There are a number of visual and functional differences between MTB and road bike helmets. We’re going to focus on only the safety-relevant differences here.
Different types of riding require different types of protection. The helmets for each discipline are designed for different purposes and common crash situations.
While mountain bike helmets generally offer more head coverage with thicker EPS foam, road helmets have better lab test results regarding brain injury prevention – but that means less than it should.
Injury Patterns in Road vs. Trail Biking
A study comparing road biking vs trail riding helmets found that head injuries were more common in road-related trauma compared to trail-related trauma (16% vs 6%). This indicates a potential difference in the protection focus of helmets used in these different biking environments.
Get-offs on steep, uneven trails often result in bikers tumbling down the mountain with lots of rotational forces from trail obstacles grabbing the helmet. Every bit of coverage helps here.
While on the road, crashed riders usually slide on the pavement after a particularly hard initial impact. It’s actually helpful if the helmet has space to move on the head to diminish some of the impact force.
(Ir)Relevance of Safety Ratings
Both helmet types need to meet the same safety standards for bike helmets but in reality, are designed to work in very different kinds of crashes.
Standardized tests are done by dropping helmets from certain heights at specific angles. This means only one initial, hard impact is tested for impact force mitigation.
There is public, transparent comparison testing like Virginia Tech’s STAR bicycle helmet ratings comparing all kinds of biking helmets.
In this ongoing study of over 220 helmet models were put to two repeatable impact tests. It’s important to note, that all of them were half-helmets and any peaks or visors were taken off.
Those tests fo not account for multiple hits, tumbling or hits towards the back of the head, jaw or face.
Road bike helmets for MTB
Research suggests that conventional bicycle helmets, typically used in road biking, are inadequate for mountain biking. They lack the necessary coverage and protection for the jaw, lower head and face, areas often exposed to injury in mountain biking.
The typical helmet visors for MTB helmets are another way to avoid face injuries. They help dodging low-hanging branches and bushes trailside and rocks / roost from any riders in front.
Only MTB helmets come in full-face variants, that are designed to protect against both head and face injuries during downhill mountain biking. Chin bars are typically not found in road bike helmets.
Crashes on smooth roads are different from impacts on steep, uneven terrain. That’s why there are specific helmet designs for specific types of biking.
Also, neck braces for MTB are designed to only work with full-face helmets.
MTB helmets for road cycling
Road bike helmets provide improved aerodynamics over any other cycling helmet. In comparison, MTB helmets have arguably the highest aero resistance.
So there is a performance gap, but mountain bike helmets can generally be used for road cycling in terms of safety.
They just may not have the improved aerodynamics, ventilation, or little weight of a dedicated road helmet.
With that compromise, MTB half-shell helmets are a great option for an all-rounder helmet: road, commuting and trail riding.
So, if you need to limit your budget and only want to own one helmet for all the riding you do, this may be the way to go.
As a matter of fact, I’ve ridden bikes on streets with my mountain bike helmets for a long time. Even on road bikes. But my road bike helmet is far more enjoyable to wear and not as hot in summer.