Condensed answer: It’s possible to install an MTB crankset on a road frame. The conversion will require an MTB bottom bracket because MTB cranks have a longer spindle/axle.
Bottom Brackets and Spindle Compatibility
The first requirement for installing an MTB crankset on a road bike is to make sure that the new MTB cranks and the bottom bracket can work together.
Unfortunately, MTB cranks aren’t compatible with most road bike bottom brackets. The only exception would be a square taper bottom bracket.
A mountain bike axle does not operate well with a road bike bottom bracket for two reasons:
1. MTB axles and bottom brackets are longer than their road equivalents.
When you combine an MTB axle with a road bottom bracket, you’re left with two side gaps between the bottom bracket’s cups and the cranks.
The result is an unstable crankset.
This problem is resolved by placing a set of extra spacers around the bottom bracket to adjust the chainline and compensate for the gap created by the longer axle.
2. The diameter of a road bottom bracket could be too small for MTB cranks.
The outer diameter of the spindle has to match the inner diameter of the bottom bracket. If it doesn’t, the axles won’t fit through the bearings.
Note: Some road bottom brackets have a different inner diameter on both sides. In that case, the axle may fit only through one entrance.
Threaded Bottom Brackets
A BSA threaded bottom bracket offers the most convenient way to install MTB cranks on a road bike.
BSA stands for Birmingham Small Arms (a British industrial combine). The same type of bottom bracket is also known as:
BSC – British Standard Cycle (a British Imperial screw thread standard)
BC – Short for BSC
ISO – International Standard Organization
Luckily, BSA is the most common threaded bottom bracket.
A BSA bottom bracket uses either internal or external bearings.
The BSA bottom brackets with internal bearings are known as “square taper” because the spindle’s ends have the shape of a square. The cranks are mounted via bolts screwed into the spindle.
If your road bike has a square taper bottom bracket, you will have to search for square taper MTB cranks.
The positive in this case is that you don’t have to worry about the spindle’s length and diameter as square taper bottom brackets use the same standard.
Square taper cranks are considered low-end, but in practice, they operate well. I’ve had Shimano Acera triple on my MTB for years and cannot say a bad word about them.
Nonetheless, it’s best to stay away from square taper cranks with non-removable chainrings as you will have to replace everything when the chainring wears down.
Conclusion 1: If you have a square taper bottom bracket, you can mount square taper MTB cranks to it right away.
If the bottom bracket has external bearings (out of the bottom bracket shell), the transition from road to MTB cranks is a bit more complicated.
Most road bikes have a 68mm bottom bracket shell whereas that of standard MTBs is 73mm. To reflect that MTB bottom brackets are а bit wider.
When you install such an MTB bottom bracket on a road frame, it does not rest neatly against the outer ring of the bottom bracket shell.
To make a 73mm bottom bracket compatible with road frames, manufacturers include small spacers that go between the bottom bracket’s cups and the shell/frame.
The spacers “eat” the gap and allow you to fully tighten the bottom bracket.
Conclusion 2: If you have an external BSD bottom bracket on your road bike, you will have to replace it with an MTB one because MTBs have longer bottom brackets and spindles.
To compensate for the narrow bottom bracket shell of the road frame, you will have to use spacers which usually come with the MTB bottom bracket.
Press-fit Bottom Brackets
The main difference between press-fit bottom brackets and threaded ones is the installation process – press-fit cups and bearings are pressed into the frame whereas threaded models are screwed into the bottom bracket shell.
If you want to install MTB cranks on a road frame with a press-fit bottom bracket, you will have to replace the bottom bracket with an MTB model due to the spacing.
You cannot use the existing road bike bottom bracket because it’s shorter than an MTB model. In consequence, the spindle will be sticking out when you install MTB cranks.
To fix this issue, you can theoretically install spacers. However, the spacers will prevent the cup from going into the frame with its full body. The aftermath will be an unstable bottom bracket.
The only way out is to use an MTB bottom bracket in conjunction with spacers. The MTB bottom bracket is longer and won’t be affected by the aforementioned stability issue.
Conclusion: 3: If you have a press-fit road bottom bracket, you will have to replace it with an MTB version and add spacers to install MTB cranks on that frame.
Front Derailleur Compatibility
If you want to run a multi-chainring set-up, you will face some front derailleur issues.
The cable ratio of a derailleur expresses how much it moves by 1mm of cable pull or release by the shifter.
Shimano MTB and road front derailleurs have a different cable pull ratio.
As a result, combining an MTB derailleur with a road shifter (e.g., STI) could result in a non-ideal shifting experience because the derailleur won’t be moving as much as envisioned for proper index shifting.
The logical way to avoid this issue would be to stick with the existing front derailleur.
Unfortunately, this path doesn’t always guarantee smooth shifting.
The possible problems are:
- Inability of the derailleur to fully reach the outer ring due to the longer spindle.
- Incompatibility with a triple crankset. (Many road front derailleurs are designed for double chainrings. You won’t be able to combine those with a triple MTB crankset.)
- The chain may fall when using the so-called granny gear because road derailleurs are designed for bigger chainrings. (You can install a chain guard to reduce the chances of that outcome.)
- The derailleur may not have the capacity to make the jump from the mid-ring to the large ring.
Clamp-оn Front Derailleurs For The Win
Based on the mounting system, front derailleurs divide into two main groups – clamp-on and braze-on.
The clamp-on derailleur connects directly to the seat tube via a clamp whereas the braze-on has a dedicated mounting spot on the seat tube.
The advantage of clamp-on front derailleurs is that their position can be moved up or down. Braze-on models, however, have a fixed location.
Braze-on derailleurs can create a problem when adding MTB cranks to a road bike.
MTB cranks are smaller and thus the front derailleur has to be lowered.
The front derailleur (FD) should be 2-4mm higher than the largest chainring. If the FD a braze-on type, it may be impossible to lower it sufficiently. The result will be a poor shifting experience.
If you have a braze-on derailleur, and it’s giving you trouble due to its high position, there are two solutions:
1. Switch to a clamp-on derailleur
2. Use a braze-on derailleur dropper (a small adapter that lowers the mounting point)
2x Drivetrains Are More Likely To Work Flawlessly
A road FD combined with a triple MTB crankset may have less than ideal performance because the set-up requires a bit more precision due to the extra shift.
Consequently, if you plan to combine a road FD with a multi-chainring set-up, a 2x drivetrain is the recommended option.
Note: The above problems manifest only if you use index shifters. If you have friction shifters, you can mix road and MTB derailleurs as you wish as the shifter’s movement isn’t pre-determined.
You Will Only Know If You Try
It’s difficult to say how well the combo (road front derailleur + MTB chainrings) will perform until you experiment because every set-up is different.
With a bit of adjustment, the outcome is often satisfactory.
Conclusion 4: Index shifters complicate the use of a front derailleur when installing MTB cranks on a road bike. However, the system can still work. The chances of success are greater if you use a double chainring and a clamp-on front derailleur.
The main difference between MTB and road cassettes is the gearing. Standard road cassettes do not offer the low end of MTB models. The largest cog has about 25 teeth.
Another dissimilarity is the depth of the splines. Road hubs tend to have deeper splines because some road cassettes have many individual gears.
Individual gear is one that isn’t mechanically connected to the cassette unless it’s on the hub. Such gears stress the rear hub at a specific location and wear down the splines. To compensate, road hubs have deeper splines.
However, this technicality does not affect the crankset at all.
Therefore, the existing road bike cassette should work just fine with MTB cranks. There is no need to replace it if you decide to do the conversion.
If you want, you can put an MTB cassette in the rear too.
However, keep in mind that you cannot use an MTB cassette over 9 speeds with an indexed road shifter because 10-speed MTB derailleurs have a different rear shift ratio than the road models.
Also, chances are that your road bike has a short cage derailleur. If you want to use a high-range MTB cassette, you will have to install a derailleur with greater capacity.
Conclusion 5: It’s not necessary to replace the cassette because it has no impact on the crank conversion.
What Are The Main Benefits Of Putting MTB Cranks On a Road Bike?
1. Low gearing
A typical road crankset has two chainrings with 52 and 39 teeth.
Meanwhile, MTB cranksets go up to 48 teeth and often have a 22-28t small chainring.
Even a set of compact road cranks (50t, 34t) offers higher gearing.
Who would benefit from the low gears?
Three main demographics:
a. People who are out of shape and need lower gears to conquer hills.
b. Touring cyclists who travel with lots of luggage.
c. Cyclocross racers who ride very demanding trails.
MTB bottom brackets and cranks are stronger than the road models.
When installed on a road bike, however, the extra strength is of questionable benefit because the machine isn’t intended for stunts.
And if you were to try some MTB or street tricks, the road frame and fork will get damaged before the cranks have had a chance to “warm-up.”
3. Spare Parts
If you want to be frugal and already have MTB cranks, it would make sense to install them instead of buying a new road-specific set.
Some people like the “turtle ninja” look of MTB cranks and would gladly put them on their road bike for “style points”.