Front derailleur problems

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ricksavery

Über Member
Location
Poole, Dorset
I have a Shimano 11-34 rear 9-speed cassette, a 48-38-28 chainset from Spa Cycles, and Shimano LX front and rear derailleurs.
The rear setup works fine, and I can shift to all gears.
I have made an error on the front. Did not realise that there must be a 12T difference between middle and large chainrings.
Front derailleur hits middle chainring when shifting to large chainring.
So, I mounted derailleur higher up tube. No rubbing on any gears whilst in middle chainring, but on smaller chainring, there is rubbing in the low gears, making it all a bit pointless.

At first, I wasn't sure why Spa would sell a chainset with a 10T difference if this was an issue, but then reasoned that it must be OK in a 7-speed setup where the range of chain angles is not so great - and a company like Spa and loads of cyclists wouldn't sell/use it if it didn't work.

My stupid fault then for not having read the label on the tin!
As a temporary fix, I was thinking of lowering the front mech so the rubbing would stop, and not use the larger chainring (which i don't really anyway)
long-term options seem to be
1 Change the front mech to one which will work with chainset (don't seem to be many if at all)
2 Replace entire chainset
3 Replace middle (and smaller ?) chainring to give a 12T middle to large difference - was thinking of this to get a better lower range anyway

If I do replace the chainrings, would 48-36-24 work (ie a 24T overall difference) with the LX mech or would I have to be content with 48-36-26 ?

Would appreciate all advice on these options - and any others - are they stupid choices ?
Thanks for staying with the (long) question !
 

raindog

er.....
Location
France
I have a Shimano 11-34 rear 9-speed cassette, a 48-38-28 chainset from Spa Cycles, and Shimano LX front and rear derailleurs.
The rear setup works fine, and I can shift to all gears.
I have made an error on the front. Did not realise that there must be a 12T difference between middle and large chainrings.
48-38-28 or 50-40-30 are standard triple set-ups. Where did you hear the thing about 12 T difference? Are you using the right front mech for that size triple?
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
Everywhere I have asked about this I have been quoted this 12T difference. The Shimano leaflets for the LX mech mention it (did i read it first ? No)
I have no idea if it is correct or not, but it would explain the problem (I think)
Not sure what you mean by trhe right size mech for that particular chainset - which possibly shows why i made the mistake in the first place :blush:
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
Sorry raindog, just re-read your reply and my original post.
What i should have said was that I didn't realise that there should be a 12T difference for my particular mech to work rather than the 12T difference as a general rule for chainsets, which would be wrong.
I thought my front mech would just ... work! But it has been a while since i bought a new one and technology has moved on.
 

Crepello

Active Member
Although Shimano spec a 12T difference between large and middle, in reality plenty of people run 48/38/28 chainsets with Shimano front mechs. Front mechs are almost always a pain to set up well and the fact that the chain rubs on the mech cage in the small ring simply means that you need to study the workings a little harder. You'll get there.
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
I'm sure you are right, but I don't think they can do it with the LX front mech. There really is no way that it will shift from middle to large chainring without hitting the middle - unless i mount it higher, which i have.
I think the rubbing occurs because with the mech now mounted higher, the chain passes through the mech in a way that it was not designed to do without rubbing on the sides.
If people do use Shimano front mechs with this chainset, then I wonder which one? Maybe I should change the front mech to this model.
 

fixedfixer

Veteran
I maybe wrong here but, I thought the issue was that manufacturers say 'not more than x tooth difference' By that I thought that the max (in this case 12) was critical and that a lower difference was ok. I'm sure some more knowledgable forum members will enlighten me.
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
This is where I am getting completely confused! I have had people say that they thought the 12T was a maximum and I have no reason to doubt this, but the LX will not clear the middle ring (with a 10T difference) and keep the 2mm gap between the mech and the large ring. I have also been told that mounting it higher is not the end of the world so this is what I did. Shifts between chainrings OK now, but this cause the chain to rub where it shouldn't.
I don't know whether to trust manufacturer's specs as the Tiagra front mech for example, is supposed to have a 20T total clearance, yet top touring bikes (Galaxy etc) seem to use this with a total clearance of 22T - maybe the smaller ring isn't that critical?
 

PpPete

Legendary Member
Location
Chandler's Ford
I've read of this 12T big-middle minimum difference too - on the Shimano tech docs.

But my OH is running 48-38-26 chainring with no probs. IIRC her front mech is a Tiagra.... I'll check when she gets in.

Whats the FD mounting on this one ricksavery? If it's old school 28.6 band-on I've got a spare Suntour triple mech which you'd be welcome to try as an alternate to the LX.
 

Cheule

New Member
Location
Coventry
Try not to worry too much - I did the same as you just last month in this thread.

I am running mine mounted way higher than it should be - but it does work, it just takes a lot more fine tuning. I do intend to replace it or the crankset eventually though as I like to be within spec.
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
Thanks porkypete. It is a 28.6 band-on. I've got my old Shimano Alivio FD that I think I'm going to try first, before I make a pain of myself
I would be interested to know whether it is a Tiagra, as I think my choice of LX might be the problem (or so I'm told)
It is really annoying to only be able to use the middle chainring without the chain rubbing on the FD somewhere.
I have also received the advice of using the other type of FD (technical name escapes me), where the clamp sits lower relative to the derailleur body.
I accept that the extremes of gears are not useable, but not to be able to use the middle chainring, and the lowest gears with the small chainring, without one or t'other combination rubbing is maddening. It seems to be something of a black art!
 
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ricksavery

ricksavery

Über Member
Location
Poole, Dorset
Glad you got yours to work Cheule. The fine tuning is elusive and seems to be a choice of which combination you prefer. Gearing shouldn't be like this!
Replacing the FD and/or the chainset/chainrings is really the point of this whole thing ... possibly
 

MacB

Lover of things that come in 3's
I have a Tiagra front mech on a 52/42/30 triple and it will also accomodate a 26t front ring, but I use bar end shifters on this bike so the FD is moved by friction only.
 

Moodyman

Legendary Member
I had the same problem with my 48-38-28.

Front mechs compatible with my front chainset, weren't compatible as they were all short cages, whereas I needed a long cage that dropped further down. Longer cage avoids the rubbing on the front ring that you describe.

I took my existing front mech off, visited a LBS and bought a mech that had the same cage. Shimano C051 in my case.
 
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