Choosing the right BMX race bike can feel overwhelming. Between sizes, components and categories, it’s easy to get crossed up so here at Lisburn BMX Club we are here to help.

You can basically race on any 20’’ BMX Race bike or cruiser , but a race specific BMX bike has certain traits that lends itself to handling a modern BMX track better than say a freestyle bike. But there’s nothing wrong with giving racing a crack on your freestyle bike. There are provisions to race smaller wheeled bikes as well. 12 – 18 inch wheel sizes.



There are some rules around this though:

  1. Brakes: You have to run a rear brake. We do not recommend the use of front brakes.
  2. Pegs, stands, bells and reflectors etc must also be removed.
  3. Your bike must be in good working order. We can help you check over your bike here at Lisburn BMX Club


The big differences between a race bike and a street dirt jumper style of BMX is the geometry of the bike and the way it is set up. Race bikes are generally sized appropriately for the rider’s height and built with lighter alloy or carbon frames. There are still chromoly frames out there too. The bikes overall are lighter and built for speed, not comfort. Ditto for the cruiser class of bikes.

At this point, we advise finding a tape measure and getting a height for your future champion. BMX fit finders use an age range and height recommendation for sizes, but height is the primary metric we’ll use here, since kids grow at different rates.

Age ranges can be helpful in getting an idea what kind of bikes will be ridden in the age group classifications at the race track.

The most important measurement in BMX sizing is Top Tube (TT) length – a distance we measure from the inner seat tube along the top tube to the inner headtube. The Top Tube length is represented as Measurement C in the drawing below.


All BMX bikes have low enough stand over height (Measurement G) and racers are never pedalling from the saddle on the track, so we don’t worry about those more traditional measurements.

However, TT lengths have huge effects on handling and stability. Too short and the bike will feel off balance and unstable at speed; too long and it will be unwieldy to handle and difficult to move around under the rider.

Small changes in TT length make big differences, which is why there are so many frame sizes to ensure proper fit for maximum performance. We’ll walk through the sizes from smallest to largest and touch on details like wheel/tire widths that change as sizes increase. Note that all BMX race bikes have either 20” or 24” wheels, even the very small bikes.

At Lisburn BMX Club we have all the below sizes for you to try out to ensure you’re getting the correct size, please just ask the next time you’re at the facility



These bikes are a great way to introduce your child to the world of  BMX Racing  without the use of training wheels or a tricycle, they are designed from the ground up to meet the needs of children and parents alike.


One of the coolest parts of BMX racing is that the whole family can participate. You can start racing at about 4 years old and race your entire life. In the spirit of starting them young, the smallest BMX bike size is Micro Mini.

Micro Mini bikes will fit kids from 3’6” – 4’ (4 – 6 y.o.) and have a 16.76” TT. Not only are the frames quite small, Micro Mini bikes use skinny rims and 20 x 1⅛” tires. The narrower wheels/tires keep the bikes light for better handling and acceleration under the little guys and gals



The next size up is Mini, which are recommended for riders 4’ – 4’5”(5 – 7 y.o.). Mini BMX bikes use the same 20 x 1⅛” wheelsets as Micro Minis, but have a slightly taller standover and most significantly, a longer TT length at 17.6”.



If your little ripper is 4’5”- 4’9”(7 – 9 y.o.), they’ll best fit into Junior sized bikes. Junior TT length is 18.3”. Junior size bikes still use the 20 x 1⅛” wheel/tires setups.



Next up in size is Expert. Expert bikes have a 18.87” TT length and fit kids from 4’8” – 5’4”(9 – 11 y.o.). Here we see another change with an increased tire size of 20 x 1 ⅜”. The kids riding these bikes start to weigh a little more and need increased tire volume for proper handling when landing jumps and pushing through turns.

****Expert is the size where some confusion happens because there is an advanced race class for each age group called “Expert”. Just remember that Expert sized frames have nothing to do with skill level or experience, they only refer to fit. All BMX manufacturers share these frame size naming conventions. ****



Most brands make a frame called Expert XL with a 20” TT. These bikes are  designed for advanced racers around 11-12 years old, who may have the height to fit, but not the strength to throw around a full adult spec bike.

The Expert XL uses mid-width tires (1.6” front and 1.5” rear) to keep the handling and accelerations quick. The wheels also get a bump in strength and stiffness to handle the more aggressive riding techniques at this level with a jump from 28 to 36 spokes per wheel.



After Expert XL, bikes meet up again with the Pro size. They have a 20.75” TT and are recommended for racers 5’6” – 5’10”, ages 13+. These are essentially full adult-sized bikes that use stiff and strong 36 spoke wheels, standard 20 x 1.95/1.75 tires and 170mm cranks.

****Pro is another size where some confusion happens because of the Pro race class. Just remember that Pro sized frames have nothing to do with skill level or experience, they only refer to size and fit.



Pro XL frames get a little longer with a 21” TT and are recommended for riders 5’9” – 6’.



Pro XXL frames are recommended for racers 6’


If you’ve already done a little exploring into the BMX world, you may have seen the term “Cruiser Bike“. Don’t worry, we’re not out there racing beach cruisers.

In BMX, Cruisers are bikes with 24″ wheels. Age groups start at 11&Under and go all the way up to 61+. The bigger wheels allow for a larger frame and taller fit at the handlebar. Cruiser classes are a great choice for parents of racers to test the waters of racing on a bike that doesn’t feel aggressively small.

However, Cruisers also offer a fun racing option for all age groups, as the bikes handle differently than their 20” siblings, requiring slightly different race techniques and strategies.


BMX Gearing

what is BMX gearing exactly, and what does it all mean in relation to you or your young rider when you’re just starting out? In very basic terms, it’s how hard the bike is to pedal, and the BMX bike you buy out of a shop, or second-hand, has its gearing set as per the combination of the sprockets.

But how do you know if it’s right for the rider, or track?

Bike Sizes and Suggested Gearing


  • Micro 18 Inch: 45-47 inches
  • Mini: 47-49 inches
  • Junior: 49-52 inches
  • Expert: 51-53 inches
  • Pro: 52-54.5 inches


Cruiser: the equivalent of the rider’s 20” race bike (maybe a little lighter by up to 0.5 inches for the larger wheel)


To work out what you gearing is on the bike you have currently, here is a link that is one of the most popular in the world for BMX racers. You punch in data such as tyre size, the front chain ring, and rear cog combo.

From here you can work up, or down the range we recommend above. This will be your bible.


BMX Crank Sizes

Crank sizes differ according too the riders height / inside leg sizes, the below calculator is ideal to give you an idea if you are on the correct sizes and help you choose –  BMX Ultra crank Size calculator