1432Vehicle lift Fig 1420

When a car travels along the road the airstream moving over the upper surface of the body from front to rear has to move further than the underside airstream which almost moves in a straight line (see Fig. 14.20). Thus the direct slower moving underside and the indirect faster moving top side air-stream produces a higher pressure underneath the car than over it, consequently the resultant vertical pressures generated between the upper and under surfaces produce a net upthrust or lift. The magnitude of the lift depends mainly upon the styling profile of both over and under body surfaces, the distance of the underfloor above the ground, and the vehicle speed. Generally, the nearer the under-floor is to the ground the greater the positive lift (upward force); also the positive lift tends to increase with the square of the vehicle speed. Correspondingly a reduction in wheel load due to the lift upthrust counteracts the downward load; this therefore produces a reduction in the tyre to ground grip. If the uplift between the front and rear of the car is different, then the slip-angles generated by the front and rear tyres will not be equal; accordingly this will result in an under- or over-steer tendency instead of more neutral-steer characteristics. Thus uncontrolled lift will reduce the vehicle's road holding and may cause steering instability.

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