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Tuesday 21 January 2020

Bike chain B-gap gauge

While setting up the 12 speed gears on Shelley's and Dean's bikes, I found that setting the B-gap distance was critical to getting the gears to change reliably.

If the gap is too far, it won't change up the gears from the largest sprocket to the next smaller. If the gap is too close, it binds trying to change down gears from the smallest to the next larger. I found that getting this gap just right was critical to indexing the 12 speeds.

SRAM think it is important because they supply a plastic gauge to help set the gap. The method described by SRAM, for a full suspension bike, either requires the rider siting on the bike to take the measurement or let out the air in the spring. The chain stay needs to be at it's longest length or near enough.

I'm sure that works in factory conditions but I found that sometimes it resulted in the gears binding, or poor changes, when the suspension was set back up and the bike ridden off-road. Perhaps I'm not following the instructions properly. I use an alternate method, that works for the full suspension bikes I have done.

My preferred method is to set the B-gap to the minimum while the suspension is fully extended. The air spring is inflated and setup for the rider. The setting is then done on the bike stand.
Typically the B-gap uncompressed is more like 11mm which is closer than the15mm measurement on the SRAM guide .

I ended up drawing an additional curved line on the SRAM guide to help with the setup.  During a spare few minutes, in the evening, I created my own variation and 3D printed it.

My version has steps for 5mm, 10mm and the SRAM suggested 15mm. All of our bikes worked best, when measured uncompressed, at just over the 10mm mark.

The tool fits the SRAM 50mm chain guide cog and it works the same way as the SRAM guide. With the chain in the lowest gear, on the largest sprocket, the tool is held by hand round the upper cog of the derailleur and aligned with the tip of the nearest tooth of the largest gear at its closest point. The B-tension screw is rotated to change the gap to the desired distance.


Download 3D models:
Chain gap gauge, Fusion 360, STEP and STL (Zip)
Licence attribution - small business exception


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