Skimming over obstacles

Maybe you’ve heard that the faster you ride through rock gardens, the less you drop down between the rocks, and the faster and easier you can ride. That idea makes some sense, but the question is: How fast do you have to go?

I wish I’d gone farther in math. If I made a mistake, I’m sure you’ll let me know!


Wiley skims at Left Hand Canyon OHV Area.

Imagine a gap three feet wide and 26 inches deep. Ignore suspension, tire flex, body English and every other variable.


When a wheel rolls off the first edge, it begins to drop. The slower you’re riding, the longer it takes to get across the gap, and the farther it drops. For example, at a snail-like 8 mph, the wheel drops almost 13 inches — and POW! — it hits the next edge dead-on. That, my friends is a dead stop.


The faster you go, the less the wheel drops. Twice as fast equals one-quarter the drop. At 8 mph you drop a stop-dead 12.6 inches; at 16 mph you drop a pond-skimming 3.2 inches.


At around 15 mph your wheel drops just less than four inches, and a cool thing happens. Four inches corresponds to a 45-degree angle from your hub. At this angle, the impact is half upward and half backward. Above this point, the impact pushes the wheel mostly backward (it mostly slows you down). Below this point, the impact pushes the wheel mostly upward (it bounces you into the air). From what I gather, this four-inch point is the threshold between getting stuck between the rocks and skimming across them.

This “four-inch speed” is directly related to the gap distance. Simplified:

Gap Threshold speed
1′ 5 mph
2′ 10 mph
3′ 15 mph
4′ 20 mph
5′ 25 mph

As you go faster than this threshold speed, the tendency to skim becomes even greater.

Better hang on

That is super cool, but there’s a catch. Kinetic energy increases with the square of velocity. Twice as fast equals four times as much kinetic energy. Four times the jolt. Four times the WHOA! Racers like Peaty and McCormack stay high in the rocks, but they also contend with serious forces. One more reason to be strong!


So what does this all mean?

1. Try to ride through rocks above the “four-inch threshold” speed.

2. Faster is better, as long as your bike and body can handle the forces. That’s why race bikes have to be so stiff, and racers have to be so strong.

Right on. Go forth and skim!

Real-life application

The Pit of Despair in Moab is pretty scary. It’s about five feet wide, with a threshold speed of about 25 mph (pretty fast!). That explains why Sacha made it easily when he pinned it, and why he got balled up when he rolled it slowly. Next year, I’ll rock it in second gear. BRAAAP!


Sacha makes it on my bike.

Sacha didn’t quite make it on Diane’s bike.

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