Size and aerodynamic efficiency

[Dieselman]

Noted in the new A-class-eco literature is a comment about the car being lowered to give a smaller frontal area, thus being more aerodynamically efficient.

The suspension has been lowered by ten millimetres, thereby exposing a smaller frontal area to the airstream.

Do people realise the effect of frontal area and aerodynamics on the overall performance and efficiency of their cars.?

Taller vehicles, eg. 4x4 and mpv have a larger frontal area due to their height, this is one reason why they have higher emissions , thus become targeted as 'un-green'.

I've been thinking about the radiator grille blind, might try it and see whether it actually works without causing overheating.

Oops, theres me being renegade again..

I can feel the flames now..

I'm also constantly surprised by how little manufacturers do to make the underside of vehicles smooth, most are still very turbulant.

I really notice this on articulated lorry trailors and wonder how much fuel could be saved by fitting aerodynamics packages.

20 years ago a freind saw a demonstration tractor/trailor combo that had a rating of 0.34Cd. That was as good as a Ford Sierra and still not bad for todays cars.

 

[KillerHERTZ]

I always found it intresting that the W203 saloon has a better Cd rating than the W203 Sports Coupe.

Drag aside, I think manufactures need to work on the weights of cars. I was stuck next to a 08 Reg Corsa the other day and I couldnt belive the sheer size of it - no doubt down to all the crumple zones and saftety features. As a guess it would ether equal or outweigh my W202!

 

[Shude]

I always found it intresting that the W203 saloon has a better Cd rating than the W203 Sports Coupe.

Drag aside, I think manufactures need to work on the weights of cars. I was stuck next to a 08 Reg Corsa the other day and I couldnt belive the sheer size of it - no doubt down to all the crumple zones and saftety features. As a guess it would ether equal or outweigh my W202!

Saloons and (proper) coupes are the best, hatches are poor, estates are really poor, SUVs are probably poorer again and cabrios with the roof down are the worst, but probably the best with it up

EDIT:

I'm also constantly surprised by how little manufacturers do to make the underside of vehicles smooth, most are still very turbulant.

Forgot to add that I saw an EV conversion of a civic (the guy has converted this car to electric three times IIRC ) and he's literally nailed a wooden board to the underside of his car to improve the aerodynamics.

I suppose if you have no major servicable parts or anything producing heat down there then you might as well cover it up and enjoy the benefits.

 

[BTB 500]

The suspension has been lowered by ten millimetres, thereby exposing a smaller frontal area to the airstream.

Of course the frontal area of the vehicle itself is unchanged by this, you just lose 1cm off the exposed tyre area. The effect on airflow under the car (by narrowing the gap between the underside and the road) is probably much more significant than the tiny change in frontal area.

 

[KillerHERTZ]

Anyone have any info on AMG bumpers and if they improve drag or not at all?

They are overall smoother and have less flat frontal area, but it would be intresting to know.

I cant say I have notice any difference in fuel efficiency since fitting my C43 front bumper

 

[television]

People were amazed at how smooth my car is underneath when it was on the ramp.

The gap under the car must work in the same way as wind that passes through corridors and high rise buildings, though the frontal mass remains the same, if you lower the car then the same wind / air must pass over or around the object

Oh goodie we get the rad blind thread back agin

 

[Dieselman]

I always found it intresting that the W203 saloon has a better Cd rating than the W203 Sports Coupe.

Drag aside, I think manufactures need to work on the weights of cars. I was stuck next to a 08 Reg Corsa the other day and I couldnt belive the sheer size of it - no doubt down to all the crumple zones and saftety features. As a guess it would ether equal or outweigh my W202!

It's because people want bigger cars all the time for more comfort.
All the add on safety enhancements also take space and add mass but in the main it's to increase occupant space.

The last VW polo was as big as a Mk1 Golf and isn't the new mondeo larger than a Granada.

Longer cars are generally more aerodynamic.

 

[Dieselman]

Forgot to add that I saw an EV conversion of a civic (the guy has converted this car to electric three times IIRC ) and he's literally nailed a wooden board to the underside of his car to improve the aerodynamics.

I suppose if you have no major servicable parts or anything producing heat down there then you might as well cover it up and enjoy the benefits.

I think it's simpler than that. There's no reason why the underside couldn't be totally covered by removable covers...other than cost.

The W210 has a space under the back seat that even has fittings for a cover...but no cover...

Only the brakes and exhaust need to shed heat, well not the exhaust if it was insulated and especially not turbo diesel ones, they stay much cooler.

 

[BTB 500]

In terms of aerodynamic drag it's the shape of the rear end that has the most impact (the ideal being the classic teardrop). Of course minimum drag often isn't the main aim, even on high speed vehicles where things like downforce may be more important.

 

[DieselE]

Drag is affected by two things. Aerodynamic frontal area and flow across the body (that's generic body not vehicle body) passing through it. As BTB says the tear drop is the classic aerodynamic shape.

The problem with SUVs/MPVs/Vans & Trucks is not just a significant physical frontal area but one that is blunt aerodynamically.

All vehicles of necessity operate close to ground level and this means that the classic tear drop shape is not possible. If you just flatten the underside of the tear drop you get a wing which will try to generate lift. That's why the rear end shape of the car is important as you try to generate a low pressure area to counteract lift. However if the flat rear end surface is too large (all the above mentioned vehicles) said low pressure area creates significant drag.

Of course at lower speeds aerodynamic forces have much less effect so for most motorists the features that attract them to MPVs etc are of greater importance than the fact that on their annual holiday the vehicle is burning more fuel than an equivalent sized saloon.

 

[DieselE]

I've been thinking about the radiator grille blind, might try it and see whether it actually works without causing overheating.

Oops, theres me being renegade again..

I can feel the flames now..

If you're planning to do that in an effort to influence the aerodynamics of the car, I would have thought it would be more effective to fix a plastic sheet to the front of the grille. That would send all the air over the front of the bonnet. The engine would probably draw enough air from the vents below the bumper to avoid adverse effects on its cooling.

If you fix the blind behind the grille and in front of the radiator then instead of air passing through the radiator you will create a build up in pressure by forcing the air to change direction. Whether this build up would be enough to reduce the air passing through the grille to just the right extent to create an aerodynamically smooth frontage at the grille is a matter of conjecture.

 

[Bobby Dazzler]

It's very difficult to be accurate regarding the effects of aerodynamics without the use of a wind tunnel, or sophisticated computer model, as there are so many inter-connected variables that influences the air flow.

It is possible to make sweeping generalisations though, so I will!!

Lowering ride height would make a contribution to reducing frontal area, as does narrower tyres, albeit by a relatively small amount. Narrower tyres also reduces the rolling resistance which is a bonus when it comes to fuel economy and emissions.

Airflow around the wheels is incredibly complex, and turbulent. Air is being drawn along the side of the vehicle, down from the bonnet, up from beneath, and through the cooling outlets from the engine bay. Add to this the effect of the spinning wheel too.

Turbulence is not good for aerodynamics. Lowering ride height reduces the void in the wheel arches, meaning that the smooth sides of the body are closer to the wheel, which should help to guide and smooth some of turbulent air flows.

This is the reason the Honda Insight and some commercial vehicles have plates covering the wheels, as it limits the potential for turbulence.

A common misconception is that sports bodykits improve aerodynamics. This can be the case, but not always. Some do, although it's rarely to reduce drag - it's usually to rebalance (or improve) downforce.

Think about F1 - there's a trade of between top speed and downforce. For fast sweeping tracks, low speed downforce is reduced in favour of maximising top speed. For tighter tracks the opposite is true.

Most bodykits are simply for aesthetics, and unless developed by a manufacturer or big-budget tuner will probably harm both aerodynamic drag and downforce. Even a bootlid lip spoiler has a considerable effect on the drag coefficient. If remember right it knock several mph from the top speed of the Astra Coupe.

To reduce aerodynamic drag you need to reduce turbulence and encourage a smooth linear flow. An abrupt tail like an estate, MPV, SUV or van will leave the flow beneath the car and above the car well separated with nothing to guide the air together again. This results in big eddy currents which cover the back of the vehicle in filth.

A saloon or coupe guides the air back together again as it approaches the tail, and as such the degree to which the flows are separated are smaller, and so eddies are less strong and settle more quickly.

Going back to F1, the rear wing is about downforce and stablity, although there will be much work to smooth the flow of air across the various wings and surfaces. However the flows from above the wing and below the diffuser will be a fair distance apart and causes turbulence behind the car, called the wake.

If the car behind can get close enough to the car in front then the pursuer can get into the void immediately behind the wing dramatically reducing the drag on their car. This allows a sudden burst of acceleration required to pass as for a moment less power is being used to overcome aerodynamic drag, and therefore top speed creeps up.

The drawback is that the downforce of the pursuer is dramatically affected by reduced downforce, and so stability can be affected. As soon as they pull out to pass, the increased drag will slow the burst of acceleration down, as the pursuer will no longer be in the void, and instead will have oncoming air flow and dirty air from the car it passes. The upshot is this can help braking performance into the turn.

Back to road cars. Some bodykits are supposed to cool brakes, radiators and so on. The actual effectiveness can be varied, but anything which improves cooling increases drag. Hence blanks on grilles - some air is directed into the cooling system which is very restrictive as far as air flow goes, and some air flows through the engine bay.

I could go on, but hopefully this offers a small insight into the sorts of things that's happening to airflow around a car.

 

[Bobby Dazzler]

PS When I refer to separation in the post above I don't mean the technical term of a flow separation. I mean that as a result of the air flowing around the car, it gets sent in different directions, and it must be re-united at the tail.

PPS Which leads me nicely into the MB 190 being quite famous in aerodynamics circles due to it's very pronounced boat tail. Imagine viewing the car from above, around the wheel arches the sides start to taper in slightly, in the same way that it would if viewing a rowing boat from above.

 

[stats007]

I always found it intresting that the W203 saloon has a better Cd rating than the W203 Sports Coupe.

The drag coefficient only describes how aerodynamic a shape is - you have to multiply this by frontal area to determine which car actually performs better in this respect.

 

[Mactech]

Here's one I made earlier.....
Cd 0.14 Frontal area 0.85sq M

My S class is just plain BIG after that and a Cd of only 0.26....

 

[Mactech]

Thought you might like to see how frontal areas can vary.
This is Dieselmax getting a push start from a Fastrac.
The other picture is something I just tripped over on the salt...

 

[Dieselman]

Lowering ride height reduces the void in the wheel arches, meaning that the smooth sides of the body are closer to the wheel, which should help to guide and smooth some of turbulent air flows.

Good point there Bobby, so why do manufacturers set the ride height so high nowadays.

It appeared typical that 4" (100mm) was typical, now it's over 6" (150mm).

 

[st4]

Good point there Bobby, so why do manufacturers set the ride height so high nowadays.

It appeared typical that 4" (100mm) was typical, now it's over 6" (150mm).

Good question, maybe to prevent the under side of the car getting beached on speed bumps etc.

Does the Porsche boxster not have a completely flat underside as the have all the moving parts protected by a metal cover? Someone here will be able to put it better than I do. I

 

[whitenemesis]

I was surprise at how flat the underside of Malcolm's SL is. Only the exhaust was exposed but the gaps were very tight!

 

[television]

I was surprise at how flat the underside of Malcolm's SL is. Only the exhaust was exposed but the gaps were very tight!

And I do drive it with the seat at the lowest point as well when the roof is down