dabac

Hi all,
A friend of mine has a bike where the derailer gear rear wheel has been swapped for a nexus 8 while retaining the 48T chainring.
Now, with increasing age (and weight!) he’d like to lower the gearing.
The current sprocket is a 22T, and I’m struggling to find any usefully bigger.
However, there is a 27T sprocket for an Inter-5 hub that looks quite similar.
It seems to have 6 tabs where the Nexus has 3 to engage the hub.
Does anyone know if it’d be as simple as grinding off the extra tabs to get an Inter-5 sprocket to fit a Nexus-8 hub?
Or if there is a source for an about 27T Nexus-compatible sprocket?

The chainring is not replaceable. And apart from cost, due to BB constraints, fitting another crankset can become complicated.

FiftySix

I looked around, seems like finding a 27T is indeed difficult.

"Nexus and Alfine hubs use the same 3-spline sprockets as other internal gear hubs from Sturmey-Archer and SRAM/Sachs, also used on most coaster-brake hubs." - from https://sheldonbrown.com/nexus-mech.html

FiftySix

BTW, the only Inter-5 photos I see on the net show a 3 tab sprocket. Can you share a link to this 6 tab sprocket?

tomtomtom123

Internal gear hubs have a max torque/gear ratio recommendation. I don't know what it would be for your hub but the inter8 manual says:
TO ENSURE SAFETY
For Installation to the Bicycle, and Maintenance:
• The cassette joint should only be used with sprockets with 16T to 23T.
• The gear ratio of the front chainring to the rear is about 2.1-to-1.
Example) For 26 inch wheels
Front 36T 38T 46T
Rear 16T 18T 22T

FiftySix

My Nexus Inter 8/7/5 manual has the same info.

In my case, my Norco (with 700 wheels and Nexus 8) came with 42 front / 20 rear, but I changed it to 42 / 21 last fall. So far, so good.

DiegoFrogs

I had a nexus 7 speed coaster brake hub, and I recall it being geared exactly at 2:1 (42/21, I think). I read later somewhere that they recommended a minimum 2:1 ratio.

It seemed like most people were riding these types of bikes at about a 30 RPM cadence, so perhaps there were a lot of high torque failures.

FiftySix

Looks like the 23T is about as large a sprocket as Shimano sells for these hubs.

gilesa

Sturmey-Archer sell a 25-tooth, which Sheldon Brown says will fit. But the 27 seems an inexpensive part, so why not try it and report back? With the 48-tooth chainwheel and OP's description of declining legs, over-torque problems seem unlikely.

dabac

https://www.ebay.de/itm/SHIMANO-Ritz...YAAOSws9Ze14Si
I’ve got others, but this is the design.

dabac

I’m aware of that. But barring further rebuilding, he has a 48T chainring. So one step up on the sprocket will not put him hugely out-of-bounds.

FiftySix

I'd order it, grind off the three of the tabs and see if it fits.

Let us know how it goes.

dabac

I am sorely tempted to suggest that as a course of action.
If it’d been my time and my money, I’d have done it w/o a second thought.

FiftySix

Someone else's equipment and money, yep it's hard to recommend something that could cause troubles. Especially considering the replacement cost of a Nexus 8.

FiftySix

That 2.1:1 minimum. So Shimano is saying don't go into 2.2:1 and above or don't go into 2.0:1 and below?

dabac

And he’s gotten a taste of this already.
He’s using a roller brake, and wouldn’t take my word for how important it is to have a proper strap for the reaction arm. So he ”secured” it with a zip tie...which tore, spun the hub and thrashed the connect for the wire already during his test ride around the block.

Racing Dan

A 48/24 combination would make for final ratio of 1:1.05 with the alfine 8 hub. is that not low enough? If not you need another hub or just ignore the manual and hope for the best.

FiftySix

Yeeouch.

FiftySix

Found this little tidbit of information, which is completely logical. Why it's so vague in IGH info manuals is beyond me.

"Some internal-gear hubs are not rated for the stresses from extremely steep climbs and a heavy load, a potential problem with do-it-yourself hybrid gearing. Usually, manufacturers rate hubs in terms of the acceptable chainwheel/sprocket ratio, but this rating really amounts only to "we will make it so hard to pedal that you will get off and walk." Pushing a lower gear up the same hill actually stresses the hub less. A small rear wheel and smooth pedaling reduce stress on the hub."

from - https://sheldonbrown.com/internal-gears.html

fietsbob

Alter the other end, Fit a Smaller chainring.


Or Get a bike with smaller wheels... I have 2 Rohloff bikes with a 16t cog
the chainring on the 26" wheel bike is 38t , the chainring on the 20" wheel (Bike Friday) is 53t
the gear range is about the same ..

There is less strain on the internal gears going faster on the level ground , than there is going up hill .. because of Gravity..





'/,

ElCani

Reviving an old thread, but just to let everyone know: this doesn’t work. The Nexus C7000 27T sprocket has a much larger diameter central hole, so will not mount on a normal Nexus or Alfine hub. I went to the trouble of buying one and even ground off three of the tabs before discovering this!