Occasionally, we pull an article from around the web-o-sphere because we feel that it is a particularly enlightening or interesting article for people interested in cargo bikes and expanding the conception of what is possible to do by bike. This article by punk astronomer Doug Reilly fits the bill. He uses science to help people understand why additional weight on a bike isn’t noticeable until encountering a steep uphill grade.
by Doug Reilly
“Isn’t that thing heavy?” I’ve had a few people ask me that already. The Yuba Mundo specs out at 48 pounds, and that’s probably calculated with no cargo and few of the common accessories like running boards and a side-loader bag. Let alone that copper bell I added! Probably my daily running weight is about ~65 pounds. Certainly it’s far heavier than the 8-pound carbon fiber wonder Jim Hogan at GBC let me hold a while back. It was so light I almost threw the bloody thing through the roof of the store, just trying to pick it up. I expected it to have some weight. It appeared to almost need to be held down.
Does it matter how much your bike weighs? Certainly if you’re a professional athlete, or an uber-serious amateur, it can matter. That $11,ooo carbon fiber frame might shave a portion of a second off your time, and that might be enough. If you’re going to compete in a mega endurance race like the 3,000 mile Race Across America*, probably a big cargo bike isn’t your first choice as well. But for most people, should bicycle weight be a big concern? And should it shy people away from grocery shopping by bike?
I was a bit worried getting such a big, heavy bike as the Yuba Mundo. My specific worry was Washington Street. On the way to work, it’s all downhill, but on the way home, well, let’s just say I do wish it was reversed. It’s somehow not fair that I can get to my office in 3 minutes but it takes me 10 on the way home. I won’t show you a picture of Washington Street, because you would see how puny it is and therefore what a weakling I am. But it’s my hill, and I wondered before the Mundo arrived how it would do on that long gradual climb. Notice that I said how “it” would do. You know, because, it’s all about the bike. (Not my legs or cardiovascular strength.)
I needn’t have worried. The Yuba does its thing, amazingly. Those gear things really help! For the last seven years I’ve been riding a single speed, now I don’t know what to do with myself and my 21 shiny new gears. I actually get home faster than I did with my old single-speed hybrid, which weighs less than half what the Mundo does.
The other day I ran across these stats, posted on the blog of Portland, Oregon based Clever Cycles:
Speed of a 20-lb bicycle at 160 watts effort : 14.8 MPH
Speed of a 60-lb bicycle at the same effort, level ground : 14.6 MPH
Speed of a 20-lb bicycle at the same effort, 5% uphill : 7.2 MPH
Speed of a 60-lb bicycle at the same effort, 5% uphill : 6.1 MPH
Speed of a 20-lb bicycle at the same effort, 5% downhill : 23.9 MPH
Speed of a 60-lb bicycle at the same effort, 5% downhill : 25.5 MPH
These figures come from an online calculator, backed up with some fancy formulas, found here.
These figures also struck me as self-evidently true given how much easier and generally faster the cargo bike is to ride than the single speed. The Yuba weights 60 pounds. The single speed about 20. So if weight isn’t the biggest determinant of how much effort it takes to pedal a bike, then what is? I decided to ask a few experts, and I got an answer I didn’t really expect.
First, I went to the source, and emailed Todd at Clever Cycles and asked him why weight makes so little difference. Here’s his response:
Lighter *is* faster, but by a far, far smaller degree than widely supposed. Most of the work of pedaling a bike above speeds of about 12mph is overcoming wind resistance, not overcoming the inertia of the bike itself. Also, heavier bikes are notably slower to accelerate. It’s just that once they’re rolling, it’s not much more effort at all to keep them going.
It’s funny that I don’t perceive the Mundo as being slower, except when it’s loaded up with a lot of cargo and I’m heading up hill. Quite the opposite. I guess this is just perceptual, since my previous bike only had one gear, and apparently a pokey one at that. The heavier Yuba Mundo is also slower to stop, and this jives with Newton’s laws of motion, too. I actually bought a louder bell (a nice brass Crane bell with a spring loaded clapper) on my bike last week because I was worried someone would walk out in front of me while the bike was loaded and I was zooming down a hill. It takes a good long while to stop, even with well adjusted disc brakes (which by the way, I would consider to be essential components of any cargo bike). Objects in motion like to stay in motion, indeed.
I asked the same question of my friend and once college roomate Phil, who is now one of the world’s top experts on the subject of athletic power. He confirmed and expanded upon Todd’s explanation, and reading his response it began to feel a bit as if we were talking more about airplanes than bikes:
1. Roughly 80% of the energy of pedaling a bike involves overcoming air resistance. Thus, under all conditions other than a strictly uphill climb, aerodynamic considerations vastly outweigh weight considerations.
2. The most important consideration with respect to how quick you will get up a hill is the power to weight ratio, i.e. how many watts can you generate versus how much mass you are moving up the hill. Very good riders (i.e. guys that contend for the Tour De France) generate maybe 5ish watts per kg body mass and can hold than for less than an hour. More pedestrian riders probably generate 2 watts per kg over a similar time frame. You make considerably more power than this in the metabolic sense, however, you are only about 20-25% efficient in terms of what actually gets delivered to the external environment. This is why you get hot when you exercise…the rest of the energy is liberated predominantly as heat.
3. The most important consideration with getting down a flat road is power versus frontal surface area. Hence the aerobars you see on time trial or triathlon bikes. You are trying to poke a smaller hole in the air.
The takeaway is that the extra mass matters, sure, but not as much as one would think. Which is another way of saying that carrying your groceries home on your bike instead of your SUV is not as crazy an idea as you might think.
Another conclusion I made form this inquiry is that I made a good decision in getting the Yuba Mundo. It’s extra length adds weight, sure, but doesn’t add anything (or not very much) to the aerodynamic cross section of the bike. Something like a bakfiets, with a wide honking cargo box on the front, is, to use Phil’s expression, punching a much bigger hole in the air.
It’s funny, but now that I’ve been riding the Yuba back and forth to work every day, getting groceries with it, and of course, lugging my telescope around on it, I can’t believe I sat in a car for long and let an engine, and way too much fossilized dinosaur poop, do the work for me. It’s not much work at all, and the cost per mile is stronger legs and lungs.
*The link is to an excellent RadioLab episode that features a story about the RAAM. RAAM’s official website is here.