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.
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.