Old front derailleur + new crankset problem

Ian B. said:

Hello,

I bought a new crankset which I thought was a close match to my old
one, new one is 26-??-48 old one is 28-??-48, but now can't seem to
get the front derailleur aligned right. If I move the derailleur up
the seat post the chain scrapes on the bottom of the derailleur, if I
move it down it doesn't clear the big chainring.

Does the extra two teeth really make such a difference that I'm going
to need to invest in a new front derailleur or larger chainring? Or
could I have another problem, like too much slack in the chain
perhaps?

First issue - do the two chainsets have the same off-set, ie, each sprocket
in the same place relative to the frame. If not, it may be a case of a
different bottom bracket axle required.

I would expect any triple front mechanism to have no difficulties over the
range of chainrings you quote.

If the chain is only scraping on the small cog when also on the smallest
rear cog, its not an issue; one shouldn't ride with the chain at such a
severe angle.

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

Nigel Cliffe said:

Ian B. said:

Hello,

I bought a new crankset which I thought was a close match to my old
one, new one is 26-??-48 old one is 28-??-48, but now can't seem to
get the front derailleur aligned right. If I move the derailleur up
the seat post the chain scrapes on the bottom of the derailleur, if I
move it down it doesn't clear the big chainring.

Does the extra two teeth really make such a difference that I'm going
to need to invest in a new front derailleur or larger chainring? Or
could I have another problem, like too much slack in the chain
perhaps?

First issue - do the two chainsets have the same off-set, ie, each sprocket
in the same place relative to the frame. If not, it may be a case of a
different bottom bracket axle required.

I would expect any triple front mechanism to have no difficulties over the
range of chainrings you quote.

If the chain is only scraping on the small cog when also on the smallest
rear cog, its not an issue; one shouldn't ride with the chain at such a
severe angle.

Hmm, I my bottom bracket has a 122mm axle, I think the new crankset is
110-113 mm though. Would this be a good explanation of my problem? It is
fairly clear that the old crankset goes further on the bottom bracket
and sits closer to the frame.

Ian B. said:

Nigel Cliffe said:

Ian B. wrote:

Quoted message said:

Quoted message said:

Quoted message said:

I bought a new crankset which I thought was a close match to my old
one, new one is 26-??-48 old one is 28-??-48, but now can't seem to
get the front derailleur aligned right. If I move the derailleur up
the seat post the chain scrapes on the bottom of the derailleur, if
I move it down it doesn't clear the big chainring.

Does the extra two teeth really make such a difference that I'm
going to need to invest in a new front derailleur or larger
chainring?

Quoted message said:

Quoted message said:

First issue - do the two chainsets have the same off-set, ie, each
sprocket in the same place relative to the frame. If not, it may be
a case of a different bottom bracket axle required.

I would expect any triple front mechanism to have no difficulties
over the range of chainrings you quote.

Quoted message said:

Hmm, I my bottom bracket has a 122mm axle, I think the new crankset is
110-113 mm though. Would this be a good explanation of my problem? It
is fairly clear that the old crankset goes further on the bottom
bracket and sits closer to the frame.

I think that is part of the explanation; most front mechs describe an arc as
they move out, lifting and pushing simultaneously. If you cannot pack the
mechanism out to match the new wider position of the chain rings, I wouldn't
expect it to work well.

I would look at changing the bottom bracket. Modern cartridge types are a
doddle to fit compared to the old ballrace and lockrings, which would take
me ages to set correctly.
Modern ones just screw in from the crank side, and a support ring (usually
plastic) screws in from the plain side. Ideally you need the correct tool
(a serrated socket) to insert and remove the cartridge.

If your frame is a standard British one, I'd expect it to have standard
British bottom bracket threads, which are the same as the current cartridge
types.

- Nigel

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

Nigel Cliffe said:

I think that is part of the explanation; most front mechs describe an arc as
they move out, lifting and pushing simultaneously. If you cannot pack the
mechanism out to match the new wider position of the chain rings, I wouldn't
expect it to work well.

I would look at changing the bottom bracket. Modern cartridge types are a
doddle to fit compared to the old ballrace and lockrings, which would take
me ages to set correctly.
Modern ones just screw in from the crank side, and a support ring (usually
plastic) screws in from the plain side. Ideally you need the correct tool
(a serrated socket) to insert and remove the cartridge.

If your frame is a standard British one, I'd expect it to have standard
British bottom bracket threads, which are the same as the current cartridge
types.

Ta for the advice, I'll try and get the bottom bracket from the LBS
tomorrow, I probably should've mentioned it was a new bottom bracket as
well (the old one was knackered), but I took it in and got a direct
replacement, d'oh! Hopefully they'll change it for the correct one,
fingers crossed.

in message <[email hidden]>,

Ian B. (') said:

Nigel Cliffe said:

Ian B. said:

Hello,

I bought a new crankset which I thought was a close match to my old
one, new one is 26-??-48 old one is 28-??-48, but now can't seem to
get the front derailleur aligned right. If I move the derailleur up
the seat post the chain scrapes on the bottom of the derailleur, if I
move it down it doesn't clear the big chainring.

Does the extra two teeth really make such a difference that I'm going
to need to invest in a new front derailleur or larger chainring? Or
could I have another problem, like too much slack in the chain
perhaps?

First issue - do the two chainsets have the same off-set, ie, each
sprocket
in the same place relative to the frame. If not, it may be a case of
a different bottom bracket axle required.

I would expect any triple front mechanism to have no difficulties over
the range of chainrings you quote.

If the chain is only scraping on the small cog when also on the
smallest rear cog, its not an issue; one shouldn't ride with the chain
at such a severe angle.

Hmm, I my bottom bracket has a 122mm axle, I think the new crankset is
110-113 mm though. Would this be a good explanation of my problem? It
is fairly clear that the old crankset goes further on the bottom
bracket and sits closer to the frame.

That's your problem. One way I've solved it in the past is to take the
rings off the new crankset and bolt them onto the cranks from the old
crankset, since that obviously sits in the right position and it's
almost always the rings, not the cranks, that are worn out. Depends on
the two sets having the same BCD/number of bolts, of course.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

Morning had broken. I found a rather battered tube of Araldite
resin in the bottom of the toolbag.