Sram GX 12 Speed Derailleur is definitely a part on your bike that will need adjusted or replaced overtime. These often take a beating on the trail and hopefully they just come out of alignment versus brake. Regardless here is how you get back to shifting smooth on the trails. While many derailleurs are similar in regards to setting up, this is specific to the Sram GX 12 Speed derailleur.
Tools/Equipment Needed
- Allen Keys
- Star Keys
- Derailleur hanger alignment tool
- B-Gap Limit tool
- New Sram GX 12 Speed Derailleur (if replacing)
- Extra shift cable (just in case)
For any tools you may have have take a look at these tools which include everything you need and more. The B-GAP limit tool will come with a new GX Sram 12 Speed derailleur.
Duration
Total time required is less than 1 hour. Ok now with that said this will take longer the first time you do it. Trust me if you ride a lot this will need done more than once so you will have plenty of time to practice.
Initial Setup
- Place your bike into a bike repair stand. If you don’t have a bike stand I highly suggest you pick one up. While this can be done without this will make it much easier otherwise you will need another set of hands to help out.
- Shift the rear derailleur to the smallest / hardest gear.
- Loosen the cable tension at the derailleur.
- Loosen the barrel adjuster on the shifter by turning it all the way towards you (sitting on bike facing handlebars). Then tighten it slightly by adjusting it 10 turns away from you.
- Remove the installed derailleur. Using the derailleur hanger alignment tool perform a four point check on the derailleur hanger to wheel. Note use the valve stem as a common point to take the measurement. Adjust hanger by bending installed tool in direction necessary to ensure all points checked are within 3mm of each other.
- Once the hanger is straight re-install (or install the new) Sram GX 12 Speed Derailleur. I have found that sometimes a new cable is required when installing a new derailleur which seem to be because of the Sram GX cable housing. The derailleur cable housing has a sharp bend that can be difficult to get the cable through. Anyways always have a spare cable on hand.
Sram GX 12 Speed Derailleur Limits
Low Limit
The low limit is set to align the derailleur with the largest cog on the cassette. This setting is adjusted by using the low limit set screw shown below.
There is still no tension on the cable therefore the derailleur will need pushed into the wheel while rotating the crank until the largest/lowest gear is reached.
Once at the largest cog the low limit screw is adjusted while holding the derailleur in place. The low limit screw shall be adjust until the upper derailleur cog and largest cassette cog are aligned as shown here. The chain line should be straight. When rotating the crank the teeth on the cassette should fall approximately in the middle of the chain.
You can now release the derailleur and rotate the crank allowing the chain to fall to the smallest/highest gear.
High Limit
With the chain on the smallest/highest gear the high limit screw shown below will now be adjusted.
Adjust the high limit screw until the upper derailleur cog and smallest cassette cog are aligned as shown here. Chain line can also be slightly to the right.
The shift cable can now be tightened to the derailleur. Important: the chain must be in the highest gear and the shifter shall be set to the highest gear as well. If you haven’t touched anything the bike should already been in this state. The cable should be pulled tight by hand and then secured. I have found that pulling with pliers provides too much tension. Stick with your hand, it doesn’t take much force.
At this point both the low and high limits have been set, which defines the maximum travel of the derailleur during shifting. Next the B limit needs set which is the distance of the upper derailleur cog and the cassette.
B Limit
For full suspension bikes this setting needs to be done while the bike is under sag. This can be down two different ways. For those with hardtails you can skip this part.
- If you have someone helping you take the bike off the rack and have them sit on it while you make the adjustment. Ensure you are at the same sag you would be at.
- When alone, let all of the air out of the rear shock (record existing PSI if not already known). The rear triangle can then be secured by a strap t hold the bike in the sag position while making the adjustment.
With bike at sag, shift into second gear and install the B-Limit tool as shown below.
The B Limit screw can now be adjusted until the arrow on the tool aligns with the center of the derailleurs upper cogs mounting bolt.
Final Steps
Place your bike on the stand and shift through all the gears. Fine adjustments can be made at the barrel adjuster on the shifter. Turn the barrel adjuster away from you (when sitting on bike) to tighten cable tension (removes lag in shifting to lower gears). Turn the barrel adjuster towards you (when sitting on bike) to loosen cable tension (removes lag in shifting to higher gears).
Finally take the bike for a ride. Again adjustment can be made at the barrel if needed when out on the trail
Tips
- Derailleurs often seem scarier than they are. Just give this a try and while it may frustrate you the first couple times you will eventually get it, hang in there.
- Chain can be removed to align the derailleur. I have found this helped the first couple times i did this but now is no longer needed.
- While I did not mention it, there is a red B-GAP Limit tool from Sram as well. Honestly the new (white) tool works so much better I suggest getting one if you don’t have it already.
- Print a copy of this and take it in the garage with you to follow along. After several you certainly wont need it anymore.
- Finally when a bike is shifting fine NEVER touch the derailleur (its like waking a sleeping baby). Let it be.
When making this I realized my drivetrain needs cleaned. Its pretty dirty from some recent winter rides and could certainly use some drivetrain maintenance.
Want a way to help keep your eyes on the trail when riding and increase your Speed? Check out TrailSIte which mounts right to your helment visor!
