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Old 03-29-2008, 04:01 PM   #24 (permalink)
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Bonneville Fusion

Originally Posted by MetroMPG View Post
OK, so it's a Bonneville car. But it's instructive about the basics what to do to improve aero:

- grille blocking
- smooth wheel covers
- narrower tires
- deleted mirrors
- deleted wipers
- lowering, in conjunction with...
- full undertray
- rear decklid extension (improved roof to trailing edge angle)
Sorry! I'm late to the party on this one,but I'll throw my two cents in.How fast did the car go? If its over 200-mph,compressibility issues would be looming up on the crew.As mentioned already,since this is a pure racing environment,safety may play as big a role as drag reduction and speed.The Fusion's roofline approximates an airfoil form,and the bellypan may be there to minimize static pressure under the car in an effort to forestall takeoff.

References state that air velocity under a car is always a fraction of road speed,therefore Bernoulli's theorem would come into play,dictating that pressure would have to be higher than the surrounding flow field.Not good! The smooth underbelly,acting like a traveling duct, would better allow low pressure from the wake,to be communicated under the car to help spoil the lift created by the more stagnant flow.

With respect to the airspeed under the car,as a function of ride height,my thought is that because of the plan radius of the front airdam,that air is being channeled around the car as much as the rulebook will allow,and while the air volume under the car will be an arithmaic function of the gap between salt and dam,the actual velocity remains a fixed fraction of groundspeed,as the dam doesn't aggressively funnel air under,but rather around.

The lowering also improves the fineness ratio,as well as the rear deck foil,both of which reduce drag,and the rear foils horizontal projection moves separation and lift completely behind and under the foil,a great boon to lift reduction and directional stability,creating a pressure weather-vane,if you will.

And I agree that the fences are also only for safety,should the car loose traction ( not an unusual event at Bonneville ),and yaw into a more wing-section attitude,sideways,as NASCAR vehicles which we may have all seen literally fly off the track.

Being a fuel-cell car,the nose can be sealed,and while not a great drag reducer for race cars ( they can be idealized for low drag ),again,it cuts off airflow "through" the car,for added stability and safety at high speed.

The 0.08 Cd reduction is really impressive and tailshadows Ford's active suspension contribution to their PROBE-IVand V concept car low drag numbers.Since the PROBE-V achieved Cd 0.137 in the 1980s,the Fusion's 0.21 doesn't carry the same wow-factor,buts its still instructive,and of great value to anyone who lives between the lines.Liked the article,thanks!
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