Why do Tandems go Faster Downhill than Single Bikes?

If you answered “gravity” you are only partly correct. Galileo showed in the 1600's that, in the absence of air resistance, two objects of different mass, when simultaneously dropped from the same height, will land at the same time. Did Galileo not ever experience a tandem rolling down hill???

At the risk of readers ignoring this article, I am going to introduce a little physics. First, the difference between mass and weight. In the imperial system pounds are used for both mass and weight—very confusing. Mass is the amount of matter present. Let’s say you have the mass of 50 kilograms (kg) (at one time I am sure this was your mass). Earth’s gravity will tug at you with a force of about 500 Newtons (N). If you were taken up into deep outer space your mass would still be 50 kg, but your weight or force due to gravity would be 0 N. You would be weightless, but with unchanging mass.

Newton ’s second law says that if you divide your force, (weight), by your mass you get your rate of acceleration.

If you compare your acceleration due to gravity versus someone with twice your mass, the person with twice your mass would be acted upon by a force of gravity twice the size of yours. But the ratio of your Force to Mass and their Force to Mass would be identical. For us folks here at sea level on Earth this acceleration would be 32 ft/s2 or 10 m/s2. This means that in every second of free fall your speed will increase by 10 m/s.

At least this is the result with no air resistance. But bicyclists deal with air resistance whenever they are moving! We ride in pace lines in order to reduce the effect of air resistance on us. A pair of riders on a Tandem would have about the same amount of air resistance as a single rider.

Now for an analogy. Let’s say that there was some “financial satisfaction” statistic that was the result of the ratio of your income to wealth. And lets also say that you have the same satisfaction ratio as Bill Gates, your ratio of income to wealth is the same, even though he has a lot more of each. Now let’s further assume that both you and Bill get a $5000 tax on your income. I bet you would notice a tax this size and your satisfaction ratio would take a hit. Bill's ratio would change so little he probably wouldn't notice.

Here is how the financial analogy applies to bikes going down hill. For the force acting on bikes it is the net force that counts; that is, the force of gravity minus the force of air resistance. Since air resistance is about the same for a tandem bike vs a single bike, the net force on the single bike takes a bigger hit than on the tandem. Tandem = Bill Gates. The accelerations, the ratios of net force to mass, on the two types of bikes are no longer equal. The acceleration on the single bike is now less noticeable than the acceleration on the tandem.

The tandem's acceleration is greater and it gets to the bottom of the hill before the single bike!!

Thus the best answer as to why a tandem beats a single bike down a hill is: “Air Resistance”.

Author's note: The steepness of the incline is assumed to be equal for single bike and tandem. As a sign of caring for my readers, I did not go off on a tangent and include this calculation.

Jay Godfrey, GSW member