Offroad helmets most commonly used to be 3/4 style. I remember in the late 80's I had one that was 3/4, but with an add-on chin bar. It would not have held up to a big impact but it would deflect rocks and roost from your mouth. There were a few helmets that were one piece and were sturdier. read more
Riding a dirt bike is very exhausting and soon rider would be gasping for air and on a regular helmet this would lead to fogging up of visor and would run hot soon. It won’t be an exaggeration if we can compare street riding to a 2-D affair and dirt bikes to 3-D, and hence it should offer better peripheral vision. read more
In these case a front impact from a rock, will hit a motocross helmet in the tip of the chin bar, a small strong area, (and not a entire arc) that with the triangle shape will distribute the impact force to the whole helmet structure more effectively than a arc shape would do, preventing an early chin bar destruction. read more