File photo: Jeff Barber

Many of us have likely seen signs at a trailhead telling mountain bikers to stay off the trails when they are wet. And yet, if you’ve been riding long enough, someone might have told you that some trails are designed with wet-weather riding in mind. So why is it OK to ride certain trails in the rain, while others suffer serious damage in wet conditions? We sat down with a trail builder to gain a deeper understanding of wet-weather trails.

Soil has a lot to do with it

“When it comes to wet riding and building for wet weather,” Mic McLane told us, “it all comes down to the soil type.”

McLane is the Eastern Washington Trails Program Manager for the Evergreen Mountain Bike Alliance (EMBA). While he now lives and works in the high desert, he began his time with EMBA on the western, and rainy, side of the state. McLane spent a large portion of his time digging and riding at Duthie Hill near Seattle, where he became known as the unofficial trail steward.

The state of Washington is an excellent example of a mix of dry and wet climates, with the western side receiving substantial rainfall. At the same time, the east remains relatively dry. McLane explained that when we see those Pacific Northwest video edits of mountain bikers riding in wet conditions, the trails are likely holding up just fine. 

While the method of trail construction is a factor, McLane explained that the soil is the most critical factor. And the soil that is likely being shown in those videos is loam.

However, loam isn’t actually what many of us may think of when we hear the word. Loam doesn’t have the moss, pine needles, or other organic matter we often see in videos and associate with soil. McLane said that would be more along the lines of a duffy topsoil.

“There are no organics in loam. Loam is a soil type that is [made from a combination of ] sand, clay, and silts,” McLane told us. 

File photo: Jeff Barber

He went on to explain that the “loam” we see in the videos and that we all desperately want to ride typically has a higher percentage of sand and a lower percentage of clay and silt. When measured, some of the best loam for wet weather trails is 60-20-20…60% sand, 20% clay, and 20% silt.

This 60-20-20 mix comprises many of the soils on the western side of Washington, Oregon, and extends into British Columbia. Higher sand percentages — ranging from 60% to 65% — allow the soil to drain more effectively. However, not every area or trail system has that perfect loam mix. McLane pointed to Post Canyon in Hood River, Oregon, as an example.

Illustration: US Department of Agriculture

“If you go to Post Canyon and try riding in the rain, you’re going to slide all the way down the trail,” McLane explained. “It’s extremely heavy clay content.” 

Clay is great for the initial building stages, as it absorbs moisture and can be sculpted into features such as lips and the landings of jumps. However, after the trails dry and become hard, it does not take much moisture to make them extremely slippery. 

If these western-area trails don’t ride well in the rain, a higher clay content is likely the culprit. 

Knowing the soil allows you to build better trails

Knowing the soil content is essential for building trails. McLane explained that this is often where good trail building starts.

“And when I teach trail building classes, [soil content] is actually in one of our slides,” he said. “We cover a little bit of soil science so people understand it.”

Ultimately, the best trail-building practices are the goal, regardless of the environment (rainy or dry) in which McLane finds himself. He used building berms as an example, explaining that a berm shouldn’t be constructed with a complete angle from top to bottom. Instead, the bottom of the berm should have an outsloping gutter that is essentially the width of a trail. McLane said this practice allows water to sheet off the trail rather than channeling along it.

A significant portion of trail building best practices involves properly managing water runoff from the trail. To prevent channeling, trail builders want to sheet water off the trails in broad, even “sheets.” This is usually achieved by outsloping the downside of the trail, encouraging water off that way. As trails become steeper, water drains at higher velocities, causing deterioration much quicker. As the water travels more quickly, it can be tempting to attempt to get the water off the trail as soon as possible, breaking the outslope that allows proper sheeting.

In the wettest areas, it may be better not to build the steepest possible trails. 

Features

Wet climates also facilitate the construction of certain features, such as jumps, more easily. McLane said they have much better luck building and maintaining lips and landings — especially steeper lips — in wetter environments.

While a bit of excessive rain can help with dirt features, it can cause issues with others. In particular, wooden features like ladder drops can be particularly slimy during certain parts of the year.

“I prefer all my wood features not to change direction,” McLane told us.
“Come into it straight; go off of it straight.”

Whether they are ladders or skinnies, McLane stresses the importance of keeping them straight. Chicken wire, cross cuts, and even expanded steel have all been used in attempts to compensate for the loss of traction with changes of direction. Yet, in McLane’s experience, they are all still slippery when wet.

Additionally, materials like chicken wire and expanded steel eventually corrode, resulting in even less traction.

But, it isn’t always water and moisture that causes slippery surfaces. Even on the drier side of Washington, fine layers of silt and clay can accumulate on wooden features, carried there by the wind. McLane said these will be just as slimy as their wet-weather counterparts.

Which is why, ultimately, McLane again stressed that, regardless of rainfall, trail building best practices must be followed.

“You know, it’s kind of a funny thing. There are a lot of parallels between dry climates and wet climates when it comes to mountain biking,” he said. “So you build the trails relatively the same.”