UKH

Front Mech Woes

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Apologies if these are really stupid questions. My wife's mountain bike has had an intermittent problem for ages and I'm finally getting round to having a look. Occasionally it won’t drop into first on the front mech. I’ve tried loosening the cable tension and adjusting the lo limit screw. Even when the end of limit screw is no longer making contact with the derailleur, the cage is just about in the right position but certainly not too far. I can’t see any problems with the derailleur. It would be good to be able to adjust the limit screw too far and then back again.
Here’s where I start getting particularly stupid. The axle on the drive side doesn’t appear to stick out very far compared to the non drive side. When I’ve measured the distance from the end of the pedal axle (is that the right phrase?) to the centre line of the frame it’s about 70mm on the drive side and 74mm on the non drive side. If these were equal I’d probably have enough adjustment. Should they be equal? Is it possible the bottom bracket is installed slightly off-centre? Thanks.
 

In reply to Thugitty Jugitty:

When you say first on the front mech do you mean it won't go from big ring to small ring?

Off-set isn't unusual (it is on all my bicycles by a few mm) in fact if I'm reading you right you'd have less to play with because it'd be a further 2mm out on the DS.

Guess I'd ask was it working recently/before? Are you sure you've got the limit screws sussed (not high and low mixed up)? Have you actually detached the cable and tried it? Nothing impeding it? Is the guide plate in-line with the chainrings i.e. it's not been hit so it's not straight? Chain or chainrings worn? 

Good luck.

 malk 13 Jul 2020
In reply to Thugitty Jugitty:

maybe twist it in a touch or adjust the height?

drive side of axle usually shorter i think..

 LastBoyScout 13 Jul 2020
In reply to Thugitty Jugitty:

Which bottom bracket is it? The Shimano ones I use on my mountain bikes come with a set of 2mm (iirc) shims depending on the bottom bracket shell size and the instructions will tell you what to use on which side.

So, it could be that whoever installed it got those back to front, so worth checking.

That, however, shouldn't affect the shifting if the limit screws and cable tension are correct.

You don't say, but I assume you've tried manually pulling and pushing the mech and there's no resistance/stickiness in the action that would be stopping it?

I've had similar issues with the front mech on 2 bikes not dropping onto the inner ring. One was caused by a mix of corrosion in the mech making it very sticky and also with the cable getting very sticky round the BB (bottom pull mech) where road crud builds up. Could be that all you need is to give the mech and cable routing a really good clean and lube. The other I still need to look at, but I'm expecting exactly the same diagnosis.

The other issue, depending on age, is that it could be worn chainring teeth and worn chain stopping it shifting.

In reply to Thugitty Jugitty:

Does it happen in both the high and low rear sprockets?

Could be the chainline is not right. Try installing a spacer on the bottom bracket.

Could be the front mech spring is tired/ filthy. Take it off and free it up (so it moves across with more of a snap).

Is the front mech alignment correct (parallel with chain)?

Is the front chain ring worn out (& hooked teeth are trapping the chain)?

Post edited at 14:47
In reply to Stuart (aka brt):

Yes, it won’t go to the smallest ring, the one nearest the frame.

Since the derailleur can’t get any closer to the frame I’m wondering if the chain rings ought to be further from the frame. At the moment the drive side is 4mm closer to the frame than the non drive side, so squaring it up should give me another 2mm of adjustment.

The problem wasn’t there from new but it has been around a few years. The hi lot screws are labelled and as can see them making a difference, so I know I have them right. The guide plate looks straight and the chainring doesn’t look worn, but the chain is a bit worn.

In reply to Thugitty Jugitty:

Yes, I had a brain fart there. Could be spacers but if it was working before I'd go with stuck or sticking on something.

Edit as I've just thought about it, the spring could also have lost tension. Is it left in the big ring when not being used i.e. the spring is extended and won't retract? 

Post edited at 15:01
In reply to LastBoyScout:

> Which bottom bracket is it? The Shimano ones I use on my mountain bikes come with a set of 2mm (iirc) shims depending on the bottom bracket shell size and the instructions will tell you what to use on which side.

I'm not sure which bottom bracket it is. It has internal splines visible on the non drive side though.

> So, it could be that whoever installed it got those back to front, so worth checking.

> That, however, shouldn't affect the shifting if the limit screws and cable tension are correct.

> You don't say, but I assume you've tried manually pulling and pushing the mech and there's no resistance/stickiness in the action that would be stopping it?

Yes, tried it with the cable detached. There does seem to be quite a bit of play. Maybe wear and hysteresis is the problem. It seems very sensitive to adjustments in the cable tension, with a limited sweet spot where it will shift into both top and bottom.

> I've had similar issues with the front mech on 2 bikes not dropping onto the inner ring. One was caused by a mix of corrosion in the mech making it very sticky and also with the cable getting very sticky round the BB (bottom pull mech) where road crud builds up. Could be that all you need is to give the mech and cable routing a really good clean and lube. The other I still need to look at, but I'm expecting exactly the same diagnosis.

I'm the household bike cleaner and I've been pretty slack. Maybe I need to eliminate that cause first.

> The other issue, depending on age, is that it could be worn chainring teeth and worn chain stopping it shifting.

In reply to Stuart (aka brt):

> Yes, I had a brain fart there. Could be spacers but if it was working before I'd go with stuck or sticking on something.

> Edit as I've just thought about it, the spring could also have lost tension. Is it left in the big ring when not being used i.e. the spring is extended and won't retract? 

Yes, it is.

In reply to everyone:

I've given the front mech a good clean and lube which has improved matters a little. I'm left with the problem that it seems very sensitive to adjustments in the cable tension, so that it is hard to adjust it so that I can get top and bottom gear. I'm going to put this down to wear and tear. There's loads on the bike that's tired, and I hate the mechanical disk breaks, so I think it might be new bike time next year. This one must be a good 10 years old. I might treat it to a new chain though.

Thanks everyone.

In reply to Thugitty Jugitty:

You put us through all that so you could justify buying a new bike!!?

 GrahamD 13 Jul 2020
In reply to Thugitty Jugitty:

An obvious one to check, how is the front mech attached to the frame ? With some mountings its possible for the whole mech to slip round. 

In reply to blurty:

> You put us through all that so you could justify buying a new bike!!?

>

Worse. Justifying my wife buying a new bike.

 Basemetal 13 Jul 2020
In reply to Thugitty Jugitty:

I don't think you've said which model front mech it is, but you might be pleasantly surprised by the generally low-ish price of front derailleurs, even if you are matching a groupset (which you don't even have to do, of course). If you go this route it's worth gchanging the cable too.

In reply to GrahamD:

> An obvious one to check, how is the front mech attached to the frame ? With some mountings its possible for the whole mech to slip round. 

Do you mean rotating around the seat tube so that the derailleur cage is no longer parallel to the chain rings? It looks ok. 

In reply to Basemetal:

The front mech is a Shimano Acera. You’re right, they do seem cheap enough to be worth a punt. I’m struggling to understand what I need to know to get the right spec though: numbers of rings, numbers of teeth?

 Basemetal 13 Jul 2020
In reply to Thugitty Jugitty:

> The front mech is a Shimano Acera. You’re right, they do seem cheap enough to be worth a punt. I’m struggling to understand what I need to know to get the right spec though: numbers of rings, numbers of teeth?


Front mechs are specced by number of chain rings  (front) AND number of sprockets (rear) and by the mounting. Here's a buyer's guide:

http://hub.chainreactioncycles.com/buying-guides/components/derailleurs-buying-guide/

In reply to Thugitty Jugitty:

Normally just 2 or 3 speed (2 or 3 rings) size of chain-rings is taken account of by height of the dérailleur - if you can find a model number, that's prob best bet. 

With the cable detached can you manually move the dérailleur and it shifts ok? I think from what you've said it will. so:

One other thing to check first though is your shifters and cables - you said a sweet spot could be found with tension. depending on model of shifter, you may have some luck with the barrel adjusters at the shifter end for fine tuning tension.

The other thing is what condition the cables & outers are? do the cables move freely by hand? are the outers well seated and not moving out of their end stops?

 ChrisJD 13 Jul 2020
In reply to Thugitty Jugitty:

If you like the bike and your wife, then just go 1x11 and dispense with the front mech (especially if you are going to replace some parts anyway).  Life is much simpler without the front mech.

You should be able to use the same cranks 

.. or just buy a new bike

1
In reply to balmybaldwin:

I can move the derailleur by hand and it feels fine, although there’s also quite a bit of the unwanted kind of movement too - it’s a bit loose in its movement, presumably worn. Cables feel ok. There’s much more range of movement available in the derailleur than is provided by the shifter - is that how it should be?

So if I want to try swapping out the derailleur, I’m after something for a triple front, eight speed at the back, 42 teeth on the outer ring and clamping to a 32mm seat tube. There’s nothing leaping out at me which is identical to the original Shimano Acera.

Edit: I think this fits the bill https://www.sjscycles.co.uk/derailleurs-front/shimano-altus-fdm310-8-speed-conventional-swing-front-derailleur-multi-286318349mm/

Post edited at 19:05
In reply to ChrisJD:

> If you like the bike and your wife, then just go 1x11 and dispense with the front mech (especially if you are going to replace some parts anyway).  Life is much simpler without the front mech.

> You should be able to use the same cranks 

> .. or just buy a new bike

Like I said, I hate the mechanical disk brakes. It’s a nice looking bike with a happy history but I don’t want to prolong its life too much...

 ChrisJD 13 Jul 2020
In reply to Thugitty Jugitty:

There's been a few MTB threads recently that are worth a look at for new bike ideas

 Monk 14 Jul 2020
In reply to Thugitty Jugitty:

Have you replaced the cable and outers recently? This can often have a surprising effect. 

In reply to Monk:

No I haven’t. I’ll have another look but they seemed in good order. 

 GerM 14 Jul 2020
In reply to Thugitty Jugitty:

Overall it does sound like it is wear in the derauilleur causing the problem. One thing that may help with getting it to shift is to reduce the clearance from the derailleur to the rings by moving the derailleur slightly closer to the axle. This may be possible if there is room to adjust the height of the derailleur, certainly should if it is a band on. Care needs to be taken not to change too much as it may then foul the teeth of the chainrings, in gear, or during the shift.

In reply to GerM:

Thanks. And thanks again to everyone else. The gap to the top of the outer ring is already only a couple of mm, so no further scope for adjusting. I compared the play in the derailleur to the one on my own bike and it does seem very wobbly, so I’ve ordered a new one. It was inexpensive so seems a sensible thing to rule out.

In reply to Thugitty Jugitty:

Short version: it was the front derailleur. Replaced and everything is fine now. Thanks all.

Long version: Obsessed with the idea that the asymmetry of the bottom bracket installation was the root of the problem I decided to remove the chainset to investigate. It’s a square taper, so using the proper tool I then proceeded to strip the inner threads from the chainset. Googling revealed that this is very common. I then decided to buy a puller (the sort for cars) off Amazon to get it off. When it arrived I couldn’t fit the three legs around the chain wheel so I started on the Amazon returns process. I ticked the “not quite what I wanted” box but the Amazon process nevertheless decided I could keep the item anyway and get a full refund. A small consolation for the stripped threads I suppose  

I then ordered a two-legged puller which fitted. I also bought a new chainset to replace the stripped one, before I attempted to use the puller. I also got a new chain whilst I was at it (the old one was very stretched) and cassette.

Then I came to my senses and read Sheldon Brown. I measured the chainline and it was spot on according to both Sheldon and Shimano: 50mm. I therefore abandoned ideas of further messing with the stripped chainset. Instead I replaced the chain (and cassette) which made everything run much more smoothly but didn’t fix the front shifting problem. Then I replaced the front derailleur. That immediately gave a much bigger range of movement so that the chain could now drop off the both the big and small cogs. After going through the proper adjustment process the front derailleur now works perfectly, possibly better than when the bike was new.

I still want to put the new chainset on but we’ve a couple of biking trips coming up and I don’t want run the risk of needing the fallback of the LBS whilst they have such a long waiting list, so I’ll leave this until winter.

I still can’t really see what’s wrong with the old derailleur. It could be slightly bent, maybe too much play in the pivots, or shit by design.

I am now slightly poorer but have some new tools and hopefully a better understanding of what I’m doing. 

Post edited at 12:11
 Martin W 15 Aug 2020
In reply to Thugitty Jugitty:

> Like I said, I hate the mechanical disk brakes.

So fit hydraulic ones.  The frame obviously already has disc brake mounts at front and rear.  You don't need to buy a whole new bike just to upgrade the brakes.  Or the drive train (as ChrisJD says, 1x11 is increasingly the way to go these days, and means that there's one less derailleur - which is usually the hardest one to troubleshoot, as you seem to have found - to go wrong).

> I therefore abandoned ideas of further messing with the stripped chainset. Instead I replaced the chain (and cassette) which made everything run much more smoothly but didn’t fix the front shifting problem.

Hmm. You said that the chain was "very stretched" so you've replaced that and the cassette, but you're still running the same chainrings.  Have you checked them for wear to be sure that they don't need replacing?

BTW, I feel your pain with the stripped extractor threads: I managed to do the same to my bike recently.  In my case it was a rogue washer which had dropped off the crank retaining bolt (it was only held in place with an o-ring) and blocked the extractor from pushing against the crank spindle.  So instead of pushing the crank off the spindle, it pushed itself out of the crank, taking the threads with it.  My new cranks have self extracting crank bolts (a truly brilliant invention IMO) with properly captive washers.  I had to take the stuck crank to the LBS to get it removed, though.  I actually broke the gear puller I bought to do the job, and after inflicting all the brutality I could bring myself to exert - including hacksaw and cold chisel - without risking trashing the spindle, I gave up and handed it over to the professionals.  I don't know how exactly what they did - though the crank was even more brutalised when they handed it back to me no longer jammed on the spindle - but they charged me less for the job than the broken gear puller cost.  Lesson learned there, I think: recognise when you've reached your personal competency limit and it's time to leave it to someone who's done it before.

In reply to Martin W:

I just spent a happy afternoon trying to get the best out of the mechanical disk brakes but they remain a pain to adjust correctly and are still prone to squealing. I’ll upgrade them to hydraulic this winter.
I just nipped out to the garage to double-check the wear on the chainring - it has some wear but doesn’t look too bad. I’ll leave it on for our next couple of trips and then have a go at removing it. I already have the replacement, it’s just that I don’t want to f*ck things up whilst the bike shops are so busy round here. I might replace the bottom bracket whilst I’m at it, in which case I can probably use an angle grinder if the puller won’t shift the chainset. I have read somewhere that riding the bike with the crank bolt loosened might also work.

The removal tool made such easy work of stripping the threads it doesn’t seem like the best design, but then I’m from a world of filing-down cotter pins for cranks. 

In reply to Thugitty Jugitty: I am quite obsessive in the way I make sure the extractor tool is properly threaded, counter rotating it  first to make sure it's lines up then proceeding with extreme caution, winding it back out again after a few turns to make sure it feels right. I guess you will have a similar level of caution instilled into you now! 

In reply to blackmountainbiker:

I don’t think it stripped the threads because it was cross-threaded, it felt fine. It’s the force needed to pull a stuck crank which stripped the fine threads. Maybe not?

Post edited at 07:44
 Ben07 16 Aug 2020
In reply to Thugitty Jugitty:

It sound like it just needs new inner and outer cable. They get clogged up and the spring doesn't have enough tension to pull the mech all the way over.   Maybe you've already done this? 👍

Post edited at 10:40
 ianstevens 16 Aug 2020
In reply to Thugitty Jugitty:

Hydraulic disc brakes can squeal too, it means to pads are contaminated or excessively worn. You can clean them (can’t remember how off the top of my head, look it up). Given what you’ve said about some of the other components I’d guess that you are due some replacement pads - again, these are inexpensive and at worse will give you slightly better braking. Could also be worth changing the cables if you’ve not done it, especially if the bike is getting on (which I suspect it is if you’ve got a triple).

 Martin W 16 Aug 2020
In reply to Ben07:

> It sound like it just needs new inner and outer cable. They get clogged up and the spring doesn't have enough tension to pull the mech all the way over.   Maybe you've already done this? 👍

Maybe you didn't read far enough down the thread to see that the OP fixed the problem by replacing the old, worn front mech.


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