A rear derailleur that won't shift on an electric mountain bike usually isn't a screw problem. The cause could be a binding cable, a bent hanger, a worn chain, mismatched drivetrain parts, or an electronic control fault. So where's the real fault hiding? Start with the symptom, not the limit screws.
Kill the motor and assist before you inspect anything. Clamp the bike in a stable repair stand, keep your hands clear of the chain, and shift with the cranks turned by hand. A repair that works only in the stand isn't finished yet.
Read the symptom before touching the derailleur
The symptom usually points at the first check. Use this table so you're not changing three settings at once.
| What you notice | Check first |
|---|---|
| No movement across the cassette | Cable connection, cable friction, shifter operation, or electronic power |
| One click does nothing, then two gears shift | Cable tension, housing seating, or a worn shifter mechanism |
| Chain falls past the smallest or largest cog | H-limit, L-limit, or hanger alignment |
| Skipping occurs in one or two cogs | Chain and cassette wear, or a bent hanger |
| Shifting works in the stand but fails under assist | Excess pedal load, drivetrain wear, or a problem that appears only under torque |
| Derailleur makes noise in every gear | Incorrect indexing, wrong B-gap, hanger alignment, or incompatible parts |
Turns out the table splits into two families. A derailleur that moves but never lands on the next cog is usually fighting a tension or alignment problem. One that doesn't move at all needs a control-side check first, because turning screws on a dead derailleur fixes nothing.
Gather tools and make the e-bike safe
You won't need every tool for every job. The basics:
- A stable bike stand, 4-6 mm hex keys, and a screwdriver
- A chain wear checker matched to your chain
- A clean rag and bicycle-safe degreaser
- A torque wrench if a mounting bolt needs tightening
- A derailleur hanger alignment tool, but only if you know how to use it
Power the bike down before your hands go anywhere near the chain, cassette, or derailleur. Some bike and battery manuals require the battery to come out for service. If yours does, follow it. Torque values also differ between manufacturers, especially around the derailleur mount, crankset, and cassette.
One hard rule: never use the motor to spin the drivetrain while your hands are close to it. Turn the cranks by hand. It's slower, and that's the point.
Check the simple mechanical faults first
Start at the rear wheel. Look at the bike from behind and compare the derailleur cage with the cassette. The cage should sit roughly in the same plane as the cogs, not tilted toward the spokes or leaning away from the wheel.
Drive-side impacts bend hangers all the time, even when the derailleur above them looks perfectly fine, and because small bends are nearly impossible to confirm by eye, the safest move is to stop adjusting the moment the cage reads crooked and either put a hanger alignment tool on it or hand the bike to a shop.
Next suspect: the cable and housing. Follow the shift housing from the shifter all the way to the derailleur and look for crushed sections, sharp bends, rusty or frayed cable, and housing ends that have pulled out of their stops. Any of these can stop the derailleur from traveling the same distance on every shift.
Put the shifter in the position that leaves the cable slack, per the bike's service instructions, then pull gently on the exposed housing. It should glide. Sticky return or obvious drag mimics bad indexing so well that friction gets blamed on indexing constantly.
Bolts come after that. Check the derailleur mounting bolt, the shifter, the cable pinch bolt, and the pulley cage, and snug anything loose to the manufacturer's specification. A loose mount throws off alignment the second you pedal.
Clean the cassette and jockey wheels before you judge shift quality at all. Thick mud can hold the chain away from a cog. Packed debris around the pulleys makes the cage feel stiff. Both will fool you.
Check drivetrain wear and compatibility
A correctly adjusted derailleur can't rescue a worn chain. Measure with a chain checker and follow the limit printed for that tool and chain. A 0.8 reading works as a replacement reference for some SRAM chains, but it isn't a universal limit for every chain or drivetrain.
Worn chains mesh poorly with the cassette, and if the cassette has grown hooked or worn teeth, a fresh chain alone may not cure the skipping. Look over the chainrings and jockey wheels while you're in there. Damaged teeth, excessive side play, heavy buildup. All worth a minute.
Thing is, adjustment cannot fix incompatible parts. Match the shifter, derailleur, cassette speed, cable-pull standard, chain, and derailleur capacity. A SRAM shifter running a Shimano derailleur is the familiar example, since a SRAM 1:1 setup and a Shimano derailleur won't index correctly just because they bolt together.
Check exact model numbers before replacing anything, because similar-looking parts can carry different cable-pull ratios or cassette ranges. High-load e-bike use also makes chain and cassette wear show up sooner, which is why rental fleets and frequently ridden mountain bikes need inspections on a shorter cycle than most owners expect.
Adjust the rear derailleur in the right order
One change at a time. Touch the limit screws, cable tension, and B-screw together and you can invent a new problem with no idea which setting caused it.
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Prepare the derailleur. Bike in the stand, motor switched off. Shift to the smallest rear cog using the procedure in the derailleur manual, turning the cranks by hand if the chain needs to move. Confirm the housing is seated and the derailleur isn't visibly bent.
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Set the H-limit. This screw controls travel toward the smallest rear cog. With the chain parked on that cog, the upper pulley should line up with it. Turn the screw in tiny increments, and confirm the chain can't fall past the smallest cog toward the frame.
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Set the L-limit. Shift carefully toward the largest rear cog. The upper pulley should line up with that cog without pushing the chain into the spokes, which is exactly what the L-limit prevents. One warning: if the hanger is bent, screw adjustment won't give you reliable alignment no matter how patient you are.
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Reconnect and index the cable. Return the shifter to the smallest cog position, check that every housing section is seated, and attach the cable by the manufacturer's procedure. Turn the cranks slowly and click through one gear at a time. Hesitation moving toward larger cogs calls for a small tension increase. Hesitation back toward smaller cogs means ease tension off slightly. Use the barrel adjuster in small steps.
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Set the B-gap. The B-screw sets the distance between the upper pulley and the cassette, and the correct value depends on the derailleur, cassette size, chain length, and the manufacturer's gauge. Some 11/12-speed mountain setups use a 5-6 mm hex key as a checking guide, but don't treat that measurement as universal. Use the value or gauge specified for your model.
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Repeat the range check. Every click should produce one clean shift. Listen for rubbing, delayed movement, chain chatter. Don't keep turning the B-screw to cure a bent hanger or a worn cassette.
For diagrams and the adjustment sequence, compare your model's instructions against Park Tool's rear derailleur adjustment guide and its advanced troubleshooting guide. SRAM owners can also pull from the SRAM Setup and Service resources.
Account for electronic shifting and motor load
Plenty of e-bikes have no shift cable at all. Electronic systems like SRAM AXS or Shimano Di2 need a charged battery, secure connections, and the correct setup or calibration procedure, and they index without a barrel adjuster. There isn't one to turn.
Follow the bike's manual to check shifter and derailleur status. No response from the derailleur? Then leave the mechanical cable settings alone, because that system doesn't have them. Battery charge, connections, system warnings, and the manufacturer's reset or pairing instructions are the actual checklist.
Motor torque changes how a shift feels, too. Out on a ride, ease off pedal pressure for a beat and shift in low assist. A hard assisted shift loads the chain and cassette heavily, which can produce a clunk or a delayed shift that never appears with the motor off.
If shifts are clean without assist but fail every time under power, note the assist mode and gear combination, then check the drive system and bike manual before adjusting anything else. The motor does not require a different limit setting.
Test the repair in stages
Stand test. Motor off, cranks rotated slowly by hand, run the chain across the full cassette. One click should mean one shift, the chain should stay on every cog, and the cage should never contact the cassette or the spokes.
Low-load ride. Take it somewhere flat and quiet. Low assist, light pedal pressure, gears worked through gradually. Leave hard standing starts, steep climbs, and maximum assist alone until the chain runs quiet.
Load check. Add moderate pedal pressure only after the low-load run passes. Listen for skips on gear changes. If the chain jumps, drops, or makes a sharp grinding sound, stop immediately.
Post-ride inspection. Look for a loose derailleur mount, fresh cable movement, or chain marks near the spokes. After any impact, or the first ride following a hanger adjustment, a short recheck is worthwhile.
Know when to stop
A bent hanger needs an alignment tool, not more indexing turns. A shop can check hanger alignment accurately and inspect the derailleur mount in the same visit. If you own the tool and the training, follow the tool manufacturer's procedure, and never straighten a hanger by forcing the derailleur.
To be honest, knowing when to quit is half the repair. Get the bike serviced when the cage is bent, the mounting threads are damaged, the shifter ratchet skips internally, or the chain keeps jumping after the basic checks pass. Excessive chain or cassette wear means parts replacement, not more adjustment.
And stop riding entirely if the chain can enter the spokes or keeps falling off the cassette. A shifting fault turns into a control problem under load.
A repeatable check for rental and fleet bikes
Operators can save real time with a short record: bike ID, affected gears, whether the fault shows with assist on or off, and whether the bike took a drive-side impact recently. That little log separates a recurring setup problem from damage from a single rental.
Tag the bike out of service if the hanger looks bent, the chain skips badly, a mounting bolt is loose, or the derailleur can't reach the cassette cleanly. A quick adjustment is not the way to send a visibly damaged bike back into circulation.
Release it only after a stand test and a low-speed ride with light assist. Keep the chain wear reading and the service action in the bike's maintenance record. The next technician should know what changed without guessing.
Common questions
Why does the derailleur shift on the stand but not on the trail?
The stand removes most drivetrain load. Under motor assistance, heavy pedal pressure can make a shift clunky or expose a worn chain, cassette, or alignment problem. Redo the test with lighter pedal pressure and low assist.
Is 0.8 the correct chain replacement limit for every e-bike?
No. Treat 0.8 as a reference tied to certain SRAM chains and specific tools. Your chain and checker manufacturer's stated limit is the one that counts.
Can I mix a SRAM shifter with a Shimano rear derailleur?
Don't assume those parts will index together. Cable-pull standards differ, and a SRAM 1:1 shifter paired with a Shimano derailleur is a known mismatch example. Verify the exact models before combining components.
Can I fix a bent derailleur hanger at home?
Only with the correct alignment tool and the skill to use it accurately. Eyesight won't confirm a small bend. When you're unsure, a bike shop settles it.
What should I adjust first, the limit screws or cable tension?
Neither, not at first. Confirm the hanger, cable, chain, and component compatibility before touching anything. Then follow the derailleur manufacturer's sequence, typically limits, then indexing, then B-gap, rather than turning all three at once.
So start simple. Bike powered off, chain on the smallest rear cog, a clean view of the hanger and cable. Find damage in those checks? Stop there and arrange service before the next ride.