BMX Crank & Chainring Size Guide for NZ Racers
A practical guide to getting your gear ratio right — whether you're racing Minis or Pros.
Getting your gearing wrong is one of the most common (and easily fixable) mistakes BMX racers make. Too heavy a gear and you'll struggle out of the gate; too light and you'll spin out before the first straight. Here's everything you need to know about choosing the right chainring, cog, and crank length for your class and riding style.
What is a Gear Ratio and Why Does It Matter?
In BMX racing, your gear ratio is determined by two things:
- Front chainring (the big sprocket at your cranks)
- Rear cog (the small sprocket on your back wheel)
The formula used to measure gearing is called Gear Inches:
Gear Inches = (Chainring ÷ Rear Cog) × Wheel Diameter
A higher gear inch number means a harder, faster gear — great for longer tracks with big straights. A lower number means easier pedalling — better for tight, technical tracks or younger riders still building leg strength.
The other measurement you'll often see is Roll Out — this is how far your bike travels with one full pedal rotation. It's calculated as:
Roll Out = (Chainring ÷ Rear Cog) × (Wheel Diameter × π)
The Classic BMX Setup: 44/16
For decades, the standard BMX race gearing has been 44 tooth chainring / 16 tooth cog on a 20" wheel — giving approximately 55 gear inches. It became the benchmark because it just works — it's fast enough off the gate without being unmanageable for most riders.
Most elite-level NZ racers still run setups close to 55 gear inches, though many now fine-tune based on track type and personal preference.
Recommended Gear Ratios by Age & Class
Here are practical starting points for NZ BMX racing classes. These assume a standard 20" wheel unless noted.
Tip: These are starting points — riders with strong leg power or who race longer tracks may prefer a slightly larger chainring. Lighter, faster tracks may suit dropping down a tooth or two.
How to Fine-Tune Your Gearing
Track Type Matters
- Short, tight track with lots of turns → Go one tooth smaller on the chainring. You want to be able to spin up quickly out of each berm.
- Long, fast track with big straights → Go one or two teeth larger. You want to hold your top speed longer.
Gate Start vs. Top Speed
- Explosive gate starter? → A slightly smaller chainring helps you spin up faster out of the gate.
- Grinder who builds speed through the track? → A larger chainring rewards riders who can maintain power over longer sections.
Crank Length — Don't Overlook This
Chainring size isn't the only factor. Crank arm length affects your power output and pedalling style too. Here's a rough guide:
Shorter cranks = faster spin (better for spin-heavy riders and tighter tracks). Longer cranks = more leverage (better for powerful, strength-based riders on longer tracks).
Common Gear Setups Used by NZ Riders
What About Half-Tooth Gearing?
Some advanced racers use decimal gearing — chainrings with half-tooth increments (like 44.5t) — to fine-tune their setup more precisely. This is popular at elite level where even fractions of a gear inch can make a difference on a fast track.
Need Help Choosing?
Not sure what gearing is right for your rider? At Trackstars BMX, we stock a wide range of chainrings, cranks, and rear cogs for all classes — from Mini right through to Pro. We're happy to help you work out the best setup for your track and riding style.
📩 Contact us here or browse our BMX parts range.
References: Gear inch calculations based on the BMX Ultra Gear Calculator. Crank and gearing recommendations are general guides — always consult with an experienced coach or mechanic for personalised advice.