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Tech tussle: Is it time to ditch the press-fit bottom bracket?

Cyclist tech editor Sam Challis thinks a return to threaded BBs is a backwards step, while web ed Matthew Loveridge is all for it

Much fanfare has accompanied a recent shift back towards ‘BSA’ threaded bottom bracket systems on road bikes.

After more than a decade of various press-fit standards, big brands such as Specialized are designing around threaded BBs again, to the point its flagship Tarmac SL7 uses one, as do the Allez Sprint and Aethos.

Previous iterations of those bikes used Specialized’s OSBB 61 press-fit standard. (Top pub factoid: BSA stands for Birmingham Small Arms, which used to make guns in the 1860s, turned to bikes parts in 1893 but which hasn’t made a BB since 1957, when it was bought by Raleigh. The terms English and ISO are often used interchangeably now when referring to the standard.) 



Photo: Lizzie Crabb

Cannondale was the brand that ostensibly started the trend for press-fit BBs in road bikes with its BB30 innovation in 2000, but even it has reverted to threads for its latest Synapse Carbon and Topstone Carbon bikes.

Yet press-fit systems were heralded as a step forward from threaded systems when they emerged, and I think those advantages still exist today. To explain what they are, it is worth detailing the design of each system.

What is a threaded bottom bracket? (AKA ISO, BSA, English...)

How to change a bottom bracket

Threaded bottom brackets consist of a pair of cups, generally housing bearings, which in carbon frames screw into a threaded alloy sleeve bonded into the bike’s bottom bracket junction, and in metal frames screw into threads cut directly into the bottom bracket shell.

ISO is the prevalent BB standard, which on road bikes requires a 68mm-wide bottom bracket shell of 1.375in diameter, and typically accepts crankset axles that are 24mm in diameter (Italian threaded BBs are another story – 70mm wide with a 36mm diameter shell).

What is a press-fit bottom bracket?

By contrast, most press-fit bottom brackets use cups that press directly into the bottom bracket shell, although in some cases (BB30, BB90) the bearing cartridges fit straight into the frame, with no cup to act as an interface.

After Cannondale’s BB30 became an open standard in 2006, all sorts of variations on the theme were developed by brands seeking to optimise the standard with respect to their own frame designs.

I will readily admit to there being a few less-than-great interpretations of the press-fit standard, and it’s these that generated the stigma that now surrounds the systems through tales of persistent creaking and short bearing life.

But I don’t think this is fair in all cases, because there is one design, released by the usually conservative Shimano, which fully exploits the potential advantages: BB86, and its more versatile cousin, BB386 EVO.

BB86 uses a bottom bracket shell that is 86.5mm in width, and fits 24mm crankset axles; BB386 EVO uses the same width but accepts 30mm axles as well as 24mm axles with a step-down adapter.

The standard therefore serves all comers in the crank department, but there are even more compelling reasons to recommend it.

Why press-fit is better

BB86 and BB386 EVO press-fit systems are an inherently simpler solution. Threaded cups require a dedicated sleeve to be bonded into a carbon frame, whereas press-fit cups directly interface with the frame.

The bearings used in headsets, wheel hubs and even pulley wheels do this, so why should the solution at the bottom bracket be any different?

As any mechanical engineer would attest, fewer parts means less potential for things to go wrong.

Not having to bond metal sleeves into carbon frames also reduces fabrication costs and speeds up production by streamlining the manufacturing process, including assembly – screwing things in takes longer than pushing things in.

Fewer subcomponents means frames can be made lighter, but there are other significant performance benefits too.

By widening the bottom bracket shell from 68mm to 86.5mm, tubes in the area can be widened too, because there is a larger BB area to bond to.

This significantly increases lateral stiffness without much increase in material, because the tubes’ second moment of area is larger.

In a similar vein, this wider BB means chainstays can be spaced further apart to increase tyre clearance without having to resort to creative solutions to squeeze everything in.

Or equally, those same solutions, such as dropped driveside chainstays, can be applied in combination to really maximise tyre clearance.

But if all this is true, why are we moving away from press-fit? It has been reported that the issue isn’t the system itself but the ability to achieve consistent and accurate manufacturing tolerances for the frames that use it.

So press-fit is better, we just need to get better at making it. And when we do, our bikes will be all the better for it.


Matthew Loveridge: Press-fit BBs can get in the sea

Photo: Canyon

Everything Sam says about press-fit bottom brackets is true, but I don’t care. 

I fully accept the engineering arguments. Press-fit is indeed a more elegant system and in a world where everything was manufactured to ideal tolerances, I’d be perfectly happy with it. 

I’d also echo Sam's feelings about BB86. Of all the press-fit standards, it’s the one I’ve had the least trouble with. And no, you don’t need a 30mm crank spindle to handle your megawatts. If 24mm is big enough for Mathieu van der Poel, it’ll do the job for the likes of you and me.

However, for all the purported advantages, press-fit will always offer a worse experience for the rider and bike owner if you’re the sort of person that maintains their own kit. 

Source: Cat3memes

While a threaded BB can be replaced in about five minutes using basic tools, a press-fit usually demands a modicum of violence to remove, while installation requires a press (duh). 

I don’t know about you, but I don't relish swinging a hammer anywhere near a carbon frame, and the use of a press demands enormous care. 

The comparison to headsets and hubs doesn’t fly for me either because the loads imposed on those components are rather different.

A bottom bracket is subjected to immense torsional forces, so even the slightest hint of movement between mating surfaces will lead to noise and potentially long-term damage.

Both types of bottom bracket can fail when they’re not manufactured correctly, but for pure ease-of-use, threaded still wins for me, particularly on aluminium, steel or titanium frames where threads can be cut directly into the metal.

Photo: Matthew Loveridge

To my mind the Shimano Hollowtech II standard in particular offers the best all-round balance of performance, longevity and ease of maintenance. Press-fit can do one.


Main photo: Matthew Loveridge

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