Thursday, May 28, 2009

aerodynamics – front splitters

Aerodynamic down-force, ground clearance & safety.
by Pierre Martins

Fat%20Bully%20Splitter%201Front end splitters on a race car produce aerodynamic down-force by creating pressure differentials in the air that the car moves through at speed.

To understand how a splitter creates down-force you have to know a little bit of fluid mechanics. One should at least be aware of the difference between static pressure and dynamic pressure. It is also instructive to know how dynamic pressure is related to flow velocity. This relationship is given by the well known Bernoulli equation.

Imagine a race car standing still with the air flow moving towards it, as opposed to the other way around. The oncoming air approaches the car at V1, which is the speed of the vehicle, and a pressure P1 which is related to V1 according to Bernoulli's equation. P1 will be somewhat less than atmospheric.

The essential point to be taken from Bernoulli's equation is that the pressure inside an air stream is related to the velocity at which the airstream is moving. As the air stream velocity goes up, the dynamic pressure within the air goes down. In layman's terms - The air travels faster in the low pressure area created under the splitter, and that "sucks" the car's nose down, creating down-force and reducing under-steer, especially on a track where you have fast flowing sweepers such as Sunset bend and the Sweep through the Mine Shaft at Kyalami.

The drawings above show what happens when incoming air reaches the front of the vehicle. The air must come to a stop before it turns to move either up and over, down and under, or around the vehicle. The area where the oncoming air flow comes to a stop is termed the "stagnation point", since the velocity has gone down and the pressure has gone up (the "stagnation" pressure). In other words, the front of the moving vehicle is an area of relatively high pressure.

Case in point -

My 928 race car is a heavy front-engine beast prone to under-steer. Apart from reducing weight, upgrading the suspension, widening the wheel track and adding fatter rubber up front, we incorporated a splitter to help eliminate under-steer.

splitter%201 The shape and design of the splitter is pretty basic - It's actually just a half-round shaped flat sheet that fits horizontally under the car.

The splitter seals off the entire area all the way from the front of the vehicle and stops just short of the the front wheels. This creates a decent low-pressure area under the car to assist down-force according to Bernoulli, but only at speeds above 120kmh. In slower corners the car still relies on mechanical grip - trick suspension and fat rubber. The splitter adjusters are light-weight and spring-loaded. We got them from Auto Style.

To construct the splitter on my 928 and make it strong and flexible in case of accidental bumps etc, we incorporated multiple layers of composites - Kevlar for flexibility, overlaid with carbon fibre for strength, but the cost didn't justify the means. We paid stupid money for the composites.

splitter%203 8mm Plywood works just as well and makes surprisingly lightweight splitters for a fraction of the cost.

Besides, if you have a mishap during Friday practice and need to replace your front splitter in a hurry, you're not likely to find Kevlar and carbon fibre at the local hardware store.

Ground clearance and safety -

This article was written with the functuality of race cars in mind, but something that's always been bugging me is the stupid aftermarket spoilers and splitters the ricer crowd put on their cars for street use, so I've decided to include this section...

splitter%204 Take a look at the ground clearance on a street legal 928.

The designers at Porsche must have given this some thought when designing the front-end and splitter on the 928. Note how low the car is, but even with such little ground clearance the stock 928 spoiler is relatively safe from unexpected speed bumps and curb-side parking.

The extra clearance is achieved by sloping the lower half of the nose cone down at an angle towards the front wheels and placing the spoiler / splitter closer to the front wheels. The reduced distance between the spoiler and front wheels lifts the spoiler out of harms way. (The old De-Tomaso Pantera and Alfetta GTV's are good examples here)

splitter%205 The pic on your left was taken when we were busy making moulds for the bodywork of my 928 racer, Fat Bully.

It illustrates the lack of ground clearance with a lowered ride height and a vertical spoiler that is flush with the font-end and mounted far ahead of the front wheels. Look closer and you'll see that the bottom of the spoiler would hit the tarmac if we moved the car a few inches forward...

It gets worse - This pic only shows the spoiler, we haven't even fitted the splitter yet. Fat Bully is a full-on race car and this set-up is obviously not safe for street use. Hell, we even have issues loading the car on a trailer.

You gotta admit, it's a tad worrying when we see riced-out cars on our roads with more radical set-ups than race cars when it comes to front spoilers and ground clearance. Please think twice before you fit radical spoilers to the front of your car... Hit something at the wrong angle and that nice rice spoiler of yours could be stuck under your front wheels and you'd be sliding along like you were on an ice rink...

That could be a helluva ride, until you kill someone.


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