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Asymmetric Bottom Brackets BB30A PF30A
Asymmetric Bottom Brackets BB30A/PF30A
As if the current crop of bottom bracket standards weren’t confusing enough both shop mechanics and consumers now must contend with even muddier waters in the form of “Asymmetric” bottom bracket shells. Simply defined an asymmetric bottom bracket shell is not identical on both sides of the frame’s center line. Among the companies using these types of shells is Cannondale. In addition to using the BB30 & PF30 standards they now bring us BB30A & PF30A. The new “standards” widen the bottom bracket shells, but unfortunately, it’s not as clear cut due to where they add the extra material. Let’s take a closer look.
The dimensions of a BB30a shell are an inner diameter of 42mm and a shell width of 73mm, while the PF30a standard uses the same shell width but an inner diameter of 46mm. This is not quite as straightforward as it seems though. The extra 5mm when compared to a standard 68mm shell gets added just to the non-drive side, giving us a bottom bracket shell (when measured from the centerline of the frame) 34mm drive-side, 39mm non-drive side.
The problems happen when you’re trying to use a Shimano or SRAM GXP crank. Non-BB30 cranksets were designed around outboard bearing bottom brackets and their spindle lengths are calculated based off this type of bottom bracket. This is related to a measurement called “Over Bearing Width” (OBW), which is defined as the width of the bearings with dust shields placed over the bearings. For example, the over bearing width of our BB30-OUT/PF30-OUT bottom brackets installed in a traditional 68mm BB30 or PF30 frame would be 90.5mm. The “Over Bearing Width” is directly related to another measurement we can call “Practical Spindle Length” which is the available spindle length when measured between the inside faces of the crankarms, illustrated in the photo below.
In the case of a standard Shimano Hollowtech 2 or SRAM GXP spindle this measurement comes out to right around 93mm. Comparing this measurement against our 90.5mm OBW and there is plenty of room to install the crank. Things change when we attempt to install these same cranks using the previously mentioned BB30a/PF30a outboard bottom brackets in a frame with a 73mm shell. In the wider frame shell, our OBW moves out to 95.5mm leaving our “practical spindle length” 2.5mm too short. If the bottom bracket was symmetrical this would be a simple problem to solve, just reduce the OBW by moving the bearings further inboard, but with the asymmetric bottom bracket shell it’s not quite that simple. While moving the bearings in solves the problem with spindle length it creates problems with chainline and chainring clearance. Since the bottom bracket shell is asymmetric it’s necessary to move the bearings inboard on just the non-drive side as seen in the photo below.
By moving the non-drive side bearing further inboard we reduce the OBW to 91mm in a 73mm shell width while preserving chainline and chainring clearance on the drive-side.
So, there you have it, a quick primer on the BB30a/PF30a standards. As we mentioned in previous articles, with any bottom bracket questions don’t rely on what it’s called, get the dimensions of the frame shell and spindle length then go from there.
Always remember that unless the frame in the stand is a Cannondale you won’t need a Bottom Bracket with an “A” in the description. As of this writing Cannondale is the only manufacturer using the BB30A/PF30A system.
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