Mountain bike handlebars are flat bars and road bike bars are curved and bent shapes which are drop bars and bull horn bars, when changing and replacing a road bike bar with an MTB bar, you have to make the following parts of your road bike compatible with the MTB bars because the length of the brake and gear cables of road biking is different from the MTB bars and you have to make levers and gear shifters of your road bars compatible with the new MTB bars, which is almost not an easy step, it takes much time in converting from MTB bar to road bar because you have to change all the parts which include, brake levers, gear shifter, stem body and headset of the road bar before inserting and installing MTB bar on a road bike.
You have to look for new brake levers, make sure levers would be compatible with the width of the MTB bars, and if you are installing old MTB levers then the compatibility between MTB levers and road bike cables, disc brake, and hosing would also matter a lot. when installing gear shifters, if the road bike gear shifters are not compatible with the MTB bar, then look for new compatible gear shifters, and also check the number of gears of the road bike and new compatible gear shifters.
Road bike derailleurs and new gear shifters are not compatible with each other, and the difference comes in their brake cables. the length of the gear shifters does not match the derailleur, you have to uninstall the drive train of your road bike and check the length of the cables if they are reaching the derailleur and working with the shifters. when buying new gear shifters or old used MTB shifters for road bikes, you have to check the compatibility between the derailleur and gear shifters, if both of them have different brands and are not compatible with the road bike derailleur then further you have to buy new shifters.
You also need a new stem body because the old MTB stem body has different thread sizing on road bike forks, there is a large difference between road and MTB stem bodies and headsets, and smaller or larger sizes of stem bodies are not compatible with different categories of bike handlebars.
Mountain bike handlebars have grips that need to be replaced when installing a bar in a road bike, replace the grips and then insert the brake levers and compatible gear shifters and then install grips on a flat bar.
You have to adjust the height of the saddle so that it should be compatible with the new flat bars, and sometimes you have to replace the bike saddle if it’s not comfortable with the flat MTB bars, it is an experiment that is done after riding with the road bike saddle MTB bars if you are comfortable with the saddle then there is no need to replace bike saddle.
Brake cables are the main brake parts that are non-compatible with the derailleur, drivetrain, and gear shifters, the shorter and longer length of the brake cable is the main issue and problem and you have to buy a new compatible brake cable.
New Front Derailleur:
Road bikes have front derailleurs, you also need new front derailleurs and the able length of the front derailleur and gear shifters do not match, you have to change the location of the front derailleur and the cable length for making the setup compatible with each other.
Speed Difference Between Flat And Drop bar:
Mounting flat bars on a road would decrease the speed of your bike as compared to drop bars because flat bars are not recommended are compatible with mountain bikes and are rarely used for road biking, and drop bars are strictly not allowed for mountain biking.
Road Bike Flat Bars For Long Rides:
You cannot ride with flat bars on a road bike for more than 100 kilometers because it will tire your hands and upper holding for a long time, flat bars are used for gripping on hard and aggressive trails for offroading but road bike trails are flat and smooth, so it is not possible to ride with flat bars for longer mileages. but if you want flat bars for shorter mileages hen they are better for road biking.
The reason is that flat bars do not have enough place for gripping but drop bars have two to three places for holding and gripping mostly used for longer rides.
Flat bars have different heights, depending on your body height and the saddle height, you can check the recommended stem height according to your own height in the mechanic shop.
Comfortable Position Of Hands:
Flat bars are much more comfortable than drop bars because you can easily stretch your hands in a straight position also flat bars give better traction and control because drop bars have narrower widths and small surfaces would tire your hands.
Road bikes have smaller-sized cassettes and mountain bikes have larger-sized cassettes, due to which the cables and hose of the road bike are non-compatible with the gear shifters.
You can easily convert and replace a road bike handlebar with a mountain bike, but you have to look for compatibility issues because the larger difference comes in the cable pulls, road bikes use shorter pull cables and mountain bikes used longer pull cables, and also the stem body, if all of these parts are compatible with your bike, then you ca easily swap and replace handlebars between road bikes and mountain bikes.