Cycling UK Forum

I have inherited a Tern folding bike. It has a 7 speed (hub) shimano nexus fitted. The problem is that the gearing is far too low - I've only used 5,6 and 7 ever. Now the normal solution would be to fit a smaller sprocket, but a quick look online seems to indicate that the smallest available is 16T (the one on it has 17T so this will make little difference).
Question is there any other make that would fit.
Otherwise it is the chainring and this is a 5-bolt 44T, but it appears that the BCD is 142mm - a very strange size.
Any ideas?

uwidavid wrote: ↑21 Jul 2021, 12:49pmOtherwise it is the chainring and this is a 5-bolt 44T, but it appears that the BCD is 142mm - a very strange size.

Yes... what's the distance between two adjacent bolts, please?

Jonathan

IIRC Sturmey Archer sprockets use a similar mounting and might fit ... something for the OP to investigate further. A quick look on eBay produced SA sprockets down to 13 teeth.

SJS sell some sprockets for Brompton’s and IIRC some of them have SA type hubs.

Edit. Sheldon has a ref chart for chainwheels : https://www.sheldonbrown.com/cribsheet-bcd.html

I'd double check that BCD, could it be 144 5Bolt? That used on the track.

The distance between adjacent bolts* is 83.5 mm and between non-adjacent 135 mm.
Changing the sprocket would be preferred (I could get away with not changing the chain). The shimano nexus sprockets seem (from pictures online) to have 3 rounded protrusions which other makes of sprocket do not have.

*I say bolts but they are actually screws.

Sheldon Brown says SA and SRAM sprockets fit but on a Nexus/Alfine you can't use smaller than 16T due to interference, so it seems you're out of luck
https://www.sheldonbrown.com/nexus-mech ... er%20chain.

Do you live in the fens? 44 x 17 seems about right to me, is that about a 30" bottom gear? I need that even for undulating urban riding.

I live in East Anglia so yes it is flat.
As I said the bike is a folder so has rather small wheels hence the resulting low gearing.
I do also favour slow rpm and high power for pedaling (maybe not fashionable these days, but that's what I am comfortable with).

Very strange BCD.

Do you think the crankset is original? Screws instead of bolts I’ve only seen in low end stuff, I wouldn’t have thought Tern used non-standard stuff like that. Online seem to show current ones have a 130 5Bolt BCD.

Not particularly helpful, but it looks like there is no easy solutions.

I thought something was a bit strange. Turns out the screws only hold on a ring that, I presume, is to stop your trousers getting caught in the chain (without using cycle clips). So the pedal end is all one unit. Oh well!

Well it took a long time, but I've finally sorted it out.
Got a 58T chainset (replacing the 44T) and it feels much better.
Had a chance to look at the sprocket and I can't see why smaller than 16T are not made - there seems plenty of space.
Ended up getting the parts from China as local parts would have cost more than the bike is worth.

Shimano Nexus, Sturmey-Archer, SRAM (Fitchel&Sachs) and KMC three-lobe IGH sprockets are 'almost' universally interchangeable. What they were thinking to allow this consumer-friendly situation versus every manufacturer having their own, proprietary design is beyond me.

These sprockets are available in 13T to 30T sizes.

IIRC Nexus hubs with their inside-the-dropouts shifting can't fit smaller than 16T without interfering with the rotary shifter, or something like that.

Sid Aluminium wrote: ↑4 Oct 2021, 4:29pm Shimano Nexus, Sturmey-Archer, SRAM (Fitchel&Sachs) and KMC three-lobe IGH sprockets are 'almost' universally interchangeable. What they were thinking to allow this consumer-friendly situation versus every manufacturer having their own, proprietary design is beyond me.

These sprockets are available in 13T to 30T sizes.

IIRC Nexus hubs with their inside-the-dropouts shifting can't fit smaller than 16T without interfering with the rotary shifter, or something like that.

I’d be glad to know where the larger sizes are available from, SRAM did a 24 but it’s hard to find now and Nexus go to 23. Where could I get a 26, 28 or 30 ?