There are two main differences
Differences in the scope of protection for the back of the head
The main difference between helmets for road biking and MTB is the range of protection for the back of the head.
In bicycles, the main role of helmets is to protect the head in the event of a fall. Unlike road bikes, which often fall sideways, helmets for MTBs, which sometimes fall while rotating vertically, such as bouncing off bumps or jumping, are more common, covering a wider area from the top of the head to the neck area. Depending on the type of ride, some models are similar to full-face helmets, covering everything from the ears to the chin.
Although the wider coverage area means heavier weight and less ventilation, it is more important to reduce the risk to the head when riding MTB on rough roads, which are more demanding than paved roads.
Visor or no visor
MTB helmets are often equipped with visors to protect the rider from the sun, since the more serious MTBers tend to enjoy riding in the mountains.
On the other hand, road biking offers few opportunities for such foreign objects to fly, and from the standpoint of ease of removal and aero, the sun is often protected by eyewear, and only a few helmets come with visors.
Although it is perfectly acceptable to use a visor on a road bike, there are not many road helmets in the lineup, partly because road helmets are often designed to be lightweight.
Choosing Between Road Bike and MTB Helmets
Road Bike Helmets are “Best for City Riding and Pavement Driving”
Road bike helmets are best suited for city riding and pavement riding, since they have a basic performance as a bicycle helmet.
Of course, if you are riding a road bike on gravel or light trails, such as a gravel road bike that is designed to be ridden on rough roads, you should choose a helmet more suitable for MTB than for road biking.
Also, if you are participating in a road race, choose a helmet for road bikes that conforms to the helmet selection criteria for each race.
For MTB, decide according to the application.
Unlike road biking, helmet selection is important for MTB.
For city riding, gravel riding, and light trail riding, a helmet that provides adequate head protection is fine, just like on a road bike, but for downhill riding and other high-risk riding, a fall is likely to cause serious injury immediately. Therefore, you should choose a MTB-specific helmet that can protect a wide area of the head.
For high-risk MTB rides, check carefully not only the shape of the helmet but also its head protection layer such as MIPS.
In this sense, it is worth remembering that you should choose an MTB helmet if you are going to do more risky rides.