E28 drag coefficient

[gidgaf]

Everything is incremental.
Dropping the car helps.
Dropping the front a wee bit more than the back- ditto.
Yes the back spoiler helps. A bigger spoiler helps less.
After 35mph, close the windows and turn on the A/C.
There's been a lot of testing on surfaces and boundary layers. For a lot of boats and planes, a super shiny surface can sometimes mean more friction than a rough or matte finish. Myth busters did a show, and a very dented car had the best aerodynamics.
There's a lot of turbulence under the car- fix that too. Smoove everything.
Those little wheel well flaps- work better for this on the front of the wheel well.
Those little hood spoilers for the wipers help a little, also.
This is also the reason all the newer cars have covered up fronts- headlights,grills, etc. Design it so the radiator is sucking air from a hi pressure area under the front of the car.
In the '60s some guys ran a couple hoses from the hi pressure area under the car, thru the trunk, and exiting at the low pressure area behind the rear window.
At some point there will be diminishing returns- but it all adds up.

[Jeremy]

So if you get the car low enough to the ground the tires will no longer matter? Right. Stance logic !

Well, yes, actually. The point at which that is realized is the point at which there is zero airflow under the car, however, so it's actually not possible. There is always SOME airflow under the car, so the width of the tires always matters to some degree.

My real point is wouldn't M-tech or similar lowering springs reduce the frontal area by reducing the amount of air that gets under the car which is affected by tires and such ?

Coefficient of drag includes many factors, frontal area being one. M-Tech euro and M-Tech US body treatments both actually increase the frontal area, but reduce the drag coefficient due to changes in how much air is channeled how smoothly and pushed to where. The underside of the car is where a lot of drag gains have been made in recent decades. Look at a typical 80s car underside compared to a modern car's underside, BMW or otherwise, HUGE differences there.

Then, where is the point the cd really starts making any big difference. The drag is exponential (not sure the power) with air velocity IIRC. I thought at 25mph insignificant, 35mph not much, 45 a little, 55 begins to matter, then above about 60-65 it starts to get significant and goes crazy at very high speeds.

It's a simple square exponential curve. y = x² If speed doubles, the force of air resistance goes up by a factor of four. The higher the speed goes, the steeper the slope of the curve becomes. At exactly what speed that force becomes "important" or "notable" is highly subjective. Keep in mind that in the 50s and 60s things like "overdrive" were largely unheard of. Keeping the engine in an efficient part of its load curve can sometimes be more important than raw speed.

[gidgaf]

Yes airdams help. They call them "splitters" for a reason.
My 528 is a box. Actually it looks like a couple of stacked boxes.
If I put an airdam on the front that splits the air to the sides,
I am presenting a lot less flat front to the air I'm compressing in front of me.
And I'm keeping a lot of the rest from going under the car.
The rear wing would break up the drag caused by the vacuum that boxy butt is causing.
It all adds up.

[Shawn D.]

Yes airdams help. They call them "splitters" for a reason.

Actually, "they" don't call air dams "splitters." Air dams are air dams -- splitters are the flat horizontal plates that bolt to the bottom of air dams. They "split" the air stream, they do not "dam" it.

[gidgaf]

Oops! I forgot about speed.

I drove from LAX to PHX and back quite a lot.
Upping my speed from @ 65 to 85mph also meant one more gas stop along the way.
So if I only counted my actual driving time, I got there way faster.
But in my total portal to portal time, that extra half hour from the extra fuel resupply drastically cut into my time decrease.
And there was an obvious decrease in responsiveness.

I used to do long rides on my Goldwing, and 70 was my cutoff point. After 75mph I was spending more money on gas but not really getting any better arrival times. At 100 it did act like a wing- that front end got real light! Mine hit that wall at 105 indicated- it just would not go any faster, just louder.

For you LA denizens, going to Palm Springs from LAX is a good example.
I've gone there with a 50mph tailwind. It was smooth, it was quiet, it was nice, and I got outrageously great gas mileage.
I've also gone there with a gusty 50mph headwind, during an alert. It was rough, it was loud, it was nasty, my groundspeed was maybe 65mph- go pedal floored- and I could see my fuel gauge drop as a I drove. It felt more like a wall of water than of air.

My '86 might be hitting that wall a lot sooner than a '96. And that more than an '06.
But here in LaLaLand, I don't think my car has gone over 50 in a year.
Maybe fins would help.

[gidgaf]

Yes airdams help. They call them "splitters" for a reason.

Actually, "they" don't call air dams "splitters." Air dams are air dams -- splitters are the flat horizontal plates that bolt to the bottom of air dams. They "split" the air stream, they do not "dam" it.

This is LaLaLand!
They used to be called spoilers.
Then they were airdams.
Now they are all splitters.
Now, maybe where YOU are they got proper grammer and terminology and stuff.
Here, anything under the front bumper is a splitter.
Anything under the rear bumper is a diffuser.
Multiple fins aid in tracking.
Loud pipes save lives.
Big wheels mean big traction.
A lot of folk have a decal that looks like a sunroof.
And if you ain't on the phone, you ain't sh*t.

[John in VA]

This is LaLaLand!

And they're called Beemers there. Doesn't make it right.

[tschultz]

Everything is incremental.
Dropping the car helps.
Dropping the front a wee bit more than the back- ditto.
Yes the back spoiler helps. A bigger spoiler helps less.
After 35mph, close the windows and turn on the A/C.
There's been a lot of testing on surfaces and boundary layers. For a lot of boats and planes, a super shiny surface can sometimes mean more friction than a rough or matte finish. Myth busters did a show, and a very dented car had the best aerodynamics.
There's a lot of turbulence under the car- fix that too. Smoove everything.
Those little wheel well flaps- work better for this on the front of the wheel well.
Those little hood spoilers for the wipers help a little, also.
This is also the reason all the newer cars have covered up fronts- headlights,grills, etc. Design it so the radiator is sucking air from a hi pressure area under the front of the car.
In the '60s some guys ran a couple hoses from the hi pressure area under the car, thru the trunk, and exiting at the low pressure area behind the rear window.
At some point there will be diminishing returns- but it all adds up.

I think this is mostly true. this is why the golf ball flies farther than a completely round ball.

Also, the coedfficient drag is related to the surface area. Sure the 535is airdam may help reduce air around the wheels, but it also has much larger frontal area than the stock valance.

Look at the new NASCAR cars this year. They have their front slitters literally on the ground. More airflow over the car for maximum grip. Splitters are known as the lower plate on the bottom of the air dam, they aren't the same.

Getting lower Cd can also be acheived by simply waxing the car. The slippery feel is directly related to how the car moves through the air. I'm sure a dirty car has more drag and reduced mileage, even though Mythbusters didn't prove it (their tests are frequently incomplete).

With additional wind tunnel testing, i'm sure aerodynamics could be greatly improved. Look at the Dodge Charger vs Dodge Daytona.

Charger 500: .42
Charger Daytona: .28

[Shawn D.]

Getting lower Cd can also be acheived by simply waxing the car. The slippery feel is directly related to how the car moves through the air. I'm sure a dirty car has more drag and reduced mileage, even though Mythbusters didn't prove it (their tests are frequently incomplete).

Incorrect. The primary source of drag on production cars is pressure drag, not friction drag. A dirty or slightly rough surface on production cars will decrease drag by keeping the boundary layer energized and allow it to flow better around the aerodynamically blunt body. That's why a golf ball flies farther with dimples and why stranded wires have less drag than solid wires on biplanes.

Yes, I am an aerospace engineer by training.

[savannah996]

So were sport springs always added to the 535is?

Not only were they not always added to the 535is, they were never added to the 535is. The only difference was shock valving.

Some folks claim the existence of a slightly-larger front sway bar on '88s (both "i" and "is"), but nobody has ever come up with one.

Well there I have it. The 535is should have the same drag as a M5 then. Everything aero wise is the same.
Im just going to use .37 as my coefficient.
EDIT unless someone has a wind tunnel I can borrow?
Thanks everyone for all the help!

[Blue Shadow]

Well there I have it. The 535is should have the same drag as a M5 then. Everything aero wise is the same.

Well you will be using the wrong number then.

The tires on an M5 are wider. More meat in the air flow so it has a higher Cd. Wrong but you will be happy with it.

[savannah996]

Well there I have it. The 535is should have the same drag as a M5 then. Everything aero wise is the same.

Well you will be using the wrong number then.

The tires on an M5 are wider. More meat in the air flow so it has a higher Cd. Wrong but you will be happy with it.

If you have been reading the thread you would have known MY tires are 205/50-16 AND its lowered. Thank you.

[Jeremy]

Well then you should have said "my M5" instead of "a M5" (which should've been *an* M5 anyway, but I digress ...). Also saying "everything aero wise is the same" completely flies in the face of everything that was just discussed over the last 2 pages. Poor wording on your part doesn't give you the right to be snarky when you're misunderstood.

English. Know it. Love it. Use it properly. You'll be well understood and admired by all.

[Blue Shadow]

Apparently his M5 also have the front of engine intake system, air handling, exhaust, transmission, engine and other bottom in the air stream devices of a 535is.

Hell the numbers are pretty close, you'll be fine using the lowest E28 number you can find. Maybe you can repeat the project using the highest E28 numbers you can find and let us know what differences it makes to the final result.

[tschultz]

Incorrect. The primary source of drag on production cars is pressure drag, not friction drag. A dirty or slightly rough surface on production cars will decrease drag by keeping the boundary layer energized and allow it to flow better around the aerodynamically blunt body. That's why a golf ball flies farther with dimples and why stranded wires have less drag than solid wires on biplanes.

Yes, I am an aerospace engineer by training.

I do not disagree with you, but I'm not sure when relating this to a car .I understand your statement which is completely accurate (and why transports jets have an optimum speed where they can fly faster with less drag). This makes sense to me.

However, on a car, if you reduce the parasitic drag, you are reducing the sum of both types of drag, right? We aren't talking supersonic...

After some quick digging:

A rough surface always has more friction than a smooth surface. However, surface friction isn't the primary consideration in the aerodynamics of a shape. What matters far more is the total turbulence that a shape creates as it moves through the air (or any fluid). Turbulence represents energy that has been used to accelerate the mass of air that is moving in a turbulent manner.

By adding selected features to an aerodynamic surface such as a rough surface finish or small protrusions called turbulators energy is added to the boundary layer air. In particular a very local area of turbulence is created. That means that some of the air (50% on average) in that turbulence will be moving in the direction of the surface it must follow. Because of boundary effect it will follow that surface and in doing so it will create a very local low pressure area near it causing the immediately adjacent air to follow as well.

This prevents flow separation which is the cause of large scale turbulence such as is present when a wing stalls. By giving up a small amount of energy to air adjacent to the surface large scale loss of energy to large scale turbulence is avoided. Large scale turbulence manifests as drag. The dimples on golf balls, the roughness of shark skin and dolphin skin and the turbulator fences on a Cessna Citation all serve this same purpose.

[Jeremy]

Depending on exactly what he wants the information for, his next revelation might be that all s38b35s don't make their exact published power numbers. Especially now that they're all over 20 years old, there's a wide range of power figures possible. Everything from "extra healthy" to "below published figures" is possible at this stage.

[Shawn D.]

Incorrect. The primary source of drag on production cars is pressure drag, not friction drag. A dirty or slightly rough surface on production cars will decrease drag by keeping the boundary layer energized and allow it to flow better around the aerodynamically blunt body. That's why a golf ball flies farther with dimples and why stranded wires have less drag than solid wires on biplanes.

Yes, I am an aerospace engineer by training.

I do not disagree with you, but I'm not sure when relating this to a car .I understand your statement which is completely accurate (and why transports jets have an optimum speed where they can fly faster with less drag). This makes sense to me.

However, on a car, if you reduce the parasitic drag, you are reducing the sum of both types of drag, right? We aren't talking supersonic...

After some quick digging:

A rough surface always has more friction than a smooth surface. However, surface friction isn't the primary consideration in the aerodynamics of a shape. What matters far more is the total turbulence that a shape creates as it moves through the air (or any fluid). Turbulence represents energy that has been used to accelerate the mass of air that is moving in a turbulent manner.

By adding selected features to an aerodynamic surface such as a rough surface finish or small protrusions called turbulators energy is added to the boundary layer air. In particular a very local area of turbulence is created. That means that some of the air (50% on average) in that turbulence will be moving in the direction of the surface it must follow. Because of boundary effect it will follow that surface and in doing so it will create a very local low pressure area near it causing the immediately adjacent air to follow as well.

This prevents flow separation which is the cause of large scale turbulence such as is present when a wing stalls. By giving up a small amount of energy to air adjacent to the surface large scale loss of energy to large scale turbulence is avoided. Large scale turbulence manifests as drag. The dimples on golf balls, the roughness of shark skin and dolphin skin and the turbulator fences on a Cessna Citation all serve this same purpose.

I don't get the comment "the sum of both types of drag." All drag is "parasitic" -- friction, pressure, or wave. I know we're not talking supersonic, which is why I didn't bring up compressible flow and wave drag in my comments earlier.

As for "transports jets have an optimum speed where they can fly faster with less drag," that's not quite correct (again, talking non-supersonic). Not counting high-lift or drag devices (flaps or spoilers, respectively) or extreme attitudes, drag will increase with speed. The more appropriate issue is the combination of specific fuel consumption, weight, lift/drag ratio, and airspeed resulting in "maximum range" and "maximum endurance" airspeeds for a given configuration. The comment would apply to all heavier-than-air aircraft anyhow, not just "transport jets."

[tn535i]

Is is safe to say that one way to reduce the drag on an e28 would be to drive it at more or less the same altitude as commercial jets where the air is less dense?

So either really low or really high will reduce drag.

[savannah996]

Is is safe to say that one way to reduce the drag on an e28 would be to drive it at more or less the same altitude as commercial jets where the air is less dense?

So either really low or really high will reduce drag.

Or activate on board gravity reduction module.

I will perform a power run on the Gtech with both .33 and .40 coefficients. And see if its even worth discussing. Im no genius but one of them here, (you know who you are) should have just told me that in the beginning. Duh.

[Blue Shadow]

[
Or activate on board gravity reduction module.

I will perform a power run on the Gtech with both .33 and .40 coefficients. And see if its even worth discussing. Im no genius but one of them here, (you know who you are) should have just told me that in the beginning. Duh.

Be interesting to see the difference between the .33 and .40 numbers but that .33 number is definitely not what your vehicles Cd is even on a great day with everything resmoothed. The windshield surround alone makes a difference. Have you seen how the windshields of new cars are flush with the body and don't have the big gasket sticking out into the airflow?

As for the lower drag at altitude, true but the Cd will be the same. It is the fluid the vehicle (airplane in this case) that effects the drag. The fluid has no effect on the coefficient of drag of a vehicle.

[867535i]

Not to drag this topic up again but would adding a skid plate type of thing or piece of sheet metal under the engine bay help reduce the overall number or would the benefits be negligible?

I was looking at the numbers compared to the e34 and e32 and they're both significantly better. Would this have to do with the rounded edges of those cars or more to do with the windshield slope and gasket profile?

[Blue Shadow]

The 32 and 34 were designed to be more aerodynamic. So lots of stuff was massaged to be smoother to the oncoming wind.

The undertray would be noticeable when you put the car in a wind tunnel to determine the Cd.

[savannah996]

Well according to Junes issue of car n driver. The new 2014 Z/28 >$56K is going to have a >.39cd. So im not worried about the E28. If Chevy cant come up with anything better than a 26 year old BMW I can sleep well tonight. LOL.

[Shawn D.]

Not to drag this topic up again but would adding a skid plate type of thing or piece of sheet metal under the engine bay help reduce the overall number or would the benefits be negligible?

That would be an "under-tray" or "belly pan." Yes, it would improve the Cd, but you'd have to do testing to see if it's worth it.

[TrakRat]

Well according to Junes issue of car n driver. The new 2014 Z/28 >$56K is going to have a >.39cd. So im not worried about the E28. If Chevy cant come up with anything better than a 26 year old BMW I can sleep well tonight. LOL.

So funny you noted that, I read that article too the other day....at least the Z/28 apparently creates downforce despite all the drag I was at my shop yesterday looking at a customer's ZL1...supposedly it doesn't make much downforce, AND, has lots of drag. silly!

As for the e28...skinny tires and lowering are probably the easiest way to make our "breadboxes" aerodynamic...kind of an oxymoron, eh?

[Blue Shadow]

Not to drag this topic up again but would adding a skid plate type of thing or piece of sheet metal under the engine bay help reduce the overall number or would the benefits be negligible?

That would be an "under-tray" or "belly pan." Yes, it would improve the Cd, but you'd have to do testing to see if it's worth it.

It was shown years ago that spoilers and air dams improved aerodynamics but the cost of the units would take hundreds of thousands of miles to repay by increased fuel economy. So even if the undertray does improve Cd and it does, the cost of testing would not be recovered in fuel economy and just add to the cost.

[Shawn D.]

Not to drag this topic up again but would adding a skid plate type of thing or piece of sheet metal under the engine bay help reduce the overall number or would the benefits be negligible?

That would be an "under-tray" or "belly pan." Yes, it would improve the Cd, but you'd have to do testing to see if it's worth it.

It was shown years ago that spoilers and air dams improved aerodynamics but the cost of the units would take hundreds of thousands of miles to repay by increased fuel economy. So even if the undertray does improve Cd and it does, the cost of testing would not be recovered in fuel economy and just add to the cost.

That's why you don't see winglets on every aircraft. For many aircraft, they don't make sense for their flight regimes. The benefits are often offset by increased structural weight and/or payload reduction, coupled with insufficient fuel saving payback to justify their addition.