Correspondingly with these changing conditions the air flow pressure on top of the body will be higher than for the underbody reduced venturi effect pressure, hence there will be a net down force (negative lift) tending to increase the contact pressure acting between the wheels and ground. As the underfloor surface moves closer to the ground the underfloor air space becomes a venturi, causing the air to move much faster underneath the body than over it, see Figs 14.21(a) and 14.22. With a large underfloor to ground clearance the car body is subjected to a slight negative lift force (downward thrust). Thus uncontrolled lift will reduce the vehicle's road holding and may cause steering instability. 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. 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. Generally, the nearer the underfloor 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. 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. Thus the direct slower moving underside and the indirect faster moving top side airstream 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. 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.
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