We do not have to look too hard to find the media flashing photos and videos of mountain bikers flying straight down a mountain. While many of us entertain the thought of doing this ourselves, a trail designed to do this is more than likely not a sustainable trail, and ultimately not ideal in most public lands that are maintained by volunteers. There is no doubt that it is much easier to fire up volunteers to build a re-route, than to repair an eroded fall line trail over and over again.
So what is fall line? Fall line is the shortest route traveling downhill, the same path that water flows. Trail users, gravity and water can erode trails, but if water is allowed to channel, this process is accelerated. I recall racing in West Virginia at a muddy ski area and followed the line of water down a trail and discovered the fastest line. While racers were slowed in mud trying to avoid the rocks and water, I flew right through on a firm eroded rock surface hidden beneath a path of water. I experienced first hand where the water went and what it could do to strip all the soil away. I may have won my race that day, but the life lesson on water was my most enlightening and vivid prize.
Building a sustainable trail that resists erosion can be done through proper design, construction and maintenance. I read in Dirt Rag that Jusitn Vander Pol, who is a long time rider, trail builder and maintainer in wet Seattle’s Tokul Creek Trails said “good trail building is 70% good routing”. Vander Pol takes pride in designing trails that drain and dry quickly in an extremely wet climate.
Ultimately, when routing a trail, the trail should be placed across the fall line with an approximate 5% out sloped tread to let the water sheet off the trail quickly before traveling down the trail. To determine maximum trail grade we use the half rule: trail grade should not exceed half the side slope. That rule is a pretty solid rule for keeping trails out of the fall line. Sometimes the rule can be stretched a bit on solid rock or with rock armoring, but if you are a trail designer, that rule should be at the front of your mind.
When choosing a route Art and I look for many things to make what we feel is the best trail. It is not just about going from A to B and adding cool technical features. Besides wanting riders to really enjoy riding the trail, we want the trail to last and be there years down the road. Some of the things we look for when planning a route: keeping the trail grade below half of the slope grade, places to add dips or grade reversals, interesting features (either for technical riding or nature enjoyment) and natural chokes to keep riders on the trail. End result: we have come to learn that some pretty amazing and sustainable trails can be built without aligning them in the fall line. When planning to build a trail, don't let yourself get sucked into the fall line lie.
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