Mountain bike stems are compatible with road bike handlebars and steerer tubes, and it is possible to install MTB stems on road bikes, but there are a few barriers and things that sometimes make MTB stems non-compatible with road bike handlebars, the inner diameter of MTB stem should be compatible with the width and thickness of the steerer tube of the road bike handlebar, before buying or inserting MTB stem ina rod bike, always measure the diameter of the MTB stem so that it should be compatible, it is necessary because if you are inserting different sized stem, then it will continuously move or rotate when you start riding on your road bike, in the end, all four screws of the stem body would vibrate and start loosing, you may face crash or accident.
The handlebar of mountain bike stems is usually size 31.8mm, now if your rad bike handlebar is of the same diameter then you can easily insert and install MTB stems, but road bike handlebars are of different sizes with different diameters and thicknesses, if the diameter of your handlebar is of 26.0mm, then it would be a difficulty for you to install MTB stems, the reason is that 26 size is non-compatible with MTB stems. old and traditional handlebars and steerer tubes of mountain bikes have a thin diameter of 25.4mm but new modern and expensive mountain bikes have greater and thicker diameters, so if you are replacing the road bike stem with an old MTB stem then there are chances of possibility because steerer tubes and stem diameters are compatible with each other.
For those road bikes which have carbon handlebar bars and steerer tubes, MTB stems are not allowed to install on these ikes, because these stems have different materials like aluminum and steel, which would cause scratches and friction with the handlebar.
Shims:
Shims are small round pieces of different materials that are used for clamping on the stem of the handlebars of both mountain and road bikes to make stem and steerer tubes compatible with each other. these shims are widely present in online stores and also in mechanic stores, beside this you can also make your own homemade shims by trimming and cutting similar-sized and compatible steel or aluminum tubes by giving them the shape of shims. these shims are small 2 to 3inches in size and have much more advantages, these will prevent you from buying a new handlebar or new stem, but the main purpose of shims is to make the non-compatible and different-sized stem and handlebars compatible with each other.
Reach And Weight Difference:
Road bike stems have a longer reach and greater length but mountain bike stems have smaller or shorter reach and smaller lengths, and also mountain bike stems are greater in weight as compared to road bike stems, these differences show that if you are installing MTB stem on a road bike, then you have to look for the reach difference between both different categories of bikes, if you are comfortable with the MTB stem then you can install it in your road bike, and also if you are a racer and you want to increase the weight of your bike then you can avoid MTB stems.
Difference In Nut Bolt Clamping:
Mountain bike stems are installed and uninstalled by screwing and screwing four bolts, while road bike Stems have 2 bolts for mounting and unmounting on the handlebars of steerer tubes, if your road bike has 2 bolt system then it is not possible to install 4 bolt stem body but if your road bike is a new and modern expensive brand which have 4 nut bolt clamping area, then it is possible to install MTB stems on the handlebar of road bikes.
Fork Compatibility For Stem:
Both mountain bikes and road bikes have different-sized forks which contain1, 1, 1 1/8, etc. if road and MTB forks are of the same size and same brand then it is possible to replace the stem body between them, but if forks are of different sizes then it is not possible to install MTB stem in a road bike.
Difference In Thickness And Shape Of Handlebar Tube:
MTB handlebars are usually butted shaped, which means these tubes are thinner in diameter in the central part of the tube but these are thicker at the edges where grips are installed on both the right and left sides of the bars, but road bikes are not thicker or thinner in diameter in the central region, if you are replacing road stem with MTB stem, then measure the clamping diameter of both road and MTB handlebars.
Aerodynamic Difference:
Road bikers usually need longer reach stem bodies, because road bikers cover larger mileage distances by traveling for two to three hours on roads, but mountain bike stems are different based on geometry difference, mountain bikers don’t need to travel for longer mileages, but their trails are hard and aggressive and the main thing their shorter reach stem provides is better gripping and better controlling on single trails, so if you want an aerodynamic position with MTB stem on your road bike, then it is possible with MTB stems.