Bosch EMTB - What's stopping me from doing this?

[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)....

[Le_Fromage_Grande]

Try it and see, sounds like the worst that could happen is you waste some zip ties, although there's a good chance you could bend all your spokes.

[MingtheMerciless]

Please film it.

[millemille]

Try it and see, sounds like the worst that could happen is you waste some zip ties, although there's a good chance you could bend all your spokes.

I don't believe for one minute that the UTS of a couple of 2mm wide nylon tie wraps is greater than that of a couple of metal spokes. The tie wraps will fail waaaaaaaaaaaaaay before the spokes will...

[Mussels]

The load on the sprockets will change, instead of the chain driving the sprocket with slack at the bottom the sprocket will drive the chain with slack at the top.
It might be ok or it might result in the chain wrapping right round the sprocket and locking the back wheel if the tie wraps don't break.

[Mr Moofo]

Is this a first world issue - or a "because I can" one?

[porter_jamie]

well, when pedalling along you are keeping the top run of the chain tight. if you freewheel, you are relying completely on the mech. might or might not be a problem - its thick thin so shouldnt fall off, but if it did, it might wrap the mech around the cassette

[Mr Moofo]

well, when pedalling along you are keeping the top run of the chain tight. if you freewheel, you are relying completely on the mech. might or might not be a problem - its thick thin so shouldnt fall off, but if it did, it might wrap the mech around the cassette

Isn't that the same for a "real" mountain bike ?

[millemille]

well, when pedalling along you are keeping the top run of the chain tight. if you freewheel, you are relying completely on the mech. might or might not be a problem - its thick thin so shouldnt fall off, but if it did, it might wrap the mech around the cassette

Isn't that the same for a "real" mountain bike ?

When you're freewheeling on an Amish bike the chain isn't moving, hence not being able to change gear when freewheeling...

[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.

[millemille]

For anyone interested, I've ridden a couple of hundred miles now in all kinds of conditions and all kinds of terrain with the fixed rear wheel set up and can report no problems.

Not lost the chain once - even when other bikes were losing theirs - and the tie wrap breaks when anything gets jammed up in the drivetrain, preventing any damage.

Being able to change gear while freewheeling is superb and I'd love to keep the set up going.

But I'm not sure how robust the freewheel/clutch in the Bosch motor actually is and how well it will stand up to long term use as a full time freewheel, so I've email Peter at performance line bearings (probably the most knowledgeable person, outside of Bosch, on the internals of e bike motors) and asked for his advice.

His feedback will let me make the decision on whether to run the setup full time or not.

[Taff]

I was pondering doing this the other evening and the only thing that stopped me was not knowing what damage could be done to the motor.,... And how much that would cost to repair!

Is be interested to know the answer from him.

[millemille]

Quick reply from yer man.

In a word, don't.

The "freewheel" in the motor is a two part mechanism. A sprag clutch...



..and a sprung pawl...



..and they are designed to work solely when the cranks are being rotated more slowly than the motor is spinning (or slowly backwards when the motor is stationary). In his words "running these at chain speeds will kill them".

So, an interesting experiment but not one to carry on with.

[Mr. Dazzle]

Lovely little 2D matrix though, Bosch clearly keep track of stuff properly on their factory floor.

[MingtheMerciless]

oh dear has something worn/broken?

[millemille]

oh dear has something worn/broken?

No.