First of all, the biggest difference is that the use of competition is different.
Mountain bike is designed for rugged natural roads, so it has a reliable and powerful braking system - disc braking system (road car only can be installed by Ruby), frame geometry is not too comfortable sitting angle is not low, do not think it is not necessary to consider the air resistance problem, handlebars are more flexible to control the car, tires are also tire particles suitable for sand road, there are Dust-proof fork.
As the name implies, a highway car is only suitable for running on the highway (except Ruby, but Ruby is no better than a mountain bike on natural roads). It has two handlebars, one is straight and the other is curved. Generally speaking, when cruising with power, it is to grasp the holding position, that is, the junction of brakes and handlebars. This ensures not only that the air resistance during riding is not too high, but also that a certain degree of comfort is guaranteed. The front fork is a hard fork, and the brake is a clamp (because the cross-wind will affect the stability of the vehicle to a certain extent when the disc brake meets, and the heavy weight does not conform to the concept of highway vehicle light), and the road wheelset. The higher the frame height, the higher the aerodynamic benefit.
Generally speaking, the whole body of the highway vehicle only tramples faster and more efficiently as the core, thus sacrificing a certain sense of riding comfort, and road restrictions are relatively large. Mountain bikes can almost adapt to most of the earth's road surface, but the impact of shock absorber system and brake, speed change kits is often much heavier than road bikes, riding posture problems will also bring greater wind resistance, trampling efficiency output power can not be maintained in a long period of time.