Most of us have experienced a foot popping off the pedal while out on the trail. Not fun. Or the arguably worse inverse – not being able to unclip when needed. In the 20-year history of clipless mountain bike pedals, spring tension snafus have provided their fair share of laughs (and cries) over a post-race beer.

The History
Most clipless mountain bike pedals use a spring-restrained jaw system to lock the cleat to the pedal. This system has to be overcome by the rider when engaging and disengaging. Created by Shimano in 1990, the pedals worked well, except in very muddy conditions when racers’ cleats would get too gummed-up to clip back in. No amount of force was going to change that, so Shimano added a small allen key adjustment to the pedal for greater spring tension release. This helped riders get back into their pedals when muddy, but also introduced the problem of inadvertently clipping out (and usually when you least wanted a foot unhooked).

Crank Brothers, which debuted a decade later, opted to leave their pedal spring tension unadjustable, relying instead on a mud-scraping micro platform and four entry points to mitigate the problem of failing to clip back in during inclement weather conditions. Further, each pedal has only one spring to overcome on entry and exit, as compared to Shimano’s two. A good solution, but Crank Bros. also made the mountain biking community realize that even with an innovative new design, riders could still fail to get back into their pedals (in extreme clay-like mud). And worse – even without the option to loosen the spring tension, they could inadvertently pop out of their pedals (particularly if the underside of the pedal hit a rock or obstacle).

The Physics
So spring-restrained jaw systems aren’t exactly perfect. The reason is best explained by physics. Hooke’s Law says that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit, or put mathematically,

F = -kx

where:
F is the restoring force exerted by the material (in newtons)
k is the force constant (or spring constant) (in force per length, so [newtons] x [meters^-1])
x is the displacement of the end of the spring from its equilibrium position (in meters)

So you can adjust the tension of a spring by displacing it (either compressing or extending it). In the case of Shimano, the allen key let’s you extend the spring, loosening the tension so it’s easier to clip in and out, or compress it, so it’s harder to get in and out. With Crank Bros., there is no ability to adjust your tension, but if you’re having trouble unclipping, Hooke’s Law states that the force holding you to your pedal (aided and abetted by a chuck of mud) is greater than what you’re producing by twisting your heel out.

The Point
The point of all this is that the physics of springs isn’t going to change. Perhaps what needs to change is using them in clipless pedals. That was Speedplay’s battle cry from the beginning. Their Frogs have a proprietary locking mechanism that doesn’t rely on spring tension for engagement or disengagement. You still give a twist of the heel to unclip, but there is no tension to overcome, so it’s effortless. And instead of jaws located on the pedal, Speedplay features a single retention device located on the cleat. Not that this system is foolproof either. My friend Dave endo’d (flipped over the handlebars) so hard and fast that he ripped the top of his Frog right off – still attached to the bottom of his shoe.

The take-away from all of this is as follows:

1). Avoid endo’s, particularly of the hard and fast variety.
2). Avoid riding in muddy conditions, particularly of the thick and clay-like variety.

If that’s impossible (which is the case for us), consider changing your perspective. The fact is, every clipless pedal, no matter what type, has the potential to behave strangely in mudslide conditions or during a crash. Even in those cases, malfunctions are rare (pro racer Rebecca Rusch told me her Crank Brothers pedals have never clogged), and so the industry isn’t exactly scrambling to reinvent the locking systems of clipless pedals systems. They’ve got their eyes fixed on something more important than spring tension design – something that can actually impact your performance based on your biomechanics. Look for an article on float coming your way soon.

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