Help needed - Replacing my chainset

[Steve H]

I've had my Carrera Vulcan moutain bike for about 4 years. The middle chain ring is bent and the teeth on all rings are quite worn, so I've decided to fit a new chainset. Went into Halfords this afternoon and described my bike. Hey presto - out comes a replacement chainset. Have got it home, whipped off the old crankset, fitted the new crankset, all was looking good.

However, the front derailleur isn't shifting properly no matter how much I adjust it. It's getting the inner and middle ring fine, but it can't stretch out far enough to engage the big ring.

After much fettling, I started searching the internet for specs of the old and new crankset and notice that they have slightly different measurements for the chainline - 47.5mm for the original chainset and 50mm for the new one. This looks to be the route of the problem - they are 2.5mm further out and therefore the front derailleur can't reach the big ring.

Is this a scenario that can be fixed? Or do I need to take my new chainset back and find a different one that will have the same chainline?

Here are the specs of the chainsets if they help.

Old Chainset


New Chainset

 

[e-rider]

if you have adjusted the H stop screw on the front mech and it simply wont move far enough your only option is to buy a new BB with a shorter axle length

 

[MacB]

if you have adjusted the H stop screw on the front mech and it simply wont move far enough your only option is to buy a new BB with a shorter axle length

Sadly this is correct though another option could be a new front derailleur

 

[Steve H]

Thanks for the responses, although this feels frustrating that I can't replace a part on a like for like basis. Are you saying that no one manufactures chainsets with a 47.5mm chainline measurement now? Is this an 'old fashioned' size that has now been upgraded? I only recently replaced the front derailleur.

If it was a choice between upgrading a bottom bracket or the front derailleur, I'll probably go for the derailleur as I don't think I have the tools for replacing the bottom bracket. Any idea what kind of front derailleur will fit this new chainline - i.e. brand and model names? I've had a look around, but can't seem to see chainline as part of the specs for these.

 

[MacB]

Have you fully released the outer limit screw? I know you probably have but worth checking first. Then I'd check the chainset specs online and see what size of BB is recommended. Next find someone with the tools to remove the existing BB, assuming you'll find it's a different size to what you now require. New BB can be had for £10-15 and installation is very easy.

 

[Steve H]

Have you fully released the outer limit screw? I know you probably have but worth checking first. Then I'd check the chainset specs online and see what size of BB is recommended. Next find someone with the tools to remove the existing BB, assuming you'll find it's a different size to what you now require. New BB can be had for £10-15 and installation is very easy.

Yes - I've fully released the outer limit screw. Even tried twisting the front derailleur to get the back end to stick out further. Still not quite enough to engage the big ring.If I go with the BB that is recommended with the crankset in the spec, won't that still give me the wider chainline? I'm guessing I'd need to go for one that is about 5mm less in order to knock 2.5mm off the chainline?

 

[RecordAceFromNew]

Mmmmm I don't understand this.

Steve according to your links the old chainset delivers a 47.5mm chainline with a 110mm bb while the new delivers a 50mm chainline with a 113mm bb. I happen to know that the 113mm bb is asymmetric, and in particular it is 3mm longer than the 110mm one on the drive side - what that means is miraculously Halfrauds appears to have sold you the right replacement and that it SHOULD deliver the same chainline as the old with the old bb!

Of course that and for it to work depends on various assumptions:
1) the old is the same as that in the linked spec
2) the new is ....
3) the new was installed and torqued correctly
4) there is no other issue unrelated to chainline to cause it to fail
5) the square taper fairy is not staying with you
etc.

I know it is a pain but if I were in your position I would reinstall and actually measure both chainlines to see that they are as expected (i.e. the same AND around 47.5mm to 50mm). If they are not then for whatever reason (e.g. manufacturing fault) they are not the same as per spec and you should take the whole lot to Halfrauds and get them to give you the right chainset since they supplied both.

If on the other hand the chainlines as measured are actually quite close then I wonder if you had to move the front mech up a little given the new chainset is 44T vs 42T for the old, and/or the front mech was knocked. Either way Park has an excellent page on front mech set up which is worth following step by step.

If it helps I might be able to find out what chainline your front mech is designed to work with if you let me know its make and precise model number. It would also be helpful to know the bb spindle length.

If it is a SRAM 3.0 like the current Vulcan then I think it is designed to work with anything between 47.5mm to 51mm.

 

[MacB]

Yes - I've fully released the outer limit screw. Even tried twisting the front derailleur to get the back end to stick out further. Still not quite enough to engage the big ring.If I go with the BB that is recommended with the crankset in the spec, won't that still give me the wider chainline? I'm guessing I'd need to go for one that is about 5mm less in order to knock 2.5mm off the chainline?

sorry Steve, my error

 

[yello]

the new chainset is 44T vs 42T

Also worth checking the maximum swing/reach of the front mech. Mech's have a range; you've gone from 20t (42 outer - 22 inner) to 22t (44 - 22). This MAY exceed the capability of the mech you have.

 

[Steve H]

Mmmmm I don't understand this.

Steve according to your links the old chainset delivers a 47.5mm chainline with a 110mm bb while the new delivers a 50mm chainline with a 113mm bb. I happen to know that the 113mm bb is asymmetric, and in particular it is 3mm longer than the 110mm one on the drive side - what that means is miraculously Halfrauds appears to have sold you the right replacement and that it SHOULD deliver the same chainline as the old with the old bb!

Of course that and for it to work depends on various assumptions:
1) the old is the same as that in the linked spec
2) the new is ....
3) the new was installed and torqued correctly
4) there is no other issue unrelated to chainline to cause it to fail
5) the square taper fairy is not staying with you
etc.

I know it is a pain but if I were in your position I would reinstall and actually measure both chainlines to see that they are as expected (i.e. the same AND around 47.5mm to 50mm). If they are not then for whatever reason (e.g. manufacturing fault) they are not the same as per spec and you should take the whole lot to Halfrauds and get them to give you the right chainset since they supplied both.

If on the other hand the chainlines as measured are actually quite close then I wonder if you had to move the front mech up a little given the new chainset is 44T vs 42T for the old, and/or the front mech was knocked. Either way Park has an excellent page on front mech set up which is worth following step by step.

If it helps I might be able to find out what chainline your front mech is designed to work with if you let me know its make and precise model number. It would also be helpful to know the bb spindle length.

If it is a SRAM 3.0 like the current Vulcan then I think it is designed to work with anything between 47.5mm to 51mm.

Yes - the front derailleur is the SRAM 3.0. Thanks for attaching the spec sheet. I can see the chainline adjustment numbers. I also see that it has an upper limit of 42T. Since the new chainset has a 44T big ring, I wonder if this is part of the problem It also says the SRAM 3.0 has a total capacity of 20t and 22T granny up to the 44T big ring would be a capacity of 22T.

I'll have another play in the morning to ensure I have everything on properly with a fresh pair of eyes!

 

[Steve H]

Also worth checking the maximum swing/reach of the front mech. Mech's have a range; you've gone from 20t (42 outer - 22 inner) to 22t (44 - 22). This MAY exceed the capability of the mech you have.

Thanks Yello - yes I think our posts must have overlapped here. Think this may well be part (if not all) of the problem.

 

[RecordAceFromNew]

It also says the SRAM 3.0 has a total capacity of 20t and 22T granny up to the 44T big ring would be a capacity of 22T.

I'll have another play in the morning to ensure I have everything on properly with a fresh pair of eyes!

In case it distracts you the 22T breach of the official 20T front mech range limit is highly unlikely to be an issue - firstly when a chainset breaches the tooth range of a front mech severely the direct consequence is that the chain will drag on the tail of the front mech when it is on small front and back, which is an unnecessary and illogical combination anyway. Secondly in this case the difference is 2T, i.e. a difference of 4mm in clearance - since front mechs are designed to accommodate all sorts of seat tube angle, chainstay length and small sprocket size etc. which all have influence on such clash, chance is it won't happen. The other constraints of a front mech's specification are usually far more critical.