One For The Mechanics

Not Zwift related for which I apologise but need to reach out to a community of knowledgeable mechanics of which there’ll be plenty here.

On an outside ride today I pinged a rear spoke. Before I could react its flicked outwards into the rear mech/chain and caused a catastrophe.

The rear mech has been ripped off completely. The hanger stayed in tact but the rear dropout has been ripped off so conversation needed with a carbon repair specialist.

The real issue is the mech is a di2 10 speed ultegra which are as rare as rocking horse poo these days. Cage bent which I can transplant from another but the b screw plate snapped so there’s no b screw anymore.

These are serviceable parts and the whole b axle can be replaced.

Question is has anybody done this and is it straightforward?

Alternatively if I swap the mech and put it on my direct drive turbo setup do I actually need the b screw or will the mech not stay in position as there is no stop.
It was screwed virtually all the way out.

Any advice greatly received.

B screw adjusts the gap between jockey wheel and cogs so they are close enough without overlapping to provide smooth/optimal shift. Lack of b screw wont impact position but are needed for tuning (gear change smoothness/speed).

Only ever done jockey wheels/cages myself - everything else is impossible to get parts where I am. But if I had old parts laying around to swap out, I’d certainly be giving it a try given how hard it is buying kit these days.

To re-use as is with your trainer, your challenge will be finding out what range your mech can travel before conflicting with your larger gears, if any (b screw will have less effect on smaller gears). For a close ratio cassette, you may well be ok (your comment suggesting it was all the way out is a good sign) but the risk grows as the ratio gets wider. Assuming you have some limitations in range, you could adjust the trainer difficulty to help with range.

Its certainly worth trying as is on your trainer, but for outdoors, I like to have things running mint so wouldn’t be trying to run without b screw.

Thanks for that.

As you can see its not the screw that’s missing but the plate that carries it.


OK pics help - I focused on the “do I actually need a B-screw” not appreciating which part had broke.

My comments still apply but it’s largely predicated on how much tension the b-screw provided in the first place. If the b screw was originally under no tension, then while using you have to consider what happens if the drivetrain has some bigger tension changes (say going from small to large front ring or even just normal gear changes) which may cause the mech to travel past the point that the tab would have stopped the travel and cause jockey wheel to clash with cassette.

As much as I like tinkering with stuff, I’d be inclined to replace the plate.

That’s my thoughts, without the screw/tab there’s nothing to stop the mech traveling all the way round and thus the cage coming into contact with the cassette.

My intention is to replace the plate using one from an old mech or to buy the complete axle kit which leads me to the original question of…

Anybody done this previously?

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I bet that was one very ugly sound.
Maybe Zwift could use that sound to indicate drop outs (no pun intended regarding your real dropout issue).