As soon as there is a few centimeters of snow, children and adults get the sleds out of cellars and garages. Even in the city you can see how small slopes in parks and playgrounds become toboggan runs. Of course, real descents are even nicer, on which the sledges can pick up speed. But how do you become particularly fast? And what role does the shape of the runners and vehicle play? Science also helps with the answers.
“The main thing you have to look at are the forces that act on the sled,” explains physics professor Oliver Natt from the Nuremberg University of Technology. First you have to take into account the downhill force, which is proportional to the weight. “If I’m twice as heavy, the downforce is twice as strong.” The slope of the terrain is also important for the downforce.
However, another role in sledding is played by the inertial force of a body, which is directed against acceleration. “The inertial force is also proportional to weight, so heavy people have a disadvantage here,” says physics professor Peter Schleper from the University of Hamburg. This means that a light and a heavy person can accelerate at the same speed with regard to these two forces – downforce and inertia.
In addition, the friction force between the sled runner and the surface is also crucial. “This is actually very complex and depends on many factors, especially the nature of the surfaces,” says Natt. How hard the sled is pressed onto the surface depends on the weight of the driver and the sled. When the snow is soft, the lighter one has a clear advantage because it doesn’t sink in as deeply and the sled doesn’t have to displace as much snow.
Air resistance slows you down
Once you go down the slope, Natt says you get faster and faster as you ride. “That’s why the force of air resistance is added to the friction between the runners and the snow.” It is not the mass that is crucial, but rather the driver’s front surface and the aerodynamics. “The value of the air resistance increases as the square of the speed. This is familiar from driving a car. If you drive faster, fuel consumption increases disproportionately,” explains Natt.
From a speed of around 20 kilometers per hour, you can clearly feel this in your face because of the wind, says Veit Senner, professor of sports equipment and sports materials at the Technical University of Munich. “You can only reduce air resistance by reducing the surface area. Many people do this intuitively by putting their upper body back while driving.” The air resistance is even lower if you lie on your stomach on the sled. However, experts advise against this because of the high risk of injury.
The ABC of steering
Ex-professional luger Marcus Grausam knows how best to steer a sled. However, it is important to differentiate between sledding and tobogganing. These expressions are often used interchangeably. But there are big differences between a sleigh and a toboggan, says Grausam, who has been building toboggans in his one-man business in Kreuth am Tegernsee for around 20 years. “You can’t steer the sleigh itself.”
In this case the framework is rigid. Before a curve you have to brake the sled and then put your foot firmly into the ground until you are around the curve. “When tobogganing, you can steer without slowing down in the curve.” Thanks to the flexible construction, you can steer it with a strap, shifting your weight and pressing your feet.
Parallels to skiing
“It’s the same principle as when skiing,” explains Senner, referring to the toboggan. In a left-hand bend, try to raise the left skid slightly by pulling on the steering belt. At the same time, the runner is edged up on the right side by pressing against the bar with the inside of your foot or lower leg. However, this is not possible with the classic sleigh because the runners are usually curved and have no edges. “The sleds can be turned more easily using leg pressure.”
“A sled is good if you go down a straight sled slope,” says Grausam. A toboggan is better suited for winding forest roads in the forest. Regardless of the vehicle, from his point of view it is important to drive with foresight and caution, sturdy shoes, gloves and, above all, a helmet. Speeds of 60 to 70 kilometers per hour are possible on some runs, while on standard toboggan runs it is at least 40 to 50 km/h.
Never without a helmet
Computer-simulated crash tests by Graz University of Technology showed that children can suffer serious head injuries in a collision with a tree without a helmet at speeds of around 10 km/h or more. The sitting position was also crucial for the risk of injury: If children sit in front of the adult on the sled, in the event of an accident the adult will essentially push the child into the tree, so that there is also a risk of injuries to the chest and thighs.
An ADAC crash test for adults showed something similar, which simulated the collision of a dummy equipped with sensors on a commercially available sled at 25 km/h with a wooden wall – without and with a helmet. According to ADAC information, it was found that the probability of a serious head injury such as a fractured skull fell from 90 percent without a helmet to 10 percent with a helmet. However, at high speeds, even when wearing a helmet, serious injuries to the neck, cervical vertebrae, hands, feet and knees can occur.
Even more physics
But why does a sled actually slide so well on snow? “This is a highly complex physics question,” says Natt. According to him, several effects play a role. One, for example, is that the top layer of ice molecules is not bound so tightly, so that there is a tiny film of liquid on the surface, says Natt. A second effect is the friction of the sled on the snow, which generates heat, which strengthens the liquid film.
