The Weirdest BMX Mod I’ve Been Asked About

from Rookie’s Keyboard

Hello, friends

Yesterday, I got an unusual request in my mailbox. A reader wanted to know whether it’s possible to transform a BMX into a multi-speed bike. 

Honestly, I’ve never thought about this issue as I’ve always seen BMX and dirt jumpers as the epitome of single-speed bikes. 

But then I began thinking and came up with the following analysis. 

The First Obstacle – Over Locknut Dimension(O.L.D.)

You are about to learn a new bike term – Over Locknut Dimension (O.L.D.) That’s the distance between the hub’s locknuts (the nuts securing the bearings and the axle).

The O.L.D. depends on the bike type. 

Rear BMX hubs have 110mm O.LD. whereas that of geared bikes varies between 120mm (old road bikes) and 135mm (basic bikes).

So, what does this tell us, rookies?

The rear dropouts of BMX frames are too narrow to accept a cassette/freewheel hub. There just isn’t enough space.

And that’s a major issue. 

That said, there are 3-4 possible “hacks”. I will describe them, but I honestly don’t recommend wasting your time performing them. The information is for educational purposes mainly.

Option 1: Cold Set the Frame

When the frame is made of steel (the case for most BMX bikes), it can be cold set a.k.a. bent to spread the dropouts further apart.

The difference between 110mm and 135mm is only 25mm, but to cold set the frame to that value, the dropouts will have to be spread far beyond the targeted number.

Remember, rookies! 

Steel is elastic and requires more effort and amplitude to be permanently deformed as it will otherwise return to a value smaller than 135mm. (You get the idea. I’m sure of it.)

“Cold setting” is fairly safe but still risky. The greater the deformation, the greater the chance of damaging the frame. Also, say goodbye to whatever warranty you had.

Option 2: A New Wheel

You can also build a new wheel around an old 120mm O.L.D. rear hub that can support a 5-speed freewheel to minimize the spreading of the dropouts.

Of course, you lock yourself into a 5-speed old-school prison. Not only that but all the parts will be used. 

Furthermore, 5-speed hubs are weak as they are built for retro road bikes and urban commuters.

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And by the way, after cold setting the frame it’s necessary to straighten the dropouts too because they will be out of alignment (not parallel to each other). This process requires a special bike tool.

Option 2: Acquire a Custom Frame

The most radical solution is to order a custom BMX frame that can accommodate a cassette hub from the get-go.

This route isn’t that bad, actually. The only downside is that you will have to pay a lot more as the frame will be handmade specifically for you.

More Issues

You thought we were done! 

Haha. There are more issues to face if you want to “gear up”.

• No Derailleur Hanger

BMX bikes don’t have an actual derailleur hanger – a.k.a. there is no way to mechanically mount the rear mech.

You can weld (or pay someone to do it) a derailleur hanger to the drive-side dropout. 

Alternatively, you could also use a chain tug that has a built-in derailleur hanger.

Internal Gear Hubs?

Technically, you could also go for an internal gear hub, although it would be a weird choice for a BMX…but it’s up to you.

Here’s what’s necessary:

a. An internal gear hub with 110mm O.L.D. (the options are limited to a few e.g., Sturmey Archer SRF5(W))

b. Cold set the frame to allow the installation of wider internal gear hubs.

Go for a hardtail?

Rookies, if you want to acquire a geared bike that can also do stunts, you can look into dirt jump frames that support derailleurs.

Those models combine aggression with efficiency (gears).

Personally, I would never bother modding a BMX into a multi-speed bike.

I think the point of BMX is found elsewhere. But I’m glad I was able to answer the inquery of a reader. 

If you have more questions, just ask in the comments below.

Until next time,

– Rookie