Condensed Answer: By default, dirt jumpers have a single-speed drivetrain and rear-facing dropouts complicating the installation of a transmission. However, with the right adapter, one can add a rear derailleur to the bike and equip it with gears.
Requirements For Installing Gears On a Dirt Jumper
1. A Derailleur Hanger
To install gears, you have to mount a derailleur to the frame.
Geared bicycles have a derailleur hanger that’s either a part of the frame or attached via bolts.
Dedicated dirt jumpers, however, come with rear-facing dropouts (the most secure set-up for a single-speed bicycle) and don’t have a derailleur hanger because the function of the bicycle does not necessitate one.
If you want to add gears, you will have to find a way to install a rear derailleur.
The possible solutions are:
- A Derailleur Hanger Adapter For Rear-facing Dropouts
There are derailleur hanger adapters that slide into the drive side dropout of the frame. Usually, they have a small inner profile that stabilizes them. The axle passes through the hole in the middle. Once the wheel is tightened to the dropouts, the hanger is secured too.
Before buying such a hanger measure the length and width of your dropouts and compare the findings to the dimensions of the particular adapter.
- Chain Tug + Derailleur Hanger Combo
Chain tugs make it easier to center the wheel and set the chain tension of a single-speed bike.
Some models come with a derailleur hanger allowing the installation of a rear derailleur too.
- A Derailleur With Its Own Hanger
Some entry-level derailleurs (e.g., Shimano Tourney TZ500) come with their own hanger which bolts to the frame. The downside is that you will be limited to 6-7 speeds.
- Weld A Derailleur Hanger To The Frame
If the frame is made of steel, one could also weld a derailleur hanger to it. The procedure eliminates the need to play with various adapters but requires welding and machining experience which most people do not have.
Your best bet is to locate a competent frame builder and pay him to do it. It won’t cost you much, but the hard part would be to find the right person for the job.
The cheaper variant of this option is to cut off the derailleur hanger of an old steel frame and weld it to the bike yourself or ask someone from your family or social circle who has some welding experience to do it for you.
If the frame is aluminum, welding is a bit more complex and most people do not recommend it.
2. Install a Cassette
To add gears to a dirt jumper, you will have to install a cassette on the rear wheel.
This step is problematic because dirt jump rear wheels are designed for a single-speed drivetrain. As a result, the hubs do not have enough space to accommodate a cassette because the driver is too short.
To solve this problem, one has to replace the rear hub with one designed for gears. The procedure will require new spokes too because the spoke length is specific to the hub and rim.
Once you have a new hub and the proper spokes, the wheel will have to re-laced. If you can’t do that procedure yourself, you will have to pay someone to do it for you (it’s not always cheap).
After calculating the expenses, you may conclude that it’s cheaper to buy a new MTB rear wheel designed for gears instead of trying to modify the current one.
A Note On Dropout Spacing
The rear dropout spacing of most dirt jumpers is 135mm and matches that of mountain bikes. This means that you can use a hub with a 135 over-locknut dimension (O.L.D.). The term O.L.D. refers to the usable part of the hub or the distance between the outer ends of the locknuts.
That said, some dirt jumpers have a BMX rear dropout spacing which is 110mm. In that case, you won’t be able to install a wheel with a cassette.
3. Cable and Housing
The frame is unlikely to have cable stoppers for the shifter’s cable and housing. The simplest workaround is to run full cable housing from the shifter to the rear derailleur and secure it with zip ties. This method works, but many consider it unsightly and sub-optimal.
A better way would be to buy clamp-on cable stoppers and install them on the top tube. The cable stoppers allow you to run the shifter cable “naked” between them as it’s done on regular MTBs.
Ideally, you will also install cable stoppers on the drive-side seat stay. However, cable stoppers of such diameter are a bit hard to find, and you may have to use zip-ties as shown in the video below:
4. A New Shifter
You will also have to install a new shifter corresponding to the number of gears in the back. If the cassette has 9 gears, you will need a 9-speed shifter.
Since the shifter will be manipulating the rear derailleur, it’s installed on the right side of the handlebars. To mount it, you will have to remove the grips and the and brake lever.
Tip: If the grips don’t use clamps, you may have a hard time removing them. You can ease the process by lifting the outer part a bit and spaying perfume underneath. Another option would be to blast compressed air under the grip.
5. A New Chain
Geared bikes use narrower chains than single-speed models to avoid rubbing on the adjacent cogs. You won’t be able to use the existing single-speed chain because it will be too wide.
The Advantages of Adding Gears To a Dirt Jump Bike
The advantages of adding gears to a dirt jump bike are:
- More efficient pedaling. Gears allow you to ride longer and spare your energy.
- More versatility. A bike with gears is more versatile than a single-speed model because it can climb better and subsequently allows the rider to cover more diverse terrain.
- Less stress on the knees. Lower gears reduce the stress on the joints when going uphill.
The Disadvantages of Adding Gears To a Dirt Jump Bike
Adding gears to a dirt jumper has the following cons:
- Another point of failure. The rear derailleur and the hanger can be damaged during riding.
- Extra weight. The derailleur, shifter, cassette, cable, and housing will add some weight to the bike. It won’t be substantial, but if you’re trying to keep your machine as light as possible, you may want to stick with a single-speed set-up.
- Extra fees. The conversion is expensive because you will need an adapter, a derailleur, a cassette, a new wheel (or at least a new hub and spokes), and a shifter.
- Different aesthetics. Once you add gears, the clean look of the bike will be lost.
FAQ: Can I run the shifter cable through a gyro?
No. A gyro system can be used only with a brake cable. A shifter cable can’t work through a gyro because the tension of the cable has to be maintained in order for the derailleur to remain in the position that you put it in. If there’s no tension, the derailleur will go to its default state.
Conversely, a brake cable works through a gyro because it’s tensioned only when you’re pressing the brake.
FAQ: Can I install a front derailleur?
You can mount a clamp-on front derailleur on a dirt jumper. You will also need new cranks that support multiple chainrings, a front shifter as well as a cable + housing.
Note: You will face the cable routing problem again since the frame won’t have cable stoppers.
Having said that, sticking with a 1x drivetrain (single chainring) has many benefits such as:
- Cheaper (you won’t have to buy a front derailleur, new cranks…etc.)
- Simple (you won’t have to bother adjusting the front mech)
- Cleaner (a single chainring gives the bike a simplistic and clean appearance)
FAQ: Can I use a dirt jumper as a commuter?
A dirt jumper makes for a poor commuter for the following reasons:
- Inefficient pedaling in a seated position (the seat tube is short, and the seat is positioned too far behind the bottom bracket)
- Small and short frame resulting in a non-aerodynamic position
- Lack of fender and rack mounts
- No gearing (at least by default)
If you want to learn more about commuting with a dirt jumper, check out the dedicated post.
FAQ: Are dirt jump bikes good for trail riding?
In general, dirt jumpers aren’t optimal for trail riding due to their geometry. You can technically ride a dirt jumper on a trail, but the performance won’t be optimal.
If you want to learn more about this topic, check out the dedicated post.