from Rookie’s keyboard,
Hello, friends
Today, I saw five BMX bikes “parked” near the bike shop where I work. They looked awesome, even though the teenagers who owned them appeared to be on the hooligan side (screaming, smoking, teasing each other, etc.).
But let’s not get carried away. This is a bicycle site, not one analyzing societal tendencies. Besides, bikes don’t choose their owners.
So, inspired by my observations, I decided to write a second daily post with a focus on BMX.
I will present some basic qualities of BMX sprockets that many rookies might not be aware of.
First, you have to know that freestyle BMX bikes (the most common model) have very small front sprockets in comparison to road and even MTBs.
This isn’t an accident or an “aesthetic choice.” The purpose is faster acceleration and increased frame-to-ground clearance. Another benefit is the lower chance of shin hits.
Gear Ratio
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.
In other words, one full rotation of the chainring, equals a 2.9 turns of the rear cog and respectively the rear wheel.
BMX bikes used to have larger chainrings but also larger rear cogs. 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 or a 2.75:1 ratio.
Whilst a more modern BMX could have a 25T chainring and 9T rear cog. The gear ratio is 2.77:1.
Both gear combinations offer the same top speed, but the smaller chainring has the following technical advantages over the big one.
- Clearance
Freestyle BMXs are thrown around and come in contact with ledges, rails…etc. 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/obstacles.
- Lower Chance of Shin Hits
Larger chainrings massively increase the chances of getting hit in the shins. Those hits are very painful and could leave an unsightly wound on your shins.
- Strength
All things being equal (material, build quality…etc.), smaller sprockets are stronger and can take more stress on impact without deformation thanks to their shorter radius.
- Lighter
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 they are built for strength. But if the goal is to come up with the lightest possible setup, a micro drivetrain will certainly help.
- Acceleration
By lowering the gearing via a smaller chainring, it becomes easier to accelerate. And that’s beneficial because BMXs are often used in tight spots.
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.
- Lower Joint Stress
Lower gearing lowers 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.
Some Downsides
- Chain wear
“Smaller” drivetrains put more stress on the chain because fewer chain links are in contact with the chainring and rear cog.
- Sprocket Wear
In general, 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.
Note: Race BMX models naturally use a larger chainring. 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 usually rely on sprockets with more than 28 teeth.
Until next time,
Rookie
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