We go into detail about each of the categories listed above to give you a better idea of what the purpose of each type of bottom bracket is. T47 Standard Oversized press fit bottom brackets have their advantages. But a significant downside is that they can creak or squeak. The T47 BB was designed to overcome the problem of creaking bottom brackets.
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SUBSCRIBE The evolution of road bikes may have provided cyclists with frames that are lighter, stiffer and more aerodynamic, but there has also been a substantial increase in the number of bottom bracket and crank axle designs. The once ubiquitous threaded bottom bracket shell has given way to larger threadless designs while the diameter of crank axles has also grown. Both may have helped elevate the performance of contemporary road bikes but consumers have been left to contend with myriad options and some frustrating incompatibilities.
In this post, Matt Wikstrom updates his original article from with a look at the range of bottom bracket and crank axle designs that are found on the market today and discusses the details that are important for matching one to the other.
When I started working as a mechanic in the mids, there were essentially two types of bottom brackets for road bikes, English- or Italian-threaded. The only other thing that varied was the length and offset of the crank axle. Two things stand out from that time: first, servicing a bottom bracket was labour-intensive; and second, there were rarely any complications when fitting a new crankset to a frame.
Few riders ever seemed to complain about clicking or creaking from the bottom bracket, however the cranks were troublesome, sometimes creaking, or more often, coming loose on the square-taper axles that had dominated the industry for a couple of decades. Things were changing, though. In , Shimano started bringing sealed bottom bracket bearings to the masses with the introduction of its innovative cartridge system, and then came Octalink in , a larger diameter crank axle with a new interface for the crank.
Meanwhile, Magic Motorcycle developed external bottom bracket bearings so that an even larger axle could be used for its ground-breaking cranks. From that point on, the weight, stiffness and reliability of cranks improved, plus, they were easier to install and service. BSA-threaded shells are a common sight for frames made from metal, be it steel, alloy, or titanium.
Cannondale has remained true to BB30 since it introduced the design in BB86 was the first threadless shell that was specifically developed to suit carbon fibre frames.
PF30 is a direct descendant of BB30 with a slightly larger shell diameter so that cups could be used with the same bearings. The first of the new wave of bottom bracket designs broke in when Cannondale unveiled BB30 at the Tour de France.
The oversized threadless shell was designed around a 30mm crank axle with bearings that were pressed directly into the frame. Rather than patent and protect the design, the company offered it openly to all frame manufacturers to encourage uptake by the industry. Importantly, none of these designs would go on to become a standard for the industry and to this day manufacturers remain free to adopt or invent any bottom bracket design that satisfies their needs.
While consumers may have grown weary of innovation in this realm, there is no indication that it is slowing down. A closer look at the range of bottom bracket designs When manufacturers started developing new bottom bracket designs, two things happened: first, the diameter of the shell increased; and second, the width of the shell tended to increase, too.
Both allowed engineers to increase the lateral stiffness of the crank and the frame, which has been embraced with enthusiasm by racers and enthusiasts alike. Figure 1: a comparison of some common bracket bracket shell designs for road bikes. A Shell diameters have increased with the introduction of new designs.
B The width of the shells has also tended to increase, and while most a symmetrical, there are some asymmetrical designs. One thing that should be obvious from comparing the various shell diameters is that they will all require different-sized cups or bearings. Unfortunately, frames are not often marked with the specifications for the bottom bracket shell, and it can be difficult to determine what they are on the basis of sight alone especially when the cranks are installed.
Table 1 details the key specifications for many, but not all, bottom bracket shells that can be found in road bike frames today. The number of threadless shells clearly outnumbers threaded shells because they are better suited to composite materials.
Nevertheless, some steel, alloy and titanium frames feature threadless bottom bracket shells, just as some composite frames have threaded alloy inserts for fitting threaded bottom bracket cups. Table 1: Overview of the specifications for common bottom bracket shell designs found in road bikes. Crankset compatibility In general terms, most cranksets can be fitted to a variety of bottom bracket shells, though much of this compatibility depends upon the availability of suitable hardware see next section.
In the past, crank axles were classified according to length and could be exchanged to suit different frames. That no longer applies to most contemporary cranksets since the axle is integrated with one of the crank arms, so if the axle is not long enough for the bottom bracket shell, then the crank will have to be replaced.
Table 2: overview of specifications for the common crank axles for road bikes. Thus, any incompatibilities may be not be obvious until an attempt is made to install the crank.
In practise, though, the matter is often decided by the availability of hardware for any given crank and shell combination. The diameter of the axle can have an impact on the compatibility of a crankset, though this is much less common. The narrow diameter of BSA- and Italian-threaded shells can also be problematic for 30mm axles, however oversized external bearings can address at least some of these difficulties.
Table 2 details the diameter of the axles for many, but not all, cranksets on the market along with known incompatibilities. For those cranks with mm axles, steel is typically used, while 30mm axles are normally made from alloy. The difference in material has some impact on the final weight of the crankset, and indeed, the lightest cranksets favour 30mm axles. While modern road cranks are compatible with many different types of bottom bracket shells, some combinations are better than others.
Shimano left and GXP right crank axles share the same diameter — 24mm — however the end of each axle is quite distinct. The diameter of the Shimano axle remains unchanged while the GXP axle steps down to 22m to provide a shoulder for the non-drive-side bearing.
The difference affects the placement of bearings on each axle and therefore requires different hardware for installation. A Praxis M30 top and BB30 bottom crank axle share the same diameter, however like GXP, M30 steps down to a smaller diameter 28mm at the end of the axle to provide a shoulder for the non-drive-side bearing. Most come to life as a proprietary feature for a new frame design. Look created its massive BB65 shell in order to accommodate its one-piece Zed cranks.
In the case of BB65, the shell has a diameter of 65mm and measures 90mm wide while the diameter of the crank axle is a massive 50mm. BB65 is still used by Look for some of its frames eg. When Colnago created the C60 , the company implemented a new bottom bracket design dubbed ThreadFit Comprising a pair of threaded rings that secure an aluminium shell within the carbon fibre bottom bracket lug, ThreadFit Finally, Cannondale has created a few variations of BB30 to suit specific models in its catalogue.
PF30A has the same asymmetrical 73mm shell as BB30A but with a larger 46mm diameter suit PF30 cups, while the variants have a 10mm wider shell to provide more tyre clearance. The adaptability of bottom bracket hardware The bottom bracket of any frame is designed to fulfil one simple, yet crucial task: housing a set of bearings for the rotation of the crank axle.
Given the size of the loads involved, reasonably large bearings with generous races are best suited to this task, though it is not strictly necessary that they be housed within the bottom bracket shell.
Some bottom bracket designs e. BB30 provide a seat for the bearings within the frame while the rest depend upon some kind of cup alloy or plastic that is either threaded e. BSA or pressed e. BB86 into the shell. A sure and accurate fit is necessary for resisting creaking under load, yet it is important that the hardware is reasonably easy to install and remove. In this regard, threaded bottom bracket shells have proven more reliable, however the industry has yet to create a foolproof system.
For those assembling a new bike or replacing the cranks, bottom bracket hardware is rarely supplied with either. Instead, it must be purchased separately, and this is where an appreciation for the specifications for each is indispensable. Even then, it can be a confounding experience due to the sheer number of potential combinations.
These cranks can be fitted in larger shells however owners will have to look elsewhere for suitable hardware. Campagnolo accommodates a wide range of possibilities with its range of Ultra-Torque cups and the 25mm axle will fit most shells. Praxis M30 axle design now permeates its range of cranks with a diverse range of bottom bracket hardware to suit a wide range of shells.
To its credit, the industry has done a good job addressing the number of potential crank and bottom bracket combinations with an enormous range of products. Now, almost every crank on the market can be installed in any given frame. Crank manufacturers have shouldered some of this work by supporting their preferred combinations, while enterprising aftermarket brands e.
C-Bear and Kogel have been founded on a commitment to tackling those that have been overlooked. Be that as it may, incompatible combinations still remain see Table 2 for some examples , and for those that are affected by them, there is very little that can be done to address the issue. As a result, it is worth considering potential incompatibilities before purchasing a new frame, crankset, or bike.
C-Bear is one company that has created an extensive catalogue of bottom bracket hardware that solves previous incompatibilities, such as fitting Shimano cranks in a BB30 shell. The first was to address the bearing durability issue that has afflicted oversized crank axles; and the second was to ensure compatibility with all of the bottom bracket shells that prevail in the current market. At the heart of the system is a 29mm axle, a seemingly trivial distinction, but according to SRAM, it makes for a better selection of bearing sizes than those available for 30mm axles.
Bottom brackets and crank axles for off-road bikes MTB have long employed wider bottom bracket shells and crank axles so as to provide more clearance for fatter tyres and shedding mud. In the beginning, an extra 5mm was added to the width of BSA to yield a 73mm shell, so there was no need to change the cups and bearings.
All that was required was a wider crank axle, which was a relatively simple demand for the industry to meet In the time since then, the industry has continued to apply this strategy, and thus, many of the crank and bottom bracket designs found in the road market have been carried over to MTB with one simple modification: an extra 5mm or more for the width of the shell and axle. Downhill and fat bikes have even wider shells, however the formats remain unchanged.
Photo: James Huang. These wider bottom bracket shells and crank axles have yet to appear on cyclocross and gravel bikes, though that may be changing. Cannondale recently opted for an 83mm bottom bracket shell BB30A when re-designing the SuperX and while the MTB-inspired width works well to provide extra tyre clearance, it does make for a couple of complications. To start with, this wider shell is incompatible with the majority of road cranks, so MTB cranks must be fitted to the bike instead, and with that comes a wider q-factor i.
For those riders with a wide stance, this will be a welcome change for a cyclocross bike, however road riders that prefer a narrow stance will have no way to compensate for the wider q-factor. These bikes already blur the distinction between traditional categories, plus, a wider q-factor may also help the general appeal of these bikes.
Road cyclists, especially racers, have been obsessed with the stiffness of frames and various components for decades. The cranks and bottom brackets of road bikes have received more attention than most parts of the bike, presumably because of the proximity to that race-winning effort. In this regard, bigger bottom shells and larger diameter axles have been important, though it is the relocation of the bottom bracket bearings so that they are much closer to the cranks that has probably had the biggest impact.
As a result, contemporary cranks now flex less than earlier designs. In contrast, there was generally much less variation in deflection for modern cranks, and interestingly, the diameter of the axle had no impact until stiffness was compared to the weight of the crankset.
However, that is based on the assumption that a flexing crank cannot return energy to the rider, which may not be the case, since Jan Heine has data that suggests that this kind of flex can reward the rider with free speed. Thus, there is little point in concentrating on crank stiffness in the current market, if only because the amount of variation between products is relatively small.
Weight, cost, and appearance do far more to distinguish the current range of cranks, so shoppers should feel free to let any and all guide their purchase decision.
A note on types of bearings and seals The bottom bracket is perhaps the most susceptible part of any bike. Water, grime, sand and mud are always quick to collect around the shell, so contamination of the bearings is inevitable.
Bottom Bracket BB30
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