Introduction

When Pivot released the Switchblade trail bike in 2016, the bike itself didn’t make much of a splash, but one small detail stirred up a lot of discussion – the 157mm rear axle spacing, which Pivot dubbed Super Boost Plus. At the time, components that supported this standard, like cranks, hubs, and chainrings, were pretty limited. Fast forward to 2020, and these parts are now everywhere. So, is the 157mm standard just a passing trend, or is it here to stay?

The Pivot Switchblade was released in 2016 as the first bike to feature the 157mm SB+ rear axle spacing.

What is Super Boost Plus?

Super Boost Plus (SB+) is essentially a rear axle spacing standard designed for trail and enduro bikes. It was introduced by Pivot in 2016 with the goal of further strengthening the rear wheel and frame, building on the Boost 148mm standard. What makes Super Boost Plus unique is that it widens the rear dropouts to 157mm. This 157mm figure isn’t random, though—it’s borrowed from the downhill bike world, where 157mm rear axle spacing has been the norm for a while, just without the extra-wide flange spacing. With the wider rear dropouts, the chainline standard is also bumped up to 56mm. Initially, this standard was exclusive to Pivot, but now brands like Evil, Knolly (who call it TRAIL157), and Devinci have adopted it as well.

Super Boost Plus and 157 DH both use a 157mm rear axle spacing. Compared to the Boost 148 spacing, the chainline is pushed further outwards.

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What’s the difference between Boost 148 and Super Boost Plus?

The biggest advantage of using the 157mm Super Boost axle spacing is that the wider hubs allow for greater flange spacing. The wider the flanges, the greater the spoke bracing angle, which improves stability. Another important factor is having the same spoke angles on both the drive and non-drive sides, which ensures even spoke tension. However, Boost 148 can’t achieve this because the cassette body takes up space on the hub, forcing the drive-side flange to move inward. This results in an asymmetrical flange setup, with the flanges shifted towards the non-drive side. While the rim is still centered over the hub, the asymmetry leads to a so-called "dish" effect.

The symmetrical flange positioning around the centerline of a 157 DH hub creates a balanced spoke bracing angle…

With Super Boost Plus, the spoke bracing angles are uneven because of the asymmetrical flange spacing.

As with all hubs, the rim is centered over the hub. However, because of the uneven flange spacing on a Super Boost Plus hub, the rim is not aligned equally between the flanges. This offset creates what's known as the "dish."

Compared to Boost 148, SB+ provides a more balanced spoke tension between the drive side and non-drive side. Super Boost Plus wheels have a dish ratio of around 60/40, while Boost 148 has a less even dish ratio of 62/38. The less dish there is, the more even the spoke tension, which results in a stronger and stiffer wheel.

Although the dish ratio of Super Boost Plus is slightly more balanced than that of Boost 148, the improvement is minimal. However, it still falls short of the ideal 50/50 ratio found in 157 DH hubs.

What are the benefits of Super Boost Plus?

The wider flange spacing of the Super Boost Plus hub ultimately makes for a stiffer, stronger wheel, which is especially beneficial for long-travel 29ers and eMTBs, where the wheel load tends to be high. While we’re all for strong, durable wheels, simply increasing stiffness isn’t always the end goal. With the chainline pushed further out, SB+ gives bike designers more room and flexibility when designing the drive-side chainstay. Depending on the design, this can increase tire clearance, boost rear-end stiffness, or shorten the chainstays. However, with the current trend toward 29×2.5” tires, tire clearance is rarely an issue, especially since super-wide 2.8” Plus tires are disappearing—honestly, we won’t miss them. As for stiffer and shorter rear ends, these factors should align with the overall geometry and intended use of the bike. The best enduro and trail bikes out there are already very stiff, and beyond a certain point, more stiffness can actually make the ride feel uncomfortable.

The purported advantages of Super Boost Plus are stronger wheels, stiffer frames, and increased tire clearance.

What are the downsides?

The first downside is something we all saw coming – it’s yet another new standard. But before we all rush to the forums to complain, let’s remember that without new standards, innovation wouldn’t be possible. The real question is whether the improvements are worth the trade-offs.

Compatibility

Of course, a new standard comes with compatibility issues. Super Boost Plus not only requires new 157mm hubs but also cranks and chainrings that fit the 56mm chainline. Fortunately for SB+ fans, most major drivetrain, wheel, and crankset/chainring manufacturers now offer options compatible with Super Boost Plus.

Q factor

Due to the wider dropouts and chainstays, and the need to achieve the 56mm chainline, manufacturers had to design wider cranks for Super Boost Plus frames. The result is a larger Q factor – the distance between the outer edges of the crank arms. The bigger the Q factor, the wider your feet will be on the pedals. For a Super Boost Plus bike with a standard 93mm press-fit or 73mm threaded bottom bracket, the Q factor is between 173-177mm, compared to 168-177mm on a Boost 148 bike. However, this change mainly affects XC riders, as enduro riders typically use wider pedals mounted on cranks with a 177mm Q factor anyway.

Heel rub

One potential downside of wider chainstays and increased tire clearance is heel rub. With the chainstays pushed out closer to your heels, there’s less space for your feet to move on the pedals, which can lead to rubbing against the frame.

We often encounter broken rims during testing, but it's hard to recall the last time we broke a spoke. So, is wheel strength really a problem? We don't think so, and neither does DT Swiss!

What does the industry think?

We reached out to several major industry players, both those in favor of SB+ and those against it, to get their thoughts on the matter. Is Super Boost Plus just a passing trend, or is it here to stay? Pivot, the company that introduced the standard in 2016, is still fully behind it, and has continued using it on their latest all-rounder trail bike, the Pivot Switchblade 2020.

“For Pivot, SB+ allows us to design a bike with better tire clearance, increased strength and stiffness in key areas — sometimes even lighter weight through better optimization. Plus, spoke tension is more even, which greatly enhances the durability of the wheel structure. I see it as the standard for trail and longer-travel bikes. There really are no downsides once you move beyond the XC category. I don’t think it’s the best standard for XC because the rest of the bike isn’t as burly, and every gram counts.”

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

Canyon stated that they are still researching the SB+ standard and see both advantages and disadvantages to it. However, it hasn’t been a necessity for them, as they are already happy with the performance they get from the Boost 148 mm standard.

Merida, on the other hand, has a very anti-SB+ stance. They argue that the wider rear spacing causes heel rub and are frustrated by the fact that it makes existing Boost wheels incompatible. To gain more tire clearance, they are focusing on cranksets with a 55 mm chainline offered by Shimano, which gives them an extra 3 mm of space between the chainring and chainstays, helping to optimize rear-end stiffness and clearance, while still using Boost 148 spacing at the dropouts.

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

We reached out to Trek, and their response was similarly against SB+. They said: "We will not be adopting 157 mm spacing in the near future. It's not due to a lack of pioneering spirit... We've looked into it closely, and the cost of moving to 157mm spacing outweighs the small gains. Through frame and drivetrain improvements, we are able to achieve the necessary performance and features with 148mm spacing."

So, what do the wheel and hub manufacturers have to say? We reached out to industry leader DT Swiss for their opinion. They said, "When we transitioned from 142mm to Boost 148mm, we were already able to achieve a flatter spoke angle, which not only made the wheel stiffer but also more stable. The 157mm downhill standard was still a bit superior to the Boost standard due to its symmetrical and wider flange spacing. With the new 157 Super Boost Plus standard, only the left flange moves significantly toward the brake interface, resulting in an even flatter spoke angle. In theory, all standards have their pros and cons, but it's important to view the bike as a complete system. From our customer service standpoint, and confirmed by our test lab and field tests, we see no reason to replace the established Boost 148mm standard. As a supplier to the bike industry, we meet the needs of bike manufacturers and customers, and of course, we don’t want to block innovation. Another new standard would only create more confusion for the end consumer. So, in the near future, we will design all 157mm hubs with the wider Super Boost flange spacing. For downhill use of these hubs, nothing changes fundamentally, except for different spoke lengths."

What do we think?

First of all, don’t judge a bike’s performance just by its rear axle standard – Super Boost Plus won’t make a bad bike good or a good bike bad. Yes, it’s true that Super Boost Plus brings stronger, more durable wheels, stiffer frames, and more tire clearance. However, it’s important to remember that these improvements are only marginal. Over the years, we haven’t seen any real issues with the current Boost 148mm standard, which has been around for quite some time. In our opinion, the changes Super Boost Plus brings aren’t worth the problem of many people ending up with useless wheels after buying an SB+ bike. There are many ways to improve a bike, but taking the shortcut of introducing a new standard with only minor changes is, in our view, not the right approach.

For all the bike enthusiasts out there, this diagram shows the relationship between dish ratio, axle spacing, and chainline. Take note of how the flange positions vary on each hub and how that affects the dish ratio.

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