Top speed vs Coefficient of drag

[old greg]

Originally Posted by serialk11r

Sealing off the bottom completely with nylon brushes and then using fans is something I want to try someday

I take it by brushes you are referring to solid sliders and not brushes of the bristled variety. F1 teams tried bristles to seal their ground effects early on and they were completely worthless. But anyway, skip the nylon and use UHMW or Delrin; both are slipperier and harder wearing than nylon and Delrin is super impact resistant, which could come in handy if your driving on uneven pavement.

My only concern would be melting the plastic while driving at high speeds.

[carbonBLUE]

Originally Posted by Sully

Downforce is also drag. You can get lots of downforce(F1), or a low C/D. Not both.

down force is drag but drag isn't always down force... example a brick has no down force but tons of drag

if the body of a car (without an aero kit) has a drag ratio of .25 but with the aero kit has a drag ratio .28(giving plenty of down force) but another car (based on its shape) has a drag ratio of .30 without any added down force.... which one would you take....

[old greg]

Originally Posted by serialk11r

The sides of the car should be completely sealed off (is almost possible with nylon brushes lol), splitter in the front as close to the ground as possible, underbody of the car slowly sloping up towards the top edge of the car, which meets the bottom of the car at as close to parallel to the ground as possible. Putting channels into the bottom of the car like Lotus is a way to mimic this although it's best to have no air escaping in through the sides. Basically have the car shaped like an enormous upside down wing.

Your understanding of undertray venturi tunnels is a little off. Firstly you need air to pass under the car, the more the better in fact (see the high noses on F1 cars, or the audi R15 for proof). So a splitter scraping the ground is counter productive (unless you are required to run a flat floor), you need the splitter to not choke the airflow going to the venturis. Following that, you need to have the undertray slope down towards the ground a bit to reduce the cross sectional area of the tunnel (but not so much as to choke the flow, that would create huge drag and lift). This forces the airflow to speed up relative to the airflow passing over the vehicle. Bernoulli's principle and dynamic pressure mean that you've just created a pressure differential. In a true ground effects car (ala F1 circa 1980) this is where the majority of the downforce is generated, and it's the reason why the FIA mandated flat floors for the whole length of the wheelbase. Contrary to popular belief, the diffuser doesn't create a negative pressure zone, it's simply there to slow the air back down and recover some of the energy (drag) that went into speeding it up in the first place. As a side effect of that process though, some of that high speed flow of air is pulled upward due to it's attachment to the surface of the diffuser, as a result of this the diffuser is pulled down (equal and opposite reactions) creating downforce. An interesting side effect of this is that the majority of the downforce generated in the diffuser is generated in the forward most portion. An airfoil mounted just behind and above the trailing edge of the diffuser (see F1 Beam Wing) will create a negative zone low and directly behind the vehicle which, while it creates drag, actually helps pull even more air through the venturis (by creating a higher front to rear pressure gradient) creating even more downforce. This is often reffered to as using a wing to "drive" the diffuser.

The reason why the sides should be sealed is to prevent air from escaping the nozzle (front) section and not speeding up like it should, and to prevent slow moving high pressure air from being pulled into the Venturi section. Sealing up the diffuser though isn't particularly critical. Air bleeding in from the sides of the car simply means that the diffuser needs (and can can have) a greater angle of attack.

Originally Posted by carbonBLUE

if the body of a car (without an aero kit) has a drag ratio of .25 but with the aero kit has a drag ratio .28(giving plenty of down force) but another car (based on its shape) has a drag ratio of .30 without any added down force.... which one would you take....

I'll pick a Corvette over a Prius with a wing any day of the week

[Sully]

Originally Posted by old greg

Ahem.

Bah, like that's at all relevant to anything to anything contemporary or useful.

But well played, nonetheless!

Originally Posted by carbonBLUE

down force is drag but drag isn't always down force... example a brick has no down force but tons of drag

You'll note that isn't what I said.

[carbonBLUE]

Originally Posted by old greg

Your understanding of undertray venturi tunnels is a little off. Firstly you need air to pass under the car, the more the better in fact (see the high noses on F1 cars, or the audi R15 for proof). So a splitter scraping the ground is counter productive (unless you are required to run a flat floor), you need the splitter to not choke the airflow going to the venturis. Following that, you need to have the undertray slope down towards the ground a bit to reduce the cross sectional area of the tunnel (but not so much as to choke the flow, that would create huge drag and lift). This forces the airflow to speed up relative to the airflow passing over the vehicle. Bernoulli's principle and dynamic pressure mean that you've just created a pressure differential. In a true ground effects car (ala F1 circa 1980) this is where the majority of the downforce is generated, and it's the reason why the FIA mandated flat floors for the whole length of the wheelbase. Contrary to popular belief, the diffuser doesn't create a negative pressure zone, it's simply there to slow the air back down and recover some of the energy (drag) that went into speeding it up in the first place. As a side effect of that process though, some of that high speed flow of air is pulled upward due to it's attachment to the surface of the diffuser, as a result of this the diffuser is pulled down (equal and opposite reactions) creating downforce. An interesting side effect of this is that the majority of the downforce generated in the diffuser is generated in the forward most portion. An airfoil mounted just behind and above the trailing edge of the diffuser (see F1 Beam Wing) will create a negative zone low and directly behind the vehicle which, while it creates drag, actually helps pull even more air through the venturis (by creating a higher front to rear pressure gradient) creating even more downforce. This is often reffered to as using a wing to "drive" the diffuser.

The reason why the sides should be sealed is to prevent air from escaping the nozzle (front) section and not speeding up like it should, and to prevent slow moving high pressure air from being pulled into the Venturi section. Sealing up the diffuser though isn't particularly critical. Air bleeding in from the sides of the car simply means that the diffuser needs (and can can have) a greater angle of attack.




I'll pick a Corvette over a Prius with a wing any day of the week

ide pick a gtr over a corvette any day of the week

[serialk11r]

@old greg
sorry, you're totally right. Airflow is also slower when it hugs the ground. Oops brainfart.

[maximumunicorn]

Another factor to keep in mind here is that while Cd can help with fuel economy at normal speeds and allow the car to go faster given the same horsepower, you may not want to. It's important to reiterate the fact that drag can come from down force which in turn can make high speed handling more stable. With a low coefficient of drag the FT-86/FRS/BRZ might be able to reach higher speed, but might be squirrelier (huh, never spelled that before, looks kind of silly).

[carbonBLUE]

Originally Posted by maximumunicorn

Another factor to keep in mind here is that while Cd can help with fuel economy at normal speeds and allow the car to go faster given the same horsepower, you may not want to. It's important to reiterate the fact that drag can come from down force which in turn can make high speed handling more stable. With a low coefficient of drag the FT-86/FRS/BRZ might be able to reach higher speed, but might be squirrelier (huh, never spelled that before, looks kind of silly).

it might be but you can design a car with plenty of down force and still be slippery through the air, the nissan gtr has a drag ratio of .27 and .26(without the spoiler) but that car can go 200mph and not be scary at all. Stability at high speeds also has to do with how well the car can handle crosswinds. my first car, 2005 corolla, is more aerodynamic than my celica in a straight line but on windy days on the freeway i was tossed around and got quite annoying... but in my celica i never have problems with crosswinds and im lighter than my corolla.

[fatoni]

Originally Posted by carbonBLUE

it might be but you can design a car with plenty of down force and still be slippery through the air, the nissan gtr has a drag ratio of .27 and .26(without the spoiler) but that car can go 200mph and not be scary at all. Stability at high speeds also has to do with how well the car can handle crosswinds. my first car, 2005 corolla, is more aerodynamic than my celica in a straight line but on windy days on the freeway i was tossed around and got quite annoying... but in my celica i never have problems with crosswinds and im lighter than my corolla.

lets not forget about things like drivetrain, wheelbase, and traction and stability control. i think the point people are making is that downforce increases drag. yeah cars can be slippery and have downforce but adding downforce adds drag

[old greg]

Originally Posted by fatoni

adding downforce adds drag

This is usually, but not always true. There are several places on a car where downforce can be created with negligible changes in drag, and at least one where you can add downforce and reduce drag.

There's the shaped undertray as previously mentioned, but lift can be reduced by venting high pressure air from the wheel wells and the engine bay without incurring any drag penalty. But the most efficient place to create downforce (and reduce drag!) is the rear windshield/trunk. The air flowing over the top of a car has a tendency to detach from the surface of the roof/rear glass, or at the very least to form a very thick boundary layer. This forms a low pressure region behind/above the car which causes both increased drag and increased lift (negative downforce) on the rear portion of the car. The addition of a spoiler (not a wing) to the trunk lid obstructs the flow of air coming off the rear of the car. You would naturally assume that this would create drag but in fact, that rise in pressure helps to prevent flow detachment over the rear windshield and causes the boundary layer to shrink. This reduces drag while at the same time, the increased pressure on the top of the vehicle causes a decrease in lift (= increase in downforce). Of course some cars are designed to prevent flow detachment, the Prius, CRZ etc. any it's likely to become more common, but many cars are simply awful in this regard.

[Longhorn248]

Originally Posted by old greg

This is usually, but not always true. There are several places on a car where downforce can be created with negligible changes in drag, and at least one where you can add downforce and reduce drag.

There's the shaped undertray as previously mentioned, but lift can be reduced by venting high pressure air from the wheel wells and the engine bay without incurring any drag penalty. But the most efficient place to create downforce (and reduce drag!) is the rear windshield/trunk. The air flowing over the top of a car has a tendency to detach from the surface of the roof/rear glass, or at the very least to form a very thick boundary layer. This forms a low pressure region behind/above the car which causes both increased drag and increased lift (negative downforce) on the rear portion of the car. The addition of a spoiler (not a wing) to the trunk lid obstructs the flow of air coming off the rear of the car. You would naturally assume that this would create drag but in fact, that rise in pressure helps to prevent flow detachment over the rear windshield and causes the boundary layer to shrink. This reduces drag while at the same time, the increased pressure on the top of the vehicle causes a decrease in lift (= increase in downforce). Of course some cars are designed to prevent flow detachment, the Prius, CRZ etc. any it's likely to become more common, but many cars are simply awful in this regard.

Great post.

I'm a bit surprised that a car company hasn't come out with a niche car designed almost completely in the wind tunnel to try and minimize drag and raise fuel economy. Even without doing any calculations or testing, you can just look at the shapes of many of the cars on the road today and tell that aerodynamics weren't really taken into consideration.

[fatoni]

Originally Posted by old greg

This is usually, but not always true. There are several places on a car where downforce can be created with negligible changes in drag, and at least one where you can add downforce and reduce drag.

There's the shaped undertray as previously mentioned, but lift can be reduced by venting high pressure air from the wheel wells and the engine bay without incurring any drag penalty. But the most efficient place to create downforce (and reduce drag!) is the rear windshield/trunk. The air flowing over the top of a car has a tendency to detach from the surface of the roof/rear glass, or at the very least to form a very thick boundary layer. This forms a low pressure region behind/above the car which causes both increased drag and increased lift (negative downforce) on the rear portion of the car. The addition of a spoiler (not a wing) to the trunk lid obstructs the flow of air coming off the rear of the car. You would naturally assume that this would create drag but in fact, that rise in pressure helps to prevent flow detachment over the rear windshield and causes the boundary layer to shrink. This reduces drag while at the same time, the increased pressure on the top of the vehicle causes a decrease in lift (= increase in downforce). Of course some cars are designed to prevent flow detachment, the Prius, CRZ etc. any it's likely to become more common, but many cars are simply awful in this regard.

i guess it was just a matter of semantics with the reducing lift and creating downforce. it makes sense because cars are kinda shaped for passengers and tha creates lift. good point though

[fatoni]

Originally Posted by Longhorn248

Great post.

I'm a bit surprised that a car company hasn't come out with a niche car designed almost completely in the wind tunnel to try and minimize drag and raise fuel economy. Even without doing any calculations or testing, you can just look at the shapes of many of the cars on the road today and tell that aerodynamics weren't really taken into consideration.

i think that would be really expensive to do and i dont think looking at the shapes of cars is as great an indicator as you think. weve already brought up the fact that my brick of an ls430 is more aerodynamic than just about everysports car including the frs

[old greg]

Originally Posted by Longhorn248

I'm a bit surprised that a car company hasn't come out with a niche car designed almost completely in the wind tunnel to try and minimize drag and raise fuel economy.

There have been a few actually. Streamlining was very fashionable in the 30's so there were are a couple of prewar cars with very low coefficients of drag. The Tatra 77 (Cd=0.212) comes to mind, and more recently there was the GM EV1 (Cd=0.19). And if it ever makes it into production, there's the Aptera (Cd=0.15).