from Rookie’s Keyboard
Hello, friends
Yesterday I overheard a conversation between two parents who were observing a group of kids making tricks on dirt jumpers. Their exchange made me realize that regular people don’t know the difference between a dirt jumper and an MTB. And how can you blame them? Dirt jumpers (DJs) are extremely similar to a hardtail (HT).
But there are MASSIVE dissimilarities nonetheless that make dirt jumpers close to unusable on regular trails.
DJs are 24 or 26-inch hardtail bikes engineered specifically for skate parks and jumping.
You can think of them as a mix between a BMX and an MTB. They are pretty good for street riding too, albeit not as capable as a BMX due to the longer wheelbase and larger size.
The Differences Between a DJ and a HT
- Head Tube Angle
The head tube angle (HTA) of a frame greatly influences a bike’s handling.
There are two HTA types – slack and steep.
А slack HTA positions the front wheel further away from the bike whilst a steep HTA brings the front wheel closer to the rider.
Most dirt jumpers come with a steep head angle(e.g., 69°) by the standards of modern MTBs.
The steep HTA improves the maneuverability of the bike at low speeds and positions the rider’s weight closer to the front wheel. This engineering makes it a lot easier to lift the front wheel for tricks.
Meanwhile, modern MTBs have slacker HTAs (e.g., 66.5°).
The table below contains the HTA of popular hardtails and dirt jumpers.
Head Тube Angle Comparison
| Hardtail | HTA | Dirt Jumper | HTA |
| Rockhopper Expert 29 | 69.8° | DMR Sect | 69º |
| Canyon Exceed 7.0 | 69.5° | Scott Voltage YZ 0.1 | 68.5° |
| Trek Stache 9.7 | 67.8° | Marin Alcatraz | 69º |
| Fuse Comp 29 | 66.5° | Commencal Absolut Dirt | 69° |
| Giant XTC Advanced | 69.5° | Santa Cruz Jackal | 68.7° |
| Santa Cruz Chameleon | 67.3° | Canyon Stitched 720 Pro | 69° |
| Rocky Mountain Growler 20 | 67° | NS Movement 1 | 69° |
| Merida BIG.TRAIL 400 | 67.5 | Specialized P3 | 69.5° |
| Kona Mahuna | 68° | Pivot Point | 68° |
| Saracen Zenith Pro | 67° | Mongoose Fireball SS | 69° |
| Average: | 67.99° | Average: | 68.87° |
So, the average HTA of hardtails is about 0.88° slacker.
The benefits of a slacker HTA are:
1. Additional stability when descending
2. Lower chances of going over the bars due to the increased wheelbase
3. Compatibility with forks of greater travel.
Note: When a suspension fork compresses, the HTA of the bike gets steeper. For that reason, bikes with a steep HTA cannot operate with long travel forks as the HTA will get too steep upon compression.
This is one of the reasons why a dirt jumper is not as safe to ride downhill. The chance of going over the bars is just too high when the terrain is demanding.
Seat Tube Length
Dirt jumpers are built for tricks rather than comfortable pedaling and have a very short seat tube. The goal is to get the saddle out of the way.
This property makes DJs awful climbers and commuters as you are always pedaling outside of the saddle. Strong riders may cover fairly long distances like that, but their efficiency will still be quite low due to the additional drag and the high energy expenditure per 1 crank spin.
MTBs (apart from downhill models), on the other hand,allow you to climb forever in a fairly comfortable position.
The next table below compares the seat tube lengths of some hardtails and dirt jumpers:
| Hardtail | Seat Tube | Dirt Jumper | Seat Tube |
| Rockhopper Expert | 370-560mm | DMR Sect | 330mm |
| Canyon Exceed 7.0 | 375-535mm | Scott Voltage YZ 0.1 | 315mm |
| Trek Stache 9.7 | 394-521mm | Marin Alcatraz | 320mm |
| Fuse Comp 29 | 340-505mm | Commencal Absolut Dirt | 310mm |
| Giant XTC Advanced | 350-520mm | Santa Cruz Jackal | 340mm |
| Santa Cruz Chameleon | 390-420mm | Canyon Stitched 720 Pro | 326mm |
| Rocky Mountain Growler 20 | 380-480mm | NS Movement 1 | 335mm |
| Merida BIG.TRAIL 400 | 390-540mm | Specialized P3 | 312mm |
| Kona Mahuna | 370-530mm | Pivot Point | 285.7mm |
| Saracen Zenith Pro | 393-533mm | Mongoose Fireball SS | 292mm |
Conclusion: The seat tubes of dirt jumpers are much shorter than those of hardtail MTBs. (what a “surprise”!?)
- Seat Tube Angle(STA)
The seat tube angle STA (image below) is important too.
A slack STA puts the rider closer to the rear wheel; a steep one pushes the rider forward.
MTBs have steeper seat tube angles as a way to compensate for the slack head tube geometry.
Dirt jumpers, however, have slacker STAs as otherwise there will be no room for the rider.
When you install a long seat post on a dirt jumper, the seat goes behind the bottom bracket by a mile and makes pedaling in a seated position uncomfortable.
The table below compares the STAs of popular hardtails and dirt jumpers.
| Hardtail | STA* | Dirt jumper | STA |
| Rockhopper Expert 29 | 73.8° | DMR Sect | 71º |
| Canyon Exceed 7.0 | 72.7° | Scott Voltage YZ 0.1 | 71° |
| Trek Stache 9.7 | 67.8° | Marin Alcatraz | 71° |
| Fuse Comp 29 | 74° | Commencal Absolut Dirt | 72° |
| Giant XTC Advanced | 73.5° | Santa Cruz Jackal | 72.7° |
| Santa Cruz Chameleon | 72.8° | Canyon Stitched 720 Pro | 72.5° |
| Rocky Mountain Growler 20 | 73° | NS Movement 1 | 72° |
| Merida BIG.TRAIL 400 | 73.5° | Specialized P3 | 70° |
| Kona Mahuna | 75° | Pivot Point | 70.2° |
| Saracen Zenith Pro | 74° | Mongoose Fireball SS | 70° |
| Average: | 73.01° | Average: | 71.24° |
Conclusion: The average STA of modern hardtails 1.77° steeper than that of the dirt jumpers.
- Chainstays
For easier execution of bunny hops and manuals, dirt jumpers have shorter chainstays than regular MTBs.
Shorter chainstays make front wheel lifts almost effortless because the rider doesn’t have to move much to get their weight behind the rear axle.
Meanwhile, hardtails come with longer chainstays for extra stability during descending and climbing.
The notably short chainstays on dirt jumpers are dangerous when covering demanding terrain as the bike may jump out in front of you.
- Bottom Bracket Drop
The bottom bracket drop represents the vertical distance between the center of the bottom bracket and an imaginary horizontal line between the axles.
Dirt jumpers have a shorter bottom bracket drop than hardtails.
Why? A small bottom bracket drop facilitates manuals and hops whilst a big bottom bracket drop increases stability.
- Frame Size
Trail bikes come in a variety of sizes designed to accommodate most riders. Meanwhile, most dirt jumpers are uni-size.
Frame Weight
Dirt jumpers have heavy frames and could end up weighing as much as a geared hardtail. Why? Because the extra strength that the frame needs to handle the impact comes with more material and more weight.
No Accessories
Dirt jump frames don’t have eyelets for fenders, racks, or water bottles. I don’t know about you, but that doesn’t sound like a fun bike to cover high distances…
The next table shows the frame differences between hardtails and dirt jumpers in a compact form.
| Hardtail MTB | Dirt Jumpers |
| Slack head tube angle | Steep head tube angle |
| Steep seat tube angle | Slack seat tube angle |
| Long seat tube | Short seat tube |
| Various sizes | One or a few sizes |
| Large Bottom Bracket Drop | Small Bottom Bracket Drop |
| Lighter | Heavier |
| Bottle cage mounts (occasionally rack and fender eyelets too) | No eyelets |
- Suspension
The travel of dirt jumps forks is quite short between 80 and 100mm.
Why?
- To keep the HTA steep(reasons are explained above)
- Stiffness
DJ forks are designed to absorb hard but relatively infrequent landings. Thus, they are kept stiff. Short travel helps with that too.
Meanwhile, the fork travel of modern hardtails often is between 120-140mm…sometimes more.
The extra travel is nice for difficult off-road terrain, but it’s not needed for dirt jumping.
The next table contains the fork travel of the models from the previous tables.
| Hardtail | Fork Travel | Dirt Jumper | Fork Travel |
| Rockhopper Expert 29 | 80-100mm | DMR Sect | 100mm |
| Canyon Exceed 7.0 | 100mm | Scott Voltage YZ 0.1 | 100mm |
| Trek Stache 9.7 | 120mm | Marin Alcatraz | 100mm |
| Fuse Comp 29 | 100-150mm | Commencal Absolut Dirt | 100mm |
| Giant XTC Advanced | 100mm | Santa Cruz Jackal | 80-100mm |
| Santa Cruz Chameleon | 120mm | Canyon Stitched 720 Pro | 100mm |
| Rocky Mountain Growler 20 | 130-140mm | NS Movement 1 | 100mm |
| Merida BIG.TRAIL 400 | 140mm | Specialized P3 | 100mm |
| Kona Mahuna | 100mm | Pivot Point | 100-120mm |
| Saracen Zenith Pro | 120mm | Mongoose Fireball SS | 100mm |
Ironically, since DJ frames are massively overbuilt, many can take forks with ultra-long travel. But the geometry of the bike isn’t engineered for such forks.
- Wheel Sizes
Most MTBs use 29-inch wheels. 27.5″ are also available but are becoming rarer. Meanwhile, dirt jumpers are equipped with 26-inch wheels and occasionally 24-inch.
Why?
29″ wheels are good for speed as each revolution covers a greater distance. They also have great “roll-over-ability” or in other words pass over obstacles easily.
But they aren’t as strong and as maneuverable as 26″.
- Transmission
DJs are single-speed for simplicity and weight reduction. Therefore, they are super inefficient commuters.
In different, MTBs offer a fairly wide gearing range allowing them to cover all sorts of terrain.
Brakes
Standard DJs typically come with a rear brake to make the execution of tricks such as bar spins possible.
A rear brake is not enough for trail riding. The front brake that DJs are missing provides more stopping power.
“Hybrid Dirt Jumpers”
Some DJs have a bit of “hardtail bone” in them and can work as multi-purpose bicycles:
Example models:
- NS Clash
- Kona Shred
- Dartmoor Shine
- Santa Cruz Chameleon
- Chromag Stylus
- Dartmoor Hornet
- NS Eccentric
- Marin San Quentin
- …etc.
That said, those still aren’t as good as the real thing when it comes to trail shredding.
Until next time,
– Rookie
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