2 new chains, one snapped and the other slipping......

tempesc

Hi all, I am hoping someone can help me with a slipping chain problem. I have 4 questions:

1. Do I need to change the front chain ring(s)
2. Why would the KMC chain be slipping but not the Shimano one?
3. Is my rear derailleur bent or does it look normal?
4. If I have to start buying parts what should I buy?

I am riding a 2013 Voodoo Bokor, with a Shimano Deore shifters and rear derailleur, Shimano Alivio (FC-M431) triple front ring and Shimano HG400 9 speed 11-32T rear cassette.

I recently serviced the bike including a new head set, bottom bracket, new rear cassette (changed like for like other than the original was an 11-34T - original had one broken tooth) and new Shimano HG53 9 speed chain (original was worn to 0.75, but not 1.0). I could not find a like for like replacement crank set in stock.

It was fine on my first ride (just a hour or so on a flat single track), but on my second ride since this service I took rode it up to the top of a downhill course and snapped the new chain just before the decent. This may have been my fault and there are two factors to be aware of.

Firstly I screwed up putting the pin into the chain first time round and snapped it. I had to get a replacement from the LBS - not sure if this was Shimano OE. Secondly, when it snapped I got caught out on a short but very steep incline in the wrong gear and tried to grunt it out. I am a 17st rugby player so put a fair amount of torque through the chain. It snapped at the point of maximum torque.

I ordered a replacement KMC x9 9 speed chain with a quick link. I adjusted this to length, indexed the gears and took it for a short (flat) ride and all seemed fine. However when I went out today and started to climb, as soon as I put any meaningful pressure through the pedals it slipped badly and consistently when in the middle front ring. Bottom and top ring were fine.

Here are the chain rings - they are not new but seem ok to me - do you agree? [Q1]


And if the issue was here surely the Shimano chain would have be slipping too? [Q2]. If that suggests to me that something has changed during the snapping process.

Here is the derailleur.


This looks out of alignment to me (bent outboard slightly)?. When I overwhelmed the chain could I have also bent this in the process? If so is there a fix, or is it just a replace job? [Q3]

If I do need to make changes what would you recommend I change and to what [Q4].

I am new to bike mechanics so don't know for example if I can just change the crank set and derailleur, but keep the current shifters or whether it is better just to buy a whole new group set. For context I am happy to spend what is needed but I am just a social rider (red run level) who rides once a week and am happy with my bike. I don't want to be an 'all the gear, no idea' guy. I have considered just getting a new bike but as we all do our bit to try and save the planet I'd prefer to make this work if I can.

Thanks for reading this far, and for any insights you can give.

Cheers
Tempesc

david37

ok the chain teeth look worn (the middle more than the outer and we cant see the inner)
this may well explain the slipping issue.

plus and not associated with chain slip, the gear hanger is bent. 9 speed is pretty forgiving though.

Change the chainset, you can buy a complete replacement for c£50

might as well change the bottom bracket whilst youre at it.

JBA

That middle chain ring is definitely worn. Slight differences in tolerances may account for one chain slipping and the other not.
If you are going to change the chain set I’d recommend going 2x9-speed, or if funds allow go the whole hog and upgrade to 1x10 or 1x11. The latter will be an expensive initial outlay (cassette, rear mech’, shifter, chain and chain set) but is more future proof.
Whatever you do your biggest problem will probably be parts availability so good luck.

tempesc

Thanks both, appreciate the help.

As you say, finding parts seems tricky. The closest i can get to like for like is this https://www.amazon.co.uk/Shimano-2092822600-Crankset-Black-17-5/dp/B00MV5QMGS/ref=dp_prsubs_1?pd_rd_i=B06Y19D5QY&psc=1.

My current crankset is M371. I gather the T is for trekking. Other than the extra weight of the outer guard any issues ordering this?

Sorry for what’s a basic question but if instead i switch to 2x9 can i still use the current shifters? If so do i just set the inner and outer limits for the front derailleur guide so as to make the 3rd shift redundant? And would you recommend a specific spec - i ride a downhill course quite often and use the 22T ring to get to the top so need to keep that low gear option.

tempesc

....or i can get an FC-M371 direct replacement with 170mm crank length. I am 6ft and currently use 175mm. Would 170mm be too short?

steve_sordy

The rear mech can be realigned. To do it properly needs a derailleur alignment gauge and tool. Depending upon which one you buy they can cost £35 - £65 and you are unlikely to use it very often. Instead, let your LBS do the job; it is a relatively quick job and should cost about £10-£15 (ask them if in doubt).

It is unlikely that you actually snapped a chain on a clean pull, that takes quite a few tons of force. What may have happened is that the chain was not straight and one of the sideplates was peeled off, perhaps one of the ones secured by the (maybe) dodgy pin?

Ref the crank length. Although crank length is part of your overall gearing, I doubt that you would notice the difference in going from 175 to 170. That will reduce the torque you can apply by less than 3%, and your saddle height may have to move up and back by the same amount (5mm). However, you will get fewer pedal strikes. Leg length is more important than height when it comes to cranks.

Ref shifters: Yes, just set the limit screws so that you cannot shift into the third front ring. I once went from 3x9 (44/32/22) to 2x9 but with a bashguard in place of the outer ring. The new 2x was an M665 SLX double and bash 36/22. Note that the 22T granny was kept, but the 2nd ring was just under half way between the previous middle and outer. I never once missed the 44t, but the bashguard sure came in handy on multiple occasions.

tempesc

Thanks, that’s helpful.

On reinspecting the Shimano chain it was a side plate (it was twisted); the original joining pin I used was still intact, it was a different link that gave out. I have refitted this chain with a second pin.

As suspected I struggled to find a 2x9 octalink crankset, but got a Shimano Alivio M370 triple with 170 cranks for £40 on ebay (no idea what the difference between M370 and M371 is). I fitted a new octalink BB last month so that should do me for now.

This seemed sensible having just bought a new 9 speed cassette and now two new chains. When this lot wear out i’ll either change the whole group set or the bike. (i have a CX bike with 1x11 and really like that, so that set up is appealing).

Thanks again all, great insights - just what I needed 👍

tempesc

Ok, so i now know the difference between the M430 and M431. My m431 works with a 68x126 BB. The M430 looks like it is designed to work with a narrower BB.

So apologies, but another question. Can i just buy a narrower bottom bracket (eg 68x118 that puts me back in the same place? When I fitted the m370 that is about the right difference. Or is that too simplistic?