from Rookie’s keyboard
Hello, friends
Dirt jumping (DJ) isn’t nearly as popular as MTB and road cycling. As a result, people have a harder time finding information when it comes to fit and size.
But today, I will make the selection process easier by giving you the most detailed guide on the topic ever.
Factors Influencing DJ Sizing
Contact Points = Essential
In the case of dirt jumpers and BMX (a.k.a. stunt bikes), the rider has two contact points with the bike rather than three – the handlebars and the pedals.
The distance between the bottom bracket(BB) and the head tube(HT) has the highest influence on how large or small a frame feels.
That distance can be measured/analyzed via two methods – by taking into account the frame’s reach or the length of the downtube.
The reach (a.k.a. “cockpit room”) is the horizontal distance between the center of the bottom bracket and the middle of the head tube’s top.
Sadly, sometimes manufacturers don’t give that value when listing their DJs.
Canyon, Norco, and Transition are among the few producers that publish the reach of their dirt jumpers.
Canyon has two sizes – small and large.
The small model has a reach of 400mm and is intended for people who are 140-170cm/4’7″-5’7″ tall.
The large model has a 417mm reach and is recommended for individuals between 160cm/5’3″ and 200cm/6’6.7″.
Canyon Dirt Jimp Sizing
| Height | Reach | Frame Size |
| 140-170cm (4’7″-5’7″) | 400mm | Small |
| 160-200cm (5’3″-6′.6.7″) | 417mm | Large |
Meanwhile, Transition offers three models based on their reach:
| Frame Size | Reach |
| Short (small) | 415mm |
| Long (large) | 440mm |
| X-Long (X-large) | 465mm |
The sizing options of both companies are a bit different. Hence the importance of analyzing the specific model that you plan on buying.
Canyon’s tall/large model has a 2mm longer reach than Transition’s short/small model.
In the meantime, Norco sizes their dirt jumpers as follows:
| Frame Size | Reach |
| Small | 380mm |
| Medium | 405mm |
| Large | 430mm |
Important conclusion:
The frame size depends greatly on the manufacturer. One company may list a dirt jumper as small while another considers a model of similar size medium or large.
For that reason, you always have to look at the technical information provided by the specific manufacturer.
Top Tube Length = The Most Used Value
In most cases, the frame size of most dirt jumpers is categorized according to the length of the top tube.
Super important: There’s a world of difference between the actual top tube and the effective top tube a.k.a. horizontal top tube.
- Effective Top Tube(ETT) = the horizontal distance between the head tube and the seat tube.
The adjective “effective” is added because this would be the length of the top tube if it was horizontal rather than sloping.
Manufacturers often list the effective top tube measurement rather than the actual top tube length.
Just like with the reach, the values vary greatly between manufacturers.
The tables below contain the top tube DJ sizing of Canyon, Transition, Norco, and Black Market:
Canyon
| Frame Size | Horizontal Top Tube |
| Small/short | 583mm/22.95″ |
| Large/long | 600mm/23.6″ |
Transition
| Frame Size | Horizontal Top Tube |
| Short/small | 600mm/23.6″ |
| Long/large | 625mm/24.6″ |
| X-long/x-large | 650mm/25.59″ |
Norco
| Frame Size | Horizontal Top Tube |
| Small | 551mm/21.7″ |
| Medium | 577mm/22.7″ |
| Large | 604mm/23.77″ |
Black Market
| Frame Size | Horizontal Top Tube Length |
| Small | 540mm/21.2″ |
| Medium | 553mm/21.7″ |
| Large | 565mm/22.2″ |
| XL | 578mm/22.7″ |
| XLL | 590mm/23.2″ |
| XXXL | 604mm/23.7″ |
| XXXL Tallcan | 616mm/24.25″ |
Important conclusion:
DJ sizes vary between manufacturers even when you compare the horizontal top tube lengths.
That said, most small, medium, and large frames fall within the following range:
| Frame Size | Horizontal Top tube |
| Small | 546-558mm/21.5″-22″ |
| Medium | 558-584mm/22″-23″ |
| Large | 584mm/23″+ |
Top Tube Length vs. Reach
Personally, I think that relying on the reach rather than the top tube length is the better way to go
Why?
The seat tube angle(STA) 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 an identical reach.
Conclusion: The reach is not affected by the STA and represents a more consistent way of evaluating the size of a frame.
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 center of the bottom bracket and the upper part of the seat tube.
This technique is 100% worthless when sizing a dirt jumper(or a BMX) because those bicycles are designed for out-of-the-saddle riding.
The length of the seat tube is practically irrelevant because the seat post isn’t a contact point (apart from the times when the rider is resting).
Many Dirt Jump Bikes Are One-size-Only
Most dirt jumpers are single-size and built to fit people of average height.
Unisizes usually have about 400mm of reach and a 600mm effective top tube.
The most popular models are Marin Alcatraz (401.84mm reach/600mm effective top tube) and Scott Voltage YZ 0.1 (403.4mm reach/600mm).
Don’t forget that DJs are designed for tricks and large jumps rather than commuting and trail riding. For that reason, they come as single-speed units with low seats.
Moreover, the relatively small market share of dirt jumpers in comparison to MTBs and road bikes reduces the incentive to produce a large variety of frame sizes.
Many People Ride Smaller Frames on Purpose
Since smaller frames are lighter and easier to manipulate, even tall riders prefer smaller sizes.
The Stem and Handlebars Can Help With The Fit
A longer stem stretches you a bit (increases the effective reach) whereas a shorter decreases the effective reach.
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 DJs are not designed for climbing and remain unaffected.
DJ stems usually vary between 30 and 50mm. If a frame feels a bit small, you can experiment with a longer stem.
The rise of the handlebars influences the geometry and fit too. Taller riders often get handlebars with a greater rise to increase the height of the bike and make the frame feel more “roomy”.
Another benefit of bars with a high rise is that they make the back angle more vertical. Tall riders like that too.
The Style Of Riding Matters
Larger frames with longer wheelbases are seen as better for jumping thanks to their greater stability.
In the meantime, frames with shorter chainstays are the norm for more technical riding as it’s easier to manipulate them around.
The Rider’s Personal Anthropometry Is Crucial
Height matters when sizing any bike, but the actual proportions (anthropometry) of the rider are super important too.
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.
Ideally, you should be able to test the bike before buying.
The Final Plan
The key points are:
An average dirt jumper with a reach of about 400mm/15.74″ and an effective top tube around 570 mm-600 mm/22.5″-23.5″ will serve people of average adult height well.
If you are shorter than average, consider frames with a 550mm/21.5″ effective top tube.
If you are a volleyball player kinda tall, search for a frame that has a top tube over 23.5″.
Size Chart
Note: The table below is just a guideline. As I’ve already said, the sport of dirt jumping is different than classic cycling.
| Height Imperial | Height (metric) | Frame Size | Effective Top Tube(imperial) | Effective Top Tube (metric) |
| 4’7″-5’7″ | 140-170cm | Small | 21.5″-22″ | 546-558mm |
| 5’7″-6’1″ | 170-185 | Medium | 22″-23″ | 558-584mm |
| 6’1″+ | 180+ | Large | 23″+ | 584mm+ |
Additional notes:
24-inch jumpers are a bit more nimble and accelerate faster due to the smaller tire size. They also work well 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.
If you have a BMX background, a 24-inch bike would have more familiar handling.
If you plan on riding big trail jumps, however, a 26-inch dirt jumper is considered more optimal.
Until next time,
Rookie
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