When modifying your car's aerodynamics, here's stuff to ignore

One thing I often notice is that people seem to want to make the subject of car aerodynamic modification more complex that it needs to be.

I am sure that this puts off lots of people, because they think that they need to know all of that stuff - stuff that actually doesn't matter much, if at all - in achieving good car modification results.

Here are some things you can largely ignore:

1. Turbulent versus laminar boundary layers

First, the boundary layer is just the 'skin' of air nearest the car's body that is flowing slower than the speed that the car is moving forward.

On a car, the boundary layer is basically all in the form of what is called a turbulent boundary layer. So forget laminar boundary layers as being relevant on any normal road car. (Exceptions? Maybe the first 300mm of car hood, and maybe the mirror housings.)

Don't confuse turbulent boundary layers with separated flow - the latter is where the car's body shape is no longer guiding the flow - a completely different idea.

So don't worry about the type of boundary layer - instead worry about getting attached rather than separated flow.

2. Reynolds numbers

In effect, this is a number that allows us to compare airflow characteristics (drag / lift) of objects of different size.

So, for example, if we were to use a 1:5 model in a wind tunnel, we'd have to ensure that the Reynolds Number was in the same ballpark as it would be for a full size car. (To achieve that, we need to use a proportionally higher airflow speed, i.e. in this case, 5 times the speed. Or change air density!)

But if we're using our car for our on-road testing, Reynolds Numbers are irrelevant - we're already dealing with the full-size model!

If you're testing your car on the road, forget Reynolds Numbers.

3. Fineness ratio

This relates to how long the object is compared with its greatest frontal projected cross-sectional area.

It's very important on objects like tear drop shapes, because at some stage, the greater pressure recovery created by a long tear drop starts to be more than offset by the frictional drag of airflow shearing over the object i.e. frictional drag. (The shearing coming from the different airflow speeds in the boundary layer.)

But if you're modifying your car, the fineness ratio is basically irrelevant - any tapering length you can realistically add at the back of the car is going to be good for drag.

Fineness ratios don't matter when you're modifying your road car.

4. Drag coefficient

If you are modifying your car, a drag coefficient (Cd) value is of very limited use. Almost, I'd suggest, relevant only in that if you're starting with a car with a high factory Cd figure, it will be easier to make improvements. 

And that's basically it. 

Calculating Cd values resulting from your home modifications is worthless - there is no really accurate way of doing so without access to a full-size wind tunnel with expert staff.

But - and here's the rub - concentrating on Cd can lead you down all sorts of rabbit holes. "That car reduced Cd by 6 per cent with [and pick your modification here]." That doesn't mean that particular modification will do the same on your car, and often nor does it even mean that the original modification reduced Cd by that percentage on that car. (So much published data is unreliable - not all tech papers are equal!)

Furthermore, applying that Cd reduction (6 per cent in this case) to your car's original Cd and saying that, for example, "I've done that modification and so I've now reduced my Cd from 0.28 to 0.2632" has got absolute knobs on it.

Why?

Well, there's no guarantee that the mod works on your car like it did on the other car (if it did even do so on that other car!). Then there's the intellectual and engineering paucity of going from two decimal places to four places - resolution and 'accuracy' where there are actually none...

Drag coefficients have almost no relevance when modifying your car for lower drag.

That will do for now, but I am sure I'll think of some others!

My book - Modifying the Aerodynamics of Your Road Car.