Since you can’t easily cut a thread into carbon, manufacturers needed to insert a threaded metal piece into the bottom bracket shell to screw in the bearings, which added weight. With the rise of carbon fibre as a frame material, threaded bottom brackets presented a problem. It’s well worth remembering that none of these external bearing systems are cross-compatible. And there’s a really solid interface between the bearings and the frame, which should eliminate creaking (see later).Īs well as Shimano, sealed external bearings are available from SRAM, in both its GXP design and under the guise of its newer DUB bottom bracket, and Campagnolo, in both Ultra Torque and Power Torque designs. The bearings are usually housed in sealed races, which makes them easy to remove or replace. The axle is hollow, which reduces the weight too. It also means that the axle diameter can be increased to 24mm, which increases axle stiffness.
Modern road bikes (opens in new tab) however use external threaded bearings and it’s what we’ll focus on here. Or if it's older still, the cranks may be connected to the axle with a bolted-on transverse cotter pin. Here a sealed unit that comprises bearings and square-tapered axles is threaded into the shell, with the crank arms then attached to the axles. Some older bikes will still use an internal bearing design. There is a variation in the types of threaded bottom brackets however. This is becoming increasingly popular in recent times with Specialized using threaded bottom brackets on both the carbon framed Aethos (opens in new tab) and Roubaix (opens in new tab) models for example. Carbon frames can also adopt this standard by using bonded metal inserts that are then threaded. The BSA standard still rules here, with a 68mm width shell used for road bikes. If you’ve got a steel, aluminium (opens in new tab) or titanium (opens in new tab) frame then it’s more than likely to have a threaded bottom bracket shell. Later in this guide we’ve touched on a few troubleshooting questions including installation. To attempt to provide some clarity in these decidedly murky waters, we’ve run through the main bottom bracket types to help you when it comes time to replace it. 'Standards' is still most certainly plural when it comes to bottom brackets, and something of an oxymoron there’s little 'standard' about the modern day bottom bracket.įor the home mechanic it can be a minefield. The decision adds a few grams, via the threaded insert, but is music to the ears of home mechanics and those who have encountered creaky press-fit systems. The result? The years 20 saw a resurgence of threads - the T47 adopted by Trek and the 68mm wide BSA by Specialized in its launches of the Tarmac SL7 and Aethos.
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