Bosch CX Gen 2 - Chain forever coming off front sproket!

[Dubreak]

Evening all; bit of a technical question for you. Does anyone know of a chainguide or bodge for a Bosch CX "Gen 2" motor that prevents the chain hopping off the front sproket and getting sucked into the space between the front sproket and the bike?
If I'm going over rock gardens fast, or the bike is bouncing around the trails, 90% of the time the chain will hop off into the gap and get stuck.
It's a great bike, but this is a really bad design. The newer motor version (Gen 4) has a proper full size front sproket and this doesn't happen because you can fit a normal E13 chain guide. But this poxy version has a tiny front sproket (18t) and it's forever happening. It's driving me mad. Chain, sproket and cassette are about a month old. 116 links I think, no chain stretch, XT drivetrain clutch is on.
I put an aftermarket chainguide on the chainstay swingarm, but doesn't seem to help. The chain is hopping off the top.
Any ideas, apart from a new bike?
Thanks

 

[Beekeeper]

I have a 15T front sprocket and never had an issue. I do change them fairly regularly but always flip them over to get double the life. They do wear quite quickly as they are so small. Was yours always an 18T ?

 

[Dubreak]

It's a tricky situation. My CUBE Stereo Hybrid Action Team 160 2020 initally came with an SRAM EX1 Drivetrain. I swapped that out to a Shimano XT 11sp XG spline setup after the initial cassette wore out. It's over €400 for a replacement XG-899 SCAM Cassete.

The bike came with a 16T, and now I've an 18T I think, but that shouldn't make a difference... if anything the chain would be more taught?

I've seen these for sale, but if you have a look at the bike frame , there is nowhere to mount the two screw holes on my frame: https://www.probikeshop.com/.../miranda-e.../227543.html
At this stage, I wouldn't mind drilling into the alu frame... If I knew that it would fix this problem happening. It's a massive headwreck.

The 116link Shimano XG chain, Miranda Bosch sproket and XT 11sp cassette are less than a month old. I did a big €1k service last month, forks, shock, drivetrain refresh.

 

[Dubreak]

Just re-checked, it's actually a 20T I have on the front now. It originally came with a 16T.

 

[Dubreak]

I just got a reply from MTB Ireland that might solve the problem:


"Hi dara, i own a 3d printer.
A potential simple fix here is to fill the space between the frame and chai ring with a custom plastic spacer that slots in there and sticks to the frame of the bike with semi permanent double sided tape. It would have a chamfered edge that would guide a chain that has been thrown onto it back down onto the chainring. Would need to be a close to the chainring as possible but not close enough to rub."

My reply - "That's exactly what I'm thinking too. It wouldn't go into the space... if there was no space for it to go into. Like Schrodinger's cat, but with the box removed from the equation. That said it does occasionally hop onto the Miranda chainguide at the outer face, but not nearly as frequently."

 

[Dubreak]

Whoever designed this, did it wrong.

This is not a 'chain-guide', it's a shitty 'bash-guard' at most.

It should be 'H' shaped in section, and at least three times the diameter.
How is that ever supposed to keep the chain in place?

 

[Beekeeper]

Just as an experiment I’d stick on the original 16T with the original number of chain links etc and if all was well after I had thoroughly tested it, try an 18T with maybe an extra chain link.

 

[Sander23]

Just re-checked, it's actually a 20T I have on the front now. It originally came with a 16T.

I've had the same when I used to run a 21t on my bike, even the smallest bumb would cause my chain getting stuck between the motor and the sprocket. I Solved it with the narrow wide chainring from 18 thooth from sram SRAM X-Sync E-MTB Kettingblad 1-speed voor Bosch aandrijvingen, zwart | Bikester.be or the 20 T from haibike Haibike XDuro E-Bike drive sprocket chainring 20 teeth silver

The haibike ones price is just absurd, but is the only option if u want a 20t narrow wide chainring