How Do I Know What Bottom Bracket I Need – From threading to snap fit, there’s everything you always wanted to know about bottom pedals but were afraid to ask.
The bottom brackets should be seen, not heard. Unfortunately, this is not always the case, and a simple component used to attach the handle bars to the frame can cause problems. When it comes time to replace the bottom bracket, trying to find the right replacement can be confusing.
How Do I Know What Bottom Bracket I Need
In an ideal world, there would be a single lower bracket standard, but in a sport that has made huge technological advances, the lower bracket has thrown itself out, sometimes kicking and screaming, and the result is a veritable field minefield. Election
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In this guide, we’ll explain all the main bracket types below to help you identify what you need to know about each system when you need to replace your bike. Fortunately, replacement bearings have become much easier to find, and while there have been some ugly designs, parts of the industry are moving toward more responsive solutions that offer better reliability and easier service. There are also several bottom bracket conversion kits that allow you to fit cranks from one standard to a frame using another standard.
There are two main types of bottom bracket design, a threaded housing where the bearings are bolted, or a snap-fit design where the bearings are pressed into the housing. Bottom brackets are distinguished mainly by the width of the housing and shaft and the diameter of the housing and shaft.
The most common are bearings that screw into the bottom bracket shell. That means the frame must be metal or a carbon frame must be fitted with metal inserts to carry the necessary threads.
The most common threaded standard used today is the BSA, or English Threaded Bottom Bracket. It uses a 68mm wide bike shell, 73mm and 34.8mm diameter shell on mountain bikes. Some bike brands use an Italian threaded bottom bracket, but these are rare.
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Bicycles used to have a bottom bracket, but today it is obsolete. They were replaced with a square taper design with a 17mm axle with a square taper point to fit the square holes in the crank. Still used on some cheap bikes, but largely redundant on modern bikes.
Splined bottom brackets were popular for a while as Shimano’s Octalink types were popular for many years. It used an axle with eight splines, which was seen on mountain and mountain bikes. The Isis drive was another design similar to Shimano’s patented design.
The most common design used today is a threaded bottom bracket with outboard bearings. Placing the bearings on the outside of the frame allowed for a larger axle for greater stiffness and reduced weight. Greater amplitude bearings also contributed to stiffness advantages.
Shimano developed Holotech in 2003 and changed the crankset world forever. He moved the bearings out of the frame while bolting them to the frame. This allowed for larger bearings and axles for stiffness and weight.
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It uses a 24mm diameter axle with a 68mm wide bike shell and a 73mm wide mountain bike shell. English thread has a left-hand thread on the drive side and a right-hand thread on the non-drive side.
Many companies still use press-fit on the face, and some brands have abandoned press-fit and gone back to it. There are now a number of conversion kits that allow you to adapt many crank standards to Holotech pedals, including 30mm axles, adding to their popularity.
Shimano’s Holotech has similar designs and follows the same principle of outboard bearings in a standard threaded housing and 24mm axle.
The Giga X Pipe, now owned by SRAM, was an evolution of ISIS, developed under the acronym GXP. He used a right side bearing that floated on the crank and a left side bearing that sat flush with the stepped shaft design.
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In 2006 Campagnolo developed an interesting design called the Ultra Torque as a response to Shimano’s Holotech II.
The 25mm diameter crankshaft was split in half and joined by a Hirth gasket and large diameter bolt. The bearings were outside the frame.
It was joined in 2011 by Power Torque, which uses outboard bearings with a one-piece shaft bolted to the right crank.
With the advent of carbon fiber and the development of lighter and stiffer racing frames, came the advent of the snap-on bottom bracket. Carbon is a wonderful material, but it has its limits, ie you can’t cut threads into it for a threaded bottom bracket, and fitting an aluminum sleeve is problematic.
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The solution was to press the bearings directly into the frame. Advantages include greater design freedom, stiffness and weight compared to threaded designs. Several flavors of snap-on bottom bracket now exist, but most are not compatible with other designs, although conversion kits are available in some cases.
Cannondale invented the BB30 in 2000 and found a solution. The BB30 saw the bearings pressed directly into a large 42mm diameter carbon shell and a large 30mm diameter bottom bracket made from aluminium. It offered more stiffness and less weight.
Cannondale developed it using the BB30A. It uses an asymmetric shell that is 5mm wide on the non-unit side to create a wider and stiffer base. The drive side crank angle has been changed to maintain the same Q factor and equalize the ankle clearance on both sides.
In 2015, Cannondale took it even further, introducing the PF30A, which moved away from clip-on bearings to plastic cups and push-fit bearings directly into the carbon bottom bracket shell. This change also required a larger bottom bracket shell, so it grew from 42mm to 46mm.
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The BB30 requires very precision machined aluminum bottom bracket shells with tight tolerances to accommodate the bearings and clips. It was presented to the world in 2009 with the SRAM-Fit 30 press.
The housing is sized from 42mm to 46mm to accommodate the plastic cups that hold the bearings. It uses the same 30mm axle as the BB30 and is 68mm wide for bikes and 73mm wide for mountain bikes.
Carbon fiber frames made it possible for frame designers to create larger bottom brackets for stiffness and weight. Compared to Cannondale’s BB30 design it doesn’t require such tight tolerances.
This caused a lot of problems with snap-fit bottom brackets, as you’d expect the fit between the plastic cup and carbon shell to be tight enough to prevent any unwanted movement that might cause crunches.
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Shimano developed their Snap Fit standard with 41mm diameter bearing cups that are 86.5mm wide for the mountain bike and 89.5 or 92mm wide for the road bike. It’s designed to use a 24mm axle, and Shimano avoids the temptation to use a 30mm axle like its main rivals.
Trek developed its bottom bracket for its bikes and mountain bikes. The bearings are pressed directly into the frame with a 90.5mm wide casing on bikes with a 37mm casing diameter and a 95.5mm wide casing on mountain bikes.
Internet forums, if anything, are certainly one of the most problematic systems in use today. No wonder Trek adopted the T47 on their new 2020 Doman bike.
This is a standard introduced by the FSA in 2011 that aims to address some of the compatibility issues it has designed to work with the current lowest bracket standards. It is an open standard and is adopted by many manufacturers.
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It uses an 86.5mm wide shell, with the bearings spaced further apart for greater stiffness, and a 30mm shaft as long as an external mechanism. Compatible with threaded bottom bracket shells.
You get a BB386EVO bottom bracket that fits a BB shell with a 68mm thread, so you can use the new standard cranks on the old standard frame.
When Colnago wanted to create a bottom bracket for its new C60 in 2015, it came up with a design that aimed to solve the snap-fit’s problems but retain key benefits. It uses bolt-on alloy cups to house the bottom bracket bearings. The cups are replaceable so there is no chance of damaging the frame. It is compatible with BB86 and BB386EVO.
The T47 was developed by Chris King in 2015 and uses a large, threaded design. It has the same diameter shell as the Pressfit30 bottom bracket, but the bottom bracket’s threaded cups with a traditional BSA thread. It is an open standard, so any framework manufacturer can use it.
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It’s not that widely adopted, with only a few niche bike brands to begin with, but it’s gaining momentum with the likes of Trek using it on their new 2020 Doman endurance bike.
Trek has hacked the quality a bit; They made the BB shell 1mm narrower so they could add 0.5mm of extra material to the tool flanges for easier removal. However, it will fit all available T47 internal BBs.
Cervelo developed the BBRight for their frames with an 11mm wide shell on the non-motor side only with bearings pressed directly into the frame. This made it 79 mm wide
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