Nuts and Bolts: A guide to bottom brackets

Words by Adam Newman, illustrations by Andrew Roberts

A generation ago, there were only one common type of bottom brackets. They were square taper. And they were good.

Yes, there were some variations, and people were trying hard to forget about cottered cranks, but the interface from crankarm to axle remained largely unchanged for decades. But as bicycle frames evolved beyond lugged steel, product designers realized they had more room to work with, better manufacturing methods and, most of all, more performance demands from the marketplace. And things began to change.

In the past few years, it has been the constant evolutions of the bottom bracket “standard” that has made the term an inside joke in the cycling community. There’s PF this and 386 that. What does it all mean? Well, attention spans are only so long, so this isn’t meant to be a definitive guide, but a quick primer on bottom brackets and their design.

The Basics

Push the pedals and the crankarms spin, right? But for things to spin they need bearings. In this case, they’re little balls between one surface and another that let them move against each other. This allows the crankarm assembly to rotate, pulling the chain and propelling you forward.

The Bearings

So where do we put those bearings? On traditional loose ball bottom brackets, the bearings are independent and hand-packed with grease and spin in a cup that was threaded into the frame. (1) Then came the sealed cartridge bottom brackets. These had the bearings inside a sealed cylinder. When they eventually wore out, you threw the whole thing out and got a new one. (2) A big leap came when engineers wanted to use bigger axles to increase performance. But there was no room in the frame for the big axle and the bearings. So they moved the bearings outside the shell and attached the axle directly to the crankarms. (3) Finally, as carbon fiber frames became common, engineers realized gluing inserts into carbon frames was expensive and kind of lame, so the carbon was molded such that the bearings sat inside it and held in place with a tight fit. (4)

The interface with crankarm

Ah yes, the square taper. Nearly all high-quality bikes used this type of mount for decades. Square peg goes in square hole and everyone was happy. (5) Well, maybe not everyone, because as bikes got more capable and riders were pushing them harder and harder, it wasn’t enough. To create a stronger interface, splines were formed in the bottom axle that mated up with the crankarm. Examples of this design include Octalink and ISIS. (6)

As the persistent pursuit of performance was pursued, engineers wanted to make the axle between the crankarms bigger. But there wasn’t room inside the bottom bracket shell for properly sized bearings. The solution: move them outside the shell, where they could retain their proper size and spec. The axle was now permanently attached to drive side crank arm instead of being a part of the bottom bracket. This is the two-piece bottom bracket designed known as Hollowgram (Shimano), GXP (SRAM) or Ultra-Torque (Campagnolo).

bottom brackets

The bottom bracket shell

This is where most of the consternation comes in. The shell is the part of the frame where the axle sits, between the crankarms. Despite the hullabaloo you hear that there used to be a common interface, it really isn’t true. Even in days of yore, there were still English, Italian, French and even Swiss bottom bracket shells, each with different threading. None were cross compatible. The English or BSA was the most common used on road bikes, with a 68 mm wide shell and reverse threading on the drive side. (1 and 2)

When the external bottom bracket was introduced, the bearings migrated from inside the shell to the outside, but were still threaded into the same old shells. (3) You could mount these on your old bike! Rejoice! Then that whole carbon craze came along and the press-fit bottom bracket came into vogue. (4) When this happened, product designers were finally like “Wait, we can change it again?” so they went to town designing frames with all sorts of new measurements. PF30, BB30, BB86, EVO386? There are many different “standards” but they are largely the same in their execution: the bearings sit directly in a channel molded into the frame, and since carbon fiber can be shaped in nearly any way, bearings can be larger and farther apart.

This article originally appeared in Bicycle Times 45. Subscribe to our weekly email newsletter to get fresh content delivered to your inbox every Tuesday!


Nuts and Bolts: Wrapping your head around headsets

Controlling your steering is an essential part of riding a bike, transferring the actions of your arms down to the front wheel.

By Adam Newman and Eric McKeegan, illustrations by Andrew Roberts

 Threaded headsets

By far the most popular headset style through the years has been the “threaded” headset. It derives its name from the threads cut into the fork steerer that are secured by a nut at the top of the headset. They are still found on many less-expensive or transportation bikes. They are easily identified by the type of “gooseneck” or quill stem that makes a bend and disappears into the head tube.

The simplest way to explain how it works is to imagine the fork steerer as a large bolt and the top of the headset is a nut. A threaded bearing race ensures the proper tightness on the bearings and the top lock nut holds it in place. Adjusting these headsets was one of the primary uses for the thin, flat wrenches that adorn nearly every bike shop service area in the universe.

Advantages: Easy to adjust handlebar height by loosening expander wedge in stem.

Disadvantages: Fork steerer needs to be just the right length for the frame with threads cut into it, making fork changes more difficult. Headset adjustments require specialized tools.

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 Threadless headsets

Most modern performance bikes used threadless headsets. You’re never going to believe this, but they are called this because they don’t have threads. Crazy, I know. Instead of the fork steerer terminating flush with the top of the headset, it extends above it to provide a space for the stem to clamp to. Because the stem can be clamped anywhere along this extended portion (within reason, not too high!) it can be adjusted up and down with the use of spacers. Some threadless headsets use external cups that you can see, while others use cups that press into the frame directly and are hidden from view. Some frames have built in cups, with bearings that drop directly into the frame.

Instead of a threaded race and nut providing the proper tension on the bearings, a bolt through the tpp cap pulls up on the star-nut or expander plug installed in the steerer tube to adjust bearing preload.. The stem holds this adjustment by clamping around the steerer tube.. In theory the top cap could then be removed, but we don’t recommend it. The fork steerer still needs to be cut to length, especially on smaller frames, but it doesn’t need to be exact as long as it is still long enough. The spacers allow for some flexibility.

Advantages: Usually lighter, allows for greater flexibility in fork design, including the use of carbon fiber or aluminum steerers. Bearing adjustment can be done with one or two hex wrenches.

Disadvantages: Very little height adjustment possible without swapping stems. More expensive? Aesthetics? Honestly we’re having trouble thinking of more than one.

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 Troubleshooting headset issues

Issue: My handlebars, stem and fork seem clunky or loose
The easiest way to test if a headset is loose is to hold the front brake, turn the handlebars to the side and rock the bike back and forth, holding the junction between the fork and the frame at the bottom of the head tube. If you can feel some play here, the headset is likely loose. With a threadless headset, you can adjust it easily with a multi-tool.

1. Loose the stem clamp bolts.
2. Tighten the top cap bolt until it’s snug, then back it out ⅛ of a turn.
3. Make sure the stem is pointed straight, and re-tighten the stem clamp bolts. Check the torque specs to avoid over-tightening.
4. Recheck adjustment.

Issue: I tightened my top cap all the way but the headset is still loose
Check to make sure the stem clamp bolts are loose so the stem can slide up and down the steerer to put tension on the bearings. Also make sure you have a few millimeters of space between the top of the steerer tube and the top of the stem.. If the top cap is bottoming out before it can push down on the stem and spacers, it won’t achieve the proper tension.

Issue: I’m trying to remove a quill stem but it’s stuck in the frame
Metal parts can corrode and stick together if not coated properly with a layer of grease, especially anything made from aluminum. Turn the bike upside down and try squirting some penetrating fluid into the underside of the steerer tube. Let it sit for a few hours and try again. It’s also a lot easier to get some leverage on the stem with handlebars installed. You can also remove the stem bolt and strike the stem with a rubber mallet.

Issue: My steering seems to have a notch in it.
This might seem like a nice feature to have, but it actually means your bearings and bearing race are worn out. It’s probably a good idea to have them replaced.

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