SuperBoost Axle Spacing: What is it Good For?

There have to be a few agreed-upon standards so that the mountain bike puzzle can all click together, and few are as hotly debated as Boost and Super Boost axle spacing.
Salsa’s Super-Boosted Spearfish. Photo: Hannah Morvay

Scrawling a list of how mountain bikes and automobiles differ would prove largely futile. The standout element might be that we buy a car as a complete system, and often we consider every component of a mountain bike before purchasing. Not many people purchase a sedan chassis and then mount components on it until they have a functional and legal road machine. Johnny Cash tried something like this with a Cadillac once, and the legal paperwork was rather cumbersome. The title weighed sixty pounds!

Bike components are made by a variety of manufacturers who don’t always work directly with frame designers, so there have to be a few agreed-upon standards so that the puzzle can all click together. As those standards shift, the former components or frames are often made obsolete, and rear-wheel spacing has suffered from standard shifts on a number of occasions.

Most early mountain bikes had 130mm axle spacing and quick-release skewers to clamp the rear wheel in the dropouts, matching road bikes of the era. Next came QR 135mm spacing, which was essentially the new MTB standard, and it was also used on some early cyclocross frames. When through-axles entered the scene, the first dropout gap was 142 x 12mm, and around that time forks switched to 100 x 15mm through axles. Take a gander at this article from Travis Engel for a deeper historical dive on fork spacing. The exception to the 142 and 100mm axle spacing was with downhill bikes, most of which had moved to thicker 20mm front axles and 150mm rear axle spacing. A collective grumble wafted over the global MTB community with the introduction of 142mm axle spacing, as it arrived while riders had a confusion of new bottom-bracket standards on our minds and a tapered-steerer fork to consider.

Then, rolling into town like a clanking luxury sedan with mismatched headlights and an ill-aligned transmission, we received the gift of Boost spacing. The 148mm axle standard promised stiffer and more symmetrically sound rear wheels, a better chainline for improved shifting and longer transmission life, and a broader platform for frame engineers to connect the tubes with. Why they didn’t use the 150mm DH standard that the industry already had, we may never know. Or maybe we do know: they wanted to sell more stuff. As that chorus of grumpy riders finished our bellyaching and changed our rear hubs once more, a few frame designers decided that 157mm SuperBoost spacing is what we should have had all along. It promised an even better solution to all of the problems that Boost spacing had addressed. But, was it actually better? And, why aren’t all bike companies switching to SuperBoost, as they did with Boost 148? We asked a few industry pros those exact questions.

Joe McEwan from Starling Cycles says that “SuperBoost is just another step further away from inclusiveness, meaning old bikes become redundant and we’re forced to buy new bikes.” Starling bikes are notably home-serviceable and practical, with pressed headset cups, classic BSA threaded bottom brackets, external cable routing, and no SuperBoost back ends that force clients to purchase new hubs.

Several larger frame brands have also steered clear of the super standard. In the case of Ibis Cycles, regular-old Boost makes sense for practical frame engineering reasons. Their design engineer, William Hilgenberg, who previously worked at a company developing hubs and cranks, is in favor of Boost 148. “SuperBoost chainstay spacing pushes the cassette outboard and allows the spoke flanges to move outward at the same time. This creates a marginally stiffer wheel and a wider swingarm and chainline. Unfortunately, if you don’t move the crank Q-factor outboard as well, you end up with a much greater chance of hitting your heel on the swingarm because of the additional width. When you use the Boost standard, you are able to retain the 52mm chainline (compared to 56mm of SuperBoost) and have a slimmer bike overall, reducing the likelihood of hitting your heel on the swingarm or your derailleur on a rock. Both options maintain their chainline with respect to the cassette but SuperBoost will end up wider because the cassette moves outboard.”

Heel rub is prevalent on Boost frames already. How can that be dealt with if the rear triangle grows wider?

While Hilgenberg clearly sees the wheel strength advantage of SuperBoost, the impact on a rider’s experience with their heels smacking the frame isn’t worth the potential benefits. On the flip, Pivot Cycles has embraced SuperBoost wholeheartedly, which makes sense since they are effectively the founders of the new standard. Pivot CEO Chris Cocalis shared some important numbers with us, as well as his reasons for adopting SuperBoost. “Drivetrains were designed around 142mm spacing with about a 49mm chainline. Boost moved the rear hub spacing to 148mm which is 6mm wider overall but moves the cassette out half of that (3mm) for a new chainline of 52mm. SuperBoost moved the rear spacing to 157mm which is another 9mm (4.5mm for the cassette) and a chainline of 56.5mm.”

Cocalis digs deeper into why that chainline shift matters, and why he sees SuperBoost as the better solution. “As the 157mm standard already existed, this was a logical stopping point without having to up-end the industry the way that Boost had done previously, and a 56.5mm chainline could still work on existing cranks even with narrow XC Q factors down to 168mm. Most trail and enduro cranks have between a 174-178mm Q factor which mates up really well with the 56.5mm chainline. It is also an easier to make a potentially lighter and stronger front chainring because the 56.5mm chainline results in a nearly zero offset chairing design. From a frame design standpoint, it is a game-changer.  We can achieve a much better balance between chainring clearance, tire clearance, chainstay length as well as critical stiffness and frame strength in a critical area because of that additional 4.5mm that SuperBoost gets us over a standard Boost set up.”

He adds that the benefits of SuperBoost extend to hubs and overall wheel systems. “With the wider 157mm hub and the adjustments that we were able to make to the hub flange spacing, we were able to find a balance that achieved more even spoke tension than a Boost 148 or standard 142mm hub for strength, and a more even feel from one side to the other as well as matching the stiffness of the front Boost wheel. Most 157mm hubs at the time had narrow flange spacing which worked for 26” DH wheels but was not great for 29” Enduro and DH setups. It took a few years, but now almost every major hub manufacturer has adjusted flange spacing on their 157mm hubs to follow the SuperBoost standard.” 

The new SuperBoost standard wasn’t just a Pivot thing. Cocalis was involved with a number of other companies that showed interest in wider rear axle spacing. “Shimano was interested in the concept early on. We sent them a prototype SuperBoost Switchblade frame two years before we launched the bike to develop parts with. It took them about five years to make it happen with the current generation of 12-speed components but it did happen. SRAM was not interested from the beginning as they were the driver behind Boost. However, both Salsa and Devinci approached me about developing new bikes using the standard. The three of us met with SRAM together and were able to convince SRAM to develop SuperBoost-specific cranks as well. Previous to this, Wolf Tooth and One Up had developed SuperBoost rings and spiders specific to SRAM and Shimano cranks that enabled SuperBoost compatibility. “

The Pivot Mach 4 SL is not SuperBoost. Photo: Cody Wethington

We also got a chance to ask Cy Turner from Cotic Cycles for his thoughts on fat hubs. As a frame builder and direct-to-consumer retailer, he has a unique perspective on the value of switching standards. “SuperBoost in my opinion is a classic example of the great getting in the way of the good. Yes it provides more chainring and tire space, and yes, the rear wheel is stiffer than Boost 148, and yes it would have been better to go straight to it from 142, but we didn’t. We have almost completely proliferated Boost 148 throughout the industry, and it works. Nobody is moaning about flexy wheels anymore, even on enduro 29ers, and it’s great for consumers and brands that some level of interchangeability has settled back into the industry. As a brand – particularly a brand that still sells a lot of frames on their own for customers to swap parts – moving again to 157mm where part availability is still limited? The juice isn’t worth the squeeze.”

“As a side note, one bad thing about SuperBoost is that by retaining regular width/Q factor cranks (in a desperate bid to keep inventory down because Boost 148 width arms are already here) there’s not actually any more space with a SuperBoost setup for chainstays between the cranks. As an avowed maker of metal bikes, the bends and shapes required to both take advantage of the extra chainring clearance, whilst still squeezing between the same width crank arms, then swinging the end out far enough to reach a wider dropout are almost impossible in steel. At least not without using a really heavy, thick wall tube that can be bent to quite extreme angles, which increases weight, and cost, at the expense of ride feel. All for a slightly stiffer rear wheel, which no one is asking for.”

So it’s clear which side of the super-debate Cy rides on. For his frames, with the current component selection, it doesn’t make sense. Over at We Are One Composite, Dustin Adams sees some important performance advantages to the wider spacing. The brand’s newly-announced bike, The Arrival, features a SuperBoost rear triangle. Adams says “I personally feel the bike is very stable at speeds with a 157mm rear axle. The disadvantage is going to the wider Q-factor of 173+ as the bike will take more body English to whip around or tip from corner to corner. So we chose to go between the two, and run with the benefits of the rear 157mm axle out back and the 168mm Q-factor [cranks] to keep the bike ready for action instead of planted all the time.”

Adams adds that in addition to affecting the bike’s ride quality, SuperBoost also allows for better drivetrain alignment which should result in longer component life. “The benefits to this for a trail bike is that in the key gears where you spend most of your time pedaling, your chainline is more optimal and puts less friction on the larger pie-plates we have all become accustomed to for our lowest gears. When we designed our bike we all agreed that bad chainlines for trail bikes are not being addressed where it counts. We were sick of wearing out drivetrains from climbing and trail riding in our lowest gears and felt it could be done better. So we took the benefits of shifting the cassette outward and keeping the front ring in place for Boost spacing.”

The Devinci Spartan uses what the company calls SuperBoost Plus. It’s still just the same 157mm SuperBoost spacing. Photo: Matt miller

Another piece of the SuperBoost puzzle is the hub, and Industry Nine VP Jacob Mcgahey shared the positive, neutral, and negative aspects of 157mm hubs. To start off with the sugar, he said that “the wider hub allows for a wider hubshell bearing spacing and a wider spoke bracing angle – together this improves the strength and lateral stiffness and of the wheel.” Additionally, “SuperBoost opens up more options with regard to frame design as the drive-side chainstay can be spaced wider near the bottom bracket allowing for additional tire clearance and/or a wider chainstay adjacent to the chainring for additional strength/stiffness. [It also] allows for a wider main pivot on many frame designs for improved torsional stiffness between the main triangle and rear swingarm.”

The neutral element of SuperBoost that Mcgahey shared is that “the wider cassette placement requires a different offset for the front chainring vs. a Boost hub in order to maintain the correct chainline.” Likely, if SuperBoost caught on, most chainring manufacturers would create offset rings to accommodate.

Finally, the bitter pill for Mcgahey echoes some of the concerns mentioned above. “The wider rear derailleur positioning can make the derailleur more susceptible to impact damage from rocks or other trail obstacles, [and there are] reduced Wheel options; some wheel companies do not offer a full range of wheels with 12x157mm hub options, particularly in their lightweight models. However, Industry Nine does offer all of our wheel models with SuperBoost hub options including our lightest 24H alloy spoke options.” And as a last look at negatives, he calls out “reduced crank options on some frames: some (but not all) SuperBoost frames require a crankset with a longer spindle which increases the Q-factor (distance between the pedals) and reduces the number of crank options.  However, most of the major crank manufacturers now offer options to fit these frames.”

To say that “there’s a lot to consider” is an understatement, and that’s likely part of the reason why the whole bike market isn’t yet super. Maybe it never will be, and these two standards will exist in tandem. If Johnny Cash could get a whole car together from parts spanning more than a decade, there’s a good chance we will be able to continue building up sweet mountain bikes regardless of any new numbers.

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