Speeding Away

Roughly 41 million speeding tickets are distributed annually in the United States—that’s about 112,000 tickets every day, 4,700 tickets every hour, and 78 tickets every minute. Although speeding is seen as a minor offense, the points can rack up on your driving record and the fines can be expensive. To help drivers avoid speeding, the speedometer was invented. Cars and other ground vehicles have all kinds of instruments to aid in driving, but the speedometer and odometer are the only ones that are legally required. But how do those instruments actually work?

Fun Fact: Over $6 billion in speeding tickets are paid every year!

There are two types of speedometers: mechanical and electronic. The first speedometer was mechanical, patented by Strasbourg inventor Otto Schulze in 1902. By measuring the rotational speed of the transmission, the speedometer can indicate how fast a vehicle is traveling. The drive cable connects the transmission to a magnet located behind the speedometer. This connection is direct and mechanical, so the magnet is rotating at the same speed as the transmission. The magnet generates a magnetic field that applies a torque (a rotational force) on the speed cup. The magnet is not touching the speed cup—the speed cup is solely rotating due to the magnetic field. The speed cup causes a rod to rotate; this rod is connected to the needle of the speedometer that indicates the speed of the vehicle. On that rod, between the speed cup and the needle, lies a hairspring, which restricts the needle’s movement (so that it only moves a little bit) and returns it to zero as the vehicle stops. Mechanical speedometers are also known as eddy-current speedometers due to the eddy currents that the magnet creates when it is turning.

The first completely electronic speedometer wasn’t invented until 1993. It bypassed most of the drawbacks of the mechanical speedometer. Mechanical speedometers can be inaccurate due to worn-out mechanical parts, and when it breaks, the maintenance is expensive. In addition, it cannot be mounted on bicycles because it is too large. Like the mechanical version though, the electronic speedometer uses magnets to measure the vehicle’s speed. The magnets are located on the vehicle’s rotating drive cable. A short pulse of electric current is produced every time the magnets go by the sensors, and an electronic circuit calculates the rate of the pulses and translates it into the displayed speed. In short, electronic speedometers count the wheel revolutions to compute the speed of the vehicle.

The odometer is linked to the speedometer—it keeps track of how far the vehicle has traveled. An electronic odometer is relatively simple since the electronic speedometer works with wheel rotations; however, the calculations depend on the size of the tire. A mechanical odometer, on the other hand, relies on a set of gears rather than calculations. As the wheels of the vehicle rotate, the tenths drum would turn steadily. Once the tenths drum reached “9,” the ones drum would turn to the next number. This occurrence is due to small gears between the drums. The odometer has up to a ten-thousands drum. Some mechanical odometers have digital displays, so the drums aren’t visible.

(for a detailed description of the mechanical odometer, watch from about 22:00 to 28:00)

If mechanical and electronic speedometers use magnets that measure the rotations of something with the vehicle, how does a speed gun check the speed of a vehicle? The device utilizes radar beams, or radio waves, to calculate the vehicle’s speed. However, shooting radar beams at reference points is not an efficient way to calculate speed if you are in the vehicle—hence the continuation of the speedometer.

Currently, the placement of the speedometer forces the driver to look away from the road, which greatly increases the chance of an accident. In the future, it could be possible to have a digital display of the vehicle’s speed somewhere on the windshield, but the display should not block the view of the road. Another idea would be to create a software that could notify the driver when they consistently exceed a certain speed above the speed limit. This would allow the driver to avoid looking away from the road. Technology is always being improved; eventually, there will be a better version of the speedometer, one that doesn’t require the driver to look away from the road. I hope you learned something new today! Keep a lookout for the next post! And be careful on the roads!