And track bikes usually have inboard bearings, with an Octalink connection to the cranks. So the explosion of bottom bracket standards is really just trying to address these three problems. There are two basic solutions to improving bottom bracket lightness, stiffness and compatibility with carbon frames: use the same 68 width shell but place the bearings further apart; or make the shell larger altogether.
Get the bearings further apart and you increase stiffness and power transfer, and can make the frame more substantial, while larger diameter axles can be made with hollow axles with thinner walls, which are stiffer and lighter.
But also coming into play is Q-Factor. This is the distance between the outside edges of the two crank arms. Shimano was an early advocate of moving the bearings outside the bottom bracket shell. It also means that the axle diameter can be increased to 24mm, which increases axle stiffness. The axle is hollow, which reduces the weight too. The bearings are usually housed in sealed races, which makes them easy to remove or replace.
As well as Shimano, sealed external bearings are available from SRAM called GXP and Campagnolo , although none of these 24mm external bearing systems are cross-compatible. With the rise of carbon fibre as a frame material, threaded bottom brackets presented a problem. So Cannondale promoted its BB30 system as an open solution which could be adopted by other manufacturers.
Rather than screwing in, the bearings are pushed into the carbon bottom bracket shell, using a threaded press, like that used for headset bearings, and a special adapter to fit the bearings and make sure that they end up parallel.
The axle diameter is increased from 24mm to 30mm too, which means that it can be made lighter without losing rigidity, although the shell width is still 68mm. Pressfit bearings are widely used for modern carbon frames. But the BB30 standard requires close tolerances in the frame to ensure that the bearings fit without play.
Early BB30 frames were notorious for creaky bearings, although Loctite and more accurate frame building have reduced the problem significantly. Another way of making BB30 bearings more robust, which is widely used, is the PF30 bearing. This has the same dimensions as BB30, but instead of using circlips to retain the bearings, these are housed in a plastic sleeve. This pushes through the bottom bracket shell, keeping the bearings aligned and reducing the risk of squeaking.
But the plastic sleeve can be prone to wear, so PF30 bottom brackets may need replacing more frequently. So Cannondale is increasingly using BB30a bearings on its more recent bikes.
This is essentially the same as BB30, but just moves the left hand bearing outwards by another 5mm, adding a bit of extra crank stability. Bottom brackets use bearings to allow for rotation and are usually considered a service item.
Some can be adjusted or rebuilt, but modern designs are more frequently designed to be replaced rather than repaired. Some designs include an axle or spindle while others simply provide a surface upon which an axle can spin. When a bottom bracket begins to fail, its bearings can make unpleasant creaking noises and develop play.
This can lead to reduced bearing lifetimes. This system has a number of different names depending on who you talk to. The key benefits are lighter weights owing to the larger, 30mm-diameter aluminium spindle and the elimination of separate bearing cups. Plus, because the shell is still just 68mm wide, the cranks can also be made to provide more heel clearance — or, depending on the crank design, an even a narrower stance width commonly referred to as Q factor.
By comparison, Dura-Ace weighs around g [we recorded g in our review — ed]. In addition to weight, another BB30 advantage is stiffness. By increasing the spindle diameter, you increase its resistance to twisting, which is where the bulk of the deflection in a crankset comes from. Moreover, with the notable exception of Felt , which uses a carbon tube, BB30 also usually employs an alloy shell insert that has to be bonded or co-moulded into an otherwise carbon frame.
According to Cannondale, this allows for wider bearing spacing and better spindle support which should improve bearing longevity. That extra 5mm can complicate crank compatibility, however. Thankfully, many newer BBcompatible cranks now feature extra-long spindles for wider compatibility and those will install just fine. Cranks of both systems are compatible with each other. They both have the same needs for concentricity and placement relative to the frame. The threads are at a disadvantage in that they need to be machined from both sides and have a hard time sharing a centerline, whereas our OSBB is machined from one side all at once — and round holes are easy to measure.
This is essentially the same as the BB30A and BB Ai Cannondale system described above, except that it uses bearing cups rather than bearings pressed directly into the frame. BBRight shares its 30mm-diameter alloy spindle, cartridge bearings, and driveside bearing location with PF30 — in fact, the bottom bracket cups are identical to PF However, the non-driveside bearing is pushed outward by 11mm — the same as conventional threaded, BB90, and PF86 bottom brackets.
This makes it something of a hybrid between the two widely accepted systems. Your feet are in exactly the same place as before.
BBRight is available in two incarnations. The direct fit version uses the same bearings from a BB30 bottom bracket, which are pressed directly into the frame. While it seems to make good engineering sense, however, other bike companies have been slow to come on board. In addition, it uses the same bearings and cups as PF30 and BBRight and, thus, the same manufacturing tolerances and required tools.
We are able to have a full carbon shell with no metal bonded into the frame, which makes for a stronger, lighter frame structure. The straighter arms also have less ankle and heel clearance than those narrower formats. Total stiffness is not a measure of crank stiffness alone, though.
Designed in conjunction with Chris King and Argonaut Cycles, T47 — announced in — is the most recent offering hoping to become the next standard. T47 bottom brackets, aka Thread Fit 30i, use the same 46mm internal diameter as PressFit 30, but they thread into frames rather than being pressed in. The design comes about as a solution to the well documented creaking issues associated with oversized press-fit systems.
While these issues are near always the result of poor tolerances, the T47 standard promises to fix the creaking issue without sacrificing the benefits of an oversized interface. As it stands, this new standard may just become the hot new thing among smaller manufacturers of metal bikes. The need for a heavier metal interface looked like a limiting factor for mass-produced carbon bikes, but with a manufacturer like Trek on now on board as mentioned above, its Domane features a T47 BB that could all be about to change.
Threadfit Rather than press the BB86 bottom bracket directly into the frame, the ThreadFit system threads an aluminium shell into the frame first, which provides the interface for the bearings to sit in.
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