Your BMX gearing and crank length need to work together. A bigger gear can feel comfortable if you struggle to spin, but that doesn’t automatically make it your fastest option. Longer cranks give you more leverage, while shorter cranks reduce the size of your pedalling circle. Finding a combination that suits you means looking at your technique, your ability to accelerate and your times, alongside how the bike feels.
Is your gear helping you, or hiding a weakness?
There’s a fine line between a gear you can accelerate effectively and one that feels strong but takes too long to get moving.
One thing I see in coaching is riders choosing bigger gearing because they struggle to spin smoothly. They’re more comfortable pushing against resistance than turning their legs over quickly.
That preference matters, but it’s worth looking at what sits behind it.
If you move to a bigger gear whenever your cadence feels uncomfortable, you might be working around a leg-speed weakness instead of developing it. Equally, dropping to a smaller gear without improving your pedalling doesn’t automatically solve the problem.
You need to look at both the setup and the rider.
Can you accelerate the gear through your first few pedals? Can you keep applying power as your cadence rises? Does the setup help you reach the first jump quicker, or does it simply feel like you have more to push on?
Those questions tell you more than copying the gearing of someone who wins races.
Your crank length is part of the picture
Crank length changes the circle your feet travel through.
A longer crank gives you more leverage, but your legs also need to move through a larger circle. A shorter crank gives you less leverage and a smaller pedalling circle, which some riders find easier to spin smoothly.
Neither choice guarantees a faster start or better first straight.
You need to consider how your crank length works with your gearing, body position and pedalling technique. Changing one part of that combination can change how the whole bike feels.
A setup that lets another rider pedal comfortably might leave you struggling to bring your legs around.
Watch your hips as you pedal
One practical check I use is watching how a rider’s hips move.
If your cranks are longer than you can comfortably manage, you may rock from side to side as you work through the larger pedalling circle. This can be particularly noticeable when a smaller rider is trying to manage longer cranks.
Hip movement is a clue to investigate, rather than a diagnosis on its own. Your position and technique also deserve a look.
Watch for excessive movement as your cadence increases. Can you keep your hips reasonably controlled and bring your pedals around smoothly, or does your movement become less consistent as you spin faster?
Video can help you and your coach see what changes between the first few pedals and the faster part of the effort.
Height doesn’t choose your crank length for you
I run 180 mm cranks. I also know taller riders, around 6'1" to 6'2", who prefer 170 mm cranks paired with smaller gearing.
That’s why I wouldn’t choose your crank length from your height alone.
Your proportions, pedalling habits, strength and personal preference all form part of the decision. You also need to be able to use the setup effectively on the track.
Looking through the crank lengths recorded in HRVfit rider profiles shows how much variation exists:
| Age recorded in the profile | Recorded crank-length range |
|---|---|
| 9–12 | 145–165 mm |
| 13–15 | 165–175 mm |
| 16 and over | 170–177.5 mm |
These are examples from a group of my riders, not recommended sizes for each age. The records were completed at different times, and riders may have changed their setups since.
The overlap matters. Some younger riders use longer cranks than older riders. Age gives you context, but it doesn’t make the decision.
Compare the complete gearing setup
Your front and rear sprockets are only part of the comparison. Tyre size also affects how far your bike travels for each pedal revolution.
Two riders using the same tooth counts can have different effective gearing if their rear tyres have different rolling circumferences.
When you record your setup, include:
- Front chainring and rear sprocket.
- Crank length.
- Rear tyre size and model.
- Pedal type.
- The conditions and distance used for your timed efforts.
A tooth-count comparison without the wheel and tyre details can give you the wrong impression.
Give yourself time to adapt to a different gear
Changing your gearing is only the first step. You also need time to adapt to pedalling it.
If you’re used to pushing a bigger gear at a lower cadence, dropping to a smaller gear can initially feel choppy or too spinny. Your legs are being asked to turn over faster than you’re accustomed to, and your timing and coordination need practice.
That unfamiliar feeling doesn’t automatically mean the gear is wrong for you. You need to spend time on it across several sessions, giving yourself a chance to develop your leg speed and become comfortable with the different resistance.
This is why swapping gears, doing a couple of sprints and asking “Which feels better?” can give you an incomplete picture. You’re comparing a familiar setup with one you haven’t yet learnt to use effectively.
Record your initial times and how the gear feels, then reassess after consistent practice. Look at how smoothly you pedal, how well you accelerate and how your times change as you adapt. The aim is to judge the gearing once you’ve given yourself a fair chance to use it.
Race gearing and training gearing serve different purposes
The gear you race on doesn’t have to be the only gear you train with.
A change in training gearing can have a specific purpose within your program. That doesn’t mean you should immediately use that gear at your next race.
Understand what the session is trying to develop, then assess your race setup separately. The HRVfit BMX sprint-training guide explains how gearing fits alongside sprint types, recovery and session planning.
Your aim is to find a combination you can accelerate, pedal smoothly and use consistently under race pressure. Personal preference belongs in that decision, supported by what your technique and times show.
If you need help connecting those pieces, BMX coaching online with HRVfit gives you a way to discuss your setup alongside your training and race goals.