage – a questionable advantage in the eyes of some. But as each change occurs, and generates leverage for other advances, the whole process evolves. Welcome to the future.