• Modelling software can be used to perform such calculations at the design stage, since a detailed model is developed for most production vehicles from large manufacturers. A detailed model will incorporate all of the vehicle's design parameters and geometries, along with the properties of the tires and road surface. The turning circle of a vehicle is also easily measured in practice. This can be done by 'drawing' a circle on the pavement using water, the diameter of which is then measured. One or more small nozzles are attached to the side of the vehicle facing the centre of the circle or set along the centreline of the vehicle, with their outlets set close to the pavement. The nozzle may consist of nothing more than a short length of narrow, rigid tubing attached to the vehicle, with the outlet positioned an inch above the pavement. The nozzle is connected to a small water supply on the vehicle. The delivery system may be gravity-fed or pressurized, if a small pump is incorporated into the system. More than one nozzle can be used to make discrete circles to mark the path of different points on a vehicle. For example, nozzles could be attached to the front and rear inside corners of the tractor and the rear inside corner of the trailer. The tire pressure, tire model, and type of pavement are also usually recorded, along with the diameter of the circle(s), because changes in these parameters will affect the end result. (I have done this with nothing more than a short length of rigid plastic tubing, a piece of flexible tubing, a spring clamp, and a 5-gallon pail of water. No pump was needed, because the water was siphoned out of the pail.) Water is often used to mark the pavement during certain types of vehicle tests. In a braking test, for example, a pressurized water jet may be used to mark the pavement the instant the driver applies the brakes. The length of the water trail is the braking distance.

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