Another snapped chainring - why?!

milese

Following on from this thread:

http://www.bikeradar.com/forum/viewtopi ... ght=broken

I've broken the replacement in the same say, it must be less than 100 miles old. This time the bit broke at both ends and pung off somewhere. I was crossing a dual carriage way this time so it really could have killed me if I hadn't managed to get back to the side of the road in time!

It broke in the section between 2 pins.

The gears seem to be set up fine. They dont rub or catch, it changes smoothly.

The chain wasn't diagonal, I wasn't changing gear, the chain is well lubed.

No one I've spoken to, nor the bike shop have seen or heard of this happening before.

Am I damn unlucky or is there something on my bike that could be causing this to happen?

topdude

You had better get a bulk pack of chainrings and carry one with you !!
Seriously though, is it possible that your chain is too worn, if it is then all the force you apply to the crank will be transferred to the chain by only one tooth. If the chainring is inherently weak it might be overstressed and break. Sounds unlikely i know but has anyone got a better suggestion ?
I think i would be replacing the whole chainset and chain if i were you.Tiagra Hollowtech chainset and bearings are quite cheap.

cougie

Bizarre. I've had a cracked chainring before but that was fine for riding on - it didnt tear away like your ring has done - cant understand why it would ?

milese

Thanks for your reply Topdude.

I've only had the bike since the end of December and the chain is fine.

Could it be that the rear mech is somehow misaligned causing the chain to run at an angle, and trying to roll its way off the chainring? If you know what I mean...

Thats only a thought I've had sat at my desk, I've not noticed, and see no reason that there would be anything wrong with the rear mech. Can you adjust that angle of the rear mech, that changes the angle the chain runs at?

Simon Notley

Any chance that the front mech could somehow be becoming involved with the chainring? If (as suggested in your previous thread, and bizarre as it sounds) it's not the original chainset, then the front mech could be in the wrong place, when you put the power down, the frame will flec a little, which might be enough to cause the two to collide...

Just a thought, could be totally wrong...

milese

Thing is, the front mech is a flimsy bit of pressed sheet metal, against a solid machined chainring. But no, I dont see any way the two can come together.

The shop will be getting onto the Giant rep about it today, but there is no way I'm keeping this chainset. I bought the bikes to do train for and ride Sportives and can't afford to risk this happening during them, or for the bike, and my training to be put out of action whilst its sorted.

The shop said that the Giant rep thought that something must have got caught in their the first time around, so god knows what he'll think this time.

Simon Notley

I take your point, I'm not sure how the two would come together without the mech coming off worst...

It does seem highly unlikely that this could happen under normal stresses, or even under abnormal ones of the scale you might expect to encounter riding a bicycle. If the force was applied via the chain, I would have expected that to break first, if something was stuck in it, I;m sure you'd have noticed...

I'm at a loss!

milese

If you look at the opposite side from the break you'll see the two pins, its the same on the broken side. The breaks have happened where the structure is weakened by having the pins.

The second ring broke at both of the pin points, leaving a clean gap!

What are the pins for anyway?

cougie

I think they are there to help in upshifting.

But where is the force to push that bit of alloy ring out ? Is it purely when you've changed from the middle to the big ring ?

robbarker

Are you making a bit of a habit of riding in the big ring on the front and the largest cog at the back? If that is combined with a bent derailleur hanger and poorly set up chain line it might go part way to explaining this, but it definitely shouldn't happen and is dangerous, as you nearly found out.

Time to take it back to the bike shop and get it properly checked - go to Trading Standards if they don't take it seriously.

milese

No. Both times I've already been on the big ring.

First time I was moving, got out of the saddle to put some power down to overtake someone on a cycle path up a slight hill and it broke.

Second time I was stopped in the middle of a dual carriage way waiting to cross, a gap in traffic appears, I pull away and it broke.

No gear changing was involved. The only way I can think of there being the force to break it is my earlier suggestion, that the chain isn't square to the chainring when it arrives from the cassette and rear mech. Hence when I put power down it is pulling / trying to twist the teeth on the ring, and eventually it gives.

Does that sound plausable and why wouldn't the chain be straight?

I'll have a look when I get home, but that wont be til late on Wednesday now.

milese

I never ride in the two biggest cogs, never go above about the 6th biggest on the cassette, and both times when its happens its been in quite small gears on the cassette.

The only time I go diagonal is when I'm trying to ride home without a top chainring!

This is the problem with buying mail order, but you pay your money you take the chance of being saddled with problems like this.

Its going to have to go into my LBS for a checkover when I presumably get new parts under warrenty.

So is the consenus that even if things aren't set up correctly, and they can't be that bad because it rides ok, for it to break is still a definate warrenty issue?

acorn_user

Time for some TA rings

peanut

you say you did not have the chain on a diagonal but its the big ring that is bust. Why were you trying to accelerate across a dual carriageway in the big ring? far too high a gear and too much stress on the ring . I suggest that you use the small chain ring when applying a lot of force through the transmission ie when climbing or accelerating from a slow speed

milese

Good question, and I'm sure your right. It was downhill, and I suppose having just crossed 2 lanes of dual carriage way traffic occupied me a touch.

Surely its still not right though? Its not like I'm super strong!

The first time I was moving along at a reasonable speed, already turning the large chainring in the saddle, and simply got out of the saddle to get a bit more pace up a slight slope.

vbc

I could be wrong here but didn't Truvativ recall some chainsets recently? Can't remember why and I may well be mistaken.

pete.whelan

the only time this has happened to me with a chainring was many, many years ago having to brake very quickly and hard whilst riding fixed (using the backwards force on the pedals to stop the bike).

Agree it is very unusual to happen at all.

Not many years ago I ripped off three teeth on a Sachs freewheel (one of the middle sprockets) on the tandem.

sub55

sorry to hear about your problems with the chainrings. never seen this happen before
although i have seen them bend out in the past. but that was on a fixed time trial bike at the start.
2 things that occured to me;
1; chainring sizes and space between them. if you are using a particular large outer or small inner ,you could be pushing the chain up under the rivets. which would then obviously force the outer ring outwards when the chain is eventually grabs. hope you are with me on that.
2; i would check the frame alighment . if the frame was twisted to the right ,it would again be pulling the chainring to the right. so drop the rear wheel out and take a piece of string ,tie it to the right hand dropout ,over the bottom bracket shell ,under the downtube back over the bottom bracket to the other dropout . it should be perfrctly symetrical to both chainstays . be interested to hear the result.

Bugly

the pins are stress concentrators in the ring thats why the break is more likely to occur there then any other part - as to why they are breaking - perhaps they are under designed or from a faulty batch. The pins seem to be very close to the bottom of a tooth. Is there any sign of a crack propagating from the pin (for the as yet unbroken ones)?

Unless you are MASSIVLEY overweight AND have the leg strength of a track sprint cyclist I can can not visualise how anything you can do would break the cog (chains are more likely to shear at the pin then your breakage). Get in contact with the cranks distributors/manufactorers and see what they say.

Good luck

Bugly

redvee

I snapped a chainring and took an arm out of the chainset at the same time :shock: It was my commute bike that could have been better cared for :oops: Accelerating away from a roundabout, changed up 1 gear and bang :!: One dropped chain, one broken chainring and a snapped spider Not too long a walk home as I managed to blag a lift home with my manager who found my circumstances amusing. Can't remember the ring or C/set as I mixed and matched.

andrew_s

Suggestion:

The front mech limit screw isn't set right, so when you stomp on the pedals the frame flexes and the chain starts to drop off the outside of the chainring.
Then you've got the inner plate on the outside of the ring on the trailing tooth whilst the leading tooth is still engaged. Add the twist in the chain to maxi-stomp, and something will go. Normally I'd expect it to be the chain or a bent chainring tooth, but if it happens at the weak point in the chainring I suppose the ring could break instead.

andy_wrx

First time, you got up out of the saddle to climb a hill.

Second time, you were doing a standing start - presumably also out of the saddle ?

Both times you're out of the saddle using your full standing weight to get maximum torque & leverage ?

How much do you weigh ? Are you 15 stone + ?

It should of course be engineered to cope with this, your weight and the maximum torque you can put down are probably a hugh amount less than say Marcus Backstedt could put down...

milese

Thanks for the suggestions. After talking to Giant and telling them that I dont want another straight replacement ring I'm going to take it into a local Giant dealer. I'll let you know what they come up with.

I weigh about 13st, and can't have put anymore pressure through the chainring than any other SCR owner, who it appears haven't have the same problem.

Its a standard 34/50 compact.

hamstercp

peanut wrote:

you say you did not have the chain on a diagonal but its the big ring that is bust. Why were you trying to accelerate across a dual carriageway in the big ring? far too high a gear and too much stress on the ring . I suggest that you use the small chain ring when applying a lot of force through the transmission ie when climbing or accelerating from a slow speed

But in that case the tension and force you apply through the chainring are actually higher (think about the leverage - long lever of crank into short lever of small chainring)

The ring bent outwards as it was forced towards the chainset centre by the tension of the chain. As someone else said, it's a chainring weakness introduced by the shifting pin.

Hugh A

I agree - It is very likely a design or manufacturing problem, perhaps with the insertion of the pin or the size - it may be too small in diameter or if there is any damage around it, this area could lead to cracks spreading quickly followed by failure. The material also lacks depth inside the pin at this point compared to say a Campag chain ring.

andy_wrx

I'll happily accept that drilling/stamping a hole in the ring and shoving a pin into it will introduce a weaker spot in the ring.

But why aren't there lots of people reporting this ?

Either milese's doing something different to all the other people out there with Truvativ Elita cranksets (an even greater number than those with Giant SCR's), or there's something different about this particular bike...

If it were me, I'd let the bikeshop check it all out and see that there isn't some major gear set-up issue problem (e.g. if as Giant insist, it should be a triple but has a compact fitted, is it running on a triple BB ?)

And if they find nothing, I'd fit some beefier rings from a different manufacturer.

milese

Thanks for the replies.

On Giants advice I took the bike to a local dealer and they've said its a bad design / faulty batch and are going to fit a Shimano 105 chainset to it under warrenty.

They voiced their displeasure to me and Giant of mail order selling of bikes, that once sold the retailers don't offer enough support and people like them are left to pick up the pieces. Fair enough really, but they are going to charge Giant for labour and I'm glad they are helping me out!