What Is a Mountain Bike Bottom Bracket? All You Need To Know

If your mountain bike is only a year or two old, chances are good it has cartridge bearings in the bottom bracket. These are relatively easy to maintain and work great, but what is a mountain bike bottom bracket?

A bottom bracket (bb) is one of the more important parts of your mountain bike. It’s used to connect the crankset to the rear hub of your ride. The bottom bracket (bb) allows you to pedal around, produce torque and go down the trails you love.

In this article, I’ll give you an overview of what is a mountain bike bottom bracket (bb), what a mountain bike bracket does, what different types of frames, and how to choose the right one for your riding style. So, stay tuned and you’ll find some useful information for yourself.

What is a mountain bike bottom bracket?

What is a mountain bike bottom bracket

A mountain bike bottom bracket (bb) is a bicycle component that is used to transmit power from the crankset to the frame of the bicycle.

It is typically composed of two bearings, which are housed in a metal shell that allows them to spin freely and remain in place as they move up and down.

Mountain bikes are built using this type of bracket because they need to be able to withstand high amounts of torque being applied to them at all times, especially when traveling over rough terrain.

This means that they need a durable, reliable system for connecting their crankset with the frame.

The mountain bike bracket is important because it allows your cranks to spin smoothly without any resistance from your pedals or crank arms.

This means that when you pedal, your legs can move freely without any resistance from your pedals or crank arms.

A smooth pedal stroke makes for an efficient ride and less wear on your body over time.

Types of bottom brackets

Brackets are the bearings that hold your crank arms to the bottom of your frame.

They can be made of steel, aluminum, or carbon fiber and come in different styles. Here’s a look at the two most common types of bottom brackets:

1. Threaded bottom bracket

The most common type of bracket is the threaded bottom bracket (also referred to as a “cup and cone”).

It uses two cones that clip onto the sides of the shell, which have threads cut into them.

A separate cup sits inside this assembly and is held in place by the cones. The cups come in various sizes to fit different-sized shells.

Brackets threaded bottom tend to be relatively inexpensive, but they also require more maintenance than other types of bottom brackets because they need regular cleaning and regreasing if you want them to last a long time.

2. Press-Fit bottom bracket

This style is becoming more popular because it allows manufacturers to make their frames lighter while also reducing their manufacturing costs.

Press-fit bottom brackets are made from aluminum or carbon fiber, and they fit into press-fit cups on either side of your frame.

Some designs require special tools to install or remove them from your frame, while others can be done with just your fingers (and maybe some grease).

There are two main types of this bottom bracket: Spanish press-fit, also known as BB30, and Mid-press-fit, also known as BB86.

3. Bottom bracket sizes

Different bottom brackets have different widths. The width of the bottom bracket design allows you to choose a crankset that fits your frame.

A wider bottom brackets threaded shell will accommodate a wider crank and vice versa. Here’s a chart with average frame bottom bracket sizes:

Bottom Bracket TypeAverage Inner DiameterAverage Shell Width
Threaded bottom bracket1.6’’ – 1.8’’ (41 – 46 mm)2.7’’ – 2.9’’ (68 – 73 mm)
Press-Fit bottom bracket1.2’’ – 1.8’’ (30 – 45 mm)3.4’’ – 3.6’’ (86 – 92 mm)

4. Bottom bracket spindle length

The bottom bracket spindle length is the distance from the center of the crank axle to the centerline of the bottom bracket (bb) shell. The average length for a modern crankset is about 100mm.

This measurement is important because it determines which frames can be used with your crankset.

If you’re using a modern Shimano/Sram crankset, you should have no problem finding a frame that fits.

If you’re using an old-style Campagnolo crankset (which had a longer axle) or some other manufacturer’s, you may need to use a shorter spindle adapter to allow your crankset to fit into your frame.

5. Bottom bracket thread direction

The bottom bracket thread direction is the direction in which a bike’s bottom bracket threads onto the crank arm.

The majority of bikes have threaded bottom brackets, but there are some exceptions.

The most common direction for bikes with threaded bottom brackets is called “English.”

This means that the threads on the outside of the bottom bracket shell are facing outward when viewed from above.

The other common direction is called “Italian” and it’s used by Cannondale and other brands. In this case, the threads on the outside of the shell face inward when viewed from above.

Bottom bracket performance and weight

The bottom bracket (bb) is the point at which the crank meets the frame. It’s a critical component, as it allows you to move your legs and spin the crankset.

When you’re considering a new bike, it’s important to understand how different bottom bracket types perform and how much weight they add to your bike.

How long does a bottom bracket last?

How long does a bottom bracket last depends on your riding style, but the average lifespan is about two years.

Most bottom brackets are made from aluminum or steel, although titanium has become an increasingly common material in recent years.

Some riders prefer titanium’s lighter weight and greater strength over aluminum or steel.

The bearings inside are usually made from either steel or ceramic (ceramic bearings last longer but cost more).

When to replace the bottom bracket on the mountain bike?

The bottom bracket can wear out over time, so it’s not uncommon for mountain bikers to replace their bottom brackets regularly.

If you’re not sure when to replace the bottom bracket on your mountain bike, here are some signs to look out for:

  • Mud or dirt gets stuck inside the frame when you remove your crank arms;
  • The bearings feel rough or gritty when you spin them with your fingers;
  • You hear squeaking noises coming from the bottom bracket area when riding;
  • The bottom bracket is visibly worn down.

How to replace a bottom bracket on a mountain bike?

If you’re wondering how to replace a bottom bracket on a mountain bike on your own, here are the steps you need to follow:

  1. First, you must remove the crank arms. This is usually accomplished by loosening the pinch bolt on the non-drive side of the crank arm. It’s best to use an Allen wrench with a 5mm head. If you don’t have one, you can use a flathead screwdriver.
  1. Next, remove the crank arm from the spindle. If it doesn’t come off easily, use a hammer to tap it off (away from the bike).
  1. Remove any spacers or washers between the spindle and crank arm. These are usually made of steel and are about 1/4″-1/2″ thick. Steel spacers can be difficult to remove because they expand when heated up by friction from being pressed against each other for so long. You may need to cut them with a hacksaw before removing them completely from the frame’s spindle threads.
  1. Remove any tapered bearings that are still attached to your old bottom bracket shell (where your pedals attach). Then use a press fit tool or rubber mallet to drive out any bearing cups that may still be stuck in place inside your new bottom bracket shell (where your pedals attach).

How to measure the bottom bracket on a mountain bike?

The best way to figure out how to measure the bottom bracket on a mountain bike is by using a tape measure and a ruler.

Measure from side to side at the center point of your bike frame, or where the pedal crank attaches to your frame.

The measurement should be less than 68 millimeters (mm). If it is larger than 68 mm, you will need a longer seat post. If it is smaller than 68 mm, then you need a shorter one.

How to tighten the bottom bracket on a mountain bike?

If you’re looking to tighten your bottom bracket, it’s important to know that this process is different for road bikes and mountain bikes.

Road Bikes

The bottom bracket is located on the back side of your bike and can be tightened using a chain tool. Remove the chain from the rear sprockets, then loosen one side of the axle bolt.

Tighten the other side until you feel resistance, then tighten it just enough so that the chain can no longer move freely inside of the sprocket without rubbing against it.

Mountain Bikes

It’s best not to adjust a mountain bike’s bottom bracket unless you have an expert on hand – this can be extremely tricky!

If you feel confident in performing this task yourself, make sure that you have an extra pair of hands and a torque wrench compatible with your specific bike model (you can find these items at most hardware stores).

You’ll also need a few tools: hex keys; a 3/8-inch wrench; a hammer; and possibly a screwdriver or pliers depending on where your specific bike is made.

How much does it cost to replace a bottom bracket?

The cost to replace a bottom bracket varies depending on the type of bike you have. If you have an older bicycle, replacing the bottom bracket may not be worth the cost.

If you have an expensive carbon fiber bike, however, it is likely worth it to replace the bottom bracket.

The cost also depends on whether or not you can do the work yourself or if you need to hire someone else to do it for you.

Some people can get their bottom brackets replaced for as little as $20 while others pay upwards of $100 for theirs.


If there are still uncertainties about bottom brackets, here’s a list of some of the most frequently asked questions about it. Read it, and you might find some useful information for yourself.

Are Shimano bottom brackets interchangeable?

Shimano bottom brackets are one of the most common types of bottom brackets, but they’re not all interchangeable. So, before you decide to replace your bottom bracket, check its type of it.

What is a Hollowtech bottom bracket?

The Hollowtech (HG) standard is used by Shimano for their road bikes and some mountain bikes. The “Hollowtech” name comes from the fact that these bottom brackets use concave cups that fit around a hollow axle with bearings inside it.

How do I know what bottom bracket fits my bike?

The easiest way to find out what bottom bracket fits your bike is by checking the model number. If you don’t have that handy, you can also search for the dimensions of your bike’s bottom bracket and use those to determine which one will fit.

Do all cranks fit all bottom brackets?

No, not all cranks fit all bottom brackets. The reason is that there are two different types of bottom bracket systems: ISIS (International Spline Interface Standard) and BB30 (Bolt-Together 30mm).

Final word

The conclusion is clear and simple: Bottom brackets are one of the most important components of the bike.

If you want to maintain your bottom bracket properly, you should examine it frequently and replace it if needed.

I am happy to share my experiences and thoughts in the comments below, so don’t hesitate, to share your thoughts and I’ll gladly respond.

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