Yes, you can use a motorcross helmet for mountain biking. Motorcross helmets can protect your head when going downhill on a mountain bike. The EPS foam inside of the helmet is designed to compress upon impact in order to slow your head and absorb the impact.
But it is not the best helmet you can use for mountain biking. Using a motocross helmet for mountain biking has some disadvantages.
Disadvantages of Using a Motocross Helmet vs a Mountain Bike Helmet
Weight
Motorcross helmets are heavier than mountain bike helmets. A motorcross helmet can be 4 pounds compared to a mountain bike helmet which can weigh 1.5 to 2 pounds. This makes a difference when you are riding on the trails and especially for downhill riding.
The heavier weight of the motocross helmet will increase the risk of neck injuries during a crash. Plus the heavier weight will reduce the speed of range of motion. The rider will not be able to move his head around as easily in order to see left and right compared to the lighter mountain bike helmet.
A mountain biker needs to be very mobile while riding over rocks, roots, and rough terrain. The rider needs to shift his weight forward, backward, left and right. A motorcross helmet would make the mountain biker more top heavy and create an imbalance when shifting weight.
Ventilation
Motorcross helmets are not as ventilated as mountain bike helmets. A typical motorcross helmet has 11 vents, 5 in front of the helmet and 2 on top of the helmet. Then there are 4 vents on the back of the helmet. These vents cool the riders head by allowing air to flow through.
A mountain biker’s helmet needs and has more vents than a motocross helmet. The main reason why is because a mountain biker is pedaling. Pedaling uses more energy than a MX rider who only sits on his moto bike.
A typical full-face mountain bike helmet has 23 vents, 4 brow ports, and 6 chin bar vents. These vents keep the mountain biker much cooler compared to a motorcross helmet. Keeping cool is very important for comfort and maximum performance.
Protection
Motorcross helmets don’t offer as much protection as a mountain bike helmet. Motorcross helmets are designed to protect the rider when they are moving at a high impact speed. Motocross impact velocities are faster than mountain bike impact velocities.
MX helmets are much heavier and sturdier than mtb helmets so they can absorb more impact at higher speeds. These helmets will not absorb as much impact at the slower mountain bike speeds. This will cause more damage to a mountain biker in a crash.
The exception to this would be mountain biking downhill. Mountain bikers going downhill can reach speeds of 30 to 40 miles an hour. In this case a motorcross helmet can offer protection for the downhill mountain biker.
But there are downhill mountain bike helmets which are designed specifically for downhill riding. These helmets have MIPS technology. MIPS stands for Multi-directional Impact Protection System. These helmets are designed to reduce rotational forces upon impact. The slip plane technology inside the helmet diverts the impact away from the brain and head.
Should You Use a Motorcross Helmet For Downhill Mountain Biking
From my research it is probably not a good idea to use a motorcross helmet for downhill mountain biking. Even though some riders have used this helmet for downhill it is not worth the risk. These riders have not crashed using a motorcross helmet so we do not know what would happen if they did crash on their head using a motocross helmet.
MX helmets are designed for riders who are riding at high speeds of 80 to 100mph. Even though motorcross helmets do have an EPS foam liner it may not be able to compress enough upon impact at slower speeds when going downhill.
If the foam is not able to compress enough at impact your brain will smack into the inside of your skull. This will cause a concussion. Plus your neck could break as well because the outer shell of a motorcross helmet is thicker, heavier and harder and will not absorb the impact as well at slower speeds.
Why Do Riders Want To Know If They Can Use a Motorcross Helmet For Mountain Biking
The only riders that want to know the answer to this question are motocross riders. Mountain bike riders would not want to know about this. They would never even think about this question in the first place. If you are a mountain biker then you already would have a mountain bike helmet.
Only motorcross riders would be interested in the answer to this question, but why. If you are a motorcross rider and you want to go downhill mountain biking, then the only reason you would be interested in knowing the answer to this question is because you don’t want to spend money on a mountain bike helmet.
This does not make sense to me because if you are a motorcross rider and you want to go mountain biking, I assume that you must have a mountain bike. Even if you don’t have a mountain bike then you will want to buy a mountain bike to go mountain biking. So why not spend a little more to get the right kind of helmet for mountain biking.
You are not going to save much money by using a motorcross helmet to go mountain biking. Plus the risk you take by not having the right helmet is not worth it. There is not a big difference in price between a motocross helmet and a downhill mountain bike helmet.
Conclusion
In the end the final choice is yours. You know that it is possible to use a motorcross helmet to go mountain biking downhill. But is it worth the disadvantages that a MX helmet has over a mtb helmet.
The weight, ventilation, and protection should all be taken into consideration for both types of helmets before making your decision. Finally think of your skill level and riding experience. All these factors will help you to make the best decision for you.