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Description of the Situation:

1. You have a mountain bike that you would like to turn into a road machine by installing a set of slick tires and increasing the gearing.

2. You have road cranks or plan to buy a pair but don’t know if it’s possible to put them on an MTB.

The condensed answer is:

Mountain bike frames aren’t designed for road cranks. The chainstays are wide to accommodate fatter tires and often create clearance issues.

Whether this problem will manifest depends on the particular frame and chainring combo, but the chances are high.

More often than not, the conversion fails miserably and is not worth the effort, money, and time.

Technical Issues

The technical issues that arise when trying to install road cranks on a mountain bike are:

1. Chainstay Clearance

Chainstays. The pair of lower tubes on a bike frame connecting to the bottom bracket and the rear fork’s ends.

Mountain bikes are designed for aggressive off-road terrain and benefit greatly from the use of wider tires. To accommodate fatter tires, MTB frames come with wide chainstays. If the chainstays are narrower, the tire would touch the frame.

The protruding chainstays create clearance issues when combined with road cranks for two reasons:

a. Road chainrings are very large (50+ teeth)

b. Road bikes operate with a shorter bottom bracket. If you install a road bottom bracket on an MTB, it will bring the cranks even closer to the chainstays.

2. Bottom Bracket Issues

Bottom bracket. A system of bearings and a spindle/axle installed in the middle of a bike frame (bottom bracket shell) to ensure smooth rotation of the cranks.

Mountain bikes have a 73mm or 68mm wide bottom bracket shell. Meanwhile, the standard for road bikes is 68mm.

If the bottom bracket shell on your MTB is 73mm, the road cranks would stand further out than envisioned and create a crooked chain line resulting in sub-optimal power transfer and gear shifting.

The common reaction would be to install a 68mm bottom bracket, but this approach doesn’t solve much because the bottom bracket shell remains wider than optimal (73mm).

Moreover, installing a 68mm bottom bracket on a 73mm frame creates its own multitude of problems such as:

1. Pushing the drive side crank out by 2.5mm and widening the chain line.

2. Pulling the non-drive side crank in by 2.5mm and reducing the chainstay clearance even further.

If you clear the problems above somehow, you will simply end up with a 68mm bottom bracket stretched to 73mm – a number that road cranks aren’t intended for anyway.

Therefore, the best scenario is to have an MTB frame with a 68mm bottom bracket shell, to begin with.

Retro MTBs such as the one in the image above fit the bill.

3. A Short Chain

Larger chainrings require a longer chain. As a consequence, you will have to buy a new chain and size it appropriately.

4. Insufficient Q-factor

In cycling, the Q-factor equals the distance between the outside of one crank arm to the outside of the other.

MTB cranks have a slight outward bent resulting in a larger Q-factor than that of road bikes.

The difference is more pronounced when comparing modern MTB cranks to retro road versions.

The Q-factor of road cranks is often insufficient to clear the chainstays of an MTB frame on both sides.

5. Front Derailleur Incompatibility

To ensure smooth index shifting between the chainrings, it will be necessary to install a road bike front derailleur.

Road front derailleurs have a longer, curved cage facilitating the shift onto the big ring.

An MTB front derailleur cannot work as efficiently with road cranks because it’s designed for chainrings with fewer teeth and has a smaller cage.

6. Shifting Issues

The conversion from MTB to Road cranks creates the following shifting problems:

a. Front Shifting

MTB and road bike index shifters are not interchangeable because they move a different amount of cable.

Consequently, the front MTB shifter will have to be replaced by a road version. If you want to stay with your current MTB bars, search for a flat bar road shifter.

If you’re doing a full-blown MTB to road bike conversion including drop bars, you will need a break-shifter (e.g., STI).

The above applies only to an indexed system. (Index shifting is the current standard. With each click, the shifter releases or pulls a pre-determined amount of gear cable.)

If you have a friction shifter, however, the game changes. Friction shifters allow cyclists to mix road and MTB parts because it’s up to the user to determine how far the shifter moves.

b. Rear shifting

Road and MTB cassettes are interchangeable, but the shifters are a different story.

If you plan to keep your MTB cassette and run flat bars, then you should also keep the derailleur and the shifter.

If you intend to switch to an STI shifter, you will have to change the derailleur too if it’s designed for 10 or more speeds.

Shimano’s 6 to 9-speed MTB rear derailleurs have the same rear shift ratio as Shimano’s 7,8,9 and 10 Road rear derailleurs. The only exception is the Tiagra 4700 series.

The rear shift ratio describes how much the derailleur moves per 1mm of cable pulled or released by the shifter. For that reason, derailleurs with identical rear shift ratios are interchangeable.

Once again, this applies only to index shifters. Friction shifters are compatible with all kinds of parts.

A Trekking Chainset =A Better Alternative

The main incentive to put road bike cranks on a mountain bike is to increase the gearing and reach a higher top speed.

However, almost the same effect can be achieved by switching to a trekking chainset which is fully compatible with MTB frames, derailleurs, and shifters.

The speed of a bicycle is a function of three elements – the rider’s force generation, the gearing, and the circumference of the wheels.

Force

A rider’s output is limited by their fitness level. Strong cyclists can maintain higher cadence in larger gears.

A cyclist’s cadence is the number of revolutions that the cranks make per 1 minute. People argue, but in general, a cadence of 90rpm is often considered optimal for maintaining a high average speed while also minimizing fatigue.

Gearing

If a bike has a 52-teeth chainring and the smallest cog on the cassette is 11-teeth, then the gear ratio in that gear is 52:11 or 4.7:1.

This means that every rotation of the cranks spins the rear cog and consequently the wheel close to 5 times.

The higher the gear ratio, the higher the potential speed because the wheel is spinning more times per minute and propelling the bike further.

Tire Size

If two bikes have the same gearing, then the one with the bigger wheels will technically be faster. 

When ridden at identical cadence/rpm and in the same gear, both bicycles will have rear wheels spinning the same number of times per minute. 

However, the bike with the larger wheels will cover more distance in the same amount of time.

Calculating The Potential Top Speed Of a Bike

To calculate the speed of a bicycle in a certain gear, we need the following data:

1. The RPM of the rider

2. The gear ratio

3. The circumference of the wheels

4. The traveled distance or the time needed to cover it.

Example A: Trekking Bike (48/36/26)

The first example calculates the max speed of a 27.5″ trekking bike equipped with 48t, 36t, and 26t chainrings and an 11t smallest cog.

The highest gear ratio is 48:11=4.36:1 and results in 4.36 rotations of the rear cog per 1 revolution of cranks.

If the rider is maintaining a 90rpm cadence, the cranks will rotate 90 times in 60 seconds.

Consequently, the rear cog/wheel will make 90×4.36=392.4 rotations in the same time frame. If we multiply the number of rotations by the circumference of the wheel, we will have the traveled distance.

If the bike has 27.5″ rims with 2.2″ tires, the wheels’ circumference will be roughly 2185mm or 218.5cm.

In that case, the traveled distance is 218.5 x 392.4 = 85379.4cm or 853.794 meters.

To find the speed, we have to divide the distance by the elapsed time which in this case is one minute:

The speed of the trekking bike is 853.794m/60s=14.229m/s = 51.22764 km/h = 31.83mph

Conclusion: A trekking chainset with a 48t large chainring gives you a top speed of 51 kph at 90rpm cadence.

Unless you’re going downhill, you’re highly unlikely to spin out in that gear.

Example B: MTB with Road Cranks

The second example calculates the top speed of an MTB with road cranks (52t large chainring) and the exact same tires and cassette.

Traveled distance = RPM x [Gear Ratio x Tire Circumference] = 90 x [52:11 x 218.5] = 90 x [4.72 x 218.5] = 90 x 1031.32 = 92818.8cm or 928.188m.

Speed = Traveled Distance/Elapsed time = 928.188m/60s = 15.4698 m/s = 55.69128 km/h = 34.60496 mi/h

Conclusion: A 52t road chainring will give you an increase of 4.46 kph (55.69128-51.22764) over a 48t trekking chainring.

The difference is too small to justify the effort and time required to perform the conversion.

Truth be told, most people will have a hard time “maxing out” even the trekking chainring.

Below is a list containing popular trekking chainsets and their number of teeth:

Summary

1. Mountain bikes have protruding chainstays designed for wide tires. In most cases, road cranks hit the chainstays when installed on an MTB frame.

2. The simplest, easiest and cheapest way to increase the gearing of an MTB is to install a trekking chainset.