Trevor Barton asked:
Hiya - congratulations on your podcasts.
Here's a question for you:
Wikipedia tells me that the men's 1 mile running record is held by Hicham El Guerrouj at 3 mins 43.13 secs. This, according to my maths, equates to 16.03 mph. Needless to say El Guerrouj is an elite athlete!
I go cycling on my (not very expensive) bike most weekends. I usually cover about 20 miles on a ride and my average speed is never less than 16 mph.
I am not an elite althlete - in fact I'm almost 50, weigh 100 kg and have a BMI which puts me well into the obese range.
Why am I able to move at the same speed as an elite athlete, for 15 times longer - especially when you consider that the bike adds 10% to my overall weight plus lots of friction from the moving parts?
Keep up the good work
Dave - There are various different things which could be limiting how fast you can move. There are various mechanical reasons, like you might not actually be able to move your muscles quickly enough or if you're pulling a very heavy weight, itís limited by the amount of force you can apply on the weight. I think in this case itís limited by the amount of power you can put out. Elite athletes are going to be able to put a lot more power out than you are, but you're using a method of transport which is much more efficient. Bikes have been optimised over the course of 150 years to be incredibly efficient. You use very, very little energy to keep going along. In fact you need hardly any energy to keep them going along, itís just to accelerate them and to overcome air resistance, as thereís no friction there. So, with a very relatively small amount of power, you can be going very, very fast.
Whereas when running, you've got to move these great big legs around all the time and you actually kind of reach a limit to how fast you can move the legs backwards and forwards. You've got to accelerate and decelerate every time you take a step.
Chris - You're basically accelerating a bag of water weighing at least 60, if not 100 kilos which you're elevating and dropping and decelerating with every step, arenít you? So, you're basically having to keep lifting this very heavy bag up and down, and itís not an efficient way to move.
Dave - And also, you've got your legs, at 10 or 15 kilos each, which you're accelerating backwards and forwards. So all of that takes a huge amount of effort whereas on a bike, all you've got to do is just push this off along through a very efficient mechanical train.
Diana - Perhaps one of the reasons why humans are so good at doing long distances for a long time is because they've only got two legs whereas four-legged animals like horses, although they can go faster, they canít go as fast for as long as humans can because they've got four things to lift up.
It's all about the gearing ratios! The governing equation here is:
It's really more to do with friction than gearing. You don't need a bicyle. All you need to do is reduce the friction between your feet and the ground in the direction that you want to travel, and you can do that by strapping a pair of roller blades or ice skates on to your feet. Geezer, Thu, 8th Mar 2012
But the question was about a bike! Not what different ways how he move faster than an elite athlete!
Well, yes : ) But the main reason a cyclist can travel so fast is also because of the lack of friction. You can coast quite a long way on a bicycle without any exertion at all. Unless you are falling off a cliff, it's pretty difficult to coast for any distance without some sort of anti-friction means.
I generally don't grease the outside of my tires.
Yup! It's all about friction, and the efficiency of the machine.
Not to disagree too much with my esteemed colleague, Geezer, but the answer to the top speed question is essentially one of gearing, as Cheese said. In both cases, friction comes into play, since it describes how your body interacts with the ground and how you provide a forward force, but top speed really comes down to how much horizontal linear velocity you can produce in the point (tire or foot) that's in contact with the ground.
I ran a few numbers to actually see how this works out. I know I can comfortably riding a stationary bike at 80 RPM for 30 minutes. I made some measurements on my actual bike and found (rounded to the nearest cm):
Clearly the gearing is the most important factor when it comes to reaching the top speed, although air resistance is also an issue - a cyclist can get into an aerodynamic tuck and reduce drag a fair bit. If you're comparing a cyclist with a runner at 16mph though, the cyclist has an easier time of things primarily because the runner is wasting a lot of energy jumping up and down. Other species have better springs in their legs which enable them to run as fast as a cyclist by not losing so much energy on each jump, the best example being the kangaroo - almost all the energy it puts into each jump is recovered for reuse when it lands, whereas we turn a lot of it into heat.
Far be it from me to argue with one so learned as you JP, but I think you have your cause and effect front to back.
Ok, ok. It involves more than just leg turnover speed/gearing ratio. There are really two separate questions here: highest possible speed and maintaining a high speed. The maximal speed you can achieve for short instants has to do with leg turnover speed/gearing ratio. Maintaining that speed for a long time has to do with both gearing ratio, friction, and the ability of a bike to make efficient use of the human body's energy for propulsion (running has to provide a constant upward force with each stride as well as forward propulsion, and you also have to engage your core muscles a lot to keep your upper body in the right place.) jpetruccelli, Sat, 10th Mar 2012
It has to be more than just gearing...
An indirect answer to this is that you can't really ride a bike up stairs and your blood vessels would get horribly tangled if your feet had wheels.
Running stores kinetic energy in the springiness of muscles and sinews. When a muscle is stretched against its own resistance, a small amount of energy can be stored and recovered when the muscle contracts. Running faster exceeds the ability of muscles and sinews to store kinetic energy, so a greater fraction of it is turned to heat. At maximum speed, most of your effort goes into generating heat in the muscles. Only a small fraction goes into stirring up the air around you. Running downhill converts gravitational potential energy into heat in the muscles.
Bluedy 'eck! It can't be Groundhog Day already? Geezer, Wed, 2nd May 2012
Well, here's a compact version of the answer for those who've come to the party late. If you're on a bicycle, you can take a complete rest while still moving at speed, but if you try doing that while running you'll have a nasty crash into the tarmac. Runners have to work hard even to run slowly, whereas cyclists don't.
I was going to add that "a bike only improves efficiency by reducing friction which allows us to muck around with gearing ratios", but there is a bug in the forum software that won't let me turn off bold once I have turned it on :) Geezer, Thu, 3rd May 2012
I know you all had this discussion two years ago, but it came up near the top of my Google search.