Condensed answer: Rear BMX hubs have 110mm O.L.D. This makes the installation of a cassette or a multi-speed freewheel hub impossible unless the frame is modified.
Another obstacle is the absence of a derailleur hanger. This further complicates the conversion of a BMX bike into a geared machine to the point where the entire venture becomes low value.
An internal gear hub can also be used, but there aren’t that many models designed for hubs with 110mm O.L.D.
Over Locknut Dimension (O.L.D.)
The term Over Locknut Dimension (O.L.D.) refers to the distance between a hub’s locknuts.
The O.L.D. of rear BMX hubs is 110mm. Meanwhile, the rear O.L.D. of bicycles that have gears varies between 120mm (old road bikes) and 135mm (basic mountain bikes).
This leads us to the following conclusion:
The rear dropouts of a BMX frame are too close to each other to accept a hub on which the user can install a modern freewheel or cassette.
There are two possible workarounds but neither is ideal nor easy to implement.
Option 1: Cold set the Frame
If the frame is made of steel (which is the case for most BMX bikes) it can be cold set (re-bend) to spread the dropouts.
The jump from 110mm to 135mm is only 25mm, but it’s bigger than it sounds. To cold set the frame to that value, the dropouts will have to be spread far beyond that number because steel is elastic, and it takes a lot of effort to permanently deform it.
Cold setting a frame is not extremely dangerous, but it’s never 100% safe either. The larger the deformation, the greater the chance to damage the frame.
Moreover, a frame that has been cold set instantly loses its warranty.
One way to reduce the risk is to build a new wheel around an old 120mm O.L.D. rear hub that can support a 5-speed freewheel. In that case, the cold setting will be minimal.
The downside of this approach is that 5-speed is old technology. Thus, you will have to purchase all the parts from the second-hand market. Also, 5-speed hubs aren’t that strong because they’re designed for road bikes and retro commuters.
Note: After cold setting a frame it’s necessary to straighten the dropouts too because they will be out of alignment in their new position. This process requires a special bike tool.
Option 2: Get a Custom Frame
Another far more radical solution is to buy a custom BMX frame that can accommodate a cassette hub.
Such a frame will cost a decent amount of money because it will be handmade and different from standard BMX frames.
No Derailleur Hanger
Another problem that the user will face when trying to install gears on a BMX bike is the absence of a derailleur hanger.
This issue can be circumvented by welding a derailleur hanger to the drive-side dropout (recommended only if the bike is made out of steel) or by using an adapter such as a chain tug that has a derailleur hanger too.
FAQ: What about internal gear hubs?
Internal gear hubs are also an option when adding gears to a single-speed bike. In this case, the user has two choices:
a. Find an internal gear hub with 110mm O.L.D.
b. Cold set the frame to allow the installation of wider internal gear hubs.
The downside of option A is that you will be highly limited because very few internal gear hubs have 110mm O.L.D. One example would be Sturmey archer SRF5(W).
The con of option B is that you will have to cold set the frame.
If you want a geared bike for stunts, you can look into dirt jump frames that support derailleurs.
Those models will allow you to perform lots of tricks while giving you the efficiency of geared drivetrains.
Summary: What You Need To Know
- BMX bikes do not allow an easy installation of a geared drivetrain because the spacing between the rear dropouts is too narrow for a multi-speed cassette or freewheel.
- It’s possible to cold set a steel from to the point where it accepts a freewheel or a cassette. However, the procedure is not free of risk and should not be done by beginners.
- Another option is to get a custom frame with wider dropouts or switch to a geared dirt jumper.
- Some internal gear hubs could be successfully installed on BMX bikes.