Saturday, 18 January 2020

Fitting a 12 speed group set

I had a chance to see a 12 speed group set being fitted to a Giant Trance bicycle, which helped me when updating Shelley's Merida to 1x12 from 3x10. In both cases the set being fitted was from the SRAM eagle range. Both with an NX derailleur and GX shifter. One with an NX cassette and the other with a sunrace cassette.


As usual, SRAM have a very good video for fitting and adjusting the gears, however we had to fettle to get a better end result.

Shimano style driver

Both the SRAM NX and the Sunrace cassettes fit on the common Shimano style driver. The SRAM GX, and above, cassettes fit on a SRAM specific driver.



Hanger

The SRAM derailleur fits in a different position to where a Shimano would usually fit.
The SRAM does not use an extension and, more importantly, it must not be offset away from the hub.

Shimano, offset and set back

SRAM, no offset and almost straight down

Chain

To get the length of chain. Wrap it round the largest sprocket and the chain ring. Add two links, one outer and one inner. Both ends of the chain should be an inner link.

The master link fits with the curved surface outwards and the arrow facing the direction of travel of the chain.

End stops

The end stop for the smallest sprocket can be done before attaching the cable.
On a SRAM, the limit screws are furthest away from the end they adjust.
Align the outer edge of the smallest sprocket with the inner edge of the guide cog.

To adjust the lowest gear end stop, attach the cable.
First turn the barrel adjuster on the shifter, full clockwise, then turn it back two full turns anti-clockwise. This is so there is enough scope for later adjustment.
Click the down shifter and pull the cable to ensure it is fully extended, then attach the cable to the derailleur.
Once attached firmly, click the other lever to pull the derailleur to the other end and adjust the end stop.
Turn the screw so the centre of the guide cog, on the derailleur, aligns with the centre of the largest sprocket.




B-Tension

This adjusts the gap between the guide pulley and the largest sprocket on the cassette.
It is the closest the tops of the teeth get to each other.
In the past it used to be fairly common to set this to 5 to 6mm. As cassettes have got more sprockets, this gap has increased.


For SRAM 12 speed they recommend a gap of 15mm. They supply a tool to help set that gap.
This was too wide for both the Giant Stance and the Giant Trance. Both of those worked better with a gap closer to about 10mm.
The Merida ONE-TWENTY would work at 15mm but was more reliable at about 11mm.


If the B gap is wrong it will affect how quickly or slowly the gears change and it might miss a gear. Except in extreme cases, the gear change will still work, even if the gap is not ideal.

Giant Stance

The mechanics who setup my Giant Stance must have ignored the SRAM tool. I would guess that they had some prior experience of the smaller gap required because it came from the shop working efficiently, with the smaller 10mm gap. It was measuring that, against the setting on the guide tool, that helped setup the Giant Trance to work.

Gear Change Adjustment

To get the gears to change up and down quickly and smoothly, it is necessary to adjust the cable tension. Typically there is a barrel adjuster at either the derailleur or the shifter end of the gear cable.
SRAM have the adjuster on the shifter.

When changing gear:

  • if it is slow moving from Large to Small sprockets turn the barrel adjuster clockwise.
  • if it is slow moving from Small to Large sprockets turn the barrel adjuster anti-clockwise.


If it won't adjust to give good up and down shifting, consider adjusting the B gap.

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Links

All the adjustments to get gear changing to work properly.
https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment

More about the B-tension adjustment.
https://bikerumor.com/2017/09/29/aasq-14-what-does-the-b-tension-screw-on-a-derailleur-do/



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