Intermittent issue onto small chainring

[Sweep]

Am a bit puzzled by an issue I seem to remember encountering before.

Before I get the bike on the repair stand, fiddling/fettling and possibly making things worse before returning to where I was (something I make a speciality of) I thought I'd ask you good folk for possible focussed clues about what is going on.

Front chainset is 28/38/48.

I have an intermittent problem - sometimes the chain drops trouble free to the 28, sometimes it doesn't. Seems to happen in runs - run of working fine, then a run of trouble, then back again.

But, if I turn the bike over, all seems to work totally smoothly pretty much all of the time.

Now of course I'm not going to be riding the bike upside down but I wondered if this curious phenomenon gives a clue as to what's going on/what the problem is.

[CREPELLO]

The fact that it works fine with the bike upside down merely suggests that it is a marginal problem, which is essentially the mech cage isn't moving the chain across enough. But that could be for all sorts of reasons and the cold weather can play badly into the mix as well (thick gunk making for stiff linkages and sluggish cables, rusting metalwork etc)To make a better diagnosis can you supply more info - chainset model, BB length, rear axle width, mech model should help.

In addition, how is the mech positioned (parallel to the chainrings? Just above them?) Also, a very worn chain will also become so flexible that it will avoid the mech's orders to move by squirming sideways without departing track.

[pwa]

I agree that the mech is just not pushing quite enough. It sounds like it is within 1mm of being right. I remember fiddling with a front mech on my wife's bike for ages before finally resorting to slightly bending the mech cage to make one side touch the chain a tiny bit earlier, giving it just the right amount of contact to shift perfectly. But you could just be looking at a slightly sticky cable that refuses, sometimes, to slide that last 2mm that completes the shift.

[Sweep]

I agree that the mech is just not pushing quite enough. It sounds like it is within 1mm of being right. I remember fiddling with a front mech on my wife's bike for ages before finally resorting to slightly bending the mech cage to make one side touch the chain a tiny bit earlier, giving it just the right amount of contact to shift perfectly. But you could just be looking at a slightly sticky cable that refuses, sometimes, to slide that last 2mm that completes the shift.

Will investigate your points further. Thanks pwa. And for the encouragement about being almost there. I did have a similar problem with a front mech a year or two ago and that proved to be simply that the mech was old/worn and didn't consistently have enough force to shift the chain. But even though this mech is 20 years old and quite a cheap one in its time (it came off the old frame I bought but had not necessarily had a great deal of use) I don't think that is the problem here. I did solve an earlier issue with this mech by turning the back of it in a bit on its tube mount so that it gave the chain a bit more of a kick. This does (when it works) cause a slight catching on the mech cage when the shift is completed but this a minor problem as I have a nice trimmable indexed shifter for it. I want to stick with this mech if possible as it appears, remarkably, to be one of only a few that suit the 28/38/48 chainset.

Off to do more oiling - I don't want to do any moving of the mech just yet as am away from full toolset and repair stand and don't want to make things worse.

Off to answer crepello's sensible questions.

[Sweep]

Many thanks for your reply crepello. Answers, as best I can, to your sensible questions.

The chainset is this:

http://www.spacycles.co.uk/m2b0s109p200 ... e-Chainset

28/38/48

The front mech is this:

http://www.ebay.co.uk/itm/SHIMANO-ALTUS ... 3641.l6368

And the shifter is a mountain bike shifter.

Just noticed that it says in that ad that that that mech is a road bike mech. This puzzles me a bit. I do know that cable pull on front mechs is different for MTB and road types but this mech came off, and is now fitted back to, a 90s flat bar Ridgeback which I would have thought used mountain bike shifters - they were taken off but they were similar vintage Altus combined shifters and canti brake units - pretty certain they are MTB units as I have a pair on another 90s Ridgeback hybrid and they work faultlessly with MTB mechs.

I fitted that mech back on the bike after issues with others - as it originally worked with an old chainset that was also 28/38/48.

I am pretty certain that the square taper bottom bracket I have installed with the chainset is 110mm as recommended by Spa.

Is there a possibility that there is an issue with chainline? - something I have never got my head round? I wonder about this as the bike is a hybrid and have the idea that this would have a different chainline to a road bike.

You will see from that Spa ad that it says that the chainset has a Q factor of 163mm. Q factors are something else I have never got my head round so I have no idea if that is relevant to my issue.

Rear mech on the bike is an Ultegra shifting a 9 speed cassette which is, I think, 11-28. That all works faultlessly.

Rear OLN is 135mm.

>>In addition, how is the mech positioned (parallel to the chainrings? Just above them?)

Pretty sure I originally set the mech up paralell as per standard procedure. But then I had an earlier slight problem with this mech not throwing the chain onto the little ring but I solved that by turning the back in a little to give the chain an extra kick - maybe more than you are strictly supposed to do but the interference this causes between chain and mech when the shift is completed can be easily solved by using the trim function on my left hand indexed shifter.

Yes, the mech is set so that it is just above the big chainring when it is pulled out.

If you need any more info, or even a pic of the built bike, just let me know.

Many thanks again

[mercalia]

could also be the low winter temperatures? In my case I have to make a tiny adjustment to the limit screw for the large ring of my RSX 46/36/26 triple during the hot summer months or it wont take. I assume some kind of heat expansion issue is at work?

[Sweep]

Don't think it's a limit screw issue mercalia. Just had another tinker while oiling - bike upside down as don't have a workstand where I am. All as before when turned upside down works fine. But, perhaps more to the point re possible limit screw issue, I tried shifting by tightening and loosening the cable by hand (ie - not using the shifter) - the mech shifts fine to the small chainring when there is still some slack available to release in the cable. So it can't be a too loose cable either. CORRECTION - can't be a too tight cable.

Did lots of oiling in the area of the cable guide under the bottom bracket.

Would it perhaps be worth replacing that? They are plastic and I suppose with age/scouring they could become a cause of friction.

Will try to get out riding the thing in the gloom later.

[Brucey]

I have seen this kind of problem many times. It is easy to get entirely focused on the mech etc (and of course it can be that) but IME the problem is just as likely to be a fundamental one to do with the way the chain fits on the chainrings.

Some issues that have caused this shift to go bad for me;

1) the chain has cambered inner side plates, and doesn't want to derail off the middle chainring

2) the chain doesn't have the best camber on the outer side plates, ditto

3) the chain has a smaller inner dimension than is correct for the chainring (eg because it is a 10s one on 9s chainrings, or a 9s one on 8s chainrings, etc etc), ditto

4) the chain tension from the rear mech is too much or not enough

5) the chainrings are worn too much

6) the chainrings are worn not enough

7) the chainring teeth have burrs on them which make the chain a snug fit

8 ) the middle chainring is not the correct type (i.e. it is an inner ring from a double instead of a middle ring from a triple) and lacks release gates.

Changing to a different type of chain can often wreak a dramatic transformation in shifting quality. For example even refitting a worn chain is a test that is OK for the workstand.

BTW the (right way up) shift on the workstand is often worse than it is when you are on the road, because the bumps in the road can help the chain to derail. Most probably the reason the shift works OK at present when the bike is upside down is because the (small) additional force of gravity is helping the chain to derail rather than hindering it.

hth

cheers

[Sweep]

Note my CORRECTION above

ah glad you popped on brucey as I was thinking of "paging" you, for the plot thickens.

First - one hell of a lot of possible causes you outline there but of course that is a reflection of your knowledge/wide experience.

I had wondered about changing the middle chain ring in case some odd bit of metal was causing issues.

The chainset including all chain rings is pretty new from Spa.

Now to the extra evidence and I fully accept that you and others may think me bonkers, but I swear this is true.

Gave the drivetrain a thorough clean and oil, and in between chainrings as well as I could - liberal use of GT85. Oiled chain.

All worked fine upside down but then it did before.

Set off on a ride, changing up and down up and down - all fine across all chainrings including the middle to small drop.

Set off up the hill - lower slopes OK - got to the steeper bit - refused to change.

Did a U turn down and immediately it changed fine.

Tootled through a nearby village changing up and down - all fine.

Back up the hill - exactly the same thing - wouldn't make the drop from middle to small ring going uphill but just seconds later after a U turn all fine. I did this too many times for it to be chance.

So basically all is fine changing into bottom gear on flat or downhill but not uphill!

Does that help?

It is a particular hill with a spooky reputation but can't think that that's the cause.

Possible that a certain looseness in the mech fittings comes into play on slopes? Though I must admit that that seems unlikely - not an extreme change on gravitational forces and there is a certain friction from gunk.

All working fine up and down on the ride back.

EDIT _ with regard to your point 3 brucey would it be an idea to try things with an 8 speed chain?

The front mech was originally used on the same frame with a 7 speed on the front but also 48/38/28.

Rear mech is a 9 speed ultegra - if it's relevant I think shimano strictly says it should be used with a max sprocket of 27 but there is plenty of evidence online that this bizarre dictat is not valid and you can use up to a 30T sprocket. My max sprocket is 28.

In any case, with regard to chain tension I use the standard Park Tools method of dteremining chain length and it has always worked fine for me.

[Brucey]

you don't say what chain you are using. I doubt an 8s chain will be ideal for your cassette (if it is 9s...?), but 8s and 9s chains vary quite a lot so there may be one that works better than the one you have....?

BTW the difference in the shifting uphill vs downhill is almost certainly because the chain tension is higher on the top run in the former instance. If the shift is baulky, it may not go unless the chain is as stack as it can be, i.e. you are still pedalling, but not keeping up with the wheels as it were.

cheers

[Sweep]

BTW the difference in the shifting uphill vs downhill is almost certainly because the chain tension is higher on the top run in the former instance. If the shift is baulky, it may not go unless the chain is as stack as it can be, i.e. you are still pedalling, but not keeping up with the wheels as it were.

cheers

sorry brucey, don't quite understand.

But, if your analysis of the issue, whether I understand it or not, is correct, does it suggest for you a solution?

[Brucey]

re the shifting; if the top run is under tension (from pedalling.....) then the shift will be more baulky than normal.

We don't know yet what the problem is..... in good part this is because (unless I missed it) you have not yet divulged what chain you are using, how many sprockets you have at the back, if the cable is worn into a groove in the BB guide, if the middle chainring has gates etc, if it is worn, burred, or whatever...

Any of these things can make a middle-to small shift go bad; it is very often the first shift to go bad whenever things are not quite right.

cheers

[CREPELLO]

Your mech was most likely designed as 7 speed. It sounds from the rear mech you're using that you're maybe using a 9 speed chain. If this is so, the mech is now too wide for the chain to work well with a indexed shifter.

In also thinking some of those aluminium chainrings can be pretty soft and that slight burring may have occurred. In which case, remove the chainring and deburr with a needle file or a sharp curve bladed Stanley knife.

I'd also suggest that the BB is too narrow for this mech. If 110mm is quoted for the chainset, that could be for a road bike rear 130mm hub and a road front mech. A 113mm BB could help. I use a113 on my Cheviot, with an xd2 chainset and 135mm rear, but also an ultegra road front mech. It does however work very well off Tiagra ski's. The opposite problem for some road front mech's is that if a wider bb is used for a wider chainline and 135mm rear hub. In this case the mech can't move far enough out.

[Paul Smith SRCC]

...Did lots of oiling in the area of the cable guide under the bottom bracket....Would it perhaps be worth replacing that? They are plastic and I suppose with age/scouring they could become a cause of friction..

Is the cable guide secure? When an issue is as marginal as this it may not take much to tip the scales between working and not, a slight movement in the BB tunnel guide maybe just enough to upset those scales.

I still have a similar issue to yours on my set up, where I use a front triple mech' where the cage is designed to follow larger chain ring sizes. I have got it working just good enough for me, but it is definitely a compromise, I try not to change down under full pressure and if possible not when in the largest rear sprocket, I get a better shift if I am in 3-4th sprocket down as the mech' engages the chain nearer where it was designed to do, as a precaution I have also fitted an ’Overshift Protector’.

Your front mech' appears to be compatible with the chainset interms of chain ring sizes, but have you adjusted the front mech' close enough to the rings, if set to high it may well result in the issue I have.

When something is a puzzle like this there is often a light bulb moment when the issue is discovered. I would suggest try one thing at a time, it could be many of the suggestions already referenced in this thread, if you do one thing at a time you will then know what that one thing was. No harm in trying the cheapest or free things first, check the bb tunnel guide is secure, make sure the cable inner runs smoothly through the outer and make sure the outer ferrules have an accurate secure interface with the stops, the cable tension is often effected if not. You should see the front mech' shifting slowly in a bike stand if the cable drags. Likewise the front mech' pivot points, set the front mech' to the small ring position and pull mech' back to the middle ring position carefully with your fingers, just to see if it springs back smoothly.

Can the front mech' stops be undone sufficiently to allow the mech' to move in far enough? In my case on the bike above I had to fit a slightly wider BB, the chainset would fit well enough with the rings clearing the stays with a 113mm, but I had to use a 116mm because of the limitations of the front mech'. The 113mm BB resulted in the front mech' fowling the seat tube and as such the mech' could not shift in far enough; the seat tube effectively acted as the inner stop.

[Sweep]

Many thanks for all these responses so far - plenty to be working with.

With regard to your point about fouling Paul, the mech when it moves inwards does at the end touch the chromplastic mudguard. It doesn't seem to take any force out of things, there is no appreciable mark on the mudguard, and of course most of the time things are working fine. Also, it wouldn't explain the spooky hill phenomenon.

I will investigate this further though.

Brief answers to some of questions above:

Brucey:

Chain - it's a 9 speed SRAM.

Sprockets on back - 11 to 28 and rear mech, quality item, works fine.

Gates on middle chainring - sorry Brucey, don't know what gates are. Something to do with helping shifting up and/or down? I stress that the chainset is new, as bought from Spa - I have not tinkered with it.

Points on burring from you and crepello noted.

Paul - the front mech is definitely low enough- no problem to get this low as, as you say, the mech fits the chainring sizes.

Crepello - I thought 7/8/9 speed front mechs were effectively interchangeable. Are very minor differences in chain dimensions going to effect things much? I thought there were plenty of instances on the forum of folks using 8 and 9 speed chains both ways on 8/9 speed rear set-ups.

Thanks again for all of your help.

It may be a fair while, for various reasons, before I can try all of the options, swap bits/get the bike on a proper stand, but you can be sure that I will report back when things are fixed. For, with something so marginal, it surely will be.