The B-gap or B-tension screw is located at the very back of your derailleur. It pushes against a tab on your frame’s derailleur hanger, allowing you to adjust the distance between your rear derailleur’s top pulley and the cassette. That distance is your B-gap.
If B-gap is correctly adjusted, your rear shifting will be quick and responsive. Mess it up and the derailleur will be hesitant or sluggish. Fortunately, this is usually a set-and-forget adjustment. Here’s how to get your B-gap dialed.
How to measure B-gap
Many drivetrains require you set the B-gap to a specific distance. We’ve compiled the specs for most modern drivetrains below.
You can measure B-gap with a pair of calipers. For smaller B-gaps (e.g. in the 4-10mm range), you can use an appropriately sized hex wrench as a measuring tool. Use your calipers or hex wrench to check the distance from the upper guide pulley of the rear derailleur to the lowest point of the large cog of your cassette.
SRAM has a couple of handy, inexpensive B-gap tools that make measuring B-gap a breeze. The latest-generation (8100/9100) Shimano mountain bike derailleurs have B-gap guides incorporated into the derailleur cage itself.
Digital calipers - $11.09
SRAM B-gap measuring tool - $14.95
SRAM Eagle 50/52t B-gap guide tool - $9.99
How to adjust B-gap
The B-gap screw is located on the upper back of the rear derailleur.
- Shift your bike into the largest rear cog. If you have a 2x or 3x crankset, shift the front into the smallest chainring.
- Most B-gap screws are adjusted using a small hex wrench, usually 3mm.
- Turn the B-gap screw clockwise to increase B-Gap. Turn it counter-clockwise to decrease B-gap. (See B-gap specs for different drivetrains below.)
- Check how your bike shifts. Re-adjust as necessary.
Setting your B-gap will be easiest if your bike is in a bike stand.
For most drivetrains, the goal is to set B-gap so that the upper guide pulley of your rear derailleur is as close as possible to the cassette cogs, without making noise or getting jammed up against the cassette.
If B-gap is too big: The derailleur will be too far away from the cassette, resulting in slow shifting. It will still shift, just not as crisply.
If B-gap is too small: The derailleur will be too close to the cassette and get jammed up against the cassette when shifting to larger cogs. It may not shift correctly at all.
Note: For full suspension mountain bikes, chain growth from rear suspension compression will affect B-gap distance. Ideally, you should set your B-gap at sag (i.e. while you’re sitting on the bike). This requires a second pair of hands to help. If you don’t have help, just set B-gap in the stand and adjust later as needed.
SRAM B-gap specs
- eTap: 6-8mm
- Mechanical: 6mm
- 33t max/no marking rear derailleur
- 26t cassette: 14mm
- 28t cassette: 10mm
- 30t cassette: 10mm
- 33t cassette: 5mm
- 36t max rear derailleur
- 28t cassette: 12mm
- 20t cassette: 10mm
- 33t cassette: 5mm
- 36t cassette: 5mm
AXS XPLR(Note: set XPLR B-gap at the second largest cog of the cassette.)
- 36t cassette: 11mm
- 44t cassette: 7mm
- AXS - use SRAM Eagle guide tool
- Mechanical - use SRAM Eagle guide tool
Shimano B-gap specs
Road Mechanical & GRX
- As close as possible without touching the cassette or producing “turbulence.”
- 11-30t: 14mm
- 11-34t: 6mm
MTB 8000/9000 series
- 42t big cog: 5-6mm
- 46t big cog: 8-9mm
MTB 8100/9100 series: Single chainring
- 12spd 10-45t
- XTR: align with line on inside of derailleur cage
- XT: align with top of derailleur cage
- 12spd 10-51t: align with line on inside of derailleur cage
- 11spd 11-50t: align with line on inside of derailleur cage
Campagnolo B-gap specs
10- and 11-speed
- As close as possible to small cog, but not angled upwards
Ekar 13-speed(Note: set Ekar B-gap at the second largest cog of the cassette.)
- 3mm max, min “noise”
Checking and adjusting your B-gap is one of the easiest ways to make sure your bike shifts as well as it can. It takes less than a minute, and is worth doing for any new bike or during regular maintenance. Now get your B-gap dialed and ride on!
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