Bottom Bracket

[obelix1664]

Hi,

I currently have a square taper bb size 68 x 110.

Now I want to change this for an octalink bb but I am not sure which size I will need or could use. I have seen several on fleabay, but these are either 68x113 or 68x121.... So which if either would be suitable if not could you suggest a suitable alternative....

This is the chainset that I will be using;
SHIMANO 105 TRIPLE CHAINSET.5505
Ta, in advance..

 

[RecordAceFromNew]

According to Shimano's spec the bb you need is 118.5mm for shell width of 68mm, delivering a chainline of 45mm.

To be sure it works the front mech needs to be suitable and has appropriate reach. If you are unsure it might be worth telling us whether you are on a triple with the same tooth range already, and if the existing chainline is the same.

 

[obelix1664]

Thanks for the help Record, I forgot to add that it was a triple with a tooth range of 22-32-44 and the one I am replacing it with is a triple with 30-42-52. Not too sure if the front mech will reach the outside (biggest) ring. Any recommendations?

 

[frank9755]

Thanks for the help Record, I forgot to add that it was a triple with a tooth range of 22-32-44 and the one I am replacing it with is a triple with 30-42-52. Not too sure if the front mech will reach the outside (biggest) ring. Any recommendations?

What is your current front mech and what shifters are you using?

 

[obelix1664]

Hi frank, its a Shimano C050 front derailleur with SRAM X-5 9V speed shifters, indexed on both sides. Hope this helps.

 

[RecordAceFromNew]

Are you switching from a mtb chainset to a road one? You did not provide a reason, but there are a couple of issues you might want to be aware of:
1) In term of differences, the 105 chainset is 52/42/30, while the existing is 44/32/22. The existing chainline is probably 47.5mm (you should check and confirm by measuring bike centreline to middle front ring), while the manufacturer recommended 118.5mm bb for the 105 chainset delivers a chainline of 45mm.
2) The C050 front mech is designed to work with a 42/34/24 chainset, with the standard 122.5mm bb delivering a 47.5mm chainline.
3) Despite mismatch in the shape of the front mech (a ~42T curve on a 52T large ring) and chainline (47.5mm vs 45mm), it might still work, but there is also the possibility it doesn't. You can check whether the difference in chainline is an issue by seeing if the inner limit screw and frame allow the front mech to move another 2.5mm inward from the current small ring position.
4) If chainline difference is not an issue, then whether the front shift is acceptable can only be checked by trying - you will need to move the front mech up the seat tube until it clears the 52T ring by 1mm to 2mm by following this.
5) If the front mech is unable to move further in by 2.5mm due to clash with the frame or limit screw bottoming out, we will have to explore options to increase chainline. If the shift remains unacceptable following proper front mech adjustment, a different mtb front mech might work better, although afaik the largest optimal front ring Shimano mtb front mechs designed for is 48T.

The 105 chainset will need a longer chain.

 

[frank9755]

I don't think that chainset will work with your front shifter. It will pull too much cable. You need a non-indexed one.

More detail here under 'Front Indexing'

 

[RecordAceFromNew]

I don't think that chainset will work with your front shifter. It will pull too much cable. You need a non-indexed one.

More detail here under 'Front Indexing'

Unless I have misunderstood what the OP said he has sram x5 front shifter and shimano C050 front mech currently, both have shimano mtb characteristics in terms of cable pull, so imho he shouldn't be experiencing any issue regarding indexing (and which he has not raised either).

The current challenge is therefore to make them work with a road (105) chainset, hence potential issues regarding chainline and front mech/chainring profile.

 

[obelix1664]

Thankyou both very much for your help and the pointers to the two pages that you both included.

The problem that I seem to be experiencing with the new set-up is that the front mech just doesn't seem to have the reach to allow the chain to meet the outer (biggest) chain ring.

I did ring my LBS today and explained my predicament, they said that they have a front mech which will allow me to use the road chainset with my current indexing. So I'll give it a try and see what happens and let you both know how I get on.

However I did find this from Sheldon Brown.

 

[RecordAceFromNew]

However I did find this from Sheldon Brown.

Good point! According to Shimano here it seems the R443 front mech might be even better in your case.

Good luck!

 

[frank9755]

I don't think that chainset will work with your front shifter. It will pull too much cable. You need a non-indexed one.

More detail here under 'Front Indexing'

I've copied the relevant bit:

Specifically: mountain-bike shifters pull 19.5mm of cable across a triple, whereas road equipment does the same with only 13.5mm. So a road shifter pulls only 69% of the cable needed to index a mountain-bike front mech, and a mountain-bike shifter pulls 44% too much to index a road mech.

 

[RecordAceFromNew]

I've copied the relevant bit:

Specifically: mountain-bike shifters pull 19.5mm of cable across a triple, whereas road equipment does the same with only 13.5mm. So a road shifter pulls only 69% of the cable needed to index a mountain-bike front mech, and a mountain-bike shifter pulls 44% too much to index a road mech.

Am not sure why this is relevant to the OP. It says pairing Shimano or Shimano compatible mtb/flatbar shifter with road/drops front mech or vice versa will not work well for a triple chainset and which I agree, but the OP has and all discussion has been about Shimano compatible mtb/flatbar shifter and Shimano mtb/flatbar front mech.

 

[obelix1664]

Here's a strange one on top of the front mech issue. I went to the LBS and he sold me the necessary front mech (shimano). When I arrive home sit down have a cup of tea, then begin. I split the chain, then just for the sake of it see if the original front mech whilst still connected to the shifter will reach all three cogs on the chainset and it did.

So what would be the reason for it not reaching the outer ring when the chain is attached?

I subsequently attached the new front mech and again with no chain, perfect all 3 cogs no problem. I have however got to go back to the LBS to purchase a new chain, because the new one that I bought about a month ago seems to have vanished from the shed!!

What would cause the front mech not to reach the furthest cog when the chain is attached?

 

[RecordAceFromNew]

The answer to your questions is in bold.

Are you switching from a mtb chainset to a road one? You did not provide a reason, but there are a couple of issues you might want to be aware of:
1) In term of differences, the 105 chainset is 52/42/30, while the existing is 44/32/22. The existing chainline is probably 47.5mm (you should check and confirm by measuring bike centreline to middle front ring), while the manufacturer recommended 118.5mm bb for the 105 chainset delivers a chainline of 45mm.
2) The C050 front mech is designed to work with a 42/34/24 chainset, with the standard 122.5mm bb delivering a 47.5mm chainline.
3) Despite mismatch in the shape of the front mech (a ~42T curve on a 52T large ring) and chainline (47.5mm vs 45mm), it might still work, but there is also the possibility it doesn't. You can check whether the difference in chainline is an issue by seeing if the inner limit screw and frame allow the front mech to move another 2.5mm inward from the current small ring position.
4) If chainline difference is not an issue, then whether the front shift is acceptable can only be checked by trying - you will need to move the front mech up the seat tube until it clears the 52T ring by 1mm to 2mm by following this.
5) If the front mech is unable to move further in by 2.5mm due to clash with the frame or limit screw bottoming out, we will have to explore options to increase chainline. If the shift remains unacceptable following proper front mech adjustment, a different mtb front mech might work better, although afaik the largest optimal front ring Shimano mtb front mechs designed for is 48T.

The 105 chainset will need a longer chain.

Road chainsets and mtb chainsets typically have different chainlines. However nobody could say categorially what your existing/old chainline is without it being measured, or whether the existing/old mtb mech definitely can traverse onto all 3 rings of the new road chainset. The different chainlines as designed is a reason why sometimes mtb mechs can't handle the inner road rings (see this e.g.), and conversely road mechs can't handle the mtb chainset outer rings perfectly, hence as indicated checks were suggested to you and could have been made before any purchase.

If you did get a R443 however, not only is it designed to work with road chainline, it would also have the suitable profile to match a 52T outer ring, which should enable sweeter shifting with your 105 chainset.

Hope that clarifies.

 

[obelix1664]

RAF thankyou yes it does clarify.

I have measured the chainline and its 50mm. I did use the 118 x 68 BB as recommended. Would it be worth fitting a 113 x 68 thereby reducing the chainline to 45?