BMX Crank Length & Gearing Guide for Kids & Racers

When you're a BMX parent, it doesn't take long before you start hearing conversations about cranks, chainrings, cogs, rollout and gearing.

When Stu and I first started getting into the finer details of bike setup, it was easy to think there must be a simple answer:

"What crank length should they be on?"

or

"What gearing should an eight-year-old run?"

The more we've learnt, the more we've realised there isn't one magic number.

Two children of the same age can have completely different leg lengths, strength, cadence and riding ability. They might also be on different wheel sizes, different tyres and completely different bikes.

So for us, the starting point is no longer:

What is everyone else running?

It's:

Does this setup actually suit our rider?

For younger riders especially, we want a bike they can accelerate, pedal smoothly and control confidently.

The goal isn't to give them the biggest gear they can physically push.

It's to give them the gear they can actually use.

Start With Crank Length

Before we start worrying about chainrings and rear cogs, we'd get the crank length right.

Crank length affects how the rider's legs move through each pedal stroke.

Longer cranks provide more leverage, but the rider's feet also have to travel around a larger circle every revolution.

Shorter cranks reduce that circle, which can help smaller riders build cadence and accelerate more naturally.

Supercross makes an important point here: crank length has to fit the rider's leg first. Putting a child on oversized cranks simply because they'll eventually grow into them can compromise their position and pedal stroke.

That's very much how we look at it.

Fit the child they are now, not the child they'll be next season.

How to Measure a Young BMX Rider for Cranks

For smaller BMX racers, we prefer using inseam measurement rather than age or overall height.

The Helium Racing crank-length method was developed specifically for Micro Mini, Mini, Junior and Expert BMX riders under 152 cm tall.

Have your rider stand straight, with their riding shoes on and their feet roughly shoulder-width apart.

Use a book, clipboard or framing square and bring it firmly up into the crotch. Measure from the floor to the top edge.

Helium's calculation is:

Inseam in inches ÷ 0.173 = approximate crank length in millimetres

What About Older and Bigger Riders?

Once riders move out of the smaller youth sizing range, crank choice becomes increasingly influenced by preference and riding style.

Lengths around 165–180 mm are common in larger BMX race setups.

The trade-off is fairly simple.

A longer crank gives more leverage.

A shorter crank gives the rider's feet a smaller circle to travel and generally favours faster cadence.

So again, there isn't one crank length that every Pro or adult rider should use.

Fit comes first.

Once the Cranks Are Right, Look at Gearing

BMX gearing is created by two parts:

Front chainring ÷ rear cog

For example:

44 ÷ 16 = 2.75

That means the rear wheel turns 2.75 times for every complete revolution of the cranks.

But this is where things get a little more interesting.

The chainring and cog don't determine the complete gear on their own.

The rear wheel and tyre matter too.

What Are Gear Inches?

Gear inches are a way of comparing different gearing combinations.

The basic formula is:

Gear Inches = Gear Ratio × Effective Rear Wheel Diameter

So on a nominal 20" wheel:

44 ÷ 16 × 20 = 55 gear inches

That's where the well-known BMX "55-inch gear" comes from.

What Is Rollout?

Rollout tells us how far the bike actually travels with one complete revolution of the cranks.

Rollout = Gear Ratio × Rear Wheel Circumference

This is particularly useful because tyres aren't all exactly the same outside diameter.

A narrow youth race tyre and a larger Pro tyre can produce different rollout even when both bikes have exactly the same chainring and rear cog.

Changing only the tyre size can change rollout by several percent, while BMX Ultra also notes that tyre wear, pressure and rim width can alter the final measurement.

That's why we wouldn't recommend gearing based purely on age or the number of teeth on the front chainring.

So What About 44/16?

This is an important one.

44/16 is the traditional BMX race reference for a Pro-size 20" bike.

On a nominal 20" wheel, it gives:

55 gear inches

It's a very useful reference point.

But it is not the target gearing for every child.

We recommend starting smaller Micro, Mini and Junior riders on lighter gearing that they can spin up quickly. Our advice is essentially to build the rider first, then add gear.

If our child is grinding their way out of the gate because somebody told us they "should" be running 44/16, we'd rather take some gear away and let them accelerate properly.

Trackstars 36T–48T Chainring Reference

We stock BMX race chainrings from 36T through to 48T, which gives plenty of room to fine-tune a setup.

The table below shows what each chainring produces with a 16T rear cog on a nominal 20" wheel.

It is there to show the progression between chainring sizes — not to tell you which chainring a particular age must use.

Remember, those figures use a nominal 20-inch wheel diameter.

Put the same 40/16 gearing on a smaller-diameter youth wheel or tyre and the actual rollout will be lower.

Smaller Chainring or Bigger Chainring?

Keeping the same rear cog:

Smaller front chainring = lighter gear

That generally makes the bike easier to accelerate and allows a higher cadence.

Larger front chainring = taller gear

That moves the bike further per pedal revolution, but the rider needs enough strength and speed to turn it effectively.

For young riders, we'd generally rather see them slightly on the lighter side than trying to grind a gear that's too big.

What Does a Gear That's Too Heavy Look Like?

A rider who's over-geared may look powerful on paper, but on the track you may notice they're slow to get the pedals moving out of the gate, their cadence is low, they grind each stroke and they struggle to regain speed after a corner.

Their technique can start to break down because they're fighting the drivetrain.

That's when we'd seriously question whether adding more strength is actually the answer.

Sometimes they simply need less gear.

What Does the Right Gear Look Like?

The rider gets the pedals moving quickly.

Their cadence looks controlled rather than frantic.

They continue accelerating through the straight and can regain their speed after corners.

Most importantly, their technique stays reasonably smooth.

That's what we're looking for.

And What If It's Too Light?

At the other end, a rider may accelerate really well but reach a point where their legs are going extremely fast without producing much more speed.

That's what people generally mean when they say they're spinning out.

When that is genuinely happening, rather than just being assumed, then it's reasonable to trial a slightly taller gear.

Fine-Tune With the Chainring

Once you're close to the right setup, a one-tooth chainring change is a relatively small adjustment.

For example:

43/16 → 44/16

is a much smaller change than:

44/16 → 44/15

A one-tooth rear-cog change has a substantially larger effect. Think of the cog as the tool for a meaningful change and the front chainring as the better fine-tuning tool.

That's another reason we're happy carrying chainrings right through from 36T to 48T — it gives us room to make small changes rather than jumping too far at once.

Do We Change Gearing for Every Track?

For an experienced racer, sometimes.

For a young developing rider, generally not.

We think consistency is valuable.

They need time to learn how their bike accelerates and how the gear feels.

Once they're more experienced, you can start looking at whether a particular track rewards slightly quicker acceleration or gives enough room for a taller gear.

But we wouldn't change gearing simply because someone in the pits says:

"You need to go up a tooth for this track."

Watch your rider first.

What We Would Do as Parents

If Stu and I were working through a child's setup from scratch, this is the order we'd follow:

1. Make sure the bike itself fits the rider.

2. Measure their inseam and choose an appropriate crank length.

3. Check the actual wheel and rear tyre they're running.

4. Write down the current chainring and rear cog.

5. Watch the rider's gate, cadence and acceleration.

6. If something needs changing, make one small adjustment.

7. Give them time to train and race on it before changing something else.

That last point is important.

If you change the cranks, chainring, cog and tyres all at once, you've got no idea which change actually helped.

Don't Chase Someone Else's Setup

This is probably our biggest piece of parent advice.

Don't assume the fastest child in the class has the setup your child needs.

They may have different leg length, strength, technique, cadence, tyre size or simply prefer a completely different feeling bike.

Their gearing is right for them.

We want to find what's right for our rider.

The aim isn't the biggest gear.

The aim is a rider who can accelerate, stay in control and use the strength they have effectively.

Need Help Working It Out?

If you're unsure, send us your rider's:

height, inseam, crank length, chainring, rear cog, rear tyre size and frame size, along with what you're actually seeing on the track.

That gives us a much better starting point than simply asking:

"What gearing should my eight-year-old run?"

At Trackstars BMX, we're BMX parents as well.

We know what it's like standing beside the bike wondering whether you're making the right decision for your child.

We're always happy to help work through the numbers and get the setup heading in the right direction.

Trackstars BMX
Elevate your ride.

Sizing and gearing information is a guide only. Rider proportions, strength, cadence, ability, crank length, wheel and tyre diameter, track conditions and personal preference can all affect the ideal final setup.