Condensed Answer: Dirt jumpers don’t have the necessary suspension, geometry and frame size to match the performance of dedicated downhill bikes. Using a dirt jumper for downhill will result in poor behavior and may even cause a fall.
Differences Between Dirt Jumpers and Downhill Bikes
The properties that make a dirt jumper good for dirt jumping make it a poor downhill bike.
Those properties are:
A Short wheelbase
The term wheelbase refers to the distance between the two axles of a bicycle.
Dirt jumpers have very short chainstays and thus short wheelbases.
Shorter chainstays make it easier to lift the front wheel and perform tricks such as bunny hops and manuals which are fundamental to dirt jumping and street riding.
The table below compares the chainstay length of popular dirt jumpers and downhill bikes:
|Dirt Jumpers||Chainstay Length||Downhill bike||Chainstay Length|
|DMR Sect||390mm||Mondraker Summum||445-460mm|
|Scott Voltage YZ 0.1||380mm||YT Industries Tues 29||440mm|
|Marin Alcatraz||395mm||Commencal Supreme DH||439mm|
|Commencal Absolut Dirt||390mm||Canyon Sender CFR||435-445mm|
|Santa Cruz Jackal||387.4mm||Scott Gambler 900 Tuned||435-450mm|
|Canyon Stitched 720 Pro||397mm||Yeti – SB 165||433mm|
Conclusion: The chainstays of dirt jumpers are about 11% shorter than what we found on downhill bikes. This is a substantial difference that really plays a role in how the bike performs.
Shorter chainstays are nice for tricks and street riding, but they’re too unstable for downhill. The chance of lifting the front wheel and “looping out” is real.
Steep Head Tube Angle
The head tube angle is the angle formed by the bike’s head tube and the ground.
Dirt jump bikes use single crown forks with 100-120mm travel. Consequently, the head tubes of the frame are steeper.
The table below compares the head tube angle difference between dirt jumpers and downhill bikes:
|Dirt Jumpers||HTA||Downhill Bikes||HTA|
|DMR Sect||69°||Mondraker Summum||63.5°|
|Scott Voltage YZ 0.1||68.5°||YT Industries Tues 29||63.5°|
|Marin Alcatraz||69°||Commencal Supreme DH||63.5°|
|Commencal Absolut Dirt||69°||Canyon Sender CFR||63°|
|Santa Cruz Jackal||68.7°||Scott Gambler 900 Tuned||62°|
|Canyon Stitched 720 Pro||69°||Yeti – SB 165||63.5°|
Conclusion: The head tube angle of downhill bikes is notably slacker – 5-6% on average. This makes downhill bikes a lot more stable when riding over diverse terrain at high speeds.
The front end is raised and has an easier time going over the irregularities of the road. Also, there’s a smaller chance to flip over the handlebars.
Ultimately, the slacker HTA of downhill bikes makes them a lot safer for the terrain that they’re designed for whereas dirt jumpers put the rider at greater risk of falling.
Another major difference between dirt jumpers and downhill bikes is the suspension.
Classic dirt bikes have suspension only at the front. Since the main purpose of dirt bike forks is to soften large impacts (from jumps), the suspension is firm. Also, dirt jump forks have small travel in comparison to downhill forks.
Conversely, downhill bikes have double crown front forks and rear suspension with tons of travel designed to operate at all times.
The main purpose of suspension on MTBs is to increase the bike’s traction. When a wheel hits an obstacle, the suspension compresses and the wheel remains on the ground.
When there is no suspension, the wheel bounces and thus the bike loses traction and stability. The same happens when the suspension is too firm or simply inadequate.
When one tries to ride a downhill trail on a dirt jump bike, the suspension acts as follows:
- The firm fork fails to absorb small hits appropriately and jumps around. One way to circumvent this issue is to ride at low PSI (if the fork operates with air)
- The rear wheel of the bike will be jumping all over the place when hitting an obstacle due to the lack of suspension. Full suspension dirt jumpers are an exception to this rule, but even they won’t provide the same traction as a downhill bike.
Dirt jumpers are designed for tricks rather than comfortable pedaling. Hence why most models come in a single size. (To learn more about dirt jump sizing check out this article.)
A dirt jumper is not designed for seated pedaling. Hence why the seat is all the way down. The short wheelbase, chainstays combined with the steeper head tube angle make out of the saddle riding more dangerous because the chance to lose balance and flip over the handlebars is greater
In different, downhill bikes come in multiple sizes and are therefore much more comfortable and stable for regular riding. It’s also worth mentioning that many newer downhill bikes allow the user to adjust the head tube angle and the effective length of the chainstays (the axle can be pushed back and forth). Dirt jump bikes rarely offer those options.
Classic dirt jumpers operate with 26″ wheels for the following reasons:
- Smaller size
The smaller the bike, the easier it is to manipulate it around. (Hence why BMX bikes have 20″ wheels.)
- Stronger wheels
All things being equal, a smaller wheel is a stronger wheel.
Smaller wheels have a shorter lever arm and thus do not experience as much spoke stress when riding. As a result, 26″ wheels have an easier time handling drops and such.
- Maneuverability and Acceleration
Smaller wheels are more maneuverable and accelerate faster than bigger wheels. This makes them ideal for dirt jumping because the sport involves a lot of technique and tight cornering (especially when riding street).
Also, many spots do not have a lot of room for acceleration. Thus, the faster acceleration that smaller wheels provide is appreciated.
That said, 26″ wheels lose to 27.5″ and 29″ when it comes to descending because larger wheels have a greater roll-over ability.
Or in simpler terms, they have an easier time running over big obstacles. This results in speed and stability.
Many dirt jumpers come with a single rear brake which is often mechanical. This choice is intentional and meant to facilitate tricks such as bar spins.
Since dirt jumpers aren’t used as regular MTBs, a front brake is not really missed when dirt jumping. However, its absence tremendously reduces the braking power of the bike.
The front brake is very important because it provides a lot more stopping power. During braking, the weight is shifted forward. This increases the traction of the front wheel with the ground. Subsequently, the wheel has fewer chances to slide upon braking. Hence why front brakes are more powerful than rear brakes.
Ultimately, dirt jumpers have a weak braking system inferior for descending.
Downhill bikes, on the other hand, have two very powerful hydraulic brakes with massive rotors which provide tremendous stopping power giving extra freedom and security to the rider.
Having said that, one can easily add a front brake to a dirt jumper since suspension forks come with front brake mounts. Therefore, this point can be negated.
Summary: What You Need To Know
- Dirt jumpers are not built for descending. The frame’s size and geometry reflect that.
- Dirt jumpers have super short chainstays which make it easier to lift the front wheel but are inherently dangerous when descending.
- Most dirt jumpers are one-size-fits-all and thus provide poor “ergonomics” and customization compared to downhill bikes.
- Dirt jumpers have steep head tube angles and thus increase the rider’s chance of flipping over the handlebars.
- Dirt jumpers use 26″ wheels which are great for street and jumps but can’t match the roll-over ability of 27.5″ and 29″ wheels found on downhill bikes.
- Dirt jumpers have suspension with little travel (100-120mm) whereas downhill bikes have massive front suspension forks with two crowns and lots of travel (e.g., 200mm). As a result, the forks on dirt jumpers fail to increase traction as much downhill bikes.
- Dirt jumpers do not have rear suspension apart from some full-suspension models. That said, even the full-suspension models have inferior suspension for downhill.
- Dirt jumpers often come with a single rear brake that provides subpar braking power for downhill terrain.