Mountain Bike Bottom Bracket and Its Uses

The mountain bike bottom bracket is an unseen but vital component at the heart of your frame. The best mountain bike bottom brackets allow your cranks to spin on a safe and balanced bearing interface.

Part 1. Introduction

The bottom bracket bearing assembly, also known as the “bottom bracket”. The mountain bike bottom bracket may consist of a spindle that is attached to the crankset and bearings that allow pedal cranks to turn the rear wheel when the cyclist pedals the bike.

bottom bracket on a mountain bike

What Can Go Wrong

When your bottom bracket is poorly adjusted, it may develop play or a wobble, causing a less efficient transfer of energy from you to your bike.

Another common problem is that when bearings get contaminated with dirt and water or wear out, there is more friction in turning the cranks, making pedaling harder. Sometimes the directions feel rough when you spin them; other times, you may hear and/or feel a click or two with each pedal stroke, especially under higher loads.

Eccentric slides accommodate a standard English threaded bottom bracket and are often suitable for bikes with an off-centre axle. Their design allows users to precisely adjust the forward and reverse crank position. What’s more, the front-to-rear position of the crank can be adjusted by sliding an eccentric plug over an enlarged shell. Another essential feature of the abnormal slides is that they are designed to fit a 73mm BSA 24 TPI bottom bracket.

The spacers allow you to adjust the height by 7 mm. An eccentric bicycle slide will fit many different bicycle frames as long as the BB pan is the same size. An eccentric push-on bracket is easier to install on your bike because you don’t have to worry about retaining rings. These brackets are used in many applications where chain tension is critical.

Such brackets are often used on tandem bikes or other applications that require precise chain tension adjustment. These brackets have a separate spindle and bearing assemblies connected by a standard interface. Many older bikes have cranks with the bearing cones facing outward, and some are three-piece, where one part of the crank supports the spindle/bearing assembly. Two-piece cranks are usually inexpensive.

Part 2: Buying Guide

A good bottom bracket can be installed and left alone, because it doesn’t need any maintenance. A sealed unit is what’s modern and in demand. They are easy to replace if they become worn out and have a long lifespan. Press-fit bearings and traditional/square taper bottom brackets are the two most popular designs. In general, the quality of the bearings and the quality of the materials improve as you move up the hierarchy of components, and this is reflected in the price.

Your choice of bottom bracket will depend, at least in part, on the type of frame you have. Only one type of Bottom Bracket is supported by specific frame materials and drivetrain configurations. The two most common types of bicycle bottom brackets available today are press-fit and threaded BBs.

If you want to get the most out of your upgrade, the bearing quality in your bottom bracket is another factor that is crucial. You should consider using ceramic bottom bracket bearings rather than, say, the steel bearings that are currently found in the majority of bottom brackets because they have a much lower resistance and a much longer lifespan. These bearings are strong and have low friction.

The quality of the bearings is crucial because it determines how much energy is lost through friction and how long they will last under heavy use. Fully sealed cartridge bearings are the best option if your budget allows for them. The perfect setup when riding in muddy and rainy conditions, these non-adjustable, non-serviceable units must be thrown away when they are worn out and require no maintenance.

Higher tolerance bearings are used in more expensive units, which reduce friction and extend bearing life. Due to the increased demands of off-road use, MTB bottom brackets frequently have more sealing.

Some of the best mountain bike bottom brackets

1. Shimano XTR

Weight: 89 gram
Bearings: Small Ball
This BB adheres to the threaded, English standard. At 89g, the brand claims that the new XTR BB is 19 grams lighter than the previous version. This means that riders will be dealing with less rotational weight, and Shimano claims that the seals have been improved as well, keeping dirt and mud out. Another advantage of buying Shimano is that it should be widely available at bike shops in case you need to replace it or have it serviced.

2. Race Face Cinch

Weight: 87 gram
Bearings: Single Row
Race Face’s Cinch bottom brackets are available in a variety of standards and sizes. This threaded BSA model weighs 87g and features 6806 retainer bearings that are loaded with waterproof grease and wiper seals to keep dirt and grime from getting inside your bike.

3. Chris King Threadfit

Weight: 89 gram
Bearings: Stainless steel, ceramic
Due to the labor-intensive manufacturing process involved in making its bottom brackets, Chris King is able to achieve exceptionally tight tolerances, which are the key to their low friction performance and impressive durability. As a result, your bottom bracket will precisely fit between your frame and crank arms, resulting in a more effective ride. Chris King manufactures its own bearings and packages the cartridges in-house to ensure unrivaled quality. All bottom brackets are covered by the King Lifetime Warranty of the company and come in a choice of nine colors. Although ceramic bearings are an option for this bottom bracket, they are twice as expensive as stainless steel ones.

Part 3. Maintenance

It is essential to maintain your bottom bracket. If you have a  bottom bracket, remove it periodically and refresh the anti-seize compound. And no matter what type of bottom bracket, check your bearings regularly to keep them running smoothly. Loose bearings in some bottom brackets can be taken apart and cleaned and re-greased, whereas, in cartridge-type bottom brackets, you’ll simply replace the cartridge bearings when they go wrong.

You can make your bottom bracket last longer by carefully cleaning your bike. Never spray high-pressure water directly at and around your bottom bracket, as it will eventually penetrate seals and compromise the integrity of the bearings.

Many different bottom bracket standards mean there are many other tools for installing and removing them. If you don’t have the proper tools and skills, you may need to take your bike to your local shop to help you fix any bottom bracket issues.

Part 4. Selection Guide

A quality mountain bike bottom bracket will help you improve your bike’s experience and increase its performance. A well-maintained mountain bike bottom bracket can reduce power loss due to friction and enhance the performance of your drivetrain.

1. Threaded or Press-fit

Mountain bike bottom bracket standards and requirements cover a wide range of topics. The most basic—yet significant—categorization is probably whether your mountain bike frame uses a press fit or threaded bottom bracket. As their name suggests, threaded bottom brackets literally have threads that screw into corresponding threads in the BB shell. Meanwhile, press-fit BBs are “pressed” into the BB shell. The more traditional and durable threaded bottom bracket design is ideal for mountain bikes and continues to be favored by many people who place a high value on durability.

On the other hand, the press-fit bottom bracket allows frame designers to meet lightweight specification requirements and is simpler to produce because it can be machine-pressed. Press-fit bottom brackets are important for riders who desire a narrow q-factor (lateral crank arm spacing). The press-fit system does not require an aluminum insert to be secured, unlike a bottom bracket with threads. As a result, it is lighter than a similar-sized bike with a threaded bottom bracket.

2. Crankset compatibility

As different brands use various axle standards, you should also think about how your crankset will work with the frame. The two major drivetrain manufacturers, Shimano and SRAM, have the most popular ones. In contrast to SRAM, who uses both the stepped 24/22mm GXP and the more recent 29mm DUB axle as standards, Shimano uses a 24mm axle. Additionally, many manufacturers use the 30mm BB30 (threaded) or PF30 (press-fit) standard. There are others, so before buying a bottom bracket, make sure it is compatible with the maker of your crankset.

3. Bearings

You have two options for bearing materials: steel or ceramic. Ceramic bearings are more expensive than steel bearings, but they have numerous advantages.

The ceramic material can be shaped to a higher manufacturing tolerance, resulting in the most spherical possible shape, which reduces friction. This enables a bearing to seat with tighter tolerances and less friction in a cartridge. All of these factors work together to reduce the total amount of energy required to turn the crankset through the bottom bracket.

Under load, ceramic bottom bracket bearings are less susceptible to structural flex than steel. This means that the bearings will retain their shape even if you crank at maximum power up a climb, providing near-perfect energy transfer when you need it most – rather than friction build-up.

The majority of bottom bracket bearings are made of steel, but there is a distinction between stainless and standard steel bearings. You could have a good steel source material, but machining it to a perfectly round shape is what makes a great bearing – and this takes engineering skill.

Part 5. Frequently Asked Questions

How Do I Know What MTB Bottom Bracket I Need

To find out the size of the bottom bracket needed, measure the inside of the bottom bracket shell in your frame. It will be 73mm, 70mm, or 68mm. Some older structures may have Italian threaded bottom brackets instead of the more modern English.

Are all MTB bottom brackets the same?

According to the manufacturer, there are many different sizes and iterations – BB30, PF30, BB90, PF86/92, to name but a few – and not all are interchangeable.

How Do I Know If My Bottom Bracket is Loose

How to spot a loose BB. Firmly hold the bike steady and attempt to move the crank from side to side – in the opposite plane of motion to its rotation. The BB system is loose if there is any play from side to side. An open/flexible BB will need its bearings adjusted (via the adjusting cup) or replaced.