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Super Boost Plus 157 rear axle standard – is it here to stay?

by Martin Yang 22 Nov 2024
Super Boost Plus 157 rear axle standard – is it here to stay?
Launched in 2016, the Pivot Switchblade was the first bike with the new 157mm SB+ spacing

 

One little element in Pivot's 2016 Switchblade trail bike—its 157 mm rear axle spacing—caused more of a stir than the bike itself. Pivot referred to this as Super Boost Plus. Super Boost Plus compatible components, including cranks, hubs, and chainrings, were rare at launch. By 2020, fast forward, these parts are easily accessible. Is the 157 mm benchmark here to stay?

 

What does Super Boost Plus mean?

For trail and enduro bikes, Super Boost Plus (shortly SB+) is a "standard" for rear axle spacing. Originally developed by Pivot Cycles in 2016, SB+ improves on the enhanced rear wheel and frame strength resulting from Boost 148 mm spacing. Super Boost Plus expands the rear dropouts to 157 mm one step further. This figure was taken from the realm of downhill bikes, who have been utilizing 157 mm rear axle spacing as their standard for some time now but without the extra-wide flange spacing. This statistic wasn't snatched from thin air. Wider rear dropouts cause the chain line standard to also rise to 56 mm. Originally only employed by Pivot, Super Boost Plus spacing has lately been added by several companies, including Evil, Knolly (who calls it TRAIL157), and Devinci.

 

Super Boost Plus and 157 DH use the same axle spacing of 157 mm. Compared to Boost 148 spacing the chainline moves further outwards.

 

How does Boost 148 differ from Super Boost Plus?

Using 157 mm Super Boost spacing has the most significant advantage since wider hubs let for broader flange spacing. The spoke bracing angle is bigger the further apart the flanges—that is, the section of the hub where spokes are fastened. Not less significant is the ratio between the drive- and non-drive-side spoke angles. Both angles should ideally be the same to produce even spoken tension. Boost 148, on the other hand, is not feasible since the cassette body occupies a reasonable portion of the hub and the drives-side flange is forced inward. The flanges thus migrate towards the non-drive side rather than resting symmetrically on the hub. A so-called "dish" occurs from the rim being centered on the hub rather than the flanges.

 

The symmetrical flange position around the centerline of a 157 DH hub makes for an even spoke bracing angle…

 

… with Super Boost Plus, the spoke bracing angles are not equal due to the asymmetrical flange spacing

 

The rim revolves over the hub, as with all hubs. But on a Super Boost Plus hub, the unequal flange spacing causes the rim to be not centered between the flanges. The supposed "dish" is this offset.

 

SB+ lets the tension ratio between the drive side and non-drive side spokes be more equal than Boost 148. Whereas Boost 148 has a less equal dish ratio of 62/38, Super Boost Plus wheels have a dish ratio of about 60/40. Less dish means more even tension ratio and stronger and stiffer the wheel will be.

 

Interesting information for those SB+ detractors: the ratio is even on 157DH hubs.

 

Super Boost Plus has a somewhat more uniform dish ratio than Boost 148, but the advantages are quite nominal. Still far from the ideal 50/50 ratio of 157 DH centers.

 

What benefits are attributed to Super Boost Plus?

Particularly helpful to long-travel 29ers and eMTBs, where wheel loads are typically rather high, the overall outcome of the wider flange spacing afforded by a Super Boost Plus hub is a stiffer and stronger wheel structure. Although we highly value robust and durable wheels, the end aim is not necessarily enhanced stiffness by itself. Using SB+ allows bike designers additional room and freedom in building the drive-side chainstay since the chain line is shifting further outside. Depending on their design, this could reduce the chainstays and either improve or decrease the stiffness of the rear end. But with the current trend of 29×2.5” we hardly ever have problems with tire clearance, particularly as the super-wide 2.8” Plus tires are vanishing from the scene—we will not miss them. Regarding shorter and stiffer rear ends, these considerations should also complement the bike's intended function and general geometry. The greatest enduro and trail bikes available now are already somewhat stiff; there comes a point when more stiffness might cause an uncomfortable ride sensation.

 

The claimed benefits of Super Boost Plus include stronger wheels, stiffer frames and more tire clearance

 

What are the cons?

The first drawback is another new criterion; it is not a shocking one. But let us not forget that innovation is impossible without new standards before we all go to the forums. The true question is whether the gains exceed the drawbacks.

 

Compatibility

A new standard naturally has compatibility problems. Super Boost Plus calls not only fresh 157 mm hubs but also cranksets and chainrings fit for the needed 56 mm chain line. Fortunately for SB+ enthusiasts and converts, all of the main drivetrain, wheel, and crankset/chainring manufacturers now have Super Boost Plus compatible choices.

 

Q factor

Wider dropouts and chainstays forced engineers to design larger cranksets for Super Boost Plus frames in order to satisfy the 56 mm chain line production had to use. The effect is a larger Q factor, or the distance between the outside of one crankarm and the outside of the other. Your feet are more widely separated on the pedals the larger the Q factor. Comparatively to a Boost 148 bike, the Q factor of a Super Boost Plus bike sits at 173-177 mm with a regular 93 mm press-fit or 73 mm threaded bottom bracket. Still, this is only truly a modification for XC riders since enduro riders typically run bigger pedals set to 177 mm Q factor cranks anyway.

 

Heel rub can be an issue

Because the chainstays butt out closer to your heels and so reduce the space you have to move your feet on the pedals, one possible disadvantage of larger chainstays and improved tire clearance is heel rub.

 

While testing, we often find cracked rims; yet, we cannot recall the last time we took a spoke-for. Does wheel strength actually present a problem? Neither DT Swiss nor ourselves think so.

 

What is the industry's opinion?

We questioned several of the largest companies in the sector, for and against SB+, about their views on the topic. Dead or here to stay is Super Boost Plus? Though they have utilized the standard for their most recent all-rounder trail bike, the Pivot Switchblade 2020, Pivot, who first proposed it in 2016, is still far behind the concept.

"For Pivot, we can design a bike with better tire clearance, better strength, and stiffness in key areas—sometimes with less weight due to better optimization." Furthermore, spoke tension is more equal, which gives the wheel structure great longevity as well. That seems to be THE trail and extended travel bike standard. Once you move past the XC category, there are essentially no drawbacks. Since every gram counts and the rest of the bike is not as burley, I do not believe it is the ideal benchmark for XC.

 

Pivot introduced the standard back in 2016, and are still behind it to this day

 

Canyon answered by saying they are currently researching and see both advantages and drawbacks to the criteria. For them, though, there is not a need since they are content with the Boost 148 mm performance.

Merida answered with quite an anti-SB+ perspective. They are put off by the fact that the wider rear spacing renders current Boost wheels obsolete and complain that it creates heel friction. Though still using Boost 148 spacing at the dropouts, they are concentrating their developments around cranksets with a 55 mm chain line, provided by Shimano, which gives them an extra 3 mm of space between the chainring and the chainstays to optimize rear end stiffness and clearance.

 

Trek are still firm believers in the Boost 148 standard they pioneered

 

Trek answered with a similarly anti-SB+ attitude when we contacted them, saying: "We will not be adding 157mm spacing in the foreseeable future. It is not for a lack of pioneering energy. After properly examining this, the slight benefits of going to 157mm spacing are not worth the suffering involved. We can acquire the features and performance with 148mm separation between frame and drivetrain enhancements.

 

From our point of view on customer service as well as validated by field tests and our test laboratory, we find no justification for changing the current Boost 148 mm criterion. – DT SWISS

What then have the manufacturers of wheels and hubs to say? Industry behemoths DT Swiss responded: "We were already able to realize a flatter spoke angle, which resulted in a not just stiffer but above all more stable wheel construction when we shifted from 142mm to Boost 148mm. The symmetrical and broad flange distance of the 157mm DH standard nevertheless made it somewhat better than the Boost standard. The left flange slides just noticeably nearer the brake contact with the new 157 Super Boost Plus standard, producing an even flatter spoke angle. Though in theory these criteria offer benefits and drawbacks, it is crucial to consider the bike as a whole system. From our customer service standpoint as well as validated by our test laboratory and field trials, we find no justification for substituting the current Boost 148 mm standard. As a provider to the bike sector, however, we satisfy consumer needs as well as those of bike manufacturers, and thus we do not intend to stifle any innovation. In the near future, we will design all 157mm hubs with the broad Superboost flange distance since another standard merely causes more uncertainty for the end user. For downhill use of these hubs, nothing but varying spoke lengths changes in theory.

 

What do we think?

First of all, Super Boost Plus won't make a bad bike good or a good bike bad based on its rear axle standard. Indeed, Super Boost Plus lets you have more tire clearance, firmer frames, and stronger, more durable wheels. Still, it's crucial to keep in mind that these improvements are very nominal. The present Boost 148 mm standard has not shown any actual problems throughout the years; it has been the norm for a considerable time. We believe that the changes Super Boost Plus brings do not offset the reality that many individuals will be out there with useless wheels following a new SB+ bike purchase. There are several methods to make a bike better; we believe that the best one is to follow the simple path and implement a new standard with just minor adjustments.

 

This picture blends dish ratio, axle spacing, and chain line for all the bike buffs out there. See the varying flange angles on every hub and the matching dish ratio.

 

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