What's stopping me?

[millemille]

Bosch Gen 4 CX motor has a "freewheel" for the chainring, so effectively my Focus has 2 freewheels.

What is stopping me securing the largest sprocket of cassette to the spokes with a couple of small/thin tie wraps so that it is fixed in position relative to the wheel and then relying on the motor's "freewheel"?

The advantage being that the chain is now constantly spinning forwards, even when freewheeling, so you can change gear when not pedalling.

If the rear mech/chain gets snagged or snarled the tie wraps should snap and allow the chain to stop rotating.

What am I missing, why is on one else doing this?

Edit to add: Like this (jump to 06:30 as for some reason the forum won't let me embed the video with the correct start time)....

 

[Mteam]

Nothing stopping you, give it a go and let us know how it works. It should work fine as long as everything else is free to rotate as it should.

. What am I missing, why is on one else doing this?

You're not missing anything, I just don't think being able to change gear when not pedalling is a priority for many people.

Plus if anything does jam up you have increased the risk of damaging either the derailleur, the spokes, or the cassette, or the chain, if the cable ties don't break as they should.

 

[millemille]

Ran the described set up at BPW yesterday.

Being able to change gear while freewheeling is all of the awesome.

However, the locking ring that secures the chainring to the motor output shaft came loose. I don't know whether this is related to the modification or not, this lock ring is known for coming loose on Bosch motors and I'm struggling to see how the modification can have contributed to it coming undone. The chainring and the motor output shaft are splined and the locking ring shouldn't be taking any drive forces through it.

I've cleaned the threads up and put it all back together with a healthy dose of medium strength threadlock and a torqueseal witness mark and will keep a close eye on it.

Going to stick with the modified set up for a few more rides and see how it pans out.

 

[emtbPhil]

Errmmmmmmmm

The lockring will have come loose because now you've got the rear casette basically driving the chainring, which is the same as you sitting and pedalling backwards the whole way down the hill.. The bike is designed for the chainring to drive the casette, not the other way around.

The lockring is designed to tighten as you pedal to stop it coming loose. Essentially back-pedalling for long periods of time will loosen it off.

I really don't think this is such a good idea, and I definitely wouldn't cable tie my casette to the spokes. That's a pretty good way to end up with bent/broken spokes and a wheel rebuild.

Also I don't know what the clutch mechanism is in the motor - but I'd definitely rather wear out a freehub which can be replaced in 10 minutes than wear out an £800 motor clutch

You can change gears while going downhill anyway, just pedal very slowly.

 

[millemille]

Errmmmmmmmm

The lockring will have come loose because now you've got the rear casette basically driving the chainring, which is the same as you sitting and pedalling backwards the whole way down the hill.. The bike is designed for the chainring to drive the casette, not the other way around.

The lockring is designed to tighten as you pedal to stop it coming loose. Essentially back-pedalling for long periods of time will loosen it off.

I really don't think this is such a good idea, and I definitely wouldn't cable tie my casette to the spokes. That's a pretty good way to end up with bent/broken spokes and a wheel rebuild.

Also I don't know what the clutch mechanism is in the motor - but I'd definitely rather wear out a freehub which can be replaced in 10 minutes than wear out an £800 motor clutch

You can change gears while going downhill anyway, just pedal very slowly.

Given that the front sprocket is splined, as is the motor output shaft, the lock ring is not being asked to work against the rotation of the sprocket as you suggest. The sprocket is unable to rotate in relation to the lock ring.

The lock rings come loose on bikes with the chain running normally.

 

[emtbPhil]

Fair enough crack on then