bosch activeline chainring lockring loose

[miklekrs]

hi, the lockring came loose on the chainring on my trek marlin8+, wonder if this is common? the lockring says 30nm on it, is that enough? or is it ok to go 35nm maybe?

 

[SteveH80]

If the designed limit is 30Nm don't go higher. Any 'nut and bolt' type fastening is basically a spring and you'll remember that elastic limit experiment from school.
That's not to say your torque wrench isn't inaccurate, it might be worth checking with another.

 

[miklekrs]

i have calibrated torque wrenches at work. The bike only has 225mile on it and still under warranty, so will try to get them fix it, if not covered for some reason i've read it on another forum that the ring might be one time use only and bosch has a tsb to increase the torque to 40-45nm (this in reality is only a few degrees difference in turn). the chainring made in a way that a normal lower bracket tool won't fit properly and the ring is made out of cheese aluminium. the thread is left handed and i think it comes loose because when you stop pedalling there is a sudden stop from the motor what causing a knock that can be felt trough the pedals.

 

[Gillstay]

If the designed limit is 30Nm don't go higher. Any 'nut and bolt' type fastening is basically a spring and you'll remember that elastic limit experiment from school.
That's not to say your torque wrench isn't inaccurate, it might be worth checking with another.

Wow you went to a very different school to me. Will look up the experiment.

 

[miklekrs]

Done some more research on this. it is fairly common problem on bosch motors trough the generations. the lock ring should have a rubber o-ring behind the lockring, this is not always fitted, either forgotten or costs. there are several part numbers for different motors for instance:
Bosch 1270016107 O-ring (BDU3XX, BDU33YY)
Bosch 1270016119 Gen 4 (BDU4XX)

if it comes loose then you need the o ring and re-torque it, this should solve the issue.

 

[SteveH80]

Wow you went to a very different school to me. Will look up the experiment.

It was a bog standard comprehensive school in the middle of a council estate - nothing posh I assure you. This was back in the 70's when schools taught physics as part of the syllabus, the other options were chemistry or biology but corrosive liquids and chopping up frogs never appealed to me 😆

 

[miklekrs]

ok, i had the new oring arrived, and got my 3/8" torque wrench ready and i have noticed something. the oring was fitted so that was not the issue. but the torque wrench only works one way and since this is left hand threaded i could not torque it properly, just wait for the click and nothing. my 1/2" torque wrench could do it at 40nm minimum, but wasn't home at the time, so my new guess is that the bike mechanic who assembled it at the shop also had no torque wrench to do 30nm left handed and just nipped it up. i went fairly tight on it i guess it is in the 40-50nm region, i will have to monitor it and see what happens in the upcoming months.