raria

Hi,

I have some basic Shimano mountain bike cranks (either M131 or M361). Its a triple 24/38/44 I recall.

They work flawlessly with the FD, RD, Cassette etc, no problems what-so ever ... except the chain slips almost every half revolution but ONLY when on the smallest ring and ONLY when climbing.

Ironically during this ride, I was forced to climb on the middle ring which put way more pressure on the cranks but no slipping what-so-ever.

When on the small ring on the flats and spinning like crazy, no slippage.

My gut tells me the smallest chain ring is defective but I've had these cranks for 2 years without a problem and I rarely use the smallest ring.

Could it be a chain issue that somehow only shows up when on the smallest ring?

Le Mechanic

Sounds like maybe a bent tooth on the small ring. If everything looks straight, could just be worn out if you ride in the granny gear a lot.

cny-bikeman

The chain has several times more links than the inner ring has teeth, so it would not occur that often - would take several revolutions of the crank to bring around the same section of chain. If it occurs only with the small cogs on the rear then the problem is "cross chaining" - you need to shift to the middle ring and a larger rear cog for about the same gear ratio If it happens even with middle cogs then the problem has to be the chainwheel. But it's not "defective", but rather damaged in some way.

Andrew R Stewart 

It could be that the small ring is worn out so that the chain doesn't mesh deeply enough on the small ring's teeth to prevent 'crawl over' under high pressures. Much like a worn rear cog. As the small ring has so few teeth engaged with the chain and the pedal pressure is leveraged so much teeth wear can happen sooner then on a middle or large ring. Also if the chain was instead well worn the same issue can happen on the small ring.

An experienced guy can check the bike out and test ride then tell you which. Of course this isn't expected to be free. The usual cost is to have the corrective work done by that guy. Andy

wesmamyke

Could also be a super worn out chain, and a barely used small ring. More commonly a problem the other way around for sure. Probably possible with fewer teeth engaging the chain and added stress from climbing.

Is it making noise when it slips or is it silent? Actual slipping from worn cogs/rings and chains is often weirdly silent, it just lets go and catches again. If it's more of a cross chaining or chain line issue, and it's catching the middle ring shift ramps slightly or something I'd expect some ticking sounds.

Racing Dan

Slipping where? On the little ring in the front or in the back? When in the little ring, chain tension can be much higher than when in the middle or big ring, thus making the chain more prone to slip in the back, if the chain or cassette is worn.

ridelikeaturtle

It could be a problem with chain wrap, not getting enough chain around the small cog on the cassette. You should be able to adjust the b-screw on the rear derailleur to address this. A Google search should show how this is done.

Andrew R Stewart 

While a good point (chain wrap around a cassette cog) the B screw is to establish the guide pulley and large cog clearance as it's first function. Andy

ridelikeaturtle

You're absolutely correct, though my experience is it can affect chain wrap across the cassette. It's worth checking in any case.

Andrew R Stewart 

I agree that chain wrap can be a concern. But I find this is more to do with what components are combined in the system.

Here's a point I made in another thread a few minutes ago but is an aspect I deal with frequently at work- When we use a system at it's maximum capacity is it no wonder that there are niggling issues and less performance then when the system is less stressed? I think of cars many times when I explain bike stuff. Take your car engine's maximum RPM limit (the red line on the tach.) By the way many people who ride and work on bikes would seem to have it it's perfectly OK, and without ant added stress/wear/performance loss, to operate the engine to that max all the time. So one is to shift the gears, on their car, when the engine reaches that red line each and every time. (This is satire for those who can't read between the lines). Andy

nfmisso

A bit of a follow on to Andy.

To minimize chain tension (wear on chain and teeth of cogs & chain rings) - ride in the LARGEST chain ring which will provide the suitable ratios for what you are doing. For example, if you have a 44/34/24 triple chain ring, and 11-13-15-18-21-24-28-32 8 speed cassette; if you can nicely get up the grade in 44/28, use it not 34/21, and especially not 24/15 - all which are about the same ratio.

From a different perspective; in normal slower riding, try to stay in the largest cogs at the back until you are in the biggest chainring at the front.

dh024

OP: When you say the chain is slipping, do you mean between gears on the rear cassette? If so, sounds like a rear derailleur alignment problem to me.

If it is slipping off your front chain ring and onto the frame, lots of good advice above - adjusting your low limit on the front derailleur will probably help prevent it.

raria

Gosh so many explanations. Let me try some of them and get back to you all.

thanks again.

Homebrew01

If the chain is skipping on the cassette while on the small ring, are you also in a heavily used cog on the cassette ? Worn cassette cogs are a common reason for skipping, especially if the chain is new.
It may be fine in the middle chainring, because at the same time, you have shifted into a lesser worn cog on the cassette.

Shimagnolo

I've worn out a few chainrings, and I've never had one skip.
Quite the opposite: They developed slightly hooked teeth, resulting in chainsuck when a new chain was installed.
A mild case of chainsuck is a rattling noise as the RD tugs the chain off the teeth of the chainring.
A severe case of chainsuck is the chain sticking to the chainring and getting jammed up against the underside of the FD.

Andrew R Stewart 

Shimagolo- I suspect your not having experienced a skip due to ring wear is because you like a smooth chain/teeth meshing (and nice shifting) and likely replace your rings before the extreme of wear that a skip comes from. Every season we replace, maybe, 10-15 rings on customer bikes due to wear. Each season there's usually 1 or 2 that are driven by a skipping situation, and it's the ring (compounded by the well worn chain too). A few are driven by the auto shifting that sometimes happens with a worn ring (for some reasons this is generally with Shimano triple middle rings from the 1990s/2000s). The rest are from general meshing/noise.roughness or our suggestions. Andy

Shimagnolo

No.
Chainsuck is the only reason I've ever replaced small and middle chainrings.

I've twice replaced large rings for a reason I've never seen discussed on BF: Worn shifting pins. The first time it happened, it took me the longest time to figure out why, and I didn't fully believe it until I swapped in a new chainring, and the problem was gone. Both times this happened on Shimano XT cranksets (with 22/36/48 rings). The large ring is not flat, but has a "shoulder" facing the middle ring, with the shift pins embedded in the shoulder. Once the pins wear from a sharp edge to a rounded one, the chain just slides off the shoulder and can't climb onto the large ring.,

DaveSSS 

Worn chain rings cause chain suck, not skipping. Worn cogs don't skip either, unless you have a brand new chain on the bike. New chains skip on worn cogs. Put a chain on those worn cogs with only a few hundred miles of use and there will be no more skip.

https://www.sheldonbrown.com/brandt/chain-care.html