Derailleur 7-8 Speed — Rear Mech Replacement for Geared Bikes and E-Bikes

What Actually Fails on a Rear Mech

Rear derailleurs rarely wear out quietly. Three things end them. The first is impact — a fall, a lean against a kerb, or a bike loaded badly in a car boot, all of which bend the parallelogram or the cage. The second is a seized pivot: grit and winter road salt work into the linkage until the mech no longer springs back cleanly, so it climbs onto the bigger sprockets but drags coming back down. The third is a worn jockey wheel, where the teeth have gone hooked and the chain rattles constantly no matter how the cable is adjusted. If your shifting has drifted so far that barrel adjustment no longer holds for more than a ride or two, and the cable and outers are sound, the mech itself is usually the answer.

Why Conversions Are Harder on Derailleurs

A mid-drive conversion — the Bafang BBS01B, BBS02B and BBSHD units in our conversion kit range — drives the chain from the bottom bracket, so motor torque loads the rear derailleur every time you shift. Shifting under power is the specific killer: the mech is trying to lift a chain that has several hundred watts pressing it into the sprocket, and the cage takes that load. The habit worth learning is to ease off the pedals for a fraction of a second as you change gear, exactly as you would on a motorbike, or use a gear sensor if your kit supports one. Riders who do this get years out of a mech; riders who shift at full assist do not.

Higher average speeds also mean the drivetrain sees more chain revolutions per mile than an unassisted bike does, which is why chain, sprockets and mech tend to reach the end of their life at roughly the same time. If you are replacing this derailleur because of poor shifting rather than damage, check the rest of the set before you fit it — a stretched 5-8 speed chain or a hooked 7 speed freewheel will make a brand new mech shift like a worn one.

Setting It Up

Fitting takes a hanger bolt, a cable and patience with two limit screws. The high limit stops the chain being thrown into the spokes at one end and off the smallest sprocket at the other, and both need setting before the cable tension is fine-tuned at the barrel adjuster. Indexing that will not hold usually points back at a tired cable, a compressed outer, or the hanger. Our workshop in Roath will fit, index and hanger-align it in one visit if you would rather it was done once and done properly.

Warranty and Returns

Every rear derailleur purchased from All4eBikes is covered by a 12-month warranty against manufacturing defects, alongside a 30-day return window.