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Bottom bracket

Das Tretlager verbindet Kurbel und Rahmen – und ist trotz seiner unauffälligen Lage eine der wichtigsten Schnittstellen am Fahrrad. Dieser Artikel erklärt alle gängigen Standards von BSA über PF30 bis T47, zeigt woran man ein verschlissenes Tretlager erkennt und was beim E-Bike besonders gilt.

The bottom bracket is an essential component on bicycles that plays a central role in the drive system. It connects the crank arms and thus the pedals to the frame of the bicycle.

The bottom bracket is a fundamental component of the bicycle, responsible for transmitting pedal power to the drive system. It consists of the bottom bracket shell, bearings, crank arms and an axle. Its functions are to transmit power, create a low-friction movement and stabilize the drive system. Regular maintenance of the bottom bracket is crucial to maintain the performance of the bicycle.

Functions of the bottom bracket

  1. Power transmission: The bottom bracket serves as the interface between the pedals and the crank. It transfers the force exerted on the pedals to the crank arms, which in turn transfer the rotational movement to the chainring and the bicycle chain.

  2. Low-friction movement: The bottom bracket allows for low-friction and smooth rotation of the crank arms, which increases pedaling efficiency and minimizes energy loss.

  3. Stability of the drive system: The bottom bracket stabilizes the position of the crank arms by anchoring them securely in the frame. This ensures reliable transmission of pedal power to the bike.

Structure of the bottom bracket

  1. Bottom bracket shell: The bottom bracket shell is a casing in the bicycle frame where the bottom bracket is mounted. It can have different shapes and standards, depending on the type and model of bicycle.

  2. Bearings: Inside the bottom bracket shell are bearings that allow the crank arms to rotate. These bearings can be ball bearings or other types of plain bearings that provide low-friction movement.

  3. Crank arms: The crank arms are the levers that connect to the pedals. They are attached to the bearings in the bottom bracket shell and create the rotational movement when the rider steps on the pedals.

  4. Axle: The axle is the connection between the two crank arms and runs through the bottom bracket shell. It enables the coordinated movement of the crank arms.

The bottom bracket is a crucial part of the bicycle drivetrain as it converts the crank movement into a rotating motion that is transmitted to the chainring and the bicycle chain. This conversion of pedal power into movement is essential for the bicycle to move forward and has a significant impact on the efficiency of cycling.

Maintenance and Care

The bottom bracket requires regular maintenance to ensure it functions smoothly. This may include cleaning, lubricating and replacing the bearings if necessary. Taking good care of the bottom bracket will contribute to the longevity and performance of the bike.

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Published: August 18, 2023  |  Updated: May 22, 2026

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Häufige Fragen about Bottom bracket

Die häufigsten Fragen und Antworten

BSA has an internal thread in the bottom bracket shell—the bottom bracket is screwed in and can be tightened with a wrench. Press Fit has a smooth shell—the bearing is pressed in and held in place by friction. BSA is easier to maintain and creaks less. Press Fit allows for larger shell diameters and therefore wider crank axles, but is more prone to noise due to settling of the press fit.

Turn the cranks by hand without any load: If it runs roughly, catches, or you feel lateral play, the bottom bracket is probably worn. Another sign is rhythmic creaking or cracking in time with the pedal movement—that is, once per crank revolution. Check the crank arms and pedals beforehand, as these often produce identical noises.

For BSA threads, yes — you'll need a suitable wrench (e.g., Shimano TL-UN74-S) and a bit of finesse when screwing it in. The left-hand thread is reversed! For press-fit systems, you'll need a press tool; without one, you risk an incorrect fit and immediate creaking.

The following are crucial: the standard of your frame (BSA, PF30, BB86, T47 – often found in the frame manual), the axle diameter of your crank (24 mm for Shimano, 30 mm for SRAM DUB and many others), and the width of the bottom bracket shell (68 mm, 73 mm, or 86/92 mm). Most crank manufacturers list compatible bottom brackets in their documentation.

With mid-drive motors (Bosch, Shimano Steps, Fazua, Brose), the bottom bracket is integrated into the motor housing and cannot be replaced separately. Replacing the bottom bracket is part of the motor overhaul. On e-bikes with rear hub motors, the bottom bracket is a normal, independently replaceable component, just like on a regular bicycle.

T47 is a modern thread standard with a 47 mm inner diameter—large enough for 30 mm crank axles, but with the proven screw-in threading principle. It is structurally creak-free, can be retightened, and is compatible with BSA tools. T47 is particularly recommended for gravel and road bikes where stiffness and low maintenance are essential.