Trail-building nerds argue about what, exactly, defines a switchback. We don’t have to be so exact.

If a turn goes downhill, and it’s tight, and you’re braking, and you’re freaking out, it’s probably a switchback.

If a turn goes uphill, and it’s tight, and you’re pedaling, and you’re freaking out, it’s probably a switchback.

If a mistake will send you hurtling down a cliff, it’s probably a switchback.

Yikes. This right here is a bona fide switchback:

For our purposes, let’s define a switchback as a a turn that:

Is tight. Because switchbacks are built onto the sides of hills, there isn’t much space for wide radii. Modern trail builders are making “switchberms” and “climbing turns” with radii of 20+ feet, but they are expensive and usually limited to newer trails. The most challenging switchbacks are found on older trails, and their radii tend to be less than 10 feet. Seven feet isn’t crazy tight. Less than five feet becomes challenging.

This turn is a lot tighter from the cockpit than through a wide angle lens!

Changes direction in a big way. The trickiest switchbacks change direction almost 180 degrees as the trail zig zags down a steep slope. If the pitch isn’t so steep, or if the trail builder was clever, the turn might change direction less: say 120 degrees.

I rode these switchbacks the other day in San Jose, CA. This is a typical way to route a trail down a steep slope.

Yes! I last rode the famous San Juan Trail in 2002. Check out the switchbacks toward the bottom.

One of San Juan Trail’s mellower switchbacks.

Gains or looses elevation. If a tight 180° turn is flat, it’s just a hairpin. Hairpins are tricky in short track racing and cyclocross, but you won’t die if you mess one up. Modern trail builders work to make turns less steep than the surrounding hillside. These “climbing turns” tend to be tame and easy (and they require a lot of earth moving). We’re most worried about “old school” trails that follow the contour in one direction, turn through the fall line (yikes! Steep!), then return to the contour in the opposite direction.

A steep switchback in Hong Kong.

And, bonus, is off camber. Modern “switchberms” have positive camber, which means they have berms to support your tires and keep you on the trail. These turns tend to be easy. The switchbacks we worry about have positive camber at the top and negative camber at the bottom. Imagine turning directly across a steep hill. That’s what I’m talking about.

Many modern trails are outsloped. This helps water drain off the trail, but it doesn’t help you carve. Especially if you’re used to berms.

As you might expect, the most challenging switchbacks combine scrunched tightness, huge direction changes, scary steepness and off-camber exits. And exposure. Let’s not forget exposure.

Here’s a proper EWS switchback. Tight, lots of direction change, steep, off camber and exposed.