Bottom Brackets

Mountain Bike Bottom Bracket Buyer's Guide

The bottom bracket, or "BB" is a component that is too often overlooked. Hidden away within the frame, it is what keeps the cranks spinning smoothly. The purpose of the bottom bracket is to securely connect the cranks to the frame, while allowing them to transfer power from the chain to the rear wheel. When in a standing position, most of the rider's weight is placed firmly on the pedals and transferred to the frame through the bottom bracket. Because of its location, big landings and out of saddle sprints place a significant amount of stress on the component, so downhillers, cross-country riders and freeriders all benefit from a stiff and efficient bottom bracket. Bottom brackets are fairly simple components, and rotate on bearings that are kept in place by cups. Manufacturers usually differentiate their bottom brackets by the intended riding discipline the component is designed for, simplifying the shopping process.

Types

Two types of bottom brackets are available depending on the crankset they are used with. The two major categories pictured above are external (left) and cartridge (right).

External - The bearings are located in two cups that thread into the frame's bottom bracket shell and sit on either side of the frame. This style fits most frames, but requires a crankset designed specifically for external bottom brackets because the spindle is part of the crankset rather than the bottom bracket. A sleeve that sits within the frame, which is usually made of plastic, links the cups. The external design was developed after the cartridge style and is now standard on many mountain bikes.

Cartridge - Cartridge style bottom brackets are housed entirely within the frame. Contrary to external style bottom brackets, cartridge style have an integrated spindle (the piece that the cranks attach to), so the cranks bolt directly to the spindles that stick out from either side of the bottom bracket. Cartridge is the more traditional style of bottom bracket and is becoming less common on modern mountain bikes. Because of their integrated spindle, it's important to know what type of spindle your cranks require. The three spindle types are Square Taper, ISIS Spline and Octalink.

Sizes

Bottom brackets are sized based on two measurements: spindle length (for cartridge bottom brackets only) and shell width. Spindle lengths vary anywhere from 102 to 140mm. Shell widths come in 68, 73 or 83mm widths. 83mm shells are found most commonly on downhill bikes.

Materials

External bottom brackets generally use aluminum cups and a plastic sleeve. Some high performance models also use carbon fiber sleeves to reduce weight. Basic cartridge designs use steel, chromoly or aluminum bodies, depending on price point. High-end cartridge bottom brackets may use titanium or scandium to save weight and increase strength. Bearings are most often made from steel, but high-end bottom brackets may use a ceramic bearing to reduce friction and weight.

Things To Look For

When looking into a new bottom bracket there are three things to take into consideration: frame and crank requirements, riding style, and budget. The bottom bracket you select must be compatible with your frame and cranks. Many companies identify their parts based on riding type, so knowing the type of riding the part will be subjected to is essential. Finally, the amount of cash you are willing to spend will dictate the performance of your new bottom bracket. Riders seeking ultimate performance, whether increased strength or decreased weight (or both), can improve their ride by spending a little more on a new bottom bracket.

How Much To Spend

Bottom brackets range in price from around $15 up to $250.

In the $15-$45 range, bottom brackets are designed to provide solid performance and durability at a competitive price. Most in this range use sealed bearings, chromoly or aluminum spindles, and are available in every size.

Moving up to the $50-$125 range, bottom brackets all use sealed bearings, and aluminum cups and spindles are standard but some may have titanium or scandium components to save weight. In this range, riders are offered bottom brackets designed to excel at a particular discipline to increase performance.

Above $130, bottom brackets offer even more performance. Some bottom brackets at this price use ceramic bearings coupled with titanium, scandium, aluminum or carbon fiber parts. These bottom brackets are designed to deliver ultimate performance.

Product Reviews

Before buying, be sure to do your research and read product reviews. Reviews are a great way to find out specifics about a particular model, user impressions, and things to watch out for. After you've purchased a product and had enough time to thoroughly test it, we encourage you to leave a review for other people to see when they are researching bikes and parts on the web.

We hope you've found this information to be helpful. If you have a question that isn't answered in this guide, our mountain bike forums are a great place to get advice from knowledgeable riders. Your local bike shop is also a great resource.