The mech snapping conundrum. « Singletrack Forum

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The mech snapping conundrum.

25 posts & 22 voices | Started 3 months ago by .duncan | Latest reply from TrevorB

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.duncan - Member

5 Months, 3 broken mechs, 3 curious circumstances.

Right.. I'm confused by all this so i'm hoping someone can shed some light on it.

Mech #3

Mech #2

Mech #2 & 1

Right so, 5 months ago i'm climbing up a gentle climb propedal dial on and on max setting and wham mech snaps off. Nothing was found in the wheel / mech and the mech hanger was bent a little. Considered it was bad luck. The chain was plenty long enough and about halfway to needing replacing.

1 month ago i'm pedalling up a gentle climb (video : http://www.facebook.com/v/10150479635906533 ) and the mech snaps again, this time there may have been a very small twig involved as seen moving at the bottom of the frame, i didn't feel anything go into the mech and personally don't think it did (however it is there moving..). I stopped pedalling as soon as the chain started slipping. Inconvinient, had a nice 90 minute walk home. Different mech hanger, different chain (plenty long enough again) and propedal turned on. Mech hanger bends slightly when it snaps.

Today... Riding up a small shallow step, about 2 inches high with the propedal on, with a new mech hanger, same chain as last time (now about 12 rides old and 3 links shorter. (I'd compressed the shock and there still seemed enough space and the shock hadn't come close to bottoming out)). The mech snaps in the same place again, this time the mech hanger is perfectly straight still.

In all three incidents the mech didn't go into the wheel / doesn't appear to have done so. I'd been trying to buy an x9 rear mech to see if this would remedy the problem however I was waiting on one to come in stock at crc so that i could spend my vouchers.

So.. does anyone have any idea as to whats going on?

Posted 3 months ago #
kaesae - Member

Hey!

I've seen this on a few forums, no one seems to know what causes it, however not much research / investigating into it has been done.

You will need to really look at how you use your gears compared to other riders and then see if anything you do would cause excessive force to be applied to the mech while shifting or riding.

Do you go down the gears quickly on descents or up them quickly whe you climb, also do you tend to put a lot of power down when riding or do you make the most of your gears. That and having a good look at your bike and it's set up!

All the advice I can offer for now, in a bit of a hurry!

Posted 3 months ago #
was - Member

What do all of these have in common?

I've never had a Shimano mech break like that.

Posted 3 months ago #
amedias - Member

What do all of these have in common?
I've never had a Shimano mech break like that.

not just the mech make and model, but also

same rider...
same bike...
roughly the same situation (ie: climbing)

not that that helps you at all, but there must be something causing it, either the way you're riding/using your gears, or something else bent/mis-adjusted causing something to bind or move and causing enough load on the mech.

Were you able to tell if the mech broke by being pulled forwards, backwards or sideways?

Posted 3 months ago #
nicko74 - Member

They're both broken in the same place/ same angle, too. It's got to be something that you're doing or the bike's doing that's causing it.

My first thought, as an in-no-way mechanical person, is that the angle of the end of the swingarm is pushing the mechs into oncoming rocks or something similar. It's hard to explain, but if the swingarm's bouncing around, that mech is pretty much being pushed into rocky stuff.

Both those mechs look to be medium cage?

Posted 3 months ago #
.duncan - Member

All 3 mechs are short cage, i wasn't shifting at any time, and you can see that in the video attached. All the climbs have been easy spins. I think its broken by being pulled forwards or backwards. There's never much bend in the mech hanger itself and it doesn't collide with the wheel.

A while ago I did break a mech from having a chain too short and it went straight into the wheel and took a few spokes with it. It also bent the hanger substantially to the extent you couldn't remove the wheel from the bike. Learnt my lesson from that one!

Posted 3 months ago #
robbieh - Member

Same happened to me only yesterday! 4week old X7 mech on my new build 29er HT. I'm using 1 x 9 and its a med.cage. I was at swindon Croft trail for the first time and its fairly flat with only a few slight inclines here and there. I'm not an aggressive hard rider and as it was sticky terrain i was just getting round as best i could. I thought that i had just been unlucky and picked up a strong stick in the claggy mud thats there. Would the mud build up in the jockey wheels cause the chain to jam that hard as to make this happen? i did'nt think it possible but who knows? After reading this thread though, i'm beginning to wonder if there's an issue with Sram? Its the first time i've used their mechs and i've never had this happen with Shimano. As i've already bought another mech the same last night i'll see how that goes but i'll be back to Shimano if the same occurs again.

Posted 3 months ago #
TandemJeremy - Member

You are sure your chain is long enough when the suspension compresses?

Posted 3 months ago #
almightydutch - Member

Cough....Shimano...cough

Posted 3 months ago #
davidtaylforth - Member

It could just be SRAM X7 mechs are crap. I had a short cage one, it didnt snap, but it bent for no apparent reason. I replaced it with an old X9 rear mech and its been fine since.

Posted 3 months ago #
franciscobegbie - Member

You are sure your chain is long enough when the suspension compresses?

+1 for this. Have you checked?

Posted 3 months ago #
andyl - Member

The bottom of the cage in one of those is very twisted suggesting a chain jam of some sort.

Do you have a picture of if fitted and not broken? Just wondering where the end of the cable after the cable clamp goes and is there any chance that in one of the gears it could get caught up in the chain and jockey wheels?

Posted 3 months ago #
randomjeremy - Member

One of my riding buddies had pretty much the exact same thing, snapped a few mechs and bent no end of mech hangers. Turns out his rear wheel was flexing so much it was causing this damage. Changed the wheel, problem solved.

Posted 3 months ago #
rickmeister - Member

Is it the b screw adjustment. Top jockey wheel not clearing the cassette by enough margin, changing causing the mech to try and rip the chain off the cassette not lift it to the next sprocket on the cassette under load?

Posted 3 months ago #
WillC9999 - Member

+ another for TandemJeremy

When you kit it out next check out the angle of the hanger. In a 'sensible' gear is it hanging straight down (like your old fella), or being pulled forward? Have all three breaks occurred with the same chain (or same length chain)?

Posted 3 months ago #
WillC9999 - Member

Or aliens.

Posted 3 months ago #
unovolo - Member

In defence of SRAM I have had 2 x X7 mechs and never snapped / broke either.
First one got sold on a bike,second one is still running and on its 2nd set of jockey wheels still changes fine although the jockeys are now pretty worn and getting rough.

This is being retired and replaced with a XO.
I agree with others in regard that it must be something specific that's happening with your bike,wheel,suspension that's causing it ,either that or if all mechs were of a identical model and age then a possibility of a bad batch from SRAM?

May be worth contacting them ,see if they are interested in testing the broken mechs for possible cause of failure.

Posted 3 months ago #
nosedive - Member

I've snapped an x7 mech exactly like that. I figured it was because the chain was a tiny bit too short but now I'm re-considering

are these all x7 mechs?

Posted 3 months ago #
landcruiser - Member

Is short cage correct and recommended for your frame ? I guess it's got to be a wrong-un ! I have run SRAM X9/X0 for years and no one can get their gears wrong like me at times..... no probs whatsoever.

Posted 3 months ago #
Speshpaul - Member

Short cage mech, single pivot.
I think you've got chain length issues.

Posted 3 months ago #
antigee - Member

the pictures wouldn't stand up in court but even with my crap eyesight look like a 100% crystalline fracture suggesting that the mechs got F---ing well loaded big time and bang, that's a one off loading beyond the strength of the metal - no evidence there of cyclical fatigue, the position of the cable tensioner isn't good but look like just means the section fails there because is thinnest rather than somewhere else adjacent-
i ride cx and hard tail - two reasons: I'm tight and secondly i can't be bothered to sort all those chain length/bearing and other issues people get on full sus' - anyway those are mechs that have gone way beyond the call of duty

edit PS was nice to see good pics

Posted 3 months ago #
FuzzyWuzzy - Member

secondly i can't be bothered to sort all those chain length/bearing and other issues people get on full sus

Lol, had 3 FS bikes in the last 5 years, never had chain length issues (just used the old big/big measurement), never had to replace pivots and the only time I've written off a rear mech was in a crash.

Posted 3 months ago #
big_scot_nanny - Member

From me, only 2 possible hypothesis I can think of, both have happened to me (and I've jsut moved down to X7 as I was getting tired of replacing X0/X9!):

1) Jockey wheels jammed solid (stone or grit in the workings), cranking on pedals would pull the mech round - but unlikely to happen to you 3 times!
2) Chain length. Sorry, but as all state above, this is most likely to be it.

I did it twice in 2 weeks before crimbo. Not an issue on my other FS bike, but on my mega all previous calculations on chain length, even giving some extra links, were not enough. running 22/36 SLX double with bash at the front, and 22-34 at the back. When on 36/34, normally going uphill, that combo plus maybe an a wee extra weight shift from me, was enough to pull the mech too tight and do pretty much exactly what happened to yours. MY X9 mechs would bend though (beyond all recognition), not snap.

Gotta say, I think this is what is going on with you.

Posted 3 months ago #
robbieh - Member

Do you think chainlength is the issue on my break then? i was running a 1x9 setup on my hardtail with what i considered enough slack chain on the biggest cog (32) and it broke while on the 26.

Posted 3 months ago #
TrevorB - Member

Can you fully compress the suspension with your current mech/chain set up?
Let the air out of the shock, or remove the spring to test it. If not, then you have found the problem. If you can get full travel and the chain still has some 'free play' in it then you need to look at the mech set up.

Posted 3 months ago #