What chainring and rear cog for Rohloff?

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Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: What chainring and rear cog for Rohloff?

Postby Brucey » 6 Jan 2021, 1:00am

steel chainrings are usually thinner than aluminium ones where the bolts go through and counterboring them is not only not required, it is arguably counterproductive, in the 'big ring' position anyway, especially if the chainring is to be reversible. Provided the bolts are the correct length then un-counterbored reversible chainrings are preferable to counterbored ones in that the clamping load of the bolt is better distributed below the bolt head.

cheers
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tommydog
Posts: 278
Joined: 11 Feb 2017, 6:48pm

Re: What chainring and rear cog for Rohloff?

Postby tommydog » 6 Jan 2021, 11:00am

Brucey wrote:steel chainrings are usually thinner than aluminium ones where the bolts go through and counterboring them is not only not required, it is arguably counterproductive, in the 'big ring' position anyway, especially if the chainring is to be reversible. Provided the bolts are the correct length then un-counterbored reversible chainrings are preferable to counterbored ones in that the clamping load of the bolt is better distributed below the bolt head.


Thanks for confirming this, I just thought that maybe it had some advantage other than ascetics, given that many manufactures seem to counterbore their chainrings.

bikepunx just sent me a drawing of one of their rings - commenting that "the teeth have a square profile like the newer Shimano 1x rings to help with chain retention" Below is the style of chainring that they do in 1/2" x 1/8"

Image


I have ordered the KMC BSRR00316 1/2" x 1/8" sprocket to go with this. Although I don't have a drawing for the KMC sprocket, the below picture on their website indicates that the profile of the teeth are different to that of the chainwheel

Image

In an optimum setup, should one always try and match the exact type of tooth profile for the chainwheel and rear sprocket, or from an engineering perspective does it not matter much as long as both are for 1/2" x 1/8" chain?

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: What chainring and rear cog for Rohloff?

Postby Brucey » 6 Jan 2021, 11:27am

it might not look it at first glance but those tooth profiles are functionally equivalent to one another. There are two obvious differences

1) the sprocket has pointed tooth tips and the chainring has truncated ones
2) the tooth flank shape on the sprocket looks different.

On the latter point, it only looks different. In fact the geometry of the tooth flanks is determined by the number of teeth on the sprocket. The lower half of the illustration below shows how the flank profile of neighbouring teeth is determined by a radius described from the roller centre in the neighbouring valley, as the chain disengages.

Image

A mirror image of this figure shows the same thing for chain engagement.

The consequence of this is that the fewer teeth there are, the more 'pointy' the tooth flanks must be. Pointed tooth tips on sprockets help engagement (the bottom run is slack when pedalling and is where many derailments originate when the chain is slack) however pointed tooth tips are not a smart idea on chainrings, in good part because they tend to make holes in people.

Anyway the tooth flank shapes on both chainring and sprocket are 100% conventional, i.e. exactly as an engineering text would recommend them to be, IMHO.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

tommydog
Posts: 278
Joined: 11 Feb 2017, 6:48pm

Re: What chainring and rear cog for Rohloff?

Postby tommydog » 6 Jan 2021, 2:51pm

Thanks Brucey, they are helpful diagrams. Out of interest are you an engineer?

Brucey
Posts: 42102
Joined: 4 Jan 2012, 6:25pm

Re: What chainring and rear cog for Rohloff?

Postby Brucey » 6 Jan 2021, 3:00pm

by training I am a materials scientist. [Never bothered filling out the forms for CEng.]

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~