Adjusting Shimano Reel: Understanding The B-Tension Screw Sizes

The B-tension screw, also known as the B-screw, is an important component of a bicycle's derailleur system, specifically affecting the rear derailleur. This screw controls the body angle of the derailleur by pulling the pulleys away from the sprockets, ensuring they do not rub against each other. The optimal adjustment of the B-tension screw is when the gap between the upper pulley and the largest cog is minimised without causing any interference or noise. This adjustment improves the shifting performance of the bicycle, making it crisp and efficient.

The B-screw is also known as the B-tension screw

The B-tension screw has two main purposes. Firstly, it ensures the correct clearance between the top derailleur pulley and the largest cog on the cassette. This is achieved by pulling the derailleur back and away from the cog, preventing the pulley from bumping into the cog and refusing to shift up onto it. Secondly, it helps to wrap any excess chain slack. By turning the B-tension screw clockwise, any excess slack in the chain can be removed.

It is important to note that the B-tension screw should only be adjusted when necessary, as over-tightening can affect shifting performance. Additionally, the B-tension screw should only be used to remove minor chain slack; if the chain is too slack, the derailleur's capacity may have been exceeded, and the chain length should be adjusted instead.

The B-tension screw is typically a standard 4mm thread, but longer B-screws may be required when using certain combinations of rear derailleurs and cassettes. For example, when using a Shimano rear derailleur with the GC42 cassette, a longer B-screw is usually needed.

In summary, the B-screw, or B-tension screw, plays an important role in ensuring smooth and efficient gear shifting by maintaining the correct clearance between the derailleur pulley and the cassette cog, as well as managing any excess chain slack.

The B-tension screw controls the gap between the guide pulley and the bottom of the cogs

The B-tension screw is the third screw on the back of your rear derailleur. Its function is to control the gap between the guide pulley (also known as the upper pulley) and the bottom of the cogs. This gap is called the B-Gap. The B-tension screw contacts the derailleur hanger or derailleur tab, either directly or indirectly.

The B-tension screw is not marked with an "L" or "H" and transcends almost all rear derailleurs, except the most inexpensive units. Some units are spring-loaded, allowing for dynamic chain tension, while others are static.

The B-tension screw is typically adjusted when the drivetrain is in the absolute easiest climbing gear. This means that the chain and derailleur are in the largest cog position. The B-Gap is measured tooth to tooth.

If the B-Gap is too small, the jockey pulley will struggle to shift. This is most notable when shifting into the largest cog. The chain is forced into a sharper angle moving from one cog to the next, causing the chain to bounce around on the edges of the teeth and miss the shifting ramps of the cassette cogs.

If the B-Gap is too large, shifting becomes unpredictable and erratic, especially when shifting into the smaller cogs. If the B-Gap is too large, the derailleur may struggle to get to the smallest cog.

The B-tension screw should be adjusted to find the "sweet spot" where the B-Gap is large enough to allow the chain to have a certain amount of flexion as it travels from the upper pulley of the derailleur to the cogs, but small enough to enable quick and clean shifts.

Shimano recommends that the B-tension screw is adjusted to create a gap of 5-6mm between the guide pulley and the bottom of the cogs. SRAM recommends a B-Gap of 6mm for their road rear derailleurs and 14mm for their 1x11 drivetrains.

The B-tension screw is insensitive once you clear the cogs

The B-tension screw is an adjustment that controls the body angle of the derailleur. It pulls the pulleys away from the sprockets, preventing them from rubbing against the cogs. The B-tension screw is typically located on the back of the rear derailleur and can be identified as the non-marked screw. It is important to adjust the B-tension screw correctly to ensure smooth and efficient shifting.

When adjusting the B-tension screw, it is recommended to start by shifting the rear derailleur to the lowest geared cog. If there is more than one chainring, shift to the smallest chainring as well. If the B-gap is too small, the upper derailleur pulley may rub against the cog, causing a rumbling sound. In this case, the B-tension screw should be adjusted to increase the gap.

On the other hand, if the B-gap is too large, shifting may become unpredictable and erratic, especially when shifting into smaller cogs. To correct this, the B-tension screw should be adjusted to reduce the gap. The ideal B-gap will allow for clean and quick shifts across all gears.

It is worth noting that the B-tension screw adjustment may not have a significant impact on shifting performance in some cases, particularly with Shimano derailleurs. However, it is still important to ensure that the derailleur pulley does not interfere with the cog to prevent rubbing and maintain smooth shifting.

Additionally, the B-tension screw plays a role in chain tension. A properly adjusted B-tension screw, along with the correct chain length, rear derailleur cage length, and cable tension, will result in optimal chain tension and shifting performance.

In conclusion, while the B-tension screw may seem insensitive once the cogs are cleared, finding the optimal B-gap by fine-tuning the B-tension screw adjustment will result in improved shifting performance and chain tension.

The B-tension screw is located on the new Dura Ace rear derailleur

The B-tension screw is located on the new Dura-Ace rear derailleur, which is designed for road cyclists seeking a lightweight, aerodynamic, and durable component for their bikes. This screw plays a crucial role in ensuring smooth and precise gear shifting by adjusting the angle of the derailleur's upper spring-loaded pivot, which controls the height of the jockey pulley.

The B-tension screw, also known as the body-angle screw, is used to fine-tune the spacing between the derailleur pulleys and the cogs. This adjustment is crucial when using a larger or smaller low-gear sprocket or when changing the length of the chain. If the B-tension screw is too loose, the jockey pulley may bump into the largest sprocket, affecting the shifting performance. Therefore, it is essential to adjust the B-tension screw according to the size of the largest rear sprocket to ensure optimal gear shifts.

When adjusting the B-tension screw, it is important to use a hex wrench to gauge the appropriate distance between the derailleur pulleys and the cogs. Most road and mountain bike derailleurs require a gap between 5 and 6 millimeters. However, it is worth noting that if the bike is already shifting well, the B-tension screw is likely within the acceptable adjustment range, and significant changes may not be necessary.

Additionally, the B-tension screw can be used to balance the tension between the two springs in the derailleur, which is crucial for achieving the best shifting performance. By loosening or tightening the B-tension screw, you can adjust the tension of the upper spring, influencing the overall smoothness and precision of gear changes.

It is worth noting that the B-tension screw is not intended to move the derailleur during normal riding conditions. Its primary purpose is to set the limits of the derailleur's movement, ensuring that the chain does not shift past the largest or smallest sprocket and into the spokes or frame of the bike. This screw acts as a safeguard, preventing potential damage to the bike and ensuring a smoother riding experience.

The B-tension screw is M4 x 12mm in size

The M4 x 12mm screw size refers to the diameter and length of the screw, respectively. M4 indicates a 4mm diameter, which is the standard size for B-tension screws on Shimano rear derailleurs. The 12mm length determines how far the screw can thread into the derailleur's body.

B-tension screws with this specific size are readily available from various retailers. They are typically made of stainless steel or alloy steel and come in different head styles, such as pan head, flat head, or socket head cap screws.

When adjusting the B-tension screw, it is important to refer to the manufacturer's guidelines and make sure the derailleur is not bent. The correct adjustment will depend on the size of the largest rear sprocket and the length of the chain.

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