Single Speed Freewheel and Fixed Sprocket Compatibility Issues

I have a track/single speed bike which I bought new recently do I need 3/32” or 1/8” cogs?

You need 1/8" almost certainly (I'd say 98% chance), as this is the usual standard for factory built single speed bikes.

I have a 16T cog, if I fit the 18T will I need a longer chain? There's a few mm spare in the rear wheel slot to move the wheel forward, but it's not much

Yes, you’ll probably need to add a link or two. I would be wary of doing this if the chain you have is well worn. In such a case it’s probably better to just replace it as the differential could cause the chain to skip around and/or wear the sprockets in an unhelpful way (this is also a reason that people often change all three – chain and cogs – at once, but I’m not saying you have to do this)

Is it feasible to run an 18T freewheel and 17T fixed cog on my wheel? That would give me a good combination of ride options.

This will probably be fine. It's basically a roll of the dice.

The issue is that the slightly greater circumference of the 18T cog will use up more chain than the 17T. So say you set the bike up to run the 17T cog and it turns out that the only way to do this is if the wheel is right at the front of the dropouts. Then when you flip over to the 18T cog it'll use up some chain and pull the wheel out (i.e. you won't have enough to get the wheel in).

Conversely if you set it up initially with the 18T and it's right at the back of the dropouts, then when you put the 17T in it will loosen the chain too much.

With the usual roughly 25mm long dropout I guess there's about a 1 in 6 chance that you'll have the problem. Which would be meaning you had to alter the chain length every time you swap cog!

It's probably worth a shot therefore.

I’d like to convert my old Raleigh Yellow Banana to a flip flop single speed. I think I’ve got a 52 tooth chainring and use the 15 tooth sprocket, but having looked at your website this may be too high gearing.  What would be typical based on my chainring?

A 20T would be equivalent to the usual off the peg bikes you get which are 42/16 and which of course are a kind of slightly low gear because they're for anyone who wants to cycle.

 Given that you’ve been riding quite  high gearing already you'll probably want one of these:

 19 - 4% higher than the above

18 - 10% higher

17 - 17% higher

 I reckon 19 or 18 unless you like going at about 22mph everywhere (which is possible of course).

You’ve sent me a lock ring which is smaller than the sprocket and doesn’t fit my hub

A fixed gear hub has two different sections on it, of different diameters and the sprocket and lock ring sizes correspond to this.

It sounds like maybe you have a standard threaded hub which will not allow you to put a proper reverse threaded lock ring on it.

We have more information on the three main different types of hub here.

But please note that if you you do not fit a proper sprocket and lock ring fitted to a fixed gear hub the sprocket will not be fixed on securely enough for some riding styles and you definitely need to have two hand brakes – front and rear – on the bike

Can I have a 14 or 15T freewheel?

No, not for one of our wheels. There are some BMX hubs which have a threaded section which is only 30mm wide and onto that you can squeeze tiny freewheels.

But on a standard hub for a non BMX bike the diameter is more like 35mm and you can’t fit a freewheel cog and mechanism of fewer than 16T - even though you can fit fixed sprockets down to at least 12T onto them.

Will your freewheels fit on my BMX?

Hi, well that depends… There are two kinds of BMX thread which we know of.

One has a small 30mm metric diameter section – sometimes it'll have a tiny freewheel on it, say 13T. We do not sell freewheels for this.

The other is 1.375" in diameter which is 34.925mm. These are what we have, and the minimum number of teeth on them is 16.