I had a SA 3 speed 177% model on my Brompton with a Schram 850 chain, 102 links since I had a 54 tooth chain ring and a 14 tooth cog and it worked fine, even when I manually shifted it to a 38 tooth chain ring. I decided to give a try to just running my 1 speed which has a 12 tooth cog. Tried to use a new 100 link chain, but it would skip when I would go up anything more than a 5% grade. In fact I used a new Schram 850, and a KMC Z51, but the skip occurred with both of them. The 38 tooth chain ring was worse than the 54 tooth. In desperation I put the old Schram 850 100 link chain I had used previously, before I set up the SA 3 speed. This chain was so worn that when I put it next to either of the new 100 link chains it was a full link longer. The old worn out chain works just fine which I find perplexing. I can also go back to my 3 speed setup but I have been enjoying the more simple setup for a little change of pace. Do any of you wiser folks have some explanation of this phenomenon? the only thing i can come up with is the two new chains are just a tiny bit narrower and bind up under the stress of steeper grades. thanks, Ron

Couple of things. Could the 12 speed cog be 1/8th instead of 3/16s that your chain is? That would cause a problem.

Second, chains, cogs and chainrings wear out together. The wear pattern is like a jigsaw puzzle and they fit together nicely. Replace one of those three and problems can occur. This is why you tend to replace at least cogs and chain at the same time.

Third it is SRAM not Schram.

Others will chime in with suggestions. Hope you get it figured out.

Blakcloud you were absolutely right. A new cog was all it took. Thanks, Ron

there are 3 spline cogs (AW3/BSR) listed in both 3/32" (5~8 speed derailleur) & 1/8" (4_32nds)

I bought spares from the Mk3 bike's Sachs made 3 by 2, they are 3/32" as well ..

I use the 15t with my 54t chainring ..


Now the current (Mk4) BWR in the new 3 by 2 uses an 9 spline cog ..

Shimano used that one in a BMX hub, 1st, its not quite the cassette spline,

which to orient the multiple cogs, has a wide and a narrow spline ..