The B-gap or B-tension screw is an important part of your bike’s rear derailleur system. It’s responsible for controlling the distance between the upper pulley of the rear derailleur and the cogs on the cassette.
The B-gap screw is located at the back of the derailleur, where it pushes against a tab on the frame’s derailleur hanger. This allows you to adjust the distance between the top pulley of the derailleur and the cassette.
Importance of setting the B-gap correctly
When the B-gap is correctly adjusted, the derailleur can move freely and quickly between gears.
An incorrectly adjusted B-gap can cause the derailleur to shift slowly or sluggishly or even cause the chain to jump between gears or drop off the chainring.
- B-gap is too big. The rear derailleur will be positioned too far away from the cassette, causing slow shifting. Although the derailleur will still shift gears, the shifting will not be as precise and crisp as it should be.
- B-gap is too small. The rear derailleur will be positioned too close to the cassette. This can cause the derailleur to jam against the cassette when shifting to larger cogs, preventing it from shifting correctly or even causing damage to the components.
While B-gap adjustment is typically a set-and-forget process, it may need to be readjusted over time due to wear or changes to the drivetrain components. You can also have a professional bike mechanic check and adjust the B-gap during regular bike maintenance to ensure that it’s functioning properly.
How to measure B-gap
To measure the B-gap, you can use a pair of calipers or an appropriately sized hex wrench.
Here are the steps to measure B-gap.
- Shift your bike into the largest cog on your cassette and the smallest front chainring.
- Locate the B-tension screw on your rear derailleur. This is the small screw at the back of the derailleur.
- Use your calipers or hex wrench to measure the distance between the upper guide pulley of the rear derailleur and the lowest point of the largest cog on your cassette. This is your B-gap measurement.
In addition to calipers or hex wrenches, some bike manufacturers offer specialized tools to measure B-gap, such as;
- SRAM B-gap tool
- Shimano GG adjustment gauge (for 12-speed only)
How to adjust B-gap
Here are the steps to measure the B-gap.
- Place the bike on a repair stand.
- Shift the gear into the largest rear cog. If you have a 2x or 3x front chainring, shift to the smallest front chainring.
- Check the B-gap specifications for your groupset. It varies according to brand, model, and cassette size.
- Use a 3mm hex to turn the B-screw.
- Turn the screw clockwise to increase the B-gap or counterclockwise to decrease it.
- Shift the chain across all the cogs if it’s smooth. Make minor adjustments accordingly.
Shimano B-gap specifications
Shimano Road (12-speed)
The table below shows the B-gap for Shimano 12-speed rear derailleur and cassette sizes combination.
| Rear derailleur model | 11-30T | 11-34T | 11-36T |
|---|---|---|---|
| RD-R9250 / RD-R8150 | 14mm | 6mm | |
| RD-R7150 | 10mm | 6mm |
Shimano Road (10, 11-speed)
Shimano doesn’t specify the B-gap distance for its 10, 11-speed road groupsets.
Instead, according to Shimano, bring the guide pulley as close to the largest sprocket as possible without any turbulence between the pulley and sprocket.
The chain and guide pulley will begin to stutter if they’re too close to each other.
More reading : Shimano Ultegra Di2 (R8100) vs Dura-Ace Di2 (R9200)
SRAM B-gap specifications
SRAM AXS Road (12-speed)
The table below shows the B-gap for SRAM AXS Road (12-speed) rear derailleur and cassette sizes combination.
There are two variants of the rear derailleur,
- Short cage, supports maximum 33T cassette
- Medium cage, supports a maximum 36T cassette
| Rear derailleur model | 10-26T | 10-28T | 10-30T | 10-33T | 10-36T |
|---|---|---|---|---|---|
| Short cage (33T max) | 14mm | 10mm | 10mm | 5mm | |
| Medium cage (36T max) | 12mm | 10mm | 5mm | 5mm |
SRAM Road (11-speed)
- SRAM Red eTap : 6-8mm
- SRAM Red22, Force22, and Rival22 : 6mm
More reading : SRAM Rival eTap AXS vs Shimano 105 Di2 (R7100)
Campagnolo B-gap specifications
- Campagnolo 10, 11-speed : 5-7mm
- Campagnolo 12-speed : Guide pulley as close as possible to small cog, but not angled upwards
- Campagnolo 13-speed : 3mm maximum