Biking is a fun way to improve our fitness, meet up with new people and absorb our environs at a slower pace than possible from a speeding automobile. Bikes are not so fast, but different bikes still have different speed. Generally speaking, road bikes are faster than mountain bikes.
There are main five reasons.
What comes to first is aerodynamics. Aerodynamics is a very big factor. Aerodynamics is the study of the motion of air, particularly its interaction with a solid object, such as an airplane wing. To demonstrate this, get a road bike and a mountain bike and put the two on a trainer and do standing start to, say, 40km/h accelerations. The results will be about the same. Generally, the drivetrain of a road bike designed to give the rider better leverage. Bike manufacturers generally give the road bike more teeth in the front crankset, and up until recently fewer teeth in the back. The rider is in a more advantaged position. When it comes to top speed, cutting down bikers’ drag coefficient is the key as it's the main limiting factor next to power output. The force of drag increases exponentially as speed increases. The road bike cuts through the air much more effectively than a mountain bike. The mare a bike cuts through the air, the less drag it has. Drag doesn't influence much as slow speeds, but it’s multiplied at higher speeds. As the rider becomes faster, the road bike has a relative advantage in gaining speed.
And the second reason is power output. The less power output is, the faster the bike is. Biking would cost bikers a great deal of strengthening. Road bikes are consistently stiffer than mountain bikes due to the lack of suspension other than tires and the flex built into the frame due to its materials and geometry. This means more of the power from bikers’ legs goes into turning the wheels. The same is true due to road tires being smaller profile, thereby decreasing rolling resistance and rotational mass. Rider comfort in these conditions can be satisfied with flexibility built in the frame material, frame geometry, and in some instances the use of elastomer decouples in bikes. A bike that is built to handle cobbled roads is designed to absorb some of the energy that contact with the rough surface produces to shield the rider and as a result will also absorb some of the power that the rider puts in the pedals instead to channel it to the drive train hence resulting in slower speeds.
What’s more, weight is not neglected. The less the weight is, the faster the bike is. While weight doesn't generally effect top speed (with the exception of descending, as mass is a direct factor in potential energy), it absolutely affects acceleration, giving a biker the feeling of being fast. For example, if a road and mountain bike are climbing an 8% paved road, and start together, climb for one hour, and the road bike weighs 7kg and the mob (hard tail to remove suspension bob losses) weighs 10kg, plus 2kg of camelback and water, plus say 1kg of heavier equipment that MTB riders carry the road rider will end ~257 seconds ahead of the MTB rider (assuming 250w at 17km/h). This is a rough and ready set of calculations, but it gives a general outline of the weight factor.
And the tire is a significant part. The smaller and slicker the tire is, the faster the bike is. Road bike tires are designed to be slicker and smaller. They don't hold onto the ground as much a mountain bike tires do; less friction and surface drag. This is a table of road tires in rolling resistance tests. As we can see there is even quite a difference between road tires. Tubes even make a small difference! MTB tires are even more diverse, but we cannot see anyone MTB tire being more efficient that any one road tire. Wide and big tires mean slow. Small wheels also have some impact as bearing resistance comes into play (small wheels have to turn more times per meter traveled and bearing resistance increases exponentially with the number of revolutions per minute).
Last but not least, don't forget about terrain. The flatter the terrain is, the faster the bike is. Terrain plays an important role in speed. The mountain bike is designed to handle a variety of terrains - rocky, sandy, muddy, logged, etc. Rider comfort and the addition of an energy storage within the bike that the rider release at will in surges mandates the use of shock absorbers hence not all the energy that the rider puts translates into horizontal linear speed but some of it is channeled into vertical motion that provides for rider comfort and the ability to execute technical jumps. Any decent roadie on the flat could easily spin a 53-14/15 gear at 90+ cadences (pretty cruisy). This is about 43km/h. This is the equivalent of a 26-inch MTB is about 105 cadences (a lot) in your 42/11 gear. In other words, in order to keep up with a roadie, bikers would have to spin their highest gear. The roadie can hold a conversation at this point, the MTB rider is into oxygen debt (i.e. cannot sustain this pace).
Different bikes have different speed, but it doesn’t mean a good biker only cares high speed. The article shares five main reasons why road bikes are faster than mountain bikes to provide some help for bikers. Only when they know more details about biking, can they find a better way to enjoy this sport.