This talk will involve some technical stuff about MTB. It may not be interesting without relevant knowledge, but we’re sorry, we are not elaborating on all that because there is just too much.
Back when MTB was born, component standards were pretty simple, with dimensions of parts unified in most cases. Like if you buy MTB stuff somewhere, you can put it on yours. It was easy to mash two bikes into one. In the last decade or two, though, bike companies started coming up with their own standards, making it a nightmare for you if you wanted to build your own bike from scratch. The kind of diversification as seen in the wheel size starting from 26″ to also include 29″ and even 27.5″ later has happened to virtually every part of the bike and fitting standards therein.
On one hand, this is great. It has made it easier to build a bike that suits the rider’s build and preferences. It’s also just necessary sometimes to step out of existing standards if you are striving to create the ultimate industrial products. Without such attitudes, the Ahead system deriving from the MTB world would not have taken over all sport bike segments including road racers and BMX. We, too, employed our original BB standard for our Tanatos frame because it would just make some industrial sense to us that way.
On the other hand, as bike designers, we are skeptical about “new” standards without reasons. Case in point: Boost standard for MTB front hubs (accompanied by the new rear hub design, which we’ll skip on as there is just too much to mention).
Front hubs of sport bikes, mainly those equipped with multiple speeds, namely road and MTB, typically used to have a 9mm shaft with quick release. The width, called the over lock nut (OLD) length, was 100mm. In line with standardization of suspension forks and disc brakes, however, mountain bikes especially those in mechanically demanding usage such as downhill saw the rise of 20mm axles in 110mm width since around the end of the 20th century. The thicker axle was to compensate for the loss of rigidity owing to the structural limitations of suspension forks. The wider hub contributed to maintaining the lateral rigidity of the wheel with its greater spoke flange widths compensating for the space given to the disc rotor.
Then about a decade ago, the next standard with 15mm shaft and 100mm OLD appeared, claiming its merit in weight saving thanks to downsizing. I would say it’s 99% nonsense, with the remaining 1% being stingy reasons about stimulating new demands out of saturated markets. That’s what we suck at anyway hence is what we don’t care much about.
To put the design problem simple, the claim that the thinner tubing makes it lighter will get you something like the Alan road frame back in the day. Sure, it’s easier to make it light with smaller diameter, as long as you don’t care about rigidity. Those 20mm and 15mm hub shafts are usually made of aluminum, a material that usually has a better balance between weight, rigidity and strength if designed with a larger diameter with thinner wall thicknesses (specifics depends on material characteristics, but aluminum performs better with ever larger diameters typically). If they were serious about ultimate cross-country performance, it would have made more sense to create a thin-wall 20mm shaft. Sorry it’s not even theoretical analysis but pure gut feeling of mine, but I’d guess that’d be lighter and stronger than regular 15mm shafts. On a side note, hub manufacturers offer a bunch of hubs compatible with both 15/100 and 20/110, which realistically means there are’t many of those optimized for either standard.
Now, the industry has come up with new stuff, the Boost standard with 15mm shaft and 110mm OLD. If I’m to elaborate, the wider flange width ensures plenty of strength for large wheels, with proven lightweight. All the better than the previous 15/100, so why not replace your current one? To put it simple in my own words, that’s the death of 15mm standards. Lightweight-minded with all the sacrifice you could ever think of, as if in a case of a boxer with his hair shaved and teeth pulled out for weight loss, and he’s saying, yeah let’s go back to 110 heavyweight, I’ll show em what I’ve got. He’ll die. The longer span means more diameter needed to ensure rigidity anyway, right?
So here is my conclusion, full of biases based on personal preference so it’s up to you whether or not to swallow. The 15mm standards have gone in a full circle and are going to die off, handing their foothold back to the good ol’ 20/110. If the industry was serious about optimization they may land on something like 25/120, but it’s going to take a lot to rewrite whatever exists. We may witness the scene revert to 20/110 in a few years, and see a new struggle for the very best that may likely entail not only the dimensions but also the hub fixing mechanism in a decade or so that follows.