Condensed Answer: It’s not recommended to convert BMX bikes into dirt jumpers because the changes to the geometry will be negative. You can make a 24″ or 26″ BMX look like a dirt jumper by putting a suspension fork on it, but in most cases, the handling of the bike will be poor.
You’re also risking structural damage to the frame due to the extra stress on the headtube.
If you want to try dirt jumping, it’s better to invest in a dedicated machine.
20″ Freestyle BMXs
Standard BMX bikes have 20″ tires and come in two categories – race and freestyle.
Racing BMX bikes are made from lighter materials (aluminum, carbon) and excel at acceleration. They can’t take as much beating as freestyle models and aren’t recommended for dirt jumping.
Freestyle 20″ BMX bikes made from Chromoly steel, on the other hand, are some of the strongest bikes on the planet and can be used for dirt jumping as they are.
Of course, 20″ BMXs aren’t going to perform as well as a dedicated dirt jumper due to the small tires and frame, but it’s still possible to get some work done.
You can’t turn a 20″ BMX into an actual dirt jumper because:
- 20″ suspension forks are a rarity. The only available options are either custom-made and expensive or taken from kids’ bikes and unable to cope with the stress that an adult would put on them.
- You can’t install larger wheels on the bike due to the lack of clearance.
24″ and 26″ Cruiser BMXs
On the surface, 24″ and 26″ BMX appear to be the perfect candidates for a BMX-to-Dirt Jumper conversion thanks to the large wheels, but reality disagrees.
Turning a 24″ or a 26″ BMX cruiser into a dirt jumper has the following downsides:
1. Poor Bike Handling
Installing a suspension fork on a cruiser built around a rigid fork creates issues.
The axle-to-crown (ATC) length (image below) of a suspension fork is notably greater than that of a rigid fork.
An average rigid fork usually has around 390mm axle-to-crown distance.
Conversely, the axle to crown length of a fork with 100mm travel is about 478mm (e.g., Manitou Circus).
If you were to put that fork on a BMX cruiser with a 390mm ATC, the front end will raise by 88mm/8.8cm when the suspension fork is in a non-compressed state.
If you reduce the travel to 80mm, the suspension fork will have a 458mm ATC length, and the front end will still gain a substantial height – 6.6cm.
As a result, the headtube angle will become super slack, and the maneuverability of the bike will suffer tremendously. The rider will lose their ability to make technical corrections.
The bottom bracket of the bike will get higher too. A taller bottom bracket raises the rider’s center of gravity and hurts stability.
Note: It will also become substantially easier to loop out (the bike sliding in front of you) because more of the rider’s weight will be shifted towards the back wheel.
2. Extra Stress On the Head Tube
If you put a suspension fork on a frame that isn’t designed for suspension, the head tube can snap due to the extra stress on the area. The longer the travel, the greater the chances of that outcome.
The frames of dedicated dirt jumpers don’t experience that problem because they have taller, reinforced head tubes.
3. Anti-stunt Geometry
Many BMX cruisers aren’t designed for tricks, to begin with. They have a comfortаble geometry (large frames, long chainstays, long top tubes…etc.) making the bikes suitable primarily for cruising as their name suggests.
You can still perform tricks on such a bike, but it will be a lot more difficult. You will be swimming against the current.
Another major downside of cruiser frames is the high seat tube. Even if you lower the seat post all the way, the seat still gets in the way when performing tricks such as bunny hops.
4. No Mounts For Disc Brakes
Dirt jumpers use disc brakes (usually only a rear one) whereas 24″ cruisers rely on rim brakes. This is not necessarily a downside because rim brakes can provide a lot of braking power. Nonetheless, disc brakes offer superior braking performance, especially in dirty and wet conditions.
Frequently Asked Questions
Can you convert a BMX into a mountain bike?
A BMX makes for a bad mountain bike for the following reasons:
- Small tires. BMX bikes have small tires that have a harder time rolling over obstacles in comparison to standard MTB wheels which are now 27.5″ and 29″.
- Low speed. Small wheels accelerate faster than bigger ones, but it’s difficult to maintain a high average speed. If the gearings of two bikes are identical, the bicycle with the bigger tires has potential for greater speed.
- Small frame. BMX frames are designed for tricks. The rider isn’t supposed to pedal seated. The saddle is there for aesthetic purposes and to rest between attempts. As a result, it’s impossible to pedal efficiently.
- No suspension. Modern MTBs have suspension whose purpose is to absorb irregularities and increase traction. BMX bikes, on the other hand, are rigid. As a consequence, the frame and fork jump around upon hitting an obstacle. The aftermath is poor stability on the trail.
Note: If you install an MTB suspension on a BMX bike, the handling will suffer due to the aforementioned changes to the geometry.
- Single-speed. BMX bikes are single-speed machines. While the simplicity of single-speed bikes is great, a gear system makes the bike a lot more versatile. Obviously, this point doesn’t apply if you intend to use the BMX bike as a dirt jumper rather than an actual MTB.
- No disc brakes. Disc brakes are a game-changer when it comes to mountain biking. Even the biggest fans of rim brakes will admit that.
Can you convert a mountain bike into a dirt jumper?
It depends on the frame’s geometry. If it’s designed for cross-country riding and has a large top tube and long chainstays, it won’t be ideal for conversion.
The long chainstays will make it harder to lift the front wheel whereas the large dimensions of the frame will complicate the execution of different tricks.
However, if you have an aggressive and compact frame with short chainstays, it’s possible to make the bike even more suitable for dirt jumping by doing the following:
- Making the fork stiffer (if that’s possible)
- Switching to a single-speed drivetrain
- Replacing the stem with a shorter one
- Lowering the seat post maximally
That said, if you’re serious about dirt jumping, it’s best to buy a dedicated machine when an opportunity presents itself.
Summary: What You Need To Know
1. A 20″ freestyle BMX bike can be used for dirt jumping as it is. However, it’s not as optimal as a dedicated dirt jumper.
2. Trying to turn a 24″ or 26″ BMX cruiser into a dirt jumper by adding a suspension fork will “confuse” the geometry of the bike and harm the handling. It’s best to avoid this practice and search for an actual dirt jumper.
3. Using a BMX bike for MTB trails will result in poor experience because the machine has small wheels, a short frame and prevents the rider from pedaling efficiently.
4. If you have an MTB with a compact frame (short chainstays, a sloping top tube…etc.), the bike could be turned into a capable dirt jumper.