You know the way that the efficieny of cars is pretty small (about 40 - 40 percent depending on whether you’re using a disel or a petrol engine), and that of a motorbike is more (don’t know how efficient that is, but I assume it’s more cause it’s smaller and whatnot, so I’m assuming it’s higher), so I was thinking - what’s the efficiency of a bicycle.
It obviously depends on the conditions, the resistance the road induces on the bike, wind resistance, hills and what not. So, anyone know how to find the power needed, and the power delivered etc.
Well, it depends on how you define efficiency. From the standpoint of how much of the pedaller’s effort is delivered to the wheels, that figure is generally regarded as over 90%. But, that’s a bit disingenuous. After all, the original fuel source isn’t the turning of the crank. An automobile delivers in the high 90s if you’re talking about efficiency from crankshaft to wheels. The true fuel source is food. Viewed this way, bicycle efficiency drops to about 20%.
Please don’t be snide. Especially to someone who went to the trouble of googling something you could have googled yourself. The question is which input.
Even if you compare the chemical energy in the food or gasoline to the power output in the wheels, you’re probably still not looking at the right measure. Most of the energy loss in a car or bicycle is to wind resistance, and that won’t be taken into account by an output power measurement: You can drive with almost any amount of power to the wheels; it’s just that if you don’t have much power and you have a lot of drag, you’ll go slowly. So two different vehicles with the same efficiency, by that standard, could nonetheless burn different amounts of fuel going five miles at thirty MPH.
But if you look at it in too much detail, you end up concluding that all vehicles, in typical conditions, have the same efficiency, exactly 0%. If I get on my bike, or into a car, and go visit a friend, or go up to the office, or whatever, then at the end of the day come back and park in my driveway, the energy of the system is the same at the beginning and end of the day, but I’ve burned a lot of fuel in the mean time. It’s just all been converted to useless forms.
To further complicate things, a car is much more efficient tansporting one person than a bus is, but a bus is more efficient at transporting a full load, than the required nuymber of cars is. I suspect a bicycle is pretty inefficient at bringing home a big load of groceries.
For what it’s worth, I remember a science film from high school in which the bicycle was described as “the most efficient form of transportation ever devised”.
Bicycle efficiency depends hugely on the road surface. One can get 99% on a fixed track, on a surface as smooth as a bowling alley, but compare that to a gravel or cobblestone road. If you want to factor in the process of converting food to muscle power, why not throw in the cost of road infrastructure?
Just doing my part to muddy the waters.
99% of what? If you’re looking at drive train efficiency, then the surface you’re riding on is irrelevant (there will be losses, just not in the drive train). If you’re looking at energy delivered by the legs compared to energy gain of the bike, then once you hit top speed, the efficiency is 0%: The top speed will just be higher on a more “efficient” surface. Where did you get that 99% figure, and what were they measuring?
I can usually follow along at a distance, but honestly I have no clue what you’re saying here. Is this some physics meta point? Could you dumb it down even more and breakout the specific points you are elaborating?
Due to the gyroscope effect, the bigger the wheel the less energy is wasted on wobbling.
Get one of those penny-farthing bikes for real efficiency. But a new one, not a replica. Some of the olden day designs had terrible power wasters like belt drives instead of chains.
Efficiency is a ratio of useful energy out of a system, divided by the amount of energy you put in. But now you have to decide what you’re counting as “the system”. For example, I could say that I put a certain amount of energy into the pedals, and I get a certain amount of energy out of the wheels. By this standard, a bicycle is very efficient, since almost no energy is lost between the pedals and the wheels. But where does that energy go? If you’re speeding up, or going up a hill, that’s easy: If you’re speeding up, then the energy your legs put into the pedals is (at least partly) getting turned into kinetic energy of the bike, and if you’re going up hill, at least part of it is going into gravitational potential energy.
What if you’re going a constant speed on a level road? Well, then, your bike has some amount of kinetic energy at the beginning, and it has that same amount at the end, and at every point in between. The bike is not gaining any kinetic energy, so ultimately, the net energy output is zero. But you’re still pedalling, and putting some amount of energy into the system. Since you’re putting in some energy, but not getting any out, your efficiency is 0%: All of the energy you’re putting in is getting wasted somewhere or another.
I pulled it out of astro’s Wiki cite, several posts ago. It’s okay to discuss how efficient bikes are in a deliberately optimal environment, but for a real-world test involving having to traverse a variety of road surfaces, the extra effort needed to push a bicycle tire over a gravel or other moderately rough surface is worth considering.
I suppose if one is contemplating one’s most efficient way to get to work and the route is made up entirely of level, well-maintained city streets, no problem. If you have to roll out of this rarified environment, however briefly, you have to recalculate.