Time for some high-geekery. Whilst climbing big hills, I noticed an occasional crunchy-slippiness coming from somewhere on the bike. A quick application of the magic chain gauge showed that the chain was indeed pretty worn, and the sprockets had gone a bit shark-tooth shaped. Since I’d already planned to get a new 22 tooth rear sprocket (for the hills), I augmented my shopping list with a new 20 tooth rear sprocket and a new chain and then set about trying to understand what the heck all these different kinds of chainrings were for, and which one I needed. Several hours later, and I think I now have a Clue – which I’m going to preserve here for Google and posterity.
I’ll use this chainring as an example, because it’s the one I’m going to buy. It’s a “Thorn 104mm PCD 4 arm reversible single chainring 3/32 inch 46teeth”. Which all meant nothing to me when I first read it. But now I can explain!
- Thorn is a product range made by SJS cycles.
- 104mm PCD means that if you draw a circle which passes through all of the mounting bolts, it’ll have a diameter of 104mm. PCD stands for ‘pitch circle diameter’. Some people say ‘BCD’, which means ‘bolt circle diameter’, but it means exactly the same thing. It can be kinda fiddly to measure this directly on your bike, so you can measure it indirectly by measuring the distance between the bolts and looking up a table like this one.
- 4 arm means that the chainring will have 4 mounting bolt, to connect to the four arm ‘spider’. The spider itself is part of the cranks. As a bonus, you can infer this from the PCD size. All 104mm PCD chainrings have 4 bolts, and all 110mm PCD chainrings have 5 bolts, etc.
- single means that this chainring has been specifically designed for bikes with a single chainring. If you have a derailleur bike with multiple chainrings, the chainrings will have ramps and pins to help lift the chain up onto the next sprocket when you are changing gears. Additionally, a few of the teeth will be short and stubby – again, to help shifting. Also, the teeth might be shaped specially to help shifting. When your bike only has one chainring, you don’t all this magic. The teeth can be much simpler (possibly stronger for it?). So that’s what a single chainring is promising – straightforward teeth with no fuss. As a bonus, they can be …
- reversible, which means that when the teeth get worn out, you can just take the chainring off, flip it over, and have a go at the other side of the teeth. Twice the lifetime!
- 3/32″ is the width of the chain it was designed for. On a derailleur bike, a narrower chain is desirable because the sprockets on the rear cassette can be closer together. But on a hub-geared bike, you don’t have that constraint. So typically you run a wider 1/8″ chain (= 4/32″) (presumably inspired by a belief that a wider chain means more contact area, therefore lower pressure on the links, therefore less friction and longer life). However, a 1/8″ chain will happily ride on a 3/32″ sprocket (it’s just a wee bit wider after all).
- 46 teeth is pretty obvious. More teeth on the front == harder to pedal.
Phew, the mystery of chainrings revealed! The only other dimension I came across was the kind of metal. Aluminium alloy is lighter than steel, but will probably wear out faster. From what I saw, large chainrings are often made of alloy and smaller ones are made from steel.