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 HG style driver. The SRAM GX, and above, cassettes fit on a SRAM XG specific driver.



Hanger

The SRAM derailleur fits in a different position to where a 10 speed 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

If the SRAM is any further out, from the frame, it will not shift in to the lowest gear. Even as it is, it might need a 0.5mm or 1mm spacer, behind the cassette, to get it in to a position that works.

The hanger also needs to be aligned. If it is bent, it will affect the gear changes.
[Re-aligning the hanger is the most common fix I have had to do after rides. to get the gears to work properly again.]

Chain

Most modern chains use a master link to join the chain. Also known as a quick link or magic link.

There are several methods to get the length of the chain. I used the method that appears the most common. For a full suspension bike, it needs to be done at the longest length, which is when the suspension is fully compressed. 

I took note of the suspension pressure and then let all the air out. I compressed the suspension and tied it up with a Velcro strap.

Without the derailleur, I wrapped the new chain round the largest sprocket and the chain ring.



With half of the quick link attached, I found where the chain could be joined and then added two rivets. I find it easier to think in rivets but often it is referred to as extra links.

Both ends of the chain should be an inner link.


Informative Park Tools Video:

The Park Tools video is much clearer at explaining this than the SRAM video. It is two links or rivets past the inner link that the end of the chain could be connected to. This means the break could be nearly 4 links past the overlap in some cases.

Some types of bike with the large 50-tooth sprocket, might need to add 4 rivets instead of 2 to the length. If full suspension bike chains are measures without compressing the suspension, they could need 6 rivets added, however this is not the preferred method.


Some master links fits a specific way round and have an arrow indicating the direction the chain normally travels. In the case of SRAM, the quick link is curved and the curved surface must be outwards with the arrow facing the direction of travel of the chain, when facing it from the drive side (the arrow viewed from the non-drive side points the wrong way!)


The master link is fitted loose. To secure it, I held the rear wheel on the brake and pedalled. That pulls the chain taught and locks the quick link in place.

Some chain has a right and a wrong way round. For example, with Shimano chain the lettering should be on the outside. If you look very closely the chamfers on each link are slightly different on each side. Some chains are symmetrical.

End stops

The limit screws are used to stop the chain coming off either end of the cassette.

The end stop for the smallest sprocket can be done before attaching the cable.
On a SRAM, the limit screw for the largest sprocket, is the one on the outside of the derailleur. The limit screw for the smallest sprocket, is closest to that sprocket.

The position for the jockey wheel, is to have the outer edge of the smallest sprocket on the cassette aligned 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.



This can be hard to see on a mountain bike with 2.3" tyres fitted. I found that looking at how central the chain was, as it was pulled on to the largest sprocket, helped to gauge when the limit screw was set correctly. If the chain looked central, rather than pulled to one side or the other, then it was about right.

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 and larger sprockets, this gap has increased.


For SRAM 12 speed they recommend a gap of 15mm. They supply a tool to help set that gap.
That is the gap set when the chain stay is at it's longest length.

That requires having someone sit on the bike or remove the air from the rear shock.

I found that a good result could be achieved without compressing the suspension but the gap, for both the Giant Stance and the Giant Trance, was 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

I got the idea from the factory setup of my Giant Stance. It came from the shop working efficiently, with just over a 10mm gap with the suspension in its resting state.

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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Cassette Drivers

After decades of one predominant system, there are now a number of different types in use.
  • Shimano HG - most common, been the standard for years. The smallest sprocket that fits is 11 tooth. SRAM SX and NX cassettes use this fitting. Sunrace also have cassettes that fit this.
  • SRAM XG - specific to SRAM Eagle, used for 11 and 12 speed cassettes from the GX range upwards. The smallest sprocket can be 10 tooth.
  • Shimano Micro Spline - Used for Shimano's 12 speed cassettes. The smallest sprocket can be 10 tooth.
Many makes of wheel hub have interchangeable drivers but not all. I've changed one on a Hope Pro 4 hub. That was relatively easy to swap over, using the correct size seal press. 

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Update: Jan 2021


The latest SRAM NX Groupset came with a different tool to measure the B-gap.


This new tool measures the gap when the chain is in the 2nd lowest gear and while the bike is at normal sag, not full compression. It was easy enough to check the pressure of the rear shock and then let a little air out to simulate someone sitting on the bike.

I found this tool very easy to use.

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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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