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Confused by bottom brackets? Wondering what the ‘BB’ in BB90 stands for? Let alone the ‘90’… Well, don’t despair. I’ll be explaining what this essential bike component does, why the BB90 is unusual, and what you can do about common issues that can develop with them. Let’s take a look.
What does a bottom bracket do?
The bottom bracket is one of the most important components in a bicycle, yet it’s often overlooked. When you’re shopping for a new road bike you might be concerned with overall bike weight or gear ratios. For a new mountain bike, it might be the travel of the suspension or the gnarliness of the tires. And, for an electric bike, it could be the torque and range that the battery and motor deliver.
But, bottom brackets? They’ll be tucked right at the bottom of the spec list and are unlikely to be a factor in most purchases.
It’s easy to forget about the bottom bracket, but it plays an integral role in your riding experience and, when it goes wrong, it can really ruin your day. So, let’s break it down: what exactly is a bottom bracket, and what does it do?
A bottom bracket (often referred to as a ‘BB’) is a small part of the bike that holds the crank arms in place and attaches them to the frame. It’s located at the bottom of the bike between the two chainrings, and it consists of two bearings (called cups) that are pressed into the frame through the bottom bracket shell. The bearings allow the crank arms to spin freely, which is crucial for smooth friction-free pedaling. A BB is part of a bike’s groupset.
The bottom bracket also determines the size of your crankset. Different sizes are available and they’re measured in millimeters (mm).
For most riders, the size of the bottom bracket isn’t something they need to worry about—they simply purchase a bike with the correct size already installed. However, if you’re planning on upgrading your crankset, you’ll need to consider the size of your bottom bracket.
Aside from these considerations, there are a few other aspects to keep in mind when selecting a bottom bracket. One is bearing type: sealed bearings are preferred because they are low-maintenance and provide smoother rotation than unsealed bearings. The other is spindle diameter—the part of the bottom bracket that your pedal crank arms attach to. The two spindle diameters most commonly used today are 24mm (used on Shimano cranksets) and 30mm (used on SRAM cranksets).
Bottom brackets are also available in different configurations and materials. For example, some are threaded into the frame, while others press-fit into it. The most common materials for bottom brackets include aluminum alloy, steel, and titanium. Each material has its own unique qualities and benefits, so it’s important to choose one that fits your riding style and budget.
What is the BB90 bottom bracket?
Intriguingly, the BB90 isn’t really a bottom bracket at all. Let me explain.
A ‘typical’ bottom bracket has a cylindrical body that takes up the entire inner space of the bottom bracket shell. It might have a spindle fitted allowing crank arms to be attached, or have a slot through the center allowing for a spindle to be pushed through – an example of this type is shown below.
In contrast, the BB90 ‘bottom bracket’ is really only the two bearing cups that you’d normally find at each end of a standard bottom bracket.
This is a press-fit style of bottom bracket, where the bearing cups push into recessed fittings on each end of the bottom bracket shell on a bike. You can see what the BB90 setup looks like in the diagram below.
Let’s look at the dimensions now.
Remember we talked about the ‘90’ in BB90? Well, this refers to the length of the BB shell cylinder, which should either be 90.5mm (on road bikes) or 95mm (on mountain bikes).
Each of the two bearing cups in the BB90 has an outer diameter of 37mm – corresponding to the inner diameter of the BB shell. They also have an inner diameter of 24mm to accept the 24mm spindle cranks that they are designed to be used with.
Which bike manufacturers use BB90 bottom brackets?
There is really only one bike manufacturer that uses BB90 and that’s Trek Bikes. BB90 is their standard for most of their road bikes and mountain bikes. These will have a BB shell that is designed to accept this style of bottom bracket.
BB90 Bottom Bracket problems
BB90 is a clever bit of design as it allows for a reduction in bike weight (through doing away with the bottom bracket ‘body’ that normally goes between the bearing cups).
But, it’s not without its problems. The chief problem that many riders complain about is a creaking noise as the cranks rotate. Whilst not necessarily a loud noise, it’s enough to set the teeth on edge of both the rider and the rest of the pack.
The most common attempt at resolving this issue is with liberal application of Loctite. Sadly, this is unlikely to give anything more than temporary relief.
A better solution is to replace the two BB90 bearing cups with what looks like a standard bottom bracket. This design gives a solid threaded-together casing that holds the bearing cups snugly in position, allowing them to spin smoothly and quietly.
BB90 bottom bracket replacement
If you’re comfortable with tricky bike repairs, then either replace the bearings, or (my preference) use a BB90-compatible thread-together bottom bracket (which is more akin to a ‘standard’ bottom bracket in that it goes all the way through from one side of the BB shell to the other holding the bearing cups in each end, and the 24mm spindle slots through the middle) – see above.
Replacement of bottom brackets is really a job best left to the pros at your local bike shop. They have the specialist tools to do it, and they’re replacing them day-in and day-out. That said, if you’re handy with complex bike repairs and don’t mind getting your hands dirty, then take a look at the details in the video below:
What’s the future of BB90 bottom brackets?
Whilst these can be a weight saver, due to the elimination of the overall bottom bracket ‘body’, the creaking problems that many riders experience with BB90 setups mean that these aren’t universally adored. This may be one of the reasons why there are signs that Trek may be looking to shift gradually away from the BB90 to a different standard: PF92/BB92.
Watch this space to see what happens.