Lower limit of a SRF3 hub

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D363
Posts: 75
Joined: 23 Aug 2014, 3:03pm
Location: Sheffield

Lower limit of a SRF3 hub

Postby D363 » 11 Jan 2018, 12:56pm

Currently I've got a bike set up with the above hub. To enable luggage carrying and use on Peak District hills the range is set up at roughly 36-48-64, but I've recently come across the efneo crank product which could potentially increase the range significantly.
I'd be looking to set up with a low gear of about 25" and can't see anything in the pdf from Sturmey Archer about a recommended lower limit on the crank / sprocket ratio, but gather they formerly stated no lower than 2:1. So, if it's not advisable I'll knock the idea on the head straight off. The frame is otherwise suitable and would be used as a tourer / commuter / moderate load lugger.
Any views from the mechanically inclined would be appreciated.

Brucey
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Re: Lower limit of a SRF3 hub

Postby Brucey » 11 Jan 2018, 1:03pm

I think the hub is strong enough to withstand a much lower gear ratio than that. Obviously it will benefit from improved lubrication in hard use.

However.... the consequences of a problem will almost certainly be worse, if you have one; under high torque conditions a slippage of any kind will be much more likely to break stuff. In an SRF 3 hub the low gear pawls will likely be the first things to fail; these parts can be bought for about £5, and the shrapnel arising won't get into the other parts of the workings (and cause collateral damage) if the repair is carried out promptly.

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

D363
Posts: 75
Joined: 23 Aug 2014, 3:03pm
Location: Sheffield

Re: Lower limit of a SRF3 hub

Postby D363 » 11 Jan 2018, 2:00pm

Thanks Brucey, helpful as ever.

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mjr
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Re: Lower limit of a SRF3 hub

Postby mjr » 11 Jan 2018, 3:16pm

Mmm, but how low would you dare to go? How low have you ever gone? I think I'm running only just over 2:1 which I suspect isn't anywhere near the limit.
MJR, mostly pedalling 3-speed roadsters. KL+West Norfolk BUG incl social easy rides http://www.klwnbug.co.uk
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Brucey
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Re: Lower limit of a SRF3 hub

Postby Brucey » 11 Jan 2018, 3:25pm

it has been reported elsewhere that in high torque tests the chain broke in a (mostly similar internally) CS-RF3 hub before the internals gave up. I believe it too.

However this is presumably with both low gear pawls already engaged; if only one engages (as happens occasionally), there will usually be a small slippage and the hub will sort itself out. If such a slippage occurs under high load, it is likely that the pawls may break; they are not intended to withstand shock loads of this sort.

Seasoned users of IGHs often 'light pedal' for a few turns after any gear shift, to make sure that the gear is properly and fully engaged, before mashing on the pedals. It is not a bad policy.

cheers
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mjr
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Re: Lower limit of a SRF3 hub

Postby mjr » 11 Jan 2018, 4:05pm

Brucey wrote:it has been reported elsewhere that in high torque tests the chain broke in a (mostly similar internally) CS-RF3 hub before the internals gave up. I believe it too.

:lol:

Brucey wrote:Seasoned users of IGHs often 'light pedal' for a few turns after any gear shift, to make sure that the gear is properly and fully engaged, before mashing on the pedals. It is not a bad policy.

I do it on all bikes, as I'm not a big fan of causing a huge clunk by slamming freewheel/freehub pawls into their slots on derailleur bikes either.
MJR, mostly pedalling 3-speed roadsters. KL+West Norfolk BUG incl social easy rides http://www.klwnbug.co.uk
All the above is CC-By-SA and no other implied copyright license to Cycle magazine.