Wobbly Front Chainring

[Blakey]

Hey all,

2 months old bike (see sig). Today out on the trails the chain fell off. Checking into why it turned out the front chainring was wobbling - and I mean really wobbling. You can hold it in your hand and wobble it by over a centimetre sideways movement. As soon as I put the bike into a gear which pulled the chain at all sideways the chain just fell off the chainring. I had to cycle home in 2nd gear to keep the chain attached.

It seems like a serious issue. Is it? Anything I should look at or is this just as case of taking the bike into the LBS?

Attached is a .MOV file recorded from my iPhone to show how bad it is (zipped up as the forum does not allow .MOVs to be attached)

Thanks!
Blakey

 

[steve_sordy]

I can't see the movie file.
Have you checked the chainring bolts for tightness? (I can't believe that the motor bolts are loose!)
Do not go out on the bike again until this looseness is resolved.

 

[Blakey]

There don't seem to be any chainring bolts. The chain ring is attached on the back of the crank arm it seems with no bolts. The crank arm itself is solid.

 

[Blakey]

Chainring.mp4

• Blakey
• Nov 5, 2020

Broken chainring?

 

[deksawyer]

You need to remove the crank arm and then use the correct spider tool for your motor to tighten the castellated lock ring (it's a left hand thread).

A tool worth having IMO: Bosch Gen 4 Lockring tool, sprocket nut tool -

 

[urastus]

Geez! Behind the crank arm there is a round thing with notches in it. If it's shimano it has 16 notches. That notched round thing holds the chainring tight (well, when it's done up) onto the crank. It needs to be tightened. You need a special tool and the crank needs to come off first. It sounds like you're pretty inexperienced re maintenance - if you take it to the bike shop they could do it in about 15mins. Hopefully they'd clean the threads up too and tighten it to torque specifications.

And yes, you do have chain ring bolts, 4 of them, but they're not the problem.

 

[Blakey]

Thanks for help and feedback all - very appreciated.

I am going to take it into the LBS. You're right I am inexperienced and have a lack of skills and tools. And this looks like something which needs doing well.

I bought the bike from Rutland Cycles (who I rate highly) online as I'm a long way from them. Given I have had this issue, plus one brake needed bleeding and the bike is only 2 months old, should I be pressing Rutland to pay for these fixes?

 

[steve_sordy]

.................... I bought the bike from Rutland Cycles (who I rate highly) online as I'm a long way from them. Given I have had this issue, plus one brake needed bleeding and the bike is only 2 months old, should I be pressing Rutland to pay for these fixes?

You can try, but it would have been better to have consulted them first.

 

[Blakey]

Okay well I have not taken it in yet and have emailed Rutland. Good plan to see what they say. Too late on the bleeding brakes but I guess it adds fuel to the fire.

 

[Jimex]

I bought the lock ring tool (bosch gen 4) when I found play in my front chain ring. Its literally a two minute job to remove the crank arm and tighten.

It's worth buying the tools as bike maintenance is straightforward. I now service my forks and shock now, quite satisfying tbh.

 

[Blakey]

I bought the lock ring tool (bosch gen 4) when I found play in my front chain ring. Its literally a two minute job to remove the crank arm and tighten.

It's worth buying the tools as bike maintenance is straightforward. I now service my forks and shock now, quite satisfying tbh.

That does sound like a good idea, if it's that simple. I'm just checking with Rutland now, hopefully they will pay for the fix if it's a simple case of tightening it up, so that gets it done short term.

 

[Jimex]

That does sound like a good idea, if it's that simple. I'm just checking with Rutland now, hopefully they will pay for the fix if it's a simple case of tightening it up, so that gets it done short term.

You'll need a crank tool too. The lock ring tools are a joke price for what they are, but required when you replace your front chain ring.

 

[Blakey]

Well Rutland Cycles have come through and were worth speaking to. They are going to reimburse me for the cost to fix this and also the brake bleed I needed last week too. Fair play to them, that makes me a happier man. I'll be super happy when the bike is actually fixed and ridable mind you!

Cheers for the feedback and advice all.

Blakey

 

[deksawyer]

Great result.

 

[linkmeaney]

Hey all,

2 months old bike (see sig). Today out on the trails the chain fell off. Checking into why it turned out the front chainring was wobbling - and I mean really wobbling. You can hold it in your hand and wobble it by over a centimetre sideways movement. As soon as I put the bike into a gear which pulled the chain at all sideways the chain just fell off the chainring. I had to cycle home in 2nd gear to keep the chain attached.

It seems like a serious issue. Is it? Anything I should look at or is this just as case of taking the bike into the LBS?

Attached is a .MOV file recorded from my iPhone to show how bad it is (zipped up as the forum does not allow .MOVs to be attached)

Thanks!
Blakey

Wow, this happened to me just yesterday on my communte to work after some rocky trail riding on the weekend. I have booked my Genius in to have the locking nut inspected and tightended. A few Q's for the forum - has anyone tried aluminium thread specific loctite or simlilar? Does it prevent the issue? Has anyone asked Bosch service if they have a fix for this as it seems a design flaw if so many of us are experiencing the issue. Takata airbag recall was worldwide, why can;t we petition Bosch to recall and fix the issue. If I'm travelling at 50 plus Km down a fire trail I don't want my chain ring wobbling and chain coming off!!!!!!! I've seen people add safety wires and even grub screws to try to fix the issue but my bikes still under waranty so I don;t want to play with anything near the motor. BTW it happend just after I had done 10000KM.

 

[Techno]

My 2021 cannondale moterra with bosch gen 4 has just done this, ive bought the tool and tightened it,25-30nm left handed thread and added two red painted dots so i can see if it moves .I havnt loctited it. if it moves i will let the blue goo loose.

 

[Planemo]

You need to remove the crank arm and then use the correct spider tool for your motor to tighten the castellated lock ring (it's a left hand thread).

A tool worth having IMO: Bosch Gen 4 Lockring tool, sprocket nut tool -

Holy shit, 40 rents for a lockring tool! I just spat out me false teeth

Gotta have one though I spose

 

[Techno]

I got a good deal from the e bike motor repair centre. Looked cheaper than eBay and I knew it would fit .

 

[MrPeaski]

BB Tool 8 Prong

Buy your LifeLine BB Tool 8 Prong - Bottom Bracket Tools from Wiggle. SAVE 16% - RRP £11.99 now only £9.99. Free worldwide delivery available.

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This is the tool I used to do my Gen 4 chainring nut.