Can someone explain what a diffuser does?

[Noisycow]

It redirects airflow under the car to reduce lift?

The whole concept is a little hardcore for a marginal sports car fan like me. When I do a web search on it, most articles talk about aromatherapy.

A rear spoiler I get - air push down (caveman voice) - good - traction at speed.

[Brennon]

Originally Posted by Noisycow

It redirects airflow under the car to reduce lift?

The whole concept is a little hardcore for a marginal sports car fan like me. When I do a web search on it, most articles talk about aromatherapy.

A rear spoiler I get - air push down (caveman voice) - good - traction at speed.

Take a piece of paper and by folding it make it look like this

Edit: okay my little drawing didn't seem to work. But if you take a piece of paper and create little 'legs' out of it by folding it in 3rds, then place it with the 'legs' supporting it on a flat surface.


Blow through the bottom half and you'll find that it will suck down to the surface it's on. By increasing air flow through the bottom portion, it creates a low pressure area causing the higher pressure above to exert downforce. At least, that is my understanding. Diffuser works the same way.

[Cegan09]

My very basic understanding of it is that it allows the air to exit the bottom of the car cleanly. This in turn helps air move quicker under the car, and if done right can create down force. Think an airplane wing, air moves faster over the top, creating a lower pressure and therefore lift. The diffuser helps to create the same effect.

[TimMullen]

Originally Posted by Noisycow

It redirects airflow under the car to reduce lift?

The whole concept is a little hardcore for a marginal sports car fan like me. When I do a web search on it, most articles talk about aromatherapy.

A rear spoiler I get - air push down (caveman voice) - good - traction at speed.

The diffuser is essentially an upside down air foil. The air under the car is expanded in a controlled area (the diffuser) which reduces the air pressure. This creates a "downforce" on the rear of the car. The verticle "fins" are to keep the air moving front to back and prevent any air from the side being "sucked in" to fill the low pressure area.

Now, rear spoilers are not necessarily air foils and don't "push air down" - "spoiler" has become an generic term.

Spoilers (wings) that are shaped like inverted airfoils generate "lift" on the lower side of the spoiler - again creating a low pressure area that creates a "downforce". Other spoilers are just that - they "spoil" the smooth air flow coming off the rear of the car. This smooth air flow can create a low pressure area and generate lift. A spoiler doesn't actually create downforce, it spoils the lift instead - the result is the equivalent of downforce on the rear. An example of a spoiler is the rear lip of an Elise. The wing on the Exige is more of an air foil.

A front air dam (splitter) also doesn't usually create down force - they prevent air from being compressed under the car and creating lift.

[milehitom]

The smooth bottom pan of the car allows air flowing underneath it to flow at a fast, smooth speed. The vanes at the back of the diffuser help to slow the air down so it can rejoin the air flowing over the top of the car, which is travelling at a slower speed because of drag. The vanes reduce turbulence and drag on the rear of the car and help increases downforce. It's based on Bernoulli's Principle, where pressure increases as velocity decreases. Low pressure below-high pressure above=downforce.

Think upside airplane wing.

tom

[Conan]

just saying what's been said in another way.....

If the air under the car goes faster than the air over the car, then the air under the car has less pressure than the air over the car. ie: Downforce (higher pressure vs lower pressure). The rear diffuser helps the air exit the bottom of the car faster, thereby increasing airflow under the car, thereby increasing downforce.

[Noisycow]

Cool tech - thanks for the answers + I tried the paper trick. The airflow through does pull down the top 1/3rd of paper.

This means stability is better than my Chevy Express van with ladders on top at 105mph? The ladders create 'lift' in the front end by pulling up and back, not great.

[snuggles1]

Originally Posted by Noisycow

It redirects airflow under the car to reduce lift?

The whole concept is a little hardcore for a marginal sports car fan like me. When I do a web search on it, most articles talk about aromatherapy.

A rear spoiler I get - air push down (caveman voice) - good - traction at speed.

Diffuser: The divergent section of a duct which slows down an airflow. On a competition car, it is an upswept panel/panels at the rear of the underbody, or at the rear of the tunnels.

And Tim, yes, airdams do create downforce, but, as you'd expect, only at the front. Downforce (at front) does increase as airdam depth increases. (Of course they also increase drag.)

[TimMullen]

Originally Posted by snuggles1

And Tim, yes, airdams do create downforce, but, as you'd expect, only at the front. Downforce (at front) does increase as airdam depth increases. (Of course they also increase drag.)

No, air dams do not create down force. They prevent the generation of lift.

An air dam redirects the air over the front and around the sides of the front of the car. It prevents the (some of the) air from passing under the front of the car. Air passing under the car slows down and gets compressed - increasing local air pressure. An increase in air pressure under the car "generates lift". An air dam simply prevents the generation of the lift. The effect is the same as "down force" but the reality is that it's different.

You can only prevent so much lift, whereas a wing can make as much down force as you want to (i.e. make the wing bigger).

You would thing that an air dam would create drag and in fact it probably does - however, it more than makes up for the additional drag that is prevented by stopping the air from compressing under the car.

None of this is to say that an air dam is a bad thing - in fact it isn't. An air dam decreases lift and at the same time (usually) reduces drag; it's a win - win.

[knucklehead]

My understanding of the diffuser combined with the solid panels under the car was NOT to act as an airfoil (air traveling at different speeds on top vs bottom), but that the air under the car was expanded in the back which would pull the car down. Like a vacuum. The wings on the diffuser (as Tim mentioned) are to keep air from entering from the sides, but the reasoning is to keep the side air from filling in that expanded air space.

Is my understanding wrong?

[fitfan]

Originally Posted by TimMullen

No, air dams do not create down force. They prevent the generation of lift.

+1

[charliex]

like tim says air dams do not create downforce, a partial vacuum is created under ther car which in turn produces more downforce, but its academic anyway since neither the elise or exige have air dams, they have splitters which do generate downforce via the leading edge.

air dams and splitters aren't the same thing.

ie an air dam is just that, a splitter is an air dam with a leading edge.

[TimMullen]

Originally Posted by knucklehead

My understanding of the diffuser combined with the solid panels under the car was NOT to act as an airfoil (air traveling at different speeds on top vs bottom), but that the air under the car was expanded in the back which would pull the car down. Like a vacuum. The wings on the diffuser (as Tim mentioned) are to keep air from entering from the sides, but the reasoning is to keep the side air from filling in that expanded air space.

Is my understanding wrong?

Nope. You have it correct. It's "like an air foil" in that the air speeds up/expands as goes "up" the diffuser. This creates a low pressure area (just as an air foil does), and that creates down force.

The smooth bottom is to reduce drag and provide good air flow. It ties right into the diffuser.

[TimMullen]

Originally Posted by charliex

air dams and splitters aren't the same thing.

ie an air dam is just that, a splitter is an air dam with a leading edge.

Actually they are pretty close. The splitter just stick out farther and "splits the air" better. An air dam tends to direct the air around the sides of the car. A splitter tends to still block the air from going under, but it directs more of the air over the car. It does a better job of controlling the air flow.

Since neither is actually creating down force, they can be rather flimsy. A flexible air dam or splitter that you can bend with your fingers can "create" hundreds of pounds of down force (actually prevent the generated lift). Now, you can also angle the splitter downward (requiring lots of bracing) to act more like a "wedge" and create rear down force (think NASCAR). But there's still a limit to how much it can make.

That's one of the wonderfulness of the Elise/Exige designs. Control the air flow to "generate down force" and do it while minimizing the additional drag.

Remember that the Elise, with just the diffuser, rear lip spoiler and the winglets on the front actually generates down force. On most cars, those aerodynamic aids just reduce the generated lift. The Exige makes even more with the splitter and rear wing.

[slaughtr]

So what, if any, benefits are derived from Reverie's 3 element and 5 element diffusers?

[TimMullen]

Originally Posted by slaughtr

So what, if any, benefits are derived from Reverie's 3 element and 5 element diffusers?

A diffuser that has vertical fins that stick closer to the ground does a better job of stopping the air from "leaking in" from the sides. A longer diffuser can create a bigger low pressure area. More vertical fins may do a better job of keeping the air flow from spilling in (but it's probably mostly for bling). All of this comes with a trade off - and a big one in reality - less ground clearance. A longer or lower diffuser is going to be scraping the ground a lot more often - not a good feature for a street car.

All of this involves trade-offs. Yes, you may get more down force, but maybe not much. Is a small increase in down force worth ripping up the diffuser? Maybe. Depends on how much extra down force, how you drive, etc. Everything in life is a compromise.

[njcu]

the diffuser simply stops you from flying away. If you remove it and drive real fast you can glide over hills like a flying squirrel....

Really though... "The Man" put it there to keep you down. Seriously.

[qball]

It diffuses

[Pestodude]

Originally Posted by qball

It diffuses

To show how effective a well engineered underbelly of a sportscar can be, look at the newer Ferrari's...no wing at all but no lift (or very little).

our cars are far from perfect aerodynamically.

[fishguy]

Originally Posted by slaughtr

So what, if any, benefits are derived from Reverie's 3 element and 5 element diffusers?

for the vendor that sells the stuff to you it is very beneficial.