One of the most common questions about BMX bikes that I hear is "I want a smaller sprocket, what should I buy?"
The first question you must ask yourself is: why do I want a small sprocket so badly? The answer probably has a lot to do with peer pressure. Peer pressure is not a phenomenon relegated to kids, we are all subject to social conformity. So when one peruses a magazine and see that every single rider in there has a sprocket that is no larger than a compact disc, one assumes that a tiny sprocket it necessary to riding bmx.The truth is that most bmx riders have small sprockets because they can. Smaller sprockets are lighter and stronger due to their size, and who does not want a lighter and stronger bike? They also make certain grinds and riding ramps a little easier because they are out of the way. Back in the dinosaur days of 16t freewheels and 45t sprockets, we old dudes bashed a lot of chains on ledges, driving us to insanities such as the KMC 415 garage-door type chain and sprockets with 3/16" teeth. (Normal road bike sprockets are known to have a 3/32" chain and bmx/single-speed bikes have 1/8" chains) Also popular were the dinner plate-esque full sprocket guards.
If you think you need a tiny sprocket, give it up. You don't. If your bike is too heavy, do some pushups. Take a dump before you go out riding. If your big ol' sprocket hits the coping when you drop in, unweight the back end of your bike as you go over. That's how Mirra and Hoffman did it for many years.
When you are ready and have the money to upgrade to a smaller drivetrain for the reasons above, there are a few things to consider: component compatibility, gear ratio, and durability.
Component compatibility: Edit
This is the most complicated part of upgrading any part on a bicycle. Thankfully, compatibility issues on a bmx bike are not nearly as complicated as components on a road or mountain bike. Here as some things to assess:
What kind of hub do I have now?
- If you have a standard freewheel hub, your choices are very limited. A standard bmx freewheel hub works by accepting a thread-on freewheel of various sizes 16-tooth and larger. The hub itself has no drive mechanism inside it but instead the freewheel has a series of ratcheting pawls and teeth which allow it engage when rolling forward and to coast. To maintain a rideable gear ratio (which I will get into in a bit) with a 16t freewheel, you need a 43-45t sprocket or chainring.
- If you have a flipflop hub, each side of the hub will have a threaded protrusion for a thread-on freewheel, but one side is larger than the other. The large side of a bmx flipflop hub accepts 16t and larger freewheels, while the smaller size accepts 13-15t freewheels. Odyssey and KHE sell 13t freewheels that are fairly reliable. It should be noted that DK made a 12t freewheel, but quickly dropped the product after it failed many riders.
- If you want that cool micro-gear ratio setup that all the cool kids have, you need a bmx cassette hub. A cassette hub (not the same as a "cassette" as roadies/mtbrs see it) has a built-in ratcheting drive mechanism similar to that of a freewheel. Because the ratcheting mechanism is inside the hub, it allows for smaller sprockets. a bmx cassette hub has a driver, which engages the drive mechanism inside the hub and is home to a cog of some sort. some drivers have separate cogs that can be changed out (this is what is referred to as a "two-piece driver"), while other drivers have cog teeth built onto the driver, known as a "one-piece driver." Two-piece drivers typically don't go smaller than 12t, while a one-piece driver can be as small as 8t.
Gear ratio Edit
Now that you have an idea what components are available, you need to take into consideration the size of your sprocket relative to the size of your driver. A typical gear ratio for a 20" bmx bike is 2.75:1. This means that the front sprocket is 2.75 times larger than the rear sprocket, or that the rear wheel will make 2.75 rotation for every full rotation of the cranks. The standard for bmx bikes for many years was 44/16. Mathematically, 16 times 2.75 is 44 and 44 divided by 2.75 is 16. So if you want a smaller sprocket you need a combination that is mathematically similar to the 44/16 combo. If you ride a combo that is much higher than that, the bike will be difficult to accelerate but will be easy to maintain a high top speed. With a ratio much lower than 2.75, the bike will accelerate quickly but soon "top out" and you will be spinning like a hamster and going nowhere. Some people prefer a non-standard ratio and that is fine, just don't end up riding a difficult gear ratio by arming yourself with knowledge before buying parts.
If all that math sounds complex, let's simplify. Stick with the common combos that most riders trust. Common gear combos that are close to the standard ratio are: 43-45/16 39/14 36/13 33/12 30/11 28/10 25/9 22-23/8
As these numbers get smaller, a change in a single tooth becomes more profound. In other words, if you go from a 43/16 to a 44/16, you won't feel as much of a difference as if you changed from 28/10 to 29/10. Also, a change in the number of teeth on the rear wheel makes a bigger difference than a change in the number of teeth on the sprocket. This is clearly illustrated if you do the arithmetic but I will spare you the calculations for now.
You will be limited to a 44/16 ratio if you have a regular freewheel hub. The smallest freewheel for this kind of hub is a 16t, so putting a smaller sprocket on your bike will lower your ratio. If you want to flail wildly like and fool and go nowhere in hamster gear, be my guest. But for the love of Pete (whoever that is), don't get a sprocket smaller than 36t if you have a 16t freewheel.
If you have a flipflop hub, your options are a little less limited. You can run a 13t freewheel (Odyssey and KHE make these) and a 36t sprocket and have the same ratio as the old 44/16 setup.
A bmx cassette hub gives you a widest array of gear ratio options. Many cassette hubs have the option of a two-piece driver with 12t and larger drivers, or a one-piece driver with 11 teeth and as low as 9t on most hubs. A few hubs go as small as 8t. A tiny one-piece driver used to be an after-market upgrade to your hub, but most modern cassette hubs and freecoaster hubs (more on those another day) come standard with a tiny one-piece driver.
Durability: Is the smallest combo always the best? Edit
I will answer that with a resounding NO! There are advantages to a tiny gear combo: it's lighter due to a smaller sprocket and shorter chain, it creates more clearance under your bike for dropping in on ramps and grinding, and the sprockets are generally stronger due to their small size. However, smaller drivers and sprockets will wear out faster. When a chain is wrapped around a sprocket, it basically engages half of the teeth on the sprocket. So on a 44/16 ratio, the front sprocket engages the chain on 22 teeth and the rear engages 8 teeth. you can mash down on that all you want and it's going to take a long time to wear out the teeth on those cogs, assuming they are quality components.
Now picture a 22/8 combo. The chain is engaging only 11 teeth on the front and 4 teeth on the rear. Mash down on that and you are putting the same amount of force as with a larger combo, but each tooth is taking more force per tooth. The result is prematurely worn teeth and a more quickly-stretching chain. If you don't mind frequently replacing your driver, sprocket, and chain, this is not an issue. If you want the peace of mind of a drivetrain that will last a while longer, compromise between a small combo and one that has more teeth. I run a 28/10 combo on my own bike and that is the lowest I feel safe.
A note about tiny drivers and bearings: most hubs with 9t drivers use tiny sealed cartridge bearings that are known to blow out more quickly than larger drivers. It seems that 9 teeth and 10t is the cut-off point where a smaller bearing must be used due to size restrictions. Anything 10t and larger probably uses a larger bearing that is not so prone to exploding when you sneeze at it. However, many companies are dealing with this issue by putting needle bearings that handle the pressure of a tiny drivetrain better than standard cartridge bearings. When buying a tiny driver, be aware of the bearings in the driver you you will be stuck with a frequently broken driver, constantly shelling out money and labor replacing them.
That is it for my compendium on bmx hubs and gear ratios. Please let me know if you have any suggestions, corrections, etc.