Condensed Answer: Modern freestyle BMX bikes have a smaller front sprocket for faster acceleration and increased frame-to-ground clearance. The smaller sprocket reduces the chances of shin hits too.
The term gear ratio describes the relation between the front chainring and rear cog. For example, if the front chainring has 32 teeth and the rear cog has 11, the gear ratio is 32:11 = 2.9:1.
This means that for each full rotation of the chainring, the rear cog and respectively the rear wheel would turn 2.9 times.
In the past, BMX bikes had larger chainrings but also larger rear cogs. Thus, the actual gear ratio was similar to what it is today.
For example, an old-school BMX bike might have а 44T chainring and a 16T rear cog. This combination provides a 2.75:1 ratio.
Meanwhile, a more modern BMX could have a 25T chainring and 9T rear cog. The gear ratio is 2.77:1.
In other words, both gear combinations offer the same top speed, but the smaller chainring has technical advantages over the big one.
The Advantages of Smaller Sprockets
Freestyle BMX bikes are thrown around and come in contact with ledges, rails…etc. Consequently, there’s a great chance to hit a bicycle component while performing a trick.
A small front ring and rear sprocket are more compact and offer greater clearance between the bike’s frame and the surroundings. As a result, the bike is less likely to “hang” on an obstacle.
- Lower Chance of Shin Hits
Larger chainrings increase the chances of getting hit in the shins. Those hits are quite painful and could leave an unsightly wound on your shins.
The smaller the sprocket is, the more difficult it is to bend it because the shorter radius reduces the leverage and thus the effort that one can exert onto the chainring.
All things being equal (material, build quality…etc.), smaller sprockets are stronger and can take more stress on impact without deformation.
A drivetrain that consists of a small chainring and rear cog saves weight because each unit needs less material for its construction. Additionally, the small chainring requires a shorter chain which also contributes to the lighter weight of the bicycle.
Weight isn’t a big factor in BMX because most bikes are built for strength. But if the goal is to come up with the lightest possible setup, a micro drivetrain will certainly help.
- Easier Acceleration
By lowering the gearing via a smaller chainring, the user makes it easier to accelerate at the expense of the top speed.
If the rear cog is also replaced with a smaller one, then this doesn’t take effect because the smaller cog increases the gearing once again.
Since freestyle BMX bikes are not used for racing, it’s not crucial to have a high top speed. Also, most spots offer very little room for acceleration. In that scenario, a smaller sprocket would be advantageous.
- Lower Joint Stress
Low gears reduce the stress on the joints because it takes less effort to rotate the drivetrain.
If you install a small sprocket while keeping the rear cog the same, the lower gearing will make the bike more joint-friendly.
The Downsides of Smaller Sprockets
- Chain wear
Smaller drivetrains put more stress on the chain because fewer chain links are in contact with the chainring and rear cog. As a result, there’s more stress on the area and the links wear down faster.
- Sprocket Wear
Smaller sprockets have a shorter lifespan because the stress that comes from the chain is spread over fewer teeth. Consequently, the teeth get thinner faster.
- Lower Top Speed
If the front ring is replaced with a smaller one while the rear cog is kept the same, the potential top speed of the bicycle will drop down.
FAQ: Why do race BMX bikes use larger sprockets?
As already mentioned, larger sprockets increase the potential top speed of the bicycle because each revolution of the chainring equals more spins of the rear wheel. For that reason, race BMXs use sprockets with more than 28 teeth.
2. No Tricks
Race BMX bikes are used on the race track rather than for street tricks. Thus, the extra clearance that a smaller sprocket provides is not terribly important.