27
Aug 2010
by don

Cargo bike stopping distance

Regular readers of my blog know that I have been worried about cargo bike brakes for the past several months.  Yesterday my wife and kids left for a multi-day camping trip, and I decided this would be the perfect time to do some braking experiments with a loaded cargo bike.

In my mind, there are two kinds of braking I’m asking my bike to do: long, steady descents where I just need to keep my speed in check, and quick emergency stops.  I have a lot of experience with the former category – practically every weekday I pick up my kids from the school at the top of our hill and carry them down the steep slope.

On the other hand, I’ve only made a couple of emergency stops during the past year, so I haven’t been as confident in the ability of my brakes to stop a heavy load.  This morning before dawn, I loaded our mini-van with the bike and containers of water weighing a total of 180 pounds.  My destination was the parking lot behind the Seattle Museum of Flight.  I was looking for a long stretch of flat pavement that would allow me space to accelerate and skid to a stop.

And now I’ll cut to the chase.  When it comes to the Hammer Truck’s emergency stopping power, the big news is that there is no big news.  The bike stopped a combined weight of 420 pounds (180 pounds of water, 160 pounds of me, and 80 pounds of bike) travelling at 20 miles per hour, and it brought me to a complete halt in about the width of a parking space (less than 10 feet).  That wasn’t significantly longer than the stopping distance for the unloaded bike.  For that matter, it was on par with the stopping performance of our mini-van, and that’s comforting given the frequency I have to ride in mixed traffic.  However, I caught the unmistakable whiff of hot brakes and skidding rubber with the increased load.  The brakes were definitely working harder, even though they still felt solidly in control.

With considerable relief and bolstered confidence, I decided to repeat the experiment on a patch of wet pavement that had been moistened by the museum’s sprinklers.  That turned out to be a mistake.  The tires locked and I went into an uncontrolled skid, followed by an awkward low-speed crash.  I ended up on the ground, chain torn off the crank, and front wheel turned backwards.  Fortunately, neither bike nor rider was injured (but I was grateful there were no spectators at that hour of the morning).

This won’t come as a surprise to experienced cyclists.  Braking on wet pavement is a completely different story, and adding a heavy and somewhat badly-balanced load makes things even worse.  Perhaps I haven’t given slick pavement the caution it deserves during our damp Seattle winter – now I will.

How applicable are my results to cargo bikes in general?  My concerns about emergency braking have been mostly mitigated by these experiments, but with disc brakes on both wheels and the BionX motor assisting with regenerative braking, my bike is probably near the top of the class.  It’s possible that less favorable results would occur if you tried to stop a heavier load travelling faster with less capable brakes. Don’t let my unscientific results make you over-confident in your brakes.  I recommend doing your own experiments to give your equipment the opportunity to earn your trust.

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