Condensed answer: If the clamping diameter of the stem matches the diameter of the handlebars’ clamp area, then an MTB stem can be used on a road bike.
Short MTB stems are very likely to make a road bike unstable at high speeds.
Each stem has a clamping diameter which describes the thickness of the bars that the stem can securely hold.
If the clamping diameter of the stem is too small, it will fail to wrap around the bars securely. If it’s too large, the bars won’t be secure.
The clamping area of modern drop bars is 31.8mm and so is the clamping diameter of modern MTB stems. Therefore, the combo is very likely to work if you’re using updated components.
However, if the stem or the bars are older, then the combination may fail.
For example, older retro drop bars have a clamp area diameter of 25.4 or 26mm. The only way to combine them with a modern 31.8mm stem is to use a shim to eat up the slack.
Also, if you have an older MTB quill stem and modern drop bars, the handlebars won’t be able to fit through the stem.
If the clamp area is too slim, one can use a shim to combine it with a stem of a larger diameter.
A shim consists of two round pieces that one puts around the handlebars before tightening the stem. Once the stem is tightened, it becomes one unit with the shim and bars.
The most popular handlebar shim is 25.4mm to 31.8mm.
If you don’t have access to a shim, it’s possible to make one from the steerer of a 1-inch fork by cutting a piece of it in two as shown in the video below:
The advantages of stem shims are:
- You can combine modern stems with older bars of smaller diameter.
The disadvantages of stem shims are:
- A potential point of failure
- More finicky stem installation (The problem is more pronounced if you have a stem without a faceplate.)
What Are The Differences Between Road and MTB Stems?
1. Different Clamp Areas
Retro MTB and road bars have different clamp areas. Retro road bars are 25.4mm or 26mm whereas retro MTB bars are 25.4mm thick.
The stems from that era had no choice but to reflect that.
Currently, however, road and MTB bars have switched to the 31.8mm standard.
Modern MTB bikes have long bars and short stems. The long bars provide extra leverage whereas the short stem makes the steering faster and snappier. This combination is beneficial for technical riding.
However, a shorter stem on a road bike could create a world of problems because it will speed up the steering to the point where the bike becomes too responsive on descents. The shorter bars will aggravate the problem because the rider won’t have a lot of leverage. This is one of the reasons why road bikes stick to longer stems.
In addition, the long stems on road bikes allow cyclists to use smaller frames. Small frames are known to be lighter, stiffer, and more responsive.
Also, some road cyclists lean on their stems when descending. A longer stem provides a greater base.
Ultimately, an average MTB stem may be too short for some road bikes.
MTB stems tend to have a greater degree of rising. Or in other words, the stem elevates the bars more. The higher handlebar position makes it easier to lift the front wheel.
However, putting such a stem on a road bike will result in a less aerodynamic position because the rider will be more upright. This effect has a positive side too – the higher position reduces the strain on the lower back.
Meanwhile, road bike stems do not offer the same degree of elevation. The goal is to keep the bars low so that the rider is in a more aerodynamic stance.
MTB stems are designed to take more abuse. Consequently, they are often made of stronger and denser alloy. The extra strength comes at a price – weight.
In different, road bike stems do not have to deal with as much stress and can be lighter.
For that reason, it’s fine to put an MTB stem on a road bike because the component is strong enough to handle road use. However, installing a road bike stem on an MTB is not advisable because it may fail when riding on off-road terrain.
Road stems often have mirrored graphics because some riders like to “slam their stem” (turn it upside down) in order to lower the handlebars and acquire a more aerodynamic profile.
The mirrored graphic ensures that the label on the stem is always readable regardless of the stem’s orientation. MTB stems, on the other hand, are rarely slammed and have non-mirrored labels.
The Downsides of Running an MTB Stem On a Road Bike
1. Too short (potentially)
If you get a modern 30-50mm MTB stem and put it on a road bike, it will more than likely be too short.
The shorter stem will make the bike less stable at high speeds.
2. Less Aerodynamic
A stem with a great rise lifts the bars and puts the rider in a less aero position. The result is drag and lower efficiency.
3. Large Clamping Diameter
Modern MTB stems are 31.8mm. Thus, if you combine one with retro bars, you will need a 25.4/26mm to 31.8mm shim.
4. Extra weight
As already mentioned, MTB stems are heavier. The extra weight, however, is marginal and would matter only in situations when the rider needs the lightest possible bike for maximizing performance.
The Advantages of Using an MTB Stem on a Road Bike
1. Money Savings
If you already have an MTB stem of the right diameter and length, you can use it on a road bike and same money.
2. Extra Strength
MTB stems are stronger. However, it’s questionable whether the extra strength matters because a road bike cannot be ridden like an MTB anyway. Thus, the use of an MTB stem on a road bike is similar to installing an off-road truck component on an F1 car.