Hubs and Gearing

There are always a lot of questions on forums and message boards about the different kinds of hubs and about gearing ratios. Here I will try to explain the different kinds of hubs and bmx gearing ratios.

Freewheel Hubs: These hubs are what come on basically every complete bike out there. The hub itself is basically the same as a front hub. There is an axle and bearings which runs inside an aluminum shell with flanges where the spokes attach. On the side of the hub there is an extra part that sticks out that is threaded. The freewheel has the ratcheting mechanism inside it and it just tightens onto the hub like a nut on a bolt. The freewheel acts just like a socket wrench. when you coast or pedal backwards the freewheel clicks. When you pedal forwards the chain tries to tighten the freewheel onto the hub, the freewheel stops up against the hub flange, it can't tighten any further so it turns the wheel.

The size of the threads and the direction of the threads on the hub determine what freewheels you can run on that hub. Some hubs have threads on both sides so that you have some options. Large size threads are for 16 tooth and larger freewheels. Right thread for right side and left thread for left side drive. If you want to run left side drive you need to run a LSD or southpaw freewheel. Small size threads are for 15t, 14t and now fit the odyssey 13t freewheels. The small threads also come in right hand thread or left hand thread depending on what side you want to run your drivetrain on. When looking for a new hub pay attention to the descriptions:

Regular Hub: Large threads for 16t and above only.

LSD Hub: Usually large threads on both sides, one side for 16t right side drive and one side for 16t left side drive.

Flip Flop: One side has large threads for 16t and the other side has small threads for 14t/13t. Both right side drive.

Hoffman/DK dual drive: Small threads on both sides so you can run 14t/13t freewheels in either right side drive or left side drive.


A freewheel and a freewheel style hub. The freewheel threads onto the hub shell.

Cassette Hubs: Basically a cassette hub is a hub with the freewheel mechanism built in. The two benifits of this system are that your drive train will run smoother with a tight chain and that you can run a smaller gear in the back. Both of these benifits are due to the fact that the driver or cog is basically sitting on a bearing on the axle. In a freewheel you have the outer ring of teeth that mesh with the chain, then inside that is the ratchet teeth, then there are some loose ball bearings and usually 2 pawls in the center. (Pawls are little spring loaded flaps that catch on the ratchet when you pedal forwards and flop down out of the way when you pedal backwards or coast.) The freewheel pawls are mounted on an inner ring that threads onto the hub, then there is the part of the hub with the threads on it, then some bearings and then the axle.

As you can tell there is a lot of junk all in a big pile on a freewheel hub and because you have to fit that stuff all in there you can only go so small before you run out of room. On a cassette hub things are side by side instead of stacked up. On the outside is the cog part of the driver which is running on a sealed bearing. You can bolt different size cogs onto the driver, from 16 down to 13 or get a one piece driver with the cog built in. That way the cog can never come lose, it's part of the driver, and you can get smaller sizes from 13 down to a 9. To the side of the driver, beside the cog, inside the hub, is the part that holds the 3 or 4 pawls. On the inside of the hub shell is the ratchet that the pawls bite into. So on a cassette hub the driver is running directly on the axle on a sealed bearing so even a tight chain won't bind it up and the pawls and ratchet that make the cassette work are over to the side, out of the way, inside of the hub. This is what allows for the smaller gearings.


On a cassette hub the driver sits on a bearing outside the hub while the ratchet and pawl mechanism is inside the hub shell.

Freewheel vs. Cassette: The only downside of the cassette system is if you grind on the drive side and bend your cassette axle. Because the driver is running directly on the axle a bent axle will cause the driver to become misaligned with the hub shell it's trying to lock into. A cassette running on a bent axle will skip or just make a bunch of nasty grinding noises as it tears itself apart. To repair your cassette or replace the axle you need to take the hub completely apart. On a freewheel hub the freewheel is threaded onto the hub shell so even a badly bent axle will make no difference. You will still be able to pedal. Unfortunatly no high quality freewheels are readily available, you can rebuild them but they are so cheap most people just buy a new one. BMX cassette hubs are meant to be taken apart and rebuilt when they break and freewheels are meant to be removed from the hub, thrown away and replaced. If you want tiny gearing, don't mind working on your hub once in a while and mainly ride dirt or park a cassette may be the way to go. If you ride mostly street, grind a lot, icepick stuff or otherwise bend axles all the time stick with the freewheel hub, especially if you run 4 pegs or grind on your drive side.

Freecoaster Hubs: A freecoaster hub is basically a modified version of a coaster brake hub or backpedal brake hub without the brake. People started using coaster brakes for freestyle because the mechanism used to drive you forward disengages completely from the inside of the hub while you are coasting. This allows you to coast backwards without the pedals backpedalling like they would with a freewheel or cassette hub. Not having your cranks move while you go backwards makes fakie tricks a lot easier and is almost a necessity for some flatland tricks where you don't want the cranks backpedalling and hitting you in the shins or something.

These hubs use either a 16t cog that is held onto the driver with a lockring or you can get 12t cogs that are welded on so they can't get loose. The end of the driver that is inside of the hub slides over the axle and has a big coarse thread on one end. The clutch cone is also inside the hub shell and it is held over to the left by a spring. When you pedal forward the driver threads into the clutch cone, dragging it over to the right and into contact with part of the inside of the hub shell. When they come in contact the clutch cone drives the hub and wheel forward. When you stop pedaling the spring helps push the clutch cone back over to the left and away from where it came in contact with the hub shell. When the clutch cone is to the left you can coast forwards, backwards, whatever you want, until you pedal forward again.

As you can tell there are a lot of moving parts inside a freecoaster and they all need to be kept well greased. Freecoasters are also unsealed due to their design adding more maintenence. Also if they get out of adjustment and the distance between the clutch cone and the driver becomes too great it can take up to a full crank to get the clutch to engage. How much you have to pedal before the clutch engages with the hub is called the 'slack'. When the slack gets too large it can be annoying, making you want to rip your hub apart even more. freecoasters are fun but only if you like working on your bike.


The Freecoaster hub shell is large and hollow in order to fit the clutch, spring and driver inside.

Gearing Ratios: With so many possible freewheels and cog sizes you can get in the back there are also just as many sprocket sizes for the front. All are made to keep the basic bmx gearing ratio of 2.75 no matter what hub you run in the back or how small your sprocket. Look at any pro's bike and no matter what sprocket they are running in the front they are also running the matching cog in the back, all to keep their gearing ratio at or near 2.75. Some pros like to go fast, for example Joe Rich used to always run a 45x16 ratio, slightly higher than normal, and recently he ran a 40x14, also slightly higher than normal.

Here's a quick chart so you can see how all the ratios compare. Basically they all feel nearly the same no matter what sprocket you are running, as long as you run the right cog to match.

(sprocket / freewheel or cog = ratio)

Regular Freewheels:
45 / 16 = 2.81
44 / 16 = 2.75
43 / 16 = 2.68

Small Freewheels:
40 / 14 = 2.85
39 / 14 = 2.78
36 / 13 = 2.76

Cassette Hub Cogs:
36 / 13 = 2.76
33 / 12 = 2.75
30 / 11 = 2.72
27 / 10 = 2.70
24 / 9 = 2.77

Half Links: BMX bikes don't have deraileurs or anything like that to pick up the slack in our chains so we are pretty much stuck with pulling the wheel back in the dropouts until the chain is tight enough. If you want to move the position of your wheel you need to remove or add a full link of chain. Chain links are made up of inside links and outside links. One inside link and one outside link together are 1" long, we call this a full link. The chain is wrapped around so half of the chain is on the bottom and half is on the top, if you take out one full link it takes 1" out of the length of the chain but only moves your wheel forward 1/2".

But what if you only want to move you wheel forward 1/4", maybe any further forward and the axle will hit the front of the dropout slot. When people ran 44t sprockets they could just switch to a 45t sprocket. That extra tooth would pull the wheel up 1/4". A 45t sprocket with a 16t freewheel makes it a little harder to pedal than a 44 / 16 but it's not that big of a difference. The problem is when you go down to smaller gears one tooth makes a bigger and bigger difference. You might not want to pedal that hard and you probably won't be able to find the right size sprocket anyway. Tthis is where the half-link comes in. A half link has a special link that is one half outside link and one half inside link. With this you can keep your gear ratio, shorten your chain by 1/2" and pull your wheel forward 1/4".


Removing 1 and a half regular links and replacing it with this half-link will shorten your chainstay length by 1/4".