Body Kits And Their Effect

April 11, 2013 By Mike Lee
Last updated on July 27, 2015

Body Kits
Although mainly used for styling purposes on road cars, their main intended purpose is for improving the cars stability. Bare in mind that many aftermarket body kits are not designed for this however, with some having no benefit or testing done on them at all. I believe Mugen test their kits in a wind tunnel, but you will pay 3-4 times as much for a wind tunnel tested and designed kit, but you may get a kit that can reduce your drag by as much as 10 percent. It’s important to remember that what ever spoilers you add to your car, you will never know unless you have it wind tunnel tested whether it’s of benefit or even a detriment. You could put cotton strips around your cars body and go for a drive and have someone follow you and film the way the cotton swirls or straightens, but unless you know what to look for it’s a bit hit and miss.

Body Kit
UK road cars will not really benefit much from spoilers, as we have a 70mph speed restriction on our motorways and any positive effect is negligible below this. But as far as styling is concerned it’s all down to personal taste and some cars can be totally transformed.

Spoiler Basics
The term spoiler I believe means to spoil the bad air flow over the cars body and was first used on racing Ferraris in 1961 and on the Plymouth Superbird in 1970 for a road car.

Super Bird Spoiler
Super Bird Spoiler

Purposeful and well designed spoilers are designed to help the cars high speed stability and increase traction. They are also designed for reducing rear lift when braking hard from high speed when the cars rear end becomes lighter with weight transfer to the front. Some spoilers are designed for high speed stability and others for slower speeds. The slower speed ones will create more drag at higher speeds. Conversley spoilers with aerofoils, ridges, vortex generators can also be designed to decrease drag, it all depends on the design required for the specific car.

Early Racing Wing

The addition of spoilers as mentioned can increase the drag on the car and many performance car manufacturers have to increase the power of the car to compensate for this extra drag, also as mentioned in the aerodynamics article, many performance manufacturers are putting active spoilers on their cars, that come into play when they reach a certain speed. Therefore creating little/no resistance when not required and a vertical load when required.

Audi TT Active Spoiler

Many modern sports cars don’t have spoilers as their body has been designed with downforce built into them. Although some could benefit from them, many manufacturers of these cars don’t like the look of them as they feel it makes the car untidy or alters the cars lines.

Front Spoiler
A front spoiler is designed to increase the downforce to keep the cars front end planted. A low front spoiler can also reduce the air going under the car and therefore reduce lift. Road cars can have a front spoiler that helps reduce drag by forcing the air around the tyres (see how many front spoilers have a larger or more prominent lower side part and more narrow middle to let the air flow under the car and around the tyres).

BMW Front Bumper Spoiler
BMW Bumper Spoiler

Rear Spoiler / Rear Wing
A rear spoiler is designed to reduces rear lift in combination with the cars aerodynamics, it does this by disrupting (spoiling) the air flow and reduces lift at the rear of the car, which can aid high speed stability and braking. A rear wing is generally higher up on the car and creates its own downforce, a little like an upside down aeroplane wing.

Nascar Rear Spoiler
Nascar Rear spoiler

Formula 1 Wing
F1 Wing

Btcc Wing

Dragster Wing
Dragster Wing

A rear wing is the ultimate downforce aid for the rear of the car especially for a track car, see touring cars etc. Cars producing significant power and are light, used for the track/strip can benefit from the extra downforce, especially when under heavy braking, as the weight is transfered to the front. But most street cars may not benefit from a wing or even really require a spoiler and may actually use more fuel and loose power caused by any additional downforce (the exception to this, are cars designed specifically with aerodynamic proportions or designs that can make the rear end light in certain situations and thus they may require the additional help – see Audi TT below).

Adjustable spoilers/wings are available for those people who that like to tweak (even bi-wings are available). Some performance cars have an active spoiler that only raises when it gets to a certain speed and some even act as an air brake as well (Mclaren Mercedes SLR).

An Audi TT is an example of how effective a rear spoiler can be. When the first TT’s came out in production, they were found to be unstable at high speed (beyond the legal limit in this country) due to their design and light rear end. Audi quickly remedied this and added a rear spoiler along with it’s stability control programme.

Hatchback cars with spoilers halfway down their window will create a large wake to the rear of their car and therefore create much drag.

Note – Disrupting the air flow causes drag which in turn reduces speed. A balance between the benefits and the extra drag needs to be made for optimal application.

Rear Wing
End Plates
Many spoilers/wings have end plates. These side fins mount at the tips of wings and can increase their lift.

Side Skirts
Side skirts
Side skirt apart from giving the car a lower look, can also help in reducing the air getting under the car and therefore reduce lift. Most body kits will include the side skirts as part of the package.

Spoilt For Choice
Choosing a spoiler will depend on what you want from it (looks or functional use). A higher angled spoiler (wing) will be more functional and also cause more drag. A low factory one will more than likely be for looks only.

Whether you go for a full body kit or individual spoilers/wings, will be down to both taste and if functionality is a factor. But remember if you increase the performance of your car you may well have upset the cars original aerodynamic capabilities, this isn’t so important for road cars (if you’re driving within the law!), but may well be an issue for track use, so maybe it’s time to think about some help with increasing your cars track stability.

Another note – (mainly for track cars on high speed tracks): as you increase/change/mess about with the cars aerodynamics, adding spoilers, body kits etc, it’s important to point out that the cars charictaristics may change, such as additional downforce can affect high speed suspension changes (sinking into the ground) and the steering may become harder, so pay attention to the changes you make.

There are many other devices that can help (have a look at motorsport for all these inventive ideas) – examples: dams, gurney flaps, venturi channels, side pods, vortex generators.

Body Kits – Old and New
Old Scirocco Body Kit
350z Body Kit

What happens when you loose that downforce at speed
Raikkonen’s rear wing breakaway

Why not checkout our Aerodynamics Article.