Aerodynamic parts lesson, including nomenclature.

[BenFenner]

Seems we need to brush up on our aerodynamic parts, what they accomplish, and by extension what they are called. And maybe more importantly what they are not called. Keep in mind many automotive aerodynamic devices carry over from airplane technologies and going back to brush up on your airplane knowledge might be a good idea. I've actually been meaning to write this thread for a few years, so I guess now is the time.

I am not an expert, but I can at least get us started and we can have a good old chat about it.

Before we start, here are some related links:

A thread were I started a discussion about front spoilers and if there are any in existence (one car so far has them): http://forums.bimmerforums.com/forum...707814-Spoiler
Advertised rear diffuser is actually not a diffuser at all: http://forums.bimmerforums.com/forum...hread-amp-List


I strongly suggest you read the topics in the order they appear in this thread (as they build on each other), but if you want to jump to a section here they are in alphabetical order:

• Active aerodynamic parts
• Aerodramatic (kabel, you should write this one)
• Air dams
• "Barge boards"
• Canards (dive planes)
• Diffusers, Rear
• Diverters
• "Duck tails"
• Dynamic elements
• End plates
• Fairings
• Ground effect(s)
• Gurney flaps
• NACA ducts
• Skirts, Side
• Splitters
• Spoilers
• Strakes
• "Tea trays"
• Tonneau covers
• Venturi tunnels
• Vortex generators
• "Whale tails"
• Wheel covers, Low drag
• Wings

[kabel]

"aerodramatic" should be included as a descriptor in this conversation

[BenFenner]

Air dams

These are flat or mostly flat sections of bodywork that sit vertical or mostly vertical to present a blunt face to the incoming air. They serve different purposes depending on where they are placed on the car and if they are paired with other aero parts. As should be obvious they can create a huge amount of drag, probably the most of any aerodynamic device. Think hard before employing something like this. It would be best to talk about a lower front air dam first. Lower front air dams at their most elemental serve a single purpose. They divert air around the car, preventing the air from going underneath the car. This creates an area of low pressure behind the air dam, creating downforce.

Lower front air dam.

Because of how much pressure is created in the front of a vehicle when a lower front air dam is used, they are often paired with front splitters to further increase downforce. The high pressure air acts on the front splitter to push down on the vehicle. This was not the original purpose of a front splitter and they have benefits even when used without an air dam (as usual, a clue is in their name). See the description of splitters for more info.

Lower front air dam with front splitter.

Often air dams are used on the rear of a vehicle, most notably in previous NASCAR setups. This type of air dam provides rear downforce by creating an area of high pressure which acts on the trunk lid to push the rear of the car downward. This is distinctly different from a rear spoiler, and distinctly different from a rear wing. Large, rear deck air dams are most often erroneously called gurney flaps. See the posts on spoilers, wings, and gurney flaps for correct examples of those elements.

Rear air dam.

Other air dams can be found in various places on vehicles. The roof is another common place. Some are there for creating downforce similar to the rear air dams, and other are blurring the line between air dam and strictly a diverter such as the one seen below which directs air around a turbulent roof rack.

Here is an air dam on a roof.

[BimmerBreaker]

Great thread... I love aerodynamics Waiting4diffusers ...and to figure out what a barge board is

[BenFenner]

Wings

Wings are elements placed in heavy air flow regions (usually separated from the skin of the vehicle by posts) and aid downforce in a combination of two ways. They are essentially identical in form and function to static airplane wings, hence the name. If you're familiar with airplane wings you should be familiar with their "delta" cross-section shape which promotes air to travel faster across one surface than the other which creates a pressure differential to cause lift on an airplane and downforce when inverted on a car. Wikipedia: Airfoil The second and much more effective way that wings provide downforce is with their angle of attack. They cause air to be diverted upward which in turn pushes the vehicle downward. Wikipedia: Angle of attack

Showing a wing cross-section (airfoil) and angle of attack oriented for lift.

Wings are distinctly different from spoilers. Wings create lift for aircraft and downforce for cars where spoilers almost exclusively reduce drag. See the description of spoilers for more info. Here is a good example of a rear wing.

Rear wing with two airfloil elements stacked for better effect.

Here is a good example of a front wing (in white).

A chassis dedicated to racing often employs a front wing.

Here is a rare example of a non-dedicated racing chassis with a front wing (it has a rear wing too).

Front and rear wings on this car.

Don't forget wings in the middle. Wings can be anywhere.

Lancia Stratos (toy model) with a wing in the middle above the cabin.

It is important to note that the downforce from a wing comes at the expense of drag (as is almost always the case with downforce), and a decent amount of drag at that. Remember that the angle of attack is the more important factor, and the more of a face to put into the wind the more drag you will experience. It is also important to remember that the downward and rearward forces acting on a wing can be taken advantage of with wing placement and support length. If you mount a wing more rearward of the rear axle it will act as a lever multiplying the effectiveness of the wing. However... ...[T]here is a penalty to be paid in that placing downforce aft or the rear axles will cause lift on the front end, as a result of the lever it creates. Placing a rear wing directly over the rear axles has no effect on the front (no lever) and could be considered an ideal location if the lifting the front is to be avoided...

All else being equal, placing the wing further back will make it more effective due to leverage. (More toy models.)

Again, because of the drag produced by a wing, it is best to place the wing where the drag will help, or at least not hinder performance. Below is an example of poor wing placement. It would be better placed much lower in the front of the car like the previous examples of front wings.

Drag leverage is making this front wing less effective.

[Z3racer701]

Front wings aren't that rare, the grid girls call this group the ankle biters. Watch closely enough and you'l even get to see a tunnel:

[BenFenner]

Spoilers

Spoilers are elements placed almost exclusively on the rear of a vehicle designed to reduce drag. The only known exception so far to this rule is the Pagani Huayra which uses dynamic front spoilers as air brakes and roll control devices. See this discussion of front spoilers if you want to get into detail. Spoilers seem to be the most confusing and misunderstood aerodynamic device used on automobiles. The name is also used incorrectly to describe all manner of aerodynamic devices. For example, there is no such thing as a "chin" spoiler if you've followed along with the first two sentences of this paragraph. Rear spoilers are traditionally close to the body of the car which places them in slow and/or turbulent air (making them almost useless for creating downforce) and their shape is such that it "spoils" the air making it even more turbulent. This turbulent air causes some of the air to flow down behind the trunk/hatch of the car and fill in the low pressure area behind the car. Filling this low pressure area raises the relative pressure behind the car, reducing the drag on the car. You can think of it either as the car using less energy to create a partial vacuum behind the car because the vacuum is less, or you can think of it as the partial vacuum being less behind the car which "sucks" the car backward less. Technically anything that is designed to spoil the air and create turbulence for an overall benefit is a spoiler. This is one of the least effective aerodynamic tactics, so true spoilers are few and far between and getting more rare each day as OEMs find better ways to reduce drag. It is usually best to keep air laminar and direct it where you want it to go with diverters versus creating turbulence. Even when advertized as a "spoiler" and mounted generally in the right place, most are ineffectual, and thus not spoilers at all.

How rear spoilers reduce drag.

Quintessential rear deck spoiler.

[yip]

Ahhh.... I thought your username looked familiar. After seeing you post a pic of the B13 and Mike's car I remembered your username from sr20forum.

[Bear Grylls]

Ah awesome. I am generally interested in designing aero parts for our cars.

Just need to get my hands on a large amount of modeling clay and some space to do it all

Sent from my Nexus 7 using BF.com

[deni2s]

Would be interesting to know real names of various z3 parts - like hamann side air foils and spoiler/wing?, ACS splitters, NDG rear diffuser...

[BenFenner]

Venturi tunnels

Venturi tunnels take advantage of the Venturi effect which causes low pressure zones where air speed increases in a confined tube. A low pressure zone can be created under the car with a carefully shaped undertray for added downforce. Originally in automotive applications the Venturi tunnel consisted of the inlet, and the "throat", but no diffuser. The throat is more commonly referred to as the Venturi tunnel itself leaving the inlet basically nameless. Soon after initial development of the modern automotive Venturi tunnel, a diffuser was added, completing the picture.

An abstract and idealized Venturi tunnel showing the three main parts. The "throat" is the main player, often called a "Venturi tunnel" itself.

The first Venturi tunnels applied to cars used a lot of vertical space to shape the throat and resembled literal interpretations of the diagram above. I can't find good pictures of any right now, but imagine a tall, slab-sided tunnel under a car with a gathering inlet. Soon after this, the outlet was shaped to match the smooth inlet and the rear diffuser was born (see the post on rear diffusers for more info). Again there was more innovation, and instead of using tall sides to form the throat, the undertray of the car served as the top of the tunnel, the road surface served as the bottom of the tunnel, and the "sides" are left mostly open.

Relatively modern Venturi tunnels (one on each side) using the road surface as a major side for the throat. Note the smooth inlet and diffuser shapes.

A good understanding of the Venturi effect and Venturi tunnel is essential to understanding a bunch of other aerodynamic devices as many of them are designed to help or enhance this main principle. It should be obvious that the main purpose and function of a rear diffuser is intimately tied to the Venturi tunnel, and so are other devices. While the rear diffuser was initially a tack-on to the Venturi tunnel it has turned into a complicated aerodynamic device all its own. Now that you're familiar with the Venturi tunnel, go on and read rear diffuser post when you're ready.

[mcoupemindy]

I approve of this thread.

[dcrothers]

You mean to say that you can't just throw aero parts on your car and be faster.
Who'd a thunk!!!

[BenFenner]

Diffuser, Rear

Please read the post on Venturi tunnels before reading this post. Diffusers were designed initially as part of a Venturi tunnel so you should know those and the history before getting into them. A diffuser is best used when in conjunction with a complete Venturi tunnel setup. The Venturi tunnel throat speeds the air producing downforce in the process, but if that air is left to exit the car untreated it will encounter the slower moving surrounding air and cause a lot of turbulence, drag, and reduce the effectiveness of the Venturi tunnel and in turn reduce downforce. The diffuser's main job is to slow the air exiting the throat of the Venturi tunnel to the same or similar speed as the air behind the car and to maintain laminar flow throughout that process. This increases the downforce in the middle of the vehicle provided by the Venturi tunnel and adds additional downforce at the rear of the car. Typically diffusers accomplish this by opening up slowly in height and sometimes in width to smoothly transition the air from under the car to the rear of the car. It's about time for some pictures.

A simple rear diffuser.

This is pretty much what the fuss is all about.

Lotus showing us how it's done.

It is important that the transition angles or gradients be smooth and maintain laminar flow across all surfaces. Having diffusers at too steep of an angle or weird shapes will separate air flow from the surfaces and ruin any gain you might have expected. There are countless ways to mess up a diffuser as with anything else, so here are a few examples.

This WRX is probably running much too steep of a diffuser if high speeds are in the cards. Additionally the muffler is ruining things on the left side completely. A much shallower diffuser would probably be of more benefit all around.

This may look like a rear diffuser to the untrained eye but this Toyota Camry SEMA Rowdy Edition faux-fuser is an abomination. There will be nothing good happening here. Turbulent city!

Rear diffusers were designed to enhance the benefit from Venturi tunnels and are best used in conjunction with a full Venturi tunnel setup, however they can provide some downforce on their own. In the absence of proper Venturi tunnel the job of the rear diffuser is similar. With a bit of rake applied, the center of the car will act as a poor-man's Venturi tunnel to make use of. Even in the absence of that, a diffuser should aid the exit of air from under the car creating a low pressure area under the rear of the car. As usual, keeping the air moving smoothly is the goal. Even the side-effect of blocking off the rear bumper from acting as a parachute is a gain. Since we've covered diffusers we should now cover strakes! Check out the next post for those.

[BenFenner]

Strakes

Strakes are the name given to vertical slats designed to keep air flowing in the desired direction. Strakes are most commonly used on rear diffusers to prevent cross-wind and other disturbances from affecting the flow of air through the diffuser. Diffusers are sensitive to any turbulence and not only do the strakes help direct the flow but they also help keep the flow attached to the upper portion of the diffuser by creating little vortices. It is not typical to see wide swaths of diffuser these days without strakes to break it up. A few pictures.

Afterhours Automotive fabricated this rear diffuser with vortex generating strakes for Mike Kojima's famous Dog II car.

A Lotus Elise sporting a BOE Fabrication rear diffuser with prominent strakes.

A 3rd generation Toyota MR2 sporting rcntype's (of spyderchat.com) rear diffuser featuring those all important strakes.

[BenFenner]

Splitters

Splitters, as the name implies (you should be getting used to the concept of aerodynamic devices describing what they accomplish in their name) are designed to split the air into two parts. Almost exclusively, splitters are used at the front of a vehicle, sometimes referred to as "chin" splitters because of the lower-front placement. They are flat or mostly flat elements with a leading edge usually (but not always) sticking out past the bodywork. The first and simplest of splitters is exactly that and no more. They give the air a clean place to separate, causing some air to go under the car and some to go up, over, and around the top of the splitter. This reduces turbulence at the front of the vehicle which mainly reduces drag. Here is are good examples of the simplest of splitters.

The most elemental of front splitters. Air will better travel above and below, reducing drag.

Another example of the simplest of front splitters.

There is a lot more to a front splitter these days than simply providing a place to split the air smoothly and reduce turbulence. We will get into those details soon. As you might be able to tell from this next picture, drag reduction continues under the car with the splitter traveling back some distance. And if you're starting to think this might also look like a good inlet for a Venturi tunnel, you're on the right track.

The bottom of a front splitter usually looks something like this. It keeps the air laminar below the car, reducing drag. Does it look like the beginnings of an inlet to a Venturi tunnel to you?

It wasn't long before the front splitter started getting used in conjunction with front air dams. Once this happened, a handful of factors collided for a big boost in downforce. This turns the front splitter from an agent of drag reduction to a serious player in the downforce game. The front splitter combined with a front air dam still provides some drag reduction benefit as before but the front air dam piles up the air molecules in the front of the car creating an area of high pressure which in turn presses down on the front splitter providing a heap of downforce.

A textbook front air dam with a textbook front splitter. High pressure at the front of the car built up by the air dam will push down on the front splitter creating downforce.

Those are the basics of splitters. If you find an example of a splitter used somewhere else other than the front of the car I would love to see it. I'm not saying they aren't out there, but I can't think of any right now. There are much more details to a front splitter than what I have covered. The angle of attack of the splitter can be important and adjustable, and the shape of the splitter can be optimized to make the splitter less sensitive to pitch, yaw, and roll. I won't be going into all the details though.

Front splitter with adjustable supports used to change the angle of attack.

Front splitter with a compound shape to help maintain effectiveness during pitch, yaw, and roll.

"Did someone say splitters?" -UOP CanAm Shadow

[BenFenner]

"Tea trays"

Oh the British. If you aren't into Formula 1, you've probably never even heard of this thing. Modern Formula 1 cars employ a splitter behind the front wing, under the nose, and in front of the side pods (so maybe the only example of a non-front splitter?). The shape of the splitter is uncharacteristically narrow and deep which gives it a rectangular shape. People have taken to calling it a tea tray because of the shape. Make no mistake about it though, it is simply an atypical splitter.

Fancy a cup of Earl Grey?

Pinkies up!

I've exhausted my tea references.

[BenFenner]

End plates

End plates are simple devices placed at the end of other aerodynamic elements usually used to keep air from spilling off of said elements. They are usually trying to keep high pressure air where it can be best taken advantage of. They also prevent cross winds from interacting with the high pressure zones. The most common place to see end plates are on wings.

A rear wing with prominent end plates.

End plates are also used in other places. Pretty much anywhere there is a high pressure zone you want to take more advantage of. And as with most aerodynamic devices, they can be complicated and endlessly optimized.

An end plate keeping high pressure air over the front splitter where it can create more downforce.

Quiz time. Can you spot the four, prominent end plates on Nobuhiro ‘Monster’ Tajima's GoPro Suzuki SX4 Pikes Peak race car? Ignore the oddly placed air dam below what should be the front splitter.

End plates do function as "pressure fences" as you have described, but one of the main benefits is to promote even mixing of the flow and increased efficiency of the wing. Because of the high pressure differential between the two wing surfaces, there is a strong tendency for flow separation around the ends, which hurts lift and generates a lot of drag. Proper end plates promote even transition from the high to low pressure side of the wing, gradually shifting pressure back to ambient, usually creating a clean vortex if done properly. This deceases induced drag, and has the simultaneous benefit of increasing the effective span of the wing, so the wing becomes more efficient all around. Properly designed end plates tend to point away from the high pressure surface, hence they point "up" on airplane wings and "down" on cars.

[BenFenner]

Thanks blackxs for helping me get these last few posts out faster by replying between them. You can hold off for now as I'm done for the day. Thanks. =D

I haven't been ignoring everyone, I'm just trying to get the informative posts laid out first so things will be organized and we can have a base knowledge to work from. Also I wanted to get it out while I had the motivation. As you might be able to tell from the first post, I still have a handful of posts to write. I'll probably get to them tomorrow.

"aerodramatic" should be included as a descriptor in this conversation

I like your style. I'll let you do the description of that one, okay? =]

Great thread... I love aerodynamics Waiting4diffusers ...and to figure out what a barge board is

If you want to find out what a barge board is before I write about it, there are other ways. =D
Frankly, I'm just including them for completeness. They are rare on street cars and I thought they were something else entirely that I was looking for the correct name to. Still looking for that one...

Front wings aren't that rare

Yah, sorry. I should have said they are rare on street cars. Of course they are common on dedicated race cars not based on street cars. It's rare to see a street chassis with one, which is what I meant. I've corrected the post.

Ahhh.... I thought your username looked familiar. After seeing you post a pic of the B13 and Mike's car I remembered your username from sr20forum.

I'm actually trying to avoid using lots of SR20 and Mike Kojima stuff. That would make it too easy. =]
From sr20forum, or sr20-forum?

Ah awesome. I am generally interested in designing aero parts for our cars.

Just need to get my hands on a large amount of modeling clay and some space to do it all

Well, hopefully this will help some. I can't imagine doing such a thing without a serious grasp on the topic.

Would be interesting to know real names of various z3 parts

We can get to those in the coming days. I'm looking forward to it.

I approve of this thread.

Thanks.

You mean to say that you can't just throw aero parts on your car and be faster.
Who'd a thunk!!!

[deni2s]

I am interested what is the purpose (if any, apart from aesthetics) of hamann side airfoils "side sill wings"... Will wait for that

[BimmerBreaker]

Would be interesting to know real names of various z3 parts - like hamann side air foils and spoiler/wing?, ACS splitters, NDG rear diffuser...

Truthfully, I am doubtful any of those parts mentioned have any real aerodynamic benefit. The hamann spoiler and ACS splitters may have a marginal effect. The NDG diffuser is really a cosmetic piece due to the fact its only 3" long or so and is placed over what is already designed to be a diffuser on the M models. Hamman side foils almost definitely dont do anything beneficial, to be honest I would think they would just disrupt airflow along the side of the car, creating turbulence.

Will let Ben chime in here but particularly regarding those side foils... I dont see how they could be functional to do anything at all.

[BenFenner]

I am interested what is the purpose (if any, apart from aesthetics) of hamann side airfoils "side sill wings"... Will wait for that

Pictures would be nice. Got any?

[BimmerBreaker]

I made my post thinking he meant these:

[Tazmanian Devil]

They are good for keeping the stone chips to a minimum. I think they are just for looks.

[BenFenner]

Those don't even resemble any properly designed aero part I've ever seen, so it is hard to classify them. They are sort of almost a... no... they really are their own thing. If I had to guess, I'd say zero aero benefit at best. However, that's just an educated guess. Sometimes the wind tunnel can surprise. However I think we can safely assume no actual wind tunnel testing nor computer simulation was key in the design of those doodads.

You want name for them? Sill warts?