Bike Selection and Setup Guide
BMX Selection and Setup Basics
When my children first stared racing BMX, I was shocked at how little information was available on the web. Something as simple as determining what size bike your rider needs can be very confusing. I decided to put this guide together as a resource for parents new to the sport. Please understand that this is all my opinion and may not be the best advice for your rider.
Anatomy of a BMX Bike
Here are all the basic parts of a BMX Bike and frame geometry
Bike Sizing
BMX bike sizing can be confusing. Bikes are sized by the length of the top tube as illustrated below.
Almost every company that manufactures BMX bikes provide a sizing guide. However, for whatever reason, these sizing guides vary greatly and hardly ever represent what successful riders are actually riding in the races. If you follow the sizing charts, you will likely purchase a bike that is too small. The chart below is a good starting point in choosing a bike that fits. It’s important to note that the top tube length can change from manufacture to manufacture but these are the common sizes.
|
Frame Size |
Top Tube Length |
Height in Racing Shoes |
|
|
Micro |
16.5" |
- |
46" |
|
Mini |
17.5" |
46" |
52" |
|
Junior |
18.5" |
50" |
55" |
|
Expert |
19.5" |
54" |
58" |
|
Expert XL |
20" |
57" |
62" |
|
Pro |
20.5" |
61" |
68" |
One mistake that parents often make is to buy a larger bike so that their child can “grow into it”. If they are within an inch or so of the next size, that is probably a safe bet but it will be hard them to properly control a bike that is too large. Fortunately, it is often easy to sell bikes once they are outgrown and many of the parts will transfer to the next size frame. Finally, I suggest that you rent a bike at that track or ask to ride someone’s bike to make sure the sizing is correct.
Bike Selection
Now that you know what size you are looking for it is time to find and purchase a bike. There are several options here. You can either buy a “complete” or “custom” bike and of course, you can purchase new or used. I will quickly layout the pros and cons of each option.
Complete Bikes
A complete bike is a bike that comes from the manufacturer ready to ride. Some of the most common brands are Redline, GT, Chase DK and Haro. These bikes are great for beginners and cost in the $450 to $650 range new. The advantages of a complete bike are the cost and the fact that you can purchase a bike and in a few hours be ready to ride. The downside of a complete is that they are generally heavier and use cheaper components. Furthermore, there is a good chance that the fit will have to be adjusted to your child with new cranks and gearing (more on that below)
Custom Bikes
Many BMX companies do not sell complete bikes ready to race. They sell the frame and other parts separately, that way you can mix and match to build exactly what you want. Some common custom frame brands are Supercross, Ssquared, Crupi and Staats. Companies that manufacture completes also often offer high-end custom frame as well. The advantage to a custom bike is the weight, quality of parts and a custom fit. The downside of a custom bike is the cost. Using all new components, a custom bike will on average cost between $1,200 and $2,500. The good news here is that it is easy to source used parts at a much lower price.
Used vs. New
As you can see from the chart above, kids can outgrow bikes very quickly. This means there are a LOT of nice used bikes for sale at any given time. A used complete can normally be found in the $200 - $600 range, depending on the upgrades. While a used custom bike can be had for $500 to $1,500. I would suggest purchasing a used complete for a beginner. These are good bikes and are easy to resell once they outgrow it or move on to a custom bike. Plus, many used completes already have at least some upgraded parts. It is very common to resell a used bike for close to what you paid for it. Also, there are many Facebook groups that specialize in BMX bikes and parts. I have shipped many bikes using bikeflights or shipbikes. It cost around $50 to ship a bike. Don’t forget to use Paypal Goods and Services to protect you from fraud.
Where to buy
If you are looking for a new or used bike or parts I would suggest talking to Frank Branum with Wheeler Dealer Bike Shop. He has a mobile bike shop and is often set up at the track. Another local bike shop that carries racing bikes and equipment is All About Bikes at 621 S Mendenhall Rd in Memphis. I have listed some other options below.
The following Facebook groups are probably the best place to buy used bikes and parts.
Shelby Farms BMX Bike and Parts Resale
Bmx, Micro, Mini, Jr, Expert Race Bikes & Parts for Sale or Trade
BMX Race Bikes, Parts and Gear
Bike Fit
Now that you have a bike, or are building a bike, you have to make sure the bike fits properly. Bike fit is very important. If the bike does not fit properly, it will be hard for your child to learn how to control the bike effectively. Bike fit is changed by either adjusting or changing components. The main components that contribute to a bike’s fit are the cranks, stem and handlebars.
Cranks
It is very important to have the rider’s cranks sized properly. Almost every complete bike comes with cranks that are considerably too long. Longer cranks give more leverage and makes the bike feel easier to pedal (for a given gearing). However, if the cranks are too long, the rider will bounce up and down when they pedal or their hips will sway from side to side. This movement is inefficient and is a sign that the rider is not going to be able to transmit maximum power to each pedal stroke. With that said, crank length is a hotly contested topic. If you surveyed 10 people about crank length, you would likely get 15 different answers. For the purpose of this guide, I will share my opinion and what has worked the best for my riders. Just be aware that others may feel differently.
Damon Sparge at Helium Racing seems to be the go-to authority for sizing crank lengths, especially for younger children. His method is outlined on his webpage and is worth reading Helium Racing - Q Factor, Crank Length (heliumbikes.com) The chart below outlines his suggestions based on inseam length. It is important to measure the inseam as described in his article or you will get the wrong recommendation.
|
Inseam Length (in) |
Crank Length (mm) |
|
|
14.71 |
15.57 |
90 |
|
15.57 |
16.44 |
95 |
|
16.44 |
17.30 |
100 |
|
17.30 |
18.17 |
105 |
|
18.17 |
19.03 |
110 |
|
19.03 |
19.90 |
115 |
|
19.90 |
20.76 |
120 |
|
20.76 |
21.63 |
125 |
|
21.63 |
22.49 |
130 |
|
22.49 |
23.36 |
135 |
|
23.36 |
24.22 |
140 |
|
24.22 |
25.09 |
145 |
|
25.09 |
25.95 |
150 |
|
25.95 |
26.82 |
155 |
Stem
The stem can be replaced with a shorter or longer stem to effectively adjust the bikes top tube length. Changing the stem can bridge the gap between frame sizes. The link below shows how to change a stem on a BMX bike.
Handlebars
Handlebars impact the fit in three ways. The width of the grip, angle of the bars and height of the bars. The width of the grip should be slightly wider than shoulder width (hands at pushup width). The bars may have to be cut to fit the rider. For the most part, the angle of the bars should match the angle of the forks. However, you can adjust them slightly forward or slightly back. This adjustment shortens or lengthens the bike, similarly to changing the stem length. The height can be adjusted in three ways. First, he easiest way would be to reposition the spacers above or below the stem. Relocating the spacers from the top to the bottom will move the bars up while relocating them from the bottom to the top will move the bars down. Note: you may not have spacers on your stem. Don’t add additional spacers as this could be dangerous. Second, if your bike is equipped with a front load stem, you could flip the stem to move the bars up or down. This is illustrated in the stem video linked above. It is not recommended to flip a top load stem (see pictures below). Finally, If the bars cannot be adjusted properly by adjusting the spacers or the stem, you will need to buy taller or shorter handlebars to fit your rider.
The Big Bike Fit Picture
The frame size, stem length, and bar height all work together to fit the rider to the bike. Below is a picture of a properly fitting bike. The hips should be directly above the bottom bracket. The chin should be over, or slightly in front of the bars and the spine should make a 30 to 40 degree angle.
Gearing
The last step in setting up your rider’s bike is to ensure the gearing is correct. The gearing of the bike will determine how hard it is to pedal the bike and how far the bike moves with each pedal. First lets discuss some definitions
Gear Ratio
The gear ratio is the ratio of the number of teeth on the front chainring and the rear cog or freewheel. A bike with a 40 tooth chainring and a 15 tooth cog will have a gear ratio of 2.67
Gear Inches
Gear inches is how the BMX world measures gearing. However, confusingly, we all call it “Rollout” (which is something entirely different). Gear inches are calculated by multiplying the Gear Ratio by the diameter of the rear tire in inches. To calculate this, you will need to know the diameter of your specific brand of rear tire or you can use the online calculator I have linked below as they already have the tire sizes factored in. For example, a bike with a 1 1/8’ Tioga Powerblock tire, a 40 tooth chainring and a 15 tooth cog will have 54.35 gear inches. Again, when someone asks for rollout, they really mean gear inches.
Gear Calculator - bmxultra.com
Setting up the gearing for your rider
Different riders prefer different gearing setups, so view this as a starting point. In general, younger riders with shorter cranks require lower gearing. As a rider grows into longer cranks and starts to get stronger, they will increase their gearing. The chart below is a good starting point for most riders.
|
Crank Length (mm) |
Rollout (Gear inches) |
||
|
- |
100 |
45 |
49 |
|
105 |
115 |
49 |
52 |
|
120 |
135 |
52 |
53 |
|
140 |
155 |
53 |
54 |
|
160 |
180 |
54 |
56 |
Social