Help - I'm confused about cranks

[AlexRa]

Hello all,
Building up a Rohloff bike. Long time Rohloff-er but fairly new to bike building.

Can someone help me with some info on crank compatibility? I've entered a worm hole of info about chain lines, chain sizes, and chain rings - and got confused. 

Here's what I've got. My Bottom Bracket takes a 24 mm spindle. I've got a Rohloff hub on the back. So I need to know what crankset is going to be compatible and give me the right chain line, and have a chain ring that is compatible with the 8 / 9 speed chain that Rohloff requires.

I'm sure this is quite obvious to more experience folk, but everything I find online seems to be wrong in some way. Am I being an idiot? Is this actually simpler than I think?

Any help much appreciated.
Thanks, Alex

[mickeg]

Chainline on a Rohloff, I assume this is on a frame with conventional 135mm dropout spacing, not an extra wide hub for a fat bike.

I am not sure the numbers, so I will only talk concepts.

The Rohloff has two different splined sprocket carriers available, one is slim and one is normal.  Thus, there are two options you have for chainline, depending on which you use.

You said 24mm bottom bracket spindle.  I am guessing that is 124mm square taper bottom bracket.

Cranksets are generally much more expensive than bottom bracket spindles, so it might be more affordable to pick a crankset you like and then buy the bottom bracket that has the spindle length you need.  Finding a crankset that works with your bottom bracket might cost more in the end.

It is easy to buy cranksets for double chainrings.  If you get one of those, you can use the chainring in the outer position, or you can use the chainring on the inner position and use a bashguard in the outer position.  I have my chainring on the inner position, bashguard on outer position, photo attached.

Make sure your bottom bracket works with the frame you have, for example my Rohloff is on a frame that has a 73mm bottom bracket shell, not the more common 68mm.  Thus, I had to shop for a 73mm bottom bracket with the spindle length I needed. 

Rohloff hubs work with 8 speed chains, but if you have a 9 or 10 speed crankset, that will work with an 8 speed chain.  I have built up several bikes, both derailleur and Rohloff.  And a couple have 10 speed cranksets and 8 speed chains.  On a Rohloff, no problem with that at all.  (That is a minor incompatibility if a front derailleur is used with an 8 speed chain and 10 speed crank, but that is not your situation.)

I am running a chainline that is about 5mm off of ideal on my Rohloff bike.  This is by choice.  I wanted my Rohloff bike to have the same Q factor (pedal width spacing) that my derailleur bikes have, thus I wanted a bottom bracket spindle that was 10mm narrower than I was supposed to have for the ideal Rohloff hub chainline.  I could have used a spacer to push the bottom bracket off center, but chose not to, that would have reduced my chainline error.

That 5mm chainline error is not a problem at all.  I do have greater wear on one side of my sprocket teeth than the other side, likely the same with chainring, but it is not a bid deal.  When you consider that it does not really matter what gear a derailleur bike is in, they will always have some chainline error, I decided not to worry about a 5mm error when I built up my Rohloff bike a decade ago.

I know I did not specifically give you an answer on what to buy, but instead laid out the concepts.  Good luck with your build.

[AlexRa]

Thanks so much for this. Super helpful and very clear.

For various complicated reasons my BB choices are quite constrained. To achieve chain tension I will be using this:

https://www.sjscycles.co.uk/bottom-brackets/trickstuff-excentriker/

Basically in need a way to achieve chain tensioning.
Given I’m already committed to a certain BB, that accepts a specific spindle size, I need to find chainset that fits the BB, rather than the other way around.

[martinf]

Unlike MickeG I prefer to have the chainline more or less spot on.

I'm still on square-taper cranks, and have a fair number of spare cranks and different length bottom bracket units to play with in order to get the chainline roughly right.

If necessary I then fine-tune with spacer washers to about 1 or 2 mm precision (using either the eyeball method or a builder's straight edge). This is easier to do before fitting the chain.

With most of the bottom bracket units I have I can fit 1 to 2 mm of spacer under the fixed cup to space the chainline outwards.

On most of the cranks I have it is possible to use washers or spacers between crank and chainring to change the chainline up to about 4 mm max.

Either outwards if the ring is outboard of the crank or inwards if the ring is inboard. This generally means using chainring bolts meant for double or triple chainring setups rather than single rings.

The process is much easier with a Thorn frame. On these frames no need for spacers - I can slide the eccentric sideways to fine tune the chainline.

[B cereus]

That eccentric BB is a bit of a game changer. It precludes using square taper axles and commits you to using a chainset designed for external bearings.

The chainline for Rohloff hubs (135mm O.L.D.) varies between 54mm for screw on sprockets and 57mm for the wider of the two splined sprocket carriers (the Narrower sprocket carrier has a chain line of 55mm)  Whether by accident or design these chainlines  coincide roughly with the outer ring position of a mtb triple chainset. Using just the outer ring position of a mtb triple chainset should give a chainline of around 54mm. 

In practice absolute accuracy is not essential; for the average length of chain stay a deviation of 5mm in chainline will result in an error of around ½°.

[JohnR]

IIRC the chainline of the outer ring on an MTB triple crankset is about 55mm. If you are getting the EBB from SJS then seek their advice as I'm sure they have the answer.

[PH]

Any MTB crankset will be fine, the outer positions on doubles and triples is almost identical. You can always push it out another mm or two with the washers that come with the hub.
I like mine perfect, with ST cranks, chainring washers and BB washers and an EBB that can be run a bit off center it's always possible.  A bit of a faff, but do it once and always replace like with like and it on;y needs doing once.  I've a feeling Rohloff have relaxed their attitude to chainline, I recall them making a bigger thing about it's importance. 

[mickeg]

Can't help with that.  All the bikes I built up have square taper.  My only non square taper crank is on my road bike that I bought as a complete bike, I have not had any reason to remove it yet.

[B cereus]

Have you purchased the Trickstuff Exzentriker?  I’ve not seen one in the flesh but it looks as though it comes with the bearings pre-installed in the eccentrics. External bearings seem to be more susceptible to water ingress than the internal bearings of conventional square taper cartridges; I trust that the bearings are both well sealed and easily replaceable.

Given also that the bearing housing is designed to thread into a standard BSA BB (1.37in X 24TPI), and the male threaded inserts will further reduce this diameter, there cannot be a great deal of adjustment to move the 24mm crank spindle backwards or forwards. Presumably you will need to use half links, or am I missing something.

It also seems that the drive side and non drive side eccentrics are independently adjustable. How do you ensure that the two bearings are concentic and the crank spindle is perpendicular to the vertical plane of the bicycle? 

All in all, it look to me to be a solution to a problem that could be solved more easily and more cheaply by other means. Is there any reason why you cannot use a chain tensioner?