Old front derailleur + new crankset problem



I

Ian B.

Guest
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?

Thanks for any help,
Ian
 
Ian B. wrote:
> 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 wrote:
> Ian B. wrote:
>
>>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. wrote:
> Nigel Cliffe wrote:
>> Ian B. wrote:


>>> 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?


>> 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.


> 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 wrote:

> 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 protected]>,
Ian B. ('[email protected]') wrote:

> Nigel Cliffe wrote:
>> Ian B. wrote:
>>
>>>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 protected] (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.
 

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