Condensed answer: It’s possible to install BMX cranks on a mountain bike. In most cases, the procedure requires the use of a British BMX bottom bracket (a.k.a. Euro BB) with an inner diameter corresponding to that of the spindle (either 19 or 22mm).
Getting The Right Bottom Bracket
Bottom Bracket Definition: The bottom bracket of a bicycle is a combination of bearings plus a spindle/axle to which the cranks attach. The bearings allow smooth rotation of the spindle and consequently the cranks.
The bottom bracket is installed in the middle of the bike frame where the seat tube, downtube, and chainstays meet.
The compartment storing the bottom bracket is known as the bottom bracket shell.
The bottom bracket needed for the conversion has to answer two requirements:
- It has to be compatible with the bottom bracket shell of the MTB frame.
- It has to support a 19mm or 22mm spindle/axle. (Most BMX cranks are designed for spindles with a 19mm or 22mm diameter.)
The vast majority of mountain bikes have a threaded bottom bracket shell. Or in other words, the bottom bracket is screwed into the frame like a bolt.
To install a set of BMX cranks on a standard MTB, you would need a threaded bottom bracket built for a 19mm or a 22mm spindle (check your crankset to know which one you have.)
The only BMX bottom bracket that answers these criteria is known as Euro BB.
This type of bottom bracket has other names too – BSA, BSC, ISO, British.
BSA – Birmingham Small Arms (a British industrial combine)
BSC – British Standard Cycle (a British Imperial screw thread standard)
ISO – International Standard Organization
Before buying a bottom bracket, you have to make sure that it’s the right width for the BB shell of your frame.
Most MTB frames have 68/73mm wide bottom bracket shells.
However, some frames have a bit wider bottom bracket shells. If you have one of those, you will need a Euro BB of greater width.
Important: In this case, we are focusing on the width of the bottom bracket without the spindle.
Sometimes the listed width of the bottom bracket will include the spindle and thus be over 100mm.
Once you have the Euro BB that you need it’s time to install it.
Installing a Euro BB Bracket
Step 1: Determine the Spacing of Bottom Bracket
This is a fundamental step meant to ensure that the bottom bracket fits the frame perfectly.
If the bottom bracket is too wide, the cranks will feel unstable.
If the bottom bracket is too narrow, the bearing will bind.
Ultimately, you want the cups of the bottom bracket resting nicely against the outer part of the bottom bracket shell.
To ensure, this outcome, one has to search for the right combination of axle sleeve + washers + spacers.
Here’s how that’s done:
1. Slide the spindle through one cup.
2. Slide a sleeve of your choice onto the spindle
3. Add a washer or two.
4. Put the other cup on the spindle.
5. Turn the frame/bike upside down and place the spindle parallel to the wide part of the bottom bracket shell.
If the cups are sticking too far out, remove a washer or use a shorter sleeve.
If the cups are too close to one another, use a longer sleeve or add a washer.
Repeat until the cups rest nicely on the outer lip of the bottom bracket shell.
Note: Don’t proceed to installation until you get this part done. If you do, you may have to remove the bottom bracket and get a new fit.
Step 2: Clean and degrease the bottom bracket shell. Wipe it with a cotton rag and let it dry.
Step 3: Grease the threads and the outer ring/face of the bottom bracket to prevent squeaking.
Step 4: Install one of the cups into the frame. Thread it slowly and use only your fingers without applying excessive force.
You should be able to get the cup down to the outer ring without tools.
Step 5: Push the spindle through the cup. Then slide the sleeve and the needed washer(s) onto the spindle from the other side.
If you use 2-piece cranks begin with the one pre-attached to the spindle.
If you have 3-piece cranks, you can start with either side.
Step 6: Install the other cup.
Step 7: Tighten both cups with a wrench.
Note: The cups tighten in opposite directions.
Step 8: Install the cranks
What To Do If You Don’t Have a Euro BB Shell
If the bottom bracket shell of your frame doesn’t have threading, then it doesn’t support Euro bottom brackets.
This situation leaves us with the following options:
1. Install an adapter
If you have а press fit BB30 bottom bracket, you can convert it to BSA/Euro bottom bracket via an adapter such as FSA’s BB30 to BSA converter.
The adapter is pressed into the bottom bracket after the bottom bracket is installed.
It’s mandatory to apply Loctite 609 or an equivalent installation compound to the adapter and the seating surfaces of the bottom bracket before installation.
Note: Before buying an adapter measure the BB shell of your bike. If it’s 68mm wide, you need a 68mm adapter. If it’s 73mm, you need a 73mm adapter.
You can find FSA’s full installation manual in this PDF file.
There are BSD adapters for PF30 (Press Fit 30) frames too. One example would be SRAM’s PF30 to 83mm Euro BB converter.
Once the adapter is mounted, you can proceed to install a Euro BB bracket designed for your BMX cranks.
2. Bottom Bracket Reducer Adapter
Another option is to use a bottom bracket reducer – an adapter making your existing bottom bracket compatible with spindles of smaller size.
Such reducers exist for both BB30 and PF30 bottom brackets.
3. Use a Race BMX Bottom Bracket
The brand Profile Racing offers a press-fit bottom bracket designed to fit BB30, BB86, and BB92 MTB bottom brackets.
The bottom bracket comes in two versions – for 19mm and 22mm spindles.
The Benefits Of BMX Cranks
The strong points of BMX cranks are as follows:
A set of quality BMX cranks is as strong as it gets. They’re made of heat treated 4130 steel a.k.a. Cr-Mo and are practically unbreakable unless you’re a professional rider.
Many riders dig the look of BMX cranks and put them on their dirt jumpers and MTBs for style points rather than strength.
Truth be told, MTB cranks are more than strong enough for most riders’ needs.
3. Shorter Cranks
In some places, shorter BMX cranks are easier to find than shorter MTB models.
If you want to, you can go down from 175mm MTB cranks to 160mm BMX cranks.
The main benefit of shorter cranks is that they are stiffer (shorter moment arm) and allegedly more knee-friendly.
Why Don’t Non-Euro Bottom Brackets Fit On Аn MTB?
Most BMX bikes use one of the following bottom brackets:
Mid bottom brackets are the current BMX standard and come on most up-to-date-models.
Mid BBs do not use cups. Instead, they have large bearings press fit directly into the frame.
Spanish bottom brackets operate on the same principle as Mid BBs except that the bearings are smaller.
The American bottom bracket has large cups pressed into the bottom bracket shell. The cups are the bearings’ home.
You can’t install the aforementioned bottom brackets on an MTB because most mountain bikes still have a threaded bottom bracket shell incompatible with press-fit BMX options.
Most MTB frames with press-fit bottom brackets are also incompatible with BMX bottom brackets due to the non-matching inner shell diameters.
The table below contains the inner diameters of MTB bottom bracket shells:
|Required Inner Shell Diameter
Meanwhile, press-fit BMX bottom bracket shells have the following inner diameters:
|Required Inner Shell Diameter
The tables above show that most press-fit MTB and BMX bottom brackets are designed for frames with different BB shells.
The only exception is the BB86/92 bottom bracket which uses the same shell size as the Mid bottom bracket (41mm).
Therefore, you may be able to install a Mid bottom bracket on a BB86/92 frame.
However, doing so could void the warranty of the frame since you’re installing a component that’s not officially compatible.
Also, if the frame is made out of carbon, pressing a Mid bottom bracket bearings into it may cause some damage. A standard BB86/92 bottom bracket uses plastic cups which are a lot kinder to the bottom bracket shell.
That said, some BB86/92 models work with 19mm spindles axles by default. If you get one of those, you can install a BMX spindle and cranks.
The length of the spindle could be a problem, though. MTB frames designed for BB86/92 bottom brackets have a 91.5mm BB shell width.
The Downside Of Running BMX Cranks On an MTB
Installing BMX cranks on an MTB has the following negative effects:
1. Single chainring (potentially)
BMXs are single-speed bikes with one chainring at the front secured either by splines on the axle or by a pinch bolt screwed into the drive side crank.
If you want to use BMX cranks in conjunction with a standard BMX chainring, you won’t be able to run a multi-chainring set-up.
Having said that, it’s possible to combine non-BMX chainrings with BMX cranks. All you need is a spider adapter.
The spider adapter connects to the cranks as if it’s a chainring. Then, you can mount all kinds of chainrings to the spider as long as they’re compatible with it.
You can even run multiple chainrings if the spider supports more than one (Profile make such spiders).
2. Gear Problems
Most people wouldn’t care about the chainring issue because 1x drivetrains are the norm on modern MTBs.
However, you won’t be able to run a multi-speed 1x with a standard BMX chainring either because BMX chainrings are thicker and spaced differently.
Thus, they cannot fit into an MTB chain designed for gears (the more gears they are, the thinner the chain has to be).
If you want to use a classic BMX chainring, you will have to run a single speed set-up.
If you want to preserve the gears at the back, you will have to use a spider and install a 1x MTB chainring.
Another option is to search for rare BMX chainrings compatible with MTB chains.
3. Chainline Issues
New cranks may hurt the chainline of your bike. Or in other words, the chain could get too much to the left or right.
The chainline can be adjusted by adding spacers that the bottom bracket.
If you run a single-speed system, you could play with the position of the rear cog too.
4. Expenses, Labor, Time
The number of needed components quickly adds up and turns the conversion into a costly adventure.
The entire procedure may end up more expensive than a good set of MTB cranks.
BMX Cranks vs. MTB Cranks
1. Material Differences
One of the major differences between BMX and MTB cranks is the used material. Most BMX cranks are made of Chromoly steel whereas MTB cranks are produced from aluminum or carbon.
Steel is nearly 2.5 times denser than aluminum. In consequence, aluminum cranks have to be notably larger to match the strength of steel cranks. (For that reason, MTB cranks are thick whereas BMX models are slim.)
The thickness and elasticity of steel make the material more resistant to impact than aluminum. Therefore, steel cranks are preferred for freestyle BMX bikes which tend to be thrown around a lot. Also, steel cranks are better for grinding.
Normally, grinding is done on the pegs, but it’s possible to touch the ledge with the cranks during training. Steel cranks cope much better with the abuse than aluminum ones.
Another bonus of steel is that it’s more likely to bend than to break whereas aluminum is brittle and more prone to snapping.
MTB cranks are “gear ready” and come with one or multiple chainrings designed for thinner, derailleur-friendly chains. Conversely, BMX cranks are single-speed by default. To run gears, you need a spider adapter which isn’t widely available.
3. Weight Comparison
The table below compares the weight of MTB and freestyle BMX cranks:
|BSD Substance XL
|Shimano XT FC-M8000-1
|Shimano SLX FC-M7000-11-1
|Profile No Boss
|SRAM NX Eagle
|Shimano SLX FC-M7000-11-B1
|Profile Race RHD
|Shimano SLX FC-M7100-1
|Race Face Atlas Cinch Cranks
|Shimano XT FC-M8000-B1
|Shimano XTR Trail FC-M9020-1
|Saltplus Pro 48
|Shimano Deore XT FC-M8000-1
|Cult Hawk V2
|Hope EVO Crank
Conclusion: On average, MTB cranks are 188g lighter than the BMX models in the table. In the bike world, this is considered a substantial difference.
Aesthetic preferences aside, the main advantage of BMX cranks is that they are very durable. MTB cranks, on the other hand, are light and allow the installation of “gear-friendly” chainrings.
If your MTB uses a threaded bottom bracket, you can install a Euro BMX bottom bracket on it. Then, you can easily add BMX cranks to the bike.
If your MTB has a press-fit bottom bracket, you can use an adapter to turn the existing bottom bracket into a threaded one.
You can also search for a press-fit bottom bracket model that supports 19mm or 22mm spindles.
If you want to use a standard BMX chainring with your new cranks, you won’t be able to keep your gears.