BMX Race Bike Size Guide - Buying Your First BMX Race Bike

Author: Bruce Morris   Date Posted:26 March 2020 


BMX Race Bike Size Guide - Buying Your First BMX Race Bike

 

If you’re new to BMX racing and been looking for a race bike for your kids, or yourself, you’ve probably had a look online, or asked around at the BMX club, for info on what bike is the right size. Unfortunately there’s not a lot of great information out there, despite BMX racing being around since the mid to late 70s. Google “bike fit” and you’ll find thousands of websites and plenty of dedicated business for sizing you up for a road or mountain bike. The same can’t be said for BMX race bikes.

LUXBMX in-house race guru Bruce Morris explains below what to look for when buying your first BMX race bike, more importantly, how to size up the right race bike, and in another article, which bike is best for you to start on, or upgrade to. He’s been racing since the start of (BMX) time, but more importantly, coaching riders for over nearly 10 years, and knows a thing or two on race bike fit and set-up. Read on to make sure you choose the right bike. 

 

Aren’t all BMX bikes the same size? 

 

Being a child of the 80s and riding the wave of the BMX bandit era, it is true that then most BMX bikes were basically the same size, with kids racing whatever they could get there hands on and size specific race bikes being exotic American manufactured frames and smaller parts coming out of small boutique fabrication companies. But as the sport progressed and BMX race bike sizing became more standardised, we are now at a point where there’s bike size specific to fit kids from 3 years old, up to the point they are fully grown. From Micro Minis with 18 inch wheels, the Pro XXXLs (and bigger). Then there are the 24 inch cruiser race bikes

         

 

 

              DK Micro - 18 inch wheels                                                   DK Pro - 20 inch wheels                                                                 DK Cruiser - 24 inch wheels

 

 

BMX race rules allow a wheel to be sized from basically 12 inches, up to just a tad under 22 inches in diameter (termed “over-sized”), but most bikes are your standard 20 inch wheel which resembles the majority of bikes in the freestyle world. What changes on smaller BMX race bikes are the frame sizes, rim/tyre combo, and parts being scaled down to fit smaller bodies. Parts like handlebars, stems and cranks sizes. Subsequently, the weight of a bike comes down as you scale down the various parts. More on the importance of this later. The exception here is the cruiser class, where wheel size is 24 inches (up to a maximum of 26 inches)

 

Why is having the right sized bike important?

 

Unlike road bikes that you spend hours on to get to the cafe, the reason you want a correct size BMX race bike isn’t for comfort, nor initially for performance (not just yet), it’s essential for skill acquisition. Both in the initial stages, and as a rider develops their skills. The right size bike will help a rider develop the basic skills of pedaling and pumping much more quickly than if they were on the wrong size bike. Weight of the bike is important too. Your first BMX race bike doesn’t need to be a carbon fibre missile, but take the two bikes above, the 18 inched wheeled micro weighs in at 7.2kg, and the Pro XL at 10.2kg. Or around 40% heavier! This weight needs to accelerate, maneuvered and be lifted in and out of the car! And obviously the smaller size aids in all of this for the young shredders. 

 

Quite often in my travels as a coach, I’ll see riders on a bike that’s too big for them, that’s been bought second hand with the reasoning that the rider will grow into it. Unfortunately this will hinder a new rider’s skill development, and confidence, as they try to come to terms with a BMX track that requires a nimble and responsive machine to get the most out of the riding experience. Frame too long, bars to big, or cranks too long. Sure there’s some great buys out there second hand, but at the end of the day, spending a little more to buy a new bike that fits will outweigh the bargain second hand bike. Even if it is spec’d like a F1.  

 

On the opposite end of the scale, I often see riders on a bike that’s too small for them as mum or dad tries to get to the end of the year before upgrading. What you’re looking for is the Goldilocks bike, the one that fits just right. 

 

So what’s with all these sizes?  

 

Let’s breakdown the various names and terms of bike sizes in the BMX race world and try to guide you to the right decision. There’s a few points to consider when you look at height/age charts online, one of them being that an “Expert” sized complete bike from one brand might be a slightly bigger (or smaller) than an “Expert'' sized bike from another. Not massively different, but enough to make a difference to how a rider would feel on it. 

BMX race bike sizes are named according to their frame size (most commonly the top-tube length) and tyre size, but there’s not an international or industry standard, and what one company might call a mini-micro, another might call just micro. A Pro XL would typically have a 21 inch top-tube, or a 20.75 inch top-tube.

 

Confused? What’s a top-tube? The chart below outlines a typical frame specs and you can see what we’re referring to with top-tube length. This measurement will set the basic architecture of the rest of the frame/bike design, with parts like handlebars and cranks falling into line with a particular company’s specifications of what they feel would suit a rider choosing their “expert” size bike (for example). Let’s take a look at an example frame dimension sheet, then a complete bike spec sheet that ties it all together. 

 

Generally, but not always, these are the ranges of lengths that top-tubes fall within commonly named sizes that you will find the major brands using. Nearly always expressed in inches, they are as follows and are beased upon LUXBMX's most popular sold complete BMX race bikes.

 

Micro (18 inch wheel) - 16.25” top-tube

Mini (20 inch wheel) - 17.25” top-tube

Junior - 18.25 to 18.75” top-tube

Expert - 19.5 to 19.75” top-tube

Expert XL - 20” top-tube

Pro - 20.5” top-tube

Pro XL - 20.7 to 21” top-tube

Pro XXL - 21.5 to 21.75" top-tube

Pro XXXL - 22” top-tube

Cruiser - 21.5 to 21.75” top-tube

 

Those lengths might not look like much of a variation from size to size, but remember, all or the parts that come with those sized bikes are scaled appropriately. Let’s break this down in the next section b looking at a couple of examples.

 

The “Spec Sheet” - Whats everything mean? 

 

                                                                                                  2020 DK Sprinter Mini                                                                                                                2020 DK Sprinter Expert


Now you have an understanding of what sets the nomenclature of a certain size To help you understand this relationship of frame size and part’s sizing, let’s compare a                      2020 DK Sprinter Mini and Expert complete bikes. The list of specs read like this:
 

Component

DK 2020 Sprinter Mini Bike

DK 2020 Sprinter Expert Bike

Frame

6061 aluminum with 3D dropouts

6061 aluminum with 3D dropouts

Top Tube

17.25"

19.5"

Fork

100% chromoly with tapered legs

100% chromoly with tapered legs

Handlebar

3” rise, alloy

6.5” rise, 100% chromoly

Grips

DUO Brand Mini

DK Tsuka with press-in bar plugs

Stem

Alloy front load, 30mm reach

Alloy front load, 30mm reach

Headset

Sealed integrated

Sealed integrated

Cranks

Forged alloy 3-piece, 140mm

Forged alloy 3-piece, 165mm

BB

Sealed European

Sealed European

Pedals

Mini alloy platform

DK Blender PC platform

Sprocket

37-tooth, alloy, 5-bolt chainring

39-tooth, alloy, 5-bolt chainring

Chain

KMC Z-610 3/32”

KMC Z-610 3/32”

Rims

Alloy, double wall, 36-hole

Alloy, double wall, 36-hole

Front Hub

DK alloy, fully sealed, 3/8” chromoly axle

DK alloy, fully sealed, 3/8” chromoly axle

Rear Hub

DK alloy, fully sealed, 3/8” chromoly axle, 14-tooth cassette

DK alloy, fully sealed, 3/8” chromoly axle, 15-tooth cassette

Seat

DK Mini 1-piece seat/post combo

DK Pro 1-piece seat/post combo

Seat Post

Integrated, 22.2mm, alloy

Integrated, 27.2mm, alloy

Seat Clamp

Alloy

Alloy

Tyres

20x1-1/8” Arisun XLR8

20x1-1/8” Arisun XLR8

Weight

7.3kg

8.4kg

Colour

Silver

Silver

 

You’ll almost always see the top-tube length listed at the top of specification sheets of bikes so that you can see straight away what you’re getting size wise, and also to compare one mini bike (or expert) with another brand’s bike. 

 

Diverging here a little, but the bike’s frame and fork material is usually laid out in the spec sheet as well. You’re looking for, in a race bike, for an aluminium frame (sometimes called alloy) and chromoly fork. No need to worry too much about the type of aluminium used, though 6061 aluminium is common amongst entry to mid level race bikes, and 7000 series aluminium used in high end frames. In the smaller bikes you may find mild-steel as the fork material, which is perfectly fine for the little racers, but from junior up, you really want to see a chromoly fork as they are stronger and a rider on this side bike will probably be jumping stuff. 

 

Next up, the parts to vary greatly from these two sized bikes and the handlebars and cranks. These are probably what sets these two examples apart along with the frame size. The Mini DK here has 3 inch bars (this refers to their height) and the Expert has 6.5 inches. That’s a significant difference. 

 

Then there’s the cranks. The Mini has 145mm cranks (this refers to their length) and the Expert has 165mm cranks. A 15mm difference might not sound a lot, but it’s a massive difference in crank length and little legs need little cranks. Longer legs, longer cranks.

 

Other notable differences are the weight, less metal on the Mini means less weight. By just on a kilogram over the Expert and will make a difference to your 5 year old chucking it about on the track. The stem lengths are the same in these examples, though these can vary on two comparable bikes from other brands/models. So while these two bikes might look very similar in photos or images, but the spec sheets show a different story. And now we’ll try to tie all this together to match you or your rider with the right sized bike. This isn’t going to be perfect, but the knowledge base that has resulted from our combined riding and coaching experience should get you pretty close.

 

Remember though, this is a guide, and you'll see that there are some cross-overs in sizing on some of the bikes. This is due to variations in frame/bike sizes from manufacturer to manufacturer, and the fact that between the sizes there just isn't that great a difference in the top tube length. 

 

 

 

 

At the end of the day, ideally you would come into LUXBMX and see the team to test ride and grab your new bike right there and then. You can arrange an appointment to see the in-house race guru here bruce@luxbmx.com

 

Next Up: Which BMX race bike should I buy?

Related: What changes can I make to my race bike to optimize fit?