Dirt jumping doesn’t receive the same attention as more popular cycling disciplines (e.g., mountain, road…etc.)
In consequence, people have a harder time finding information in regards to dirt jumper sizing.
The purpose of this post is to make the selection process easier by presenting the parameters that should be taken into consideration when sizing a dirt jumper.
Factors Influencing Dirt Jumper Sizing
1. The Contact Points Are Key
When riding a dirt jumper or a BMX, the rider has two contact points with the bike – the handlebars and the pedals.
Therefore, the distance between the bottom bracket and the head tube has the highest influence on how big or small a frame feels.
There are two main ways to measure that distance – by taking into account the frame’s reach or the length of the down tube.
The reach of a bike is the horizontal distance between the center of the bottom bracket and the middle of the head tube’s upper portion. (image above)
The reach (a.k.a. “cockpit room”) provides a good way of measuring the size of a dirt jumper because it describes the distance between the two contact points.
However, not all manufacturers provide that number when presenting their models.
The bike companies Canyon, Norco, and Transition are among the producers that list the reach of their dirt jumpers.
Canyon offers two sizes – small and large. The small model has a reach of 400mm and is designed for people who are 140-170cm/4’7″-5’7″ tall.
The large model has a 417mm reach and is recommended to individuals between 160cm/5’3″ and 200cm/6’6.7″.
Canyon Dirt Jimp Sizing
Transition offers three models based on reach:
As you can see, the sizing options of both companies are a bit different.
Canyon’s large size has a 2mm longer reach than Transition’s short/small model.
Meanwhile, Norco sizes their dirt jumpers as follows:
Conclusion: The frame size varies according to the manufacturer. A company may list a dirt jumper as small even though the model is medium or large in the line-up of another producer.
Thus, it is very important to look at the technical information provided by the manufacturer.
2. The Size of Dirt Jump Frames Is Often Based On Top Tube Length
Just like BMX models, the frame size of most dirt jumpers is often categorized according to the length of the top tube.
Note: There’s a difference between the actual top tube and the effective top tube a.k.a. horizontal top tube.
The effective top tube is the horizontal distance between the head tube and the seat tube and is longer than the actual top tube.
The quality “effective” is added because this would be the length of the top tube if the actual top tube wasn’t sloping (inserting at an angle).
Most manufacturers provide the effective top tube measurement of their bikes rather than the actual top tube length.
However, just like with the reach, the values vary between manufacturers.
Below is the dirt jump sizing of Canyon, Transition, Norco, and Black Market according to the horizontal top tube length.
|Frame Size||Horizontal Top Tube|
|Frame Size||Horizontal Top Tube|
|Frame Size||Horizontal Top Tube|
|Frame Size||Horizontal Top Tube Length|
Conclusion: As expected, the sizes of dirt jump bikes vary between manufacturers even when you compare the horizontal top tube lengths.
Having said that, most small, medium and large frames fall within the following range:
|Frame Size||Horizontal Top tube|
Top Tube Length vs. Reach
Even though the top tube’s length is often used as an indication of frame size, the method isn’t as accurate as relying on the reach.
Because the angle of the seat tube instantly changes the length of the top tube without actually affecting the set-up.
Two frames can have non-matching top tube lengths due to dissimilar seat tube angles while offering the same reach.
Conclusion: The reach is not affected by the seat tube angle and represents a more consistent way of evaluating the size of a frame.
2. The Length of the Seat Tube Is Irrelevant
When sizing a traditional bicycle (e.g., a road bike), most people use the center to top method and measure the distance between the middle of the bottom bracket and the upper part of the seat tube.
This method does not work when sizing a dirt jumper or a BMX because “stunt” bicycles are engineered for out-of-the-saddle riding.
The length of the top tube is not the most important factor because the seat post isn’t part of the contact points during riding.
The seat on dirt jumpers and BMX bikes is there to rest between tricks and jumps rather than to pedal in a seated position.
3. Many Dirt Jump Bikes Come In One Size
Not every manufacturer offers different sizes. Most dirt jumpers come in a single size designed to fit most people of average height.
When a model is available in a single size, it usually has about 400mm of reach and a 600mm effective top tube.
Two popular examples of such dirt jumpers would be Marin Alcatraz (401.84mm reach/600mm effective top tube) and Scott Voltage YZ 0.1 (403.4mm reach/600mm).
Keep in mind that dirt jumpers are designed for tricks and large jumps. You’re not expected to use them as commuters or trail bikes. For that reason, they come as single-speed units with low seats.
This peculiarity plus the relatively small market share of dirt jumpers in comparison to MTBs and road bikes reduces the incentive to produce a large variety of dirt jumpers.
4. Many People Ride Smaller Frames on Purpose
Since smaller frames are lighter and easier to manipulate, many riders stick to smaller sizes despite being tall enough for larger models.
5. The Fit Can Be Adjusted via The Stem and Handlebars
The length of the stem matters too. A longer stem stretches you a bit (increases the reach) whereas a shorter one shortens the cockpit.
Most dirt jump bikes come with a shorter stem because it makes the handling snappier and adds stability.
The downside of shorter stems is that they reduce some of the weight on the front wheel and make climbing harder, but since dirt jumpers are not designed for any sort of climbing, short stems stand supreme.
Dirt jumper stems usually vary between 30 and 50mm. If you feel that the frame is a bit too small for you, you can experiment with the longer values.
The rise (elevation) of the handlebars has an impact on the geometry and the fit too. In general, tall riders often use handlebars with a greater rise as a method to increase the height of the bike and make the frame feel more “roomy”.
Another benefit of bars with a greater rise is that they allow you to keep a more vertical back angle (you don’t have to bend as much). This is beneficial for taller riders.
6. The Style Of Riding Matters Too
Larger frames with longer wheelbases are considered better for jumping due to the stability that they add.
Meanwhile, frames with shorter chainstays are the norm for more technical riding.
Having said that, those rules aren’t set in stone. In this case, one’s personal preference matters more.
7. The Rider’s Anthropometry Is Important Too
Height matters when sizing a dirt jumper, or any bike, but a person’s individual proportions (anthropometry) are of high importance too.
For example, a chart may say that you need a certain bike for your height, but then you may end up feeling uncomfortable on it because your arms or legs are shorter or longer than what’s expected for your stature.
Hence why it’s important to test ride a bicycle before buying if that’s an option.
Summary: Making Sense of It All
The information above may be confusing if you’re new to the sport of cycling and dirt jumping in particular.
At the bottom of it, however, dirt jump sizing is kinda versatile and doesn’t have to be as precise as a road bike fit for example.
The key points are:
An average dirt jumper with a reach of about 400mm/15.74″ and an effective top tube around 570mm-600mm/22.5″-23.5″ will fit most adults of average height just fine.
If you are shorter than average, consider looking into frames with a 550mm/21.5″ effective top tube.
If you are basketball player kinda tall, search for a frame that has a top tube over 23.5″.
Dirt Jumper Size Chart
Note: The table below shouldn’t be followed as a mandatory rule. It’s just a guideline. As already mentioned, the sport of dirt jumping is different than classic cycling.
|Height Imperial||Height (metric)||Frame Size||Effective |
Top Tube (imperial)
Top Tube (metric)
Note: There’s some overlap because a rider at the end of the bracket may fit just fine on a bigger frame.
Frequently Asked Questions
Should I get a 26-inch or a 24-inch dirt jumper?
24-inch jumpers are a bit more nimble and accelerate faster due to the smaller tire size.
In addition, 24-inch dirt jumpers can operate with a very small frame – a beneficial trait for youngsters and adults of shorter stature.
The downside of 24-inch jumpers is that they don’t absorb bumps as well as 26-inch models and can feel small and slow when used for jumps.
This leaves two incentives to use a 24-inch bike:
- You want to do more technical tricks (park riding).
- 26-inch dirt jumpers feel too big to you.
If you come from a BMX background, a 24-inch bike would have more familiar handling.
If you plan on riding big trail jumps, a 26-inch dirt jumper is considered more optimal.
I am a teenager who is still growing. What size should I go for?
If money isn’t a problem, you can purchase a dirt jumper that currently fits you and replace it upon outgrowing it.
If you’re on a budget, it makes more financial sense to buy a standard size dirt jumper and keep riding it as you grow.
Another option is to go for a cheap small dirt jumper (maybe second-hand?) and eventually replace it with a higher-end model of a larger size and greater quality as your height and skills increase.