Which BCD?

[Phileas]

I need to buy a Hollowtech II compatible crankset for a single speed project.

I seem to have choices with regard to BCD i.e. 110 or 104 and also 4 or 5 arm.

Since this is for commuting, I expect to replace the chainring occasionally. So, from cost saving, future-proofing and convenience perspectives, can anyone suggest which options might be better?

Also I will need 44t chainrings to suit my Hebe Chainglider.

[Brucey]

shimano FC-S501 is a reasonable choice. You will need to accept a 45T chainring or replace it though. There is also a chance that a chainglider will foul on the chainset, even once the chainguard is removed, because the ring is mounted on the inner position on the spider. You will have above average Q as well.

There is much, much, more choice in singlespeed chainsets if you stick with a square taper BB. 130BCD is the default choice; a big stack of spare chainrings in another BCD would be a good reason for looking elsewhere though.

BTW chainsets in a smaller BCD tend to be for single-ring MTBs, and may have a chainline of ~50mm. This makes them problematic for use with typical singlespeed equipment.

One reason for choosing square taper is that you can usually adjust the chainline by fitting a different BB. Choices with HT-II are more limited. The alfine chainset offers a chainline that will work with nexus and alfine hubs.

cheers

[Phileas]

Thanks Brucey.

I realise I will have to buy a 44t ring separately, but I mentioned it because it might be relevant to my query.

I also know a traditional square taper gives many more options but it has to be Hollowtech because I'll be using a frame with a vertical dropout in conjunction with a Trickstuff Exzentriker bottom bracket.

My main concern is probably that I don't want to get a crankset which is cheap just because it is soon to be obsolete and uses a BCD/arm configuration which is going out of fashion.

[Brucey]

you need to get your chainline right, otherwise the whole thing is a non-starter. What hub/chainline are you planning to use?

BTW surly do 130BCD stainless steel chainrings, which are very durable.

cheers

[Phileas]

Well, my current single-speed has an XT rear hub with spacers so I can put the chain line where I like.

I realise this all seems a bit silly but I'm indulging a whim. It's going to cost me a lot but I don't want it to cost more than it needs to.

I can get a Shimano Zee M640/645 for £64. It has 104bcd 4 arm. If this configuration is liable to be popular I might go that way.

[Brucey]

I see. It is just that singlespeed chainlines run from ~42mm (or less) right up to about 54mm (for a freewheel on a 135mm SS disc hub). Sounds like you are going to be OK for now but there might be knock-ons in the future if you start changing stuff.

I saw the Zee chainset and it looks like good value on special offer. Then again the alfine chainset is only a few quid more from some retailers (and they crop up on e-bay used, too).

Personally I'd be swayed by a lower Q value (if that is possible).... but I am a bit fussy like that....

cheers

[Phileas]

Brucey

That Alfine set could be perfect because I already have an unused 130bcd 44t ring.

Thanks

Edit. It's actually a 42t ring I need and have a spare - not the first time I've got that wrong.