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Axle Wars: Is There Really Room for Two Rear-Hub Standards?

Super Boost Plus hasn't been widely adopted yet. Will it be, or will Boost 148 continue its dominance?

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Dropout spacing was one of the most stable standards on bikes, until it wasn’t. In 135×10’s 20-year run, bottom brackets, head tubes, steerer tubes, seatpost diameters, handlebar diameters, wheel sizes, and pretty much everything else, all changed. Sure, there were some axle variations, but 135 was by far the dominant rear-end spacing from around 1990 through 2010. Hell, in that year, Specialized’s Demo 8 DH bike still had 135x10mm open dropouts.

The 135 held on for as long as it could, but like everything around us, progress got the best of it. We just couldn’t stop stuffing things between the dropouts: More gears—always more gears. Then came disc brakes, all these things pushed hub flanges inboard, which actually made wheels less laterally stiff and more prone to taco. And if that wasn’t enough, wheels and tires were getting bigger and beefier as bikes struggled to keep up with the rapidly changing world of mountain biking.

Thru-axles came onto the trail bike scene roughly a decade ago and when they did, an upheaval ensued. The first blip was 135×12, which was quickly replaced by 142×12. All that did was make installing wheels into frames easier by adding 3.5mm to each axle cap and a recessed area for the hub to locate and drop into. With 135×12, you had to line the hole in the hub with the holes in the frame yourself, and hover the wheel in place while trying to stab the axle through. 142 took care of that problem but didn’t address the fact that 29ers were all the rage even if the wheels were limited by the still-narrow hub-flange spacing. Oh, and we were still sticking more gears on the cassette. We’re never not doing that.

A New Solution

To solve the wheel strength and real estate problems plaguing 29ers, Trek and SRAM partnered to develop Boost 148, which debuted in 2014. Boost pushed the hub flanges out by a few millimeters on each side, which resulted in enough improvement in wheel stability for modern bikes and the way we ride them. Most people would agree that the key elements we’re looking for to make strong, reliable, shreddable bikes can be accomplished with Boost 148 rear ends. This was evidenced by the fact that Boost gained industry-wide acceptance almost overnight.

“When we went from 135/142 to Boost, we had simply run out of space,” explains Dylan Howes, director of MTB frame technology at Trek. “Wheels and tires were getting bigger in width and diameter, bike and component design was evolving, we simply had to make changes to keep improving bikes.”

But it was a new standard—the third one in almost as many years—so when it came out, the obvious question was: If you want to go wider, then why not just make all bikes use the downhill axle standard that had already been established? 157 was a thing. Surely we could just adapt it for use on trail bikes. Howes explains why they didn’t: “In order to get the chain line correct at the time, we couldn’t push the cassette out any further. 3x drivetrains were already fading at that point, but even with 2x rings, in order to use a 157 hub, we would have had to move the chainrings out further. But within existing cranks, we’d have needed to widen the cranks and the Q-factor to fit those further out rings, which we didn’t want to do.”

Not Far Enough?

We thought we’d finally established the next long-running axle standard. We had a couple good years with it, everyone jumped on board, and Boost front spacing followed closely behind, solving the wheel strength issue at the business end of the bike. Then Pivot threw a wrench in the works when it released the 2016 Firebird with Super Boost Plus (SB+).

It was a good punny name and it answered the question of 157mm spacing viability on trail bikes. The answer is yes, it can be done, and it has some advantages over 148. You can move the hub flanges out even farther, making wheels stronger again. Frames can be made stiffer, chain- stays shorter, and there’s more room for tire clearance as well.

This all seems better. If wide is good, then wider is clearly better. And maybe it is. Maybe we should all be on SB+ bikes. The thing is, we’re not. Since the introduction of SB+ five years ago, only a handful of companies have hopped on the Super Boost bandwagon. How come?

The market is funny that way. Take Shimano’s Center Lock disc rotor mounting standard. It makes 6-bolt look ridiculously outdated. I think most of us can agree that from an engineering and convenience stand- point, Center Lock is better. But it just hasn’t managed to take over as the preferred way to stick rotors on hubs, and do we really need two different rotor mount standards floating around? No, we don’t. Six-bolt might be less neat, but it works. When there are two good options, may- be one just has to go. Even if it’s the better one. Am I suggesting that SB+ should go to the standards graveyard? I don’t know, but it doesn’t appear to be beneficial as it might have seemed when it came online. Howes likely has some bias, but I have a hard time arguing his points when he explains why:

“Shorter stays don’t matter as much any more. Bikes are getting longer and need longer chainstays to balance weight distribution. No one is worried about being able to have the same chainstay length as their old 26-inch bike anymore. Yes, SB+ can offer more tire clearance, but as we seem to have leveled out at 2.5×29 tires being the best balance of charac- teristics we all like, and 2.6 and 2.8 tires aren’t a thing any more, combined with the shortest chainstay length possible not being a desirable trait, this is pretty much a problem that can be solved with good chainstay design. Stiffer wheels? Yes, a well-designed 157 hub can create this, but 148 frames and wheels aren’t suffering because of a lack of stiffness.”

Standard Fatigue

He continues: “Once we had Boost, SB+ was more of a reaction of, ‘Wait, we can still make a stiffer wheel, we can still do more.’ But Boost was already established by then. Wheels weren’t getting bigger again. As we already had a good solution with Boost, going to SB+ didn’t add enough real true advantage to justify yet another standard.”

Keep in mind, this is coming from a man who helped create at least a couple of them. Still, the majority of the industry seems to be in agreement by continuing to mostly produce Boost bikes. But who knows what bikes will look like in 10 years. Maybe SB+ is playing the long game. Even Howes admits that with the benefit of hindsight, perhaps they’d have gone slightly wider than 148mm. “Not all the way to 157, but somewhere in between”.

Super Boost Minus 154. You heard it here first.


Photo: Anthony Smith