Author Topic: Counting the cogs  (Read 496 times)

fergusd

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Counting the cogs
« on: April 18, 2020, 07:43:55 AM »
Various sites e.g. Cycle Monkey and including Rohloff's own provide the 14 gear ratios to 3 d.p. as follows
  1     2     3     4     5     6     7     8     9     10    11    12    13    14
0.279 0.316 0.360 0.409 0.464 0.528 0.600 0.682 0.774 0.881 1.000 1.135 1.292 1.467

Can anybody point me to the actual cog:cog integer ratios that are engaging within the hub to get these numbers?
I tried fiddling about with a few e.g. Gear 14 = 44:30 - possibly; also noticed that gears 8,9,10 are respectively the reciprocals of gears 12,13,14 - at least, to 3 d.p.; finally wondered whether the internals might be sufficiently complex for any or all of the gears to involve not just two cogs n1:n2 but three n1:n2:n3; .. ..?  Anybody?

leftpoole

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Re: Counting the cogs
« Reply #1 on: April 18, 2020, 11:29:48 AM »
Hello,
I cannot point you to anything inside this hub.
Why on Earth is this interesting?
Well, it must be boredom or being something of a gear freak nerd?  :o :o :o :D
I hope someone other than Rohloff can enlighten because I am going to be worrying if we all should know these things......
Happy days,
John

fergusd

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Re: Counting the cogs
« Reply #2 on: April 18, 2020, 02:11:45 PM »
>> Why on Earth is this interesting?
Implication: it isn't? or shouldn't be?
Just from an engineering point of view. And given the column-miles of print on derailleur cogs and ratios, surprising that there's nothing. The Wikipedia entry on Rohloff is quite enlightening and explains not just the mutual relationships 8/14, 9/13, 10/12 pivoting around 11 as suggested above, but an identical symmetry 1/7, 2/6, 3/5 around 4. The gears are clearly stepped so that each load (effort) increase n to n+1 is as near as can be contrived identical for all n from 1 to 13.
For me. It would be nothing short of Completely Fascinating to know what cog numbers have been designed into the hub to achieve this remarkable ambition. Maybe this is freaky geeky but, crikey, every purchaser of a Thorn bike, and every single contributor to any of the Thorn forums is somewhere on a spectrum. As is all too evident. And all the better for that .. ..

PH

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Re: Counting the cogs
« Reply #3 on: April 18, 2020, 04:21:02 PM »
I'm not sure I've fully understood the question, the term cogs is possibly misleading, the hub being made up of sun and planetary gears.  There is no need to understand it, which is a good job as my grasp is limited. The Rohloff website has the detail of which gears/cogs are engaging in each selection, it is fascinating to watch, though it's done more to increase my appreciation than understanding

https://www.rohloff.de/en/experience/technology-in-detail/gear-steps-1-14/

martinf

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Re: Counting the cogs
« Reply #4 on: April 18, 2020, 09:19:55 PM »
As far as I understand it, the Rohloff is basically a seven-speed hub with an "underdrive" that reduces all seven of the topmost gears to give an identically (in percentage terms) spaced range of much lower gears.

As you say that gears 8,9,10 are respectively the reciprocals of gears 12,13,14, the "seven-speed" is almost certainly a simple one like the old SRAM Spectro 7 rather than a more complicated arrangement like the Shimano Nexus 7.

I did once have enough curiosity to count the teeth on the sun and planet gears (and the gear ring) during disassembly of a Sturmey Archer S5/2, but have since forgotten.

As far as taking hub gears to bits is concerned:

- Sturmey Archer and SRAM three-speeds are easy to completely dismantle. Sturmey Archer S5/2 is a little more complicated as it is necessary to take the timing into account on reassembly, but still relatively straightforward. I strip these hubs down every few years to thoroughly clean out the old oil and check that the internal parts are undamaged. Until recently (not sure if this is still true), it was possible to get any necessary spare part for a Sturmey Archer three or five speed hub relatively cheaply, not that I needed to very often.

- SRAM Spectro 7 doesn't come completely apart like a Sturmey Archer S5/2, for example the planet cage and planets are a modular unit, but is still relatively easy to strip down, clean, lubricate and reassemble so long as you don't try to dismantle the bits that aren't supposed to be taken apart.

- Shimano Nexus 7 and Nexus/Alfine 8 are more complicated, so on these I haven't yet tried to do anything more than removing the internal from the shell and wiping it clean, lubricating it, then reassembling it into the shell. I might have a try one day if/when I break one, but if that happens, given the prices of Shimano spare parts it probably won't be worth repairing, cheaper to replace the entire hub, or at least the internal.

- With Rohloff, I reckon the idea is that it is supposed to be reliable enough never to require being taken to bits by a home mechanic, and too complicated to be done without the factory tools if by bad luck it does break. So I am not curious enough to try and take one to bits to count the teeth on the internal parts.

PH

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Re: Counting the cogs
« Reply #5 on: April 19, 2020, 01:12:02 AM »
- With Rohloff, I reckon the idea is that it is supposed to be reliable enough never to require being taken to bits by a home mechanic, and too complicated to be done without the factory tools if by bad luck it does break. So I am not curious enough to try and take one to bits to count the teeth on the internal parts.
I'm nothing like brave or stupid enough to try, but stripping a Rohloff  doesn't seem to require any more tools than a pick and a magnet.  It's well designed and made, but not really that complex apart from the gear selector.  There's a few youtube videos of it being done, I think this is the clearest
https://www.youtube.com/watch?v=lS6kj8g5On8