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A speedometer or speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to , they started to be available as options in the early 20th century, and as standard equipment from about 1910 onwards. Other vehicles may use devices analogous to the speedometer with different means of sensing speed, eg. boats use a pit log, while aircraft use an airspeed indicator.
Charles Babbage is credited with creating an early type of a speedometer, which was usually fitted to .
The electric speedometer was invented by the Croats Josip Belušić in 1888 and was originally called a velocimeter.
Some early mechanical speedometers operated on the governor principle where a rotating weight acting against a spring moved further out as the speed increased, similar to the governor used on steam engines. This movement was transferred to the pointer to indicate speed.
This was followed by the Chronometric speedometer where the distance traveled was measured over a precise interval of time (Some Smiths speedometers used 3/4 of a second) measured by an escapement. This was transferred to the speedometer pointer. The chronometric speedometer is tolerant of vibration and was used in motorcycles up to the 1970s.
When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts with a small aluminium cup (called a speedcup) attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, which itself produces another magnetic field. The effect is that the magnet exerts a torque on the cup, "dragging" it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.
The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup are balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed, the pointer will remain motionless and point to the appropriate number on the speedometer's dial.
The return spring is calibration such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tail shaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.
One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction – in this scenario, the needle would be driven against its mechanical stop pin on the zero position.
A computer converts the pulses to a speed and displays this speed on an electronically controlled, analogue-style needle or a digital display. Pulse information is also used for a variety of other purposes by the ECU or full-vehicle control system, e.g. triggering ABS or traction control, calculating average trip speed, or increment the odometer in place of it being turned directly by the speedometer cable.
Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavours to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.
Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above. These do not require battery power, but can be relatively bulky and heavy, and may be less accurate. The turning force at the wheel may be provided either from a gearing system at the hub (making use of the presence of e.g. a hub brake, cylinder gear, or dynamo) as per a typical motorcycle, or with a friction wheel device that pushes against the outer edge of the rim (same position as rim brakes, but on the opposite edge of the fork) or the sidewall of the tire itself. The former type is quite reliable and low maintenance but needs a gauge and hub gearing properly matched to the rim and tire size, whereas the latter requires little or no calibration for a moderately accurate readout (with standard tires, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel size, almost as if it were rolling along the ground itself) but are unsuitable for off-road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming.
Excessive speedometer errors after manufacture can come from several causes, but most commonly is due to nonstandard tire diameter, in which case the error is:
Nearly all tires now have their size is shown as "T/A_W" on the side of the tire (See: Tire code), and the tires.
For example, a standard tire is "185/70R14" with diameter = 2*185*(70/100)+(14*25.4) = 614.6 mm (185x70/1270 + 14 = 24.20 in). Another is "195/50R15" with 2*195*(50/100)+(15*25.4) = 576.0 mm (195x50/1270 + 15 = 22.68 in). Replacing the first tire (and wheels) with the second (on 15" = 381 mm wheels), a speedometer reads 100 * ((614.6/576) - 1) = 100 * (24.20/22.68 - 1) = 6.7% higher than the actual speed. At an actual speed of 100 km/h (60 mph), the speedometer will indicate 100 x 1.067 = 106.7 km/h (60 * 1.067 = 64.02 mph), approximately.
In the case of wear, a new "185/70R14" tire of 620 mm (24.4 inch) diameter will have ≈8 mm tread depth, at legal limit this reduces to 1.6 mm, the difference being 12.8 mm in diameter or 0.5 inches which is 2% in 620 mm (24.4 inches).
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers are similar to the UNECE regulation in that they specify that:
The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process. For example, the test measurements should be made (for most vehicles) at , and at a particular ambient temperature and road surface. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
The UNECE regulation relaxes the requirements for vehicles mass-produced following type approval. At Conformity of Production Audits the upper limit on indicated speed is increased to 110 percent plus for cars, buses, trucks, and similar vehicles, and 110 percent plus for two- or three-wheeled vehicles that have a maximum speed above (or a cylinder capacity, if powered by a heat engine, of more than ). European Union Directive 2000/7/EC, which relates to two- and three-wheeled vehicles, provides similar slightly relaxed limits in production.
The speedometers in vehicles manufactured before these dates but after 1 July 1995 (or 1 January 1995 for forward control passenger vehicles and off-road passenger vehicles) must conform to the previous Australian design rule. This specifies that they need only display the speed to an accuracy of ±10% at speeds above 40 km/h, and there is no specified accuracy at all for speeds below 40 km/h.
All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules. The state and territory governments may set policies for the tolerance of speed over the posted speed limits that may be lower than the 10% in the earlier versions of the Australian Design Rules permitted, such as in Victoria. This has caused some controversy since it would be possible for a driver to be unaware that they are speeding should their vehicle be fitted with an under-reading speedometer.
The Motor Vehicles (Approval) Regulations 2001 permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However, it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually traveling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.
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