Shimano chainwheel

[One Louder]

Hi.

I recently purchase a fairly old Raleigh Pioneer and i want to change the chainwheel (think that's what it's called) so I bought a shimano nexave from ebay. I should have really done a bit more research before purchase, as this is a oktalink and will require a bit more work and a new BB to fit. Also, on the box, it says it is "SPCM Power Charge Mechanism Compatible", which i think is some sort of gear system which my bike doesnt have, it just has those old style SIS gears with thumbshifts that were popular during the 90's.

Can anyone tell me if this will work with my current gears? Or should i cut my loses and get another set?

thanks in advance

 

[PpPete]

Never hear of SPCM Power Charge
Is the spacing between the chainwheels same as your old one? If so it should work just fine.

 

[RecordAceFromNew]

Can you provide the model number of your new/old chainsets? Do you have existing bottom bracket's spec?

There are a few issues involved when updating the chainset. Firstly there is bottom bracket compatibility, and secondly the chainline has to be roughly right, and more importantly that there is adequate clearance between the chain rings/crank arms and chain stays. It can get a little tricky not least because some "standards" are not standard.

SPCM is Shimano's "new" system to enable button/electronically operated shifting (e.g. there is a magnet in the chainset).

 

[One Louder]

think I'm a bit out of my depth here...the new one has got Shimano SG-X 9S 46-34 on the side. Is that the model number? It also has FC-C900 34x36 G 170 on the side of the box.The old one has Ofmega Sierra wrote on it, but i can't find anything that could be a model number.

 

[RecordAceFromNew]

Before saying anything else I must say you should take any of my suggestions below with a big lump of salt.

The only reasonably authoritive comment on Ofmega chainset I have found in a quick search is from here, and it says:

Ofmega (Avocet)
Ofmega cranks, including those branded "Avocet" use a non-standard spindle, thinner than either J.I.S. or ISO spindles. As far as I know, nothing else works.

If that is true, and if you wish to change your crankset, the bb should probably be replaced (irrespective of whether you are using the one you bought) unless a) somehow you found another equally non-standard Ofmega chainset, or b) if you want to use a new square-tapered chainset and make your own shim.

Regarding whether you should/could keep the new FC-C900, Shimano's spec suggests that it should be used with BB-C900, an octalink V2 bb with a spindle length of 112mm for an spcm system and to result in an unusual chainline of 52.5mm for 68mm shell width for Shimano (when most if not all other Shimano mtb systems have design chainlines of 47.5mm/50mm for 68mm/73mm bb shell width respectively).

The question here, however, is what is the reasonable chainline for your bike? You might want to look at this if you wish to work it out.

Once you have worked it out, then you should be able to deduce what bb spindle length delivers it with your new chainset. Octalink bb's are widely available. Your challenge is to find the right one that fits your bb shell (English thread most likely, but is it 68mm/73mm?) and delivers the chainline you want with the chainset.

One other thing - if you change your chainset, you might have to move your front mech up/down the seat tube, and/or change your front/rear mech, and/or shorten/lengthen your chain if the number of chain rings and/or their number of teeth are different. Again further info is needed.

Good luck!

 

[One Louder]

Before saying anything else I must say you should take any of my suggestions below with a big lump of salt.

The only reasonably authoritive comment on Ofmega chainset I have found in a quick search is from here, and it says:

Ofmega (Avocet)
Ofmega cranks, including those branded "Avocet" use a non-standard spindle, thinner than either J.I.S. or ISO spindles. As far as I know, nothing else works.

If that is true, and if you wish to change your crankset, the bb should probably be replaced (irrespective of whether you are using the one you bought) unless a) somehow you found another equally non-standard Ofmega chainset, or b) if you want to use a new square-tapered chainset and make your own shim.

Regarding whether you should/could keep the new FC-C900, Shimano's spec suggests that it should be used with BB-C900, an octalink V2 bb with a spindle length of 112mm for an spcm system and to result in an unusual chainline of 52.5mm for 68mm shell width for Shimano (when most if not all other Shimano mtb systems have design chainlines of 47.5mm/50mm for 68mm/73mm bb shell width respectively).

The question here, however, is what is the reasonable chainline for your bike? You might want to look at this if you wish to work it out.

Once you have worked it out, then you should be able to deduce what bb spindle length delivers it with your new chainset. Octalink bb's are widely available. Your challenge is to find the right one that fits your bb shell (English thread most likely, but is it 68mm/73mm?) and delivers the chainline you want with the chainset.

One other thing - if you change your chainset, you might have to move your front mech up/down the seat tube, and/or change your front/rear mech, and/or shorten/lengthen your chain if the number of chain rings and/or their number of teeth are different. Again further info is needed.

Good luck!

Ok, thanks for the info RaceAce. A lot to take in for a noob like me.

I measured the bb shell with a ruler, and it looks like its a 68mm.

I counted the teeth on my old cog and they are the same amount as the new.

I bought the Shimano FC-900 off ebay and the seller said it is best suited to a 113mm bb.

I'm worried to start this job myself incase i can't finished it. :S

*EDIT*

Don't know if it helps, but I Just measured the current chainline, and it looks about 48mm.

 

[RecordAceFromNew]

According to Shimano’s spec, we know that the special 121mm (I mistyped it as 112mm in my earlier post) spindle BB-C900 your new chainset is designed to use delivers a 52.5mm chainline for a 68mm shell. Without specific dimensions of that bb and your chainset I can not be certain what spindle length will deliver your existing chainline and be clash free. The relevant page of Shimano’s compatibility document is attached below, and there is a possibility that a standard, commonly available Octalink v2 bb with spindle length of 113mm will do the job, but we are working beyond specified, matched, components here, so I can't be certain. However, it only costs a fiver to find out at the moment.


What I do know however is fitting an Octalink v2 crank to a v1 bb (or vice versa) will ruin the crank for good, so please make sure they are of the same version.<br style=""> <br style="">

Extracting crank/bb and installing new ones require special tools. You might want to see if there are willing experienced CC'ers near you to lend you a hand if you want to do it yourself, or just let a LBS to get it done.

As I said before, changing crankset/bb can be a tricky exercise.

 

[One Louder]

I've bought a Shimano ES25 Bottom Bracket Octalink - 68-118mm (in a previous post i had mistakenly said that the ebay seller had told me the crank was best with a 113mm, he never, he said 118mm).


Does this sound like it might work?

 

[RecordAceFromNew]

If its matched 121mm bb delivers a 52.5mm chainline with your new chainset, then a 118mm bb will likely deliver more than the 48mm existing you have measured. How much more depends on the symmetry or otherwise of these two bbs' spindles, but is likely to be around 2mm and 3.5mm, which is why I guesstimated and suggested the 113mm one in my earlier post.

However, given that for a geared bike what the best chainline is in practice is not an exact science, if your front mech's outer limit screw adjustment can accommodate the extra reach, what you have will probably still work.

 

[One Louder]

If its matched 121mm bb delivers a 52.5mm chainline with your new chainset, then a 118mm bb will likely deliver more than the 48mm existing you have measured. How much more depends on the symmetry or otherwise of these two bbs' spindles, but is likely to be around 2mm and 3.5mm, which is why I guesstimated and suggested the 113mm one in my earlier post.

However, given that for a geared bike what the best chainline is in practice is not an exact science, if your front mech's outer limit screw adjustment can accommodate the extra reach, what you have will probably still work.

I have the 113mm BB too (I mistakenly ordered both 118 and 113)

I suppose the next step would be to try 'em? *nervous*

 

[PpPete]

it can't to put them in - see which once gives you the better chainline.