General Bottom Bracket Technical Information
This guide has been written to provide a general overview of how bottom bracket systems work and how to troubleshoot and diagnose problems. These problems mainly revolve around creak.
|BB Standard||Size (diameter x length)||Download Shell Drawing||Download Hambini BB System Drawing Shimano|
|BB30||42x68||Download Here||Download Here|
|PF30||46x68||Download Here||Download Here|
|BB30A||42x73||Download Here||Download Here|
|PF30A||46x73||Download Here||Download Here|
|BBright||46x79||Download Here||Download Here|
|BB386 EVO||46x86.5||Download Here||Download Here|
|BB86 / PF86||41x86.5||Download Here||Download Here|
|OSBB||42x61||Download Here||link in here|
Bottom Bracket Mechanics
Bottom brackets are not a particularly complicated engineering entity, they usually consist of two bearings and some sort of housing. Unfortunately, in recent times, they have gotten a reputation for creaking and this is mainly down to some fit issues around the interface between bearings and the housings. Mechanically, the bottom bracket needs to contain radial and axial loads. The radial loads are the forward motion generated by the rider pedalling. The axial loads are caused by the riders pedals being offset from the centerline of the bike, this causes a 3D vector of force to be generated which has an axial component. The axial force component of this vector is unwanted but must be constrained
In General, the best design of bottom bracket is one that has big bearings that are widely spaced. In this respect BB386 is the best solution of all of the standards that are currently available. It uses large 6806 bearings at a nominal 90mm spacing, it is technically superior.
Bottom Bracket Issues
This next section will discuss some of the issues associated with bottom brackets
Bottom Bracket Creaking
Bottom bracket creaking is almost always caused by one of the following
- Tolerance or Fit issue
- Improper Installation
- Brinelling of Bearings
Tolerance or Fit issue
A tolerance or fit issue is extremely common, this is exacerbated by cheap far eastern frame manufacture with the installation of even worse far eastern bearings. For any Bottom bracket system to work correctly, there must be an interference fit between the bearings and the frame or cups, usually this is an ISO M7 fit. The amount of interference is carefully controlled and has a big impact on whether the bearings spin freely or move around causing creak. The movement of the bearing is movement occuring somewhere in the audible range 20Hz - 20kHz. Some general tips are as follows:
- Avoid FSA Bearings like the plague, especially the ones with green seals. They have very poor manufacturing tolerances
- Make sure you buy genuine bearings and not fakes (NTN, NSK, FAG, INA, Timken, SKF, Nachi)
- If you remove your bearings, re-greasing them and reinstalling them is often prescribed advice given by bike shops and forums. This is poor advice as the removal of a bearing will invariably cause it to brinell (the removal force causes Herzian Impact Damage), hence always replace with new bearings.
- Do not use grease on the outside of any new bearings to install them. None of the bearing manufacturers recommend it and as it can attack the epoxy in carbon frames.
- Use Loctite 641 (EU) and Activator (7649) on carbon interfaces
Misalignment between the two bearings on a bottom bracket is commonplace. This stems from the way modern bikes are made and is pertinant to carbon as opposed to metal manufacture. Carbon Bikes are often glued together in two halves which results in misalignment or the bottom bracket relaxes after it has been manufactured. Once this occurs two types of misalignment can persist, the first is parallel misalignment. Parallel misalignment occurs when the bearings are parallel with each other but they are offset (one bearing may be slightly forward or backward of the other). The second type of misalignment is angular misalignment, this is where the bearing centers are pefectly aligned but one bearing is effectively tilted
The drawing below shows parallel misalignment
The drawing below shows angular misalignment
There is very little that can be done about misalignment without doing some sort of conversion or machining which is not for the feint of heart. Some bike frames are much more prone to misalignment than others. Cannondale, Boardman and Cervelo all suffer from (in my opinion) unacceptable levels of misalignment. Although I'm not an expert, I believe this is something to do with their carbon layup and relaxation that takes place after making it. Conversely Look and Time carbon frames are extremely good, they are unlikely to creak.
Bearings which are improperly installed is a common problem. Bearings and bottom brackets are often inserted with substandard tooling using presses that do not guide the items in accurately. These install tools do not have to be expensive but they must have the correct dimensions to prevent damage to the parts during installation.
As an example one of the easiest and often overlooked issues is on BB30/OSBB/BB386 Bottom brackets where eyeleted circlips or snap rings are used. Eyeleted snap rings in this application can often impinge on the seal and rub as each ball goes around. This will damage the bearing and lead to premature failure. A simple easy fix is to install the correct eyeletless snap ring.
Brinelling of Bearings
Brinelling of bearings is a common occurence in industry, it is caused by overload but isolated DURING operation on a bike. However it commonly occurs during installation and removal of bearings. Quite common and poor practice is to remove bearings from their enclosure, regrease them and reinstall them. This removal process loads bearings up beyond their limit and the resulting Herzian contact stress causes micropitting on the bearing races. This causes skipping under load and can be heard as creaking. Hence, if bearings are removed, replacing them is recommended. The process of regreasing the bearings by removing them can cause creak.