28
Aug 2010
by don

Cargo bike brakes with kids on board

To avoid developing brake monomania, I’m promising myself that this will be my last post (at least for awhile) on the topic of cargo bike brakes.

But I noticed something interesting during my emergency braking tests yesterday: my bike had approximately the same stopping distance when carrying 180 pounds of cargo as it did with no cargo.  How could I explain the physics of that?

I think it’s because stopping performance depends on the friction generated between the tires and the pavement.  On an unloaded bike, the front tire does extra duty as the weight of the rider bears down on it – just like the front of a car dips down during hard braking.  In this scenario, the unloaded back tire can’t produce as much friction, and it’s easier to lock into a skid.

With 180 extra pounds over the rear wheel, the back tire carries more weight, and the job of slowing down the bike will be more equitably distributed between the two wheels.  The greater momentum of the extra weight just about matches the extra friction exerted by the back tire, and stopping distances remain about the same.

That was comforting to me until I realized that the bike’s center of gravity is also an important consideration.  I carried my heavy containers of water relatively low.  If I were carrying a passenger on the cargo deck, the center of gravity would be higher, and once again a lot of that weight would be loaded onto the front wheel during hard braking.  Stopping distance would probably lengthen.

Trying to stop on a downward slope would also increase the load on the front wheel.  For some combination of slope, speed, weight, center of gravity, brake type and condition, and road slickness, there will be safety issues.

Skidding

As my experience shows, things get dicey if your tires begin to skid.  A skidding rear tire isn’t too bad, except that it might indicate less-than-optimal weight distribution and diminished stopping power.  There’s also a slight loss of maneuverability; it’s easier to steer the bike if both wheels are rotating.

A skidding front tire is another story.  It’s nearly impossible to steer when your front tire is skidding.  If you try, the tire is likely to catch the pavement, at which point your handlebars will be wrenched from your hands or you will be removed from your seat.  The outcome of that situation is up to God’s mercy.

Moral of the story

The main thing that motivated me to write about this topic again is the realization that emergency stops with human cargo may be riskier than heavy loads with a lower center of gravity.  Especially if you’re carrying kids, you should practice a series of quick stops at progressively higher speeds until you find your comfort limit, and then you should stay below it.  Holding on during quick braking is good practice for your kids as well.

In addition, you should be extra, extra careful on hills or wet streets.

With kids on board, I am planning to reduce my speed and rethink my transport strategy in wet weather.

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  • DL

    Please don’t stop writing about this! So many people writing about cargo bikes seem to have utter confidence in their own and their bike abilities to haul their kids around without a care. For those of us who are not as perfectly fit or as experienced with cargo biking, it is really a welcome discussion!

    • http://mycargobike.net Don

      Thanks for the encouragement! I love the idea of my kids seeing my commitment to environmental sustainability every time we ride together. They love the experience – it’s easier for them to see out and wave to friends than from the back seat of our mini-van. But these advantages won’t mean much if they don’t arrive in the future whole and happy. So that’s my first priority as a dad.

  • http://mark.stosberg.com/bike Mark Stosberg

    I appreciate the posts as well. Recently we’ve been experimenting with two human passengers, plus about 80 pounds of cargo of top of that. My wife and I have been concerned about adequate braking, but haven’t tried controlled experiments like this yet.

  • http://www.stridersports.co.uk Leslie

    I have to jimmy up a cargo kid bike for my son… he wants to earn extra money so maybe he will use it transport stuff around the neighborhood. all I was looking for were ideas, and you have some cool pictures. :)