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This could be a long thread, this could be a short thread when somebody points me to an article explaining the answer but I keep pondering the thought and wondering why this isn't the case.
This probably applies more to electric cars but I don't see why it couldn't apply to diesel/petrol cars as well. There's a lot of design that goes in to efficiency these days, especially when we're talking about electric cars range and companies trying to nail a higher and higher battery range. From my limited physics knowledge the biggest amount of energy goes into pushing air out the way so why don't we see slicker/lower to the ground cars? If it is regulation, why don't we see regulation in this area changing to be more 'green'?
Whilst I don't expect to see F1 style cars on the road, I would imagine there's a huge amount we could do to improve the way modern cars are punching holes in the air to make it more efficient?
Because you'd be probably talking of very low single figure fuel efficiency benefits, and if that was important to people, everyone would drive conservatively.
In reality, folk are more interested in boot space, seating, all that shit.
Some cars are already very aerodynamic and are probably already pushing what is possible within the confines of seating 4 people and so on.
Whilst I don’t expect to see F1 style cars on the road, I would imagine there’s a huge amount we could do to improve the way modern cars are punching holes in the air to make it more efficient?
I think they probably are about as aerodynamic as they're going to get without making major trade-offs in usability for very marginal gains. You could lower them, but then you'd whack the front on every speed bump. You could squish them down at the back, but then you can't get anything in the boot. You could get rid of the door mirrors, but camera alternatives are inconvenient and expensive.
I'm sure there is more to do, but there are other constraints to consider if you are thinking of radical changes;
Pedestrian safety
Size of vehicles
Image/marketing
Luggage space
Headroom
Visibility of corners of the car
Manufacturers first priority is to make money, everything else is lower priority. Even your greenest most eco-conscious packaging free shampoo soap manufacturer... Unless I suppose it's got a Philanthropic backer to sink cash into the business knowing it's never going to be commercially viable.
Slowing down would surely reduce fuel consumption more than an aero car...?
Can't fit shit in really aero cars!
Although, saying that. I have a Ford SMax, which I've always thought looked pretty aero* and you can fit shit loads of shit in it.
*It probably isn't though....
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Some cars are already very aerodynamic and are probably already pushing what is possible within the confines of seating 4 people and so on.
This. The only way to make a meaningful dent in aero efficiency now is to make the cars compromised in other areas that the consumer won't like. Car manufacturers already employ lots of tricks (like underfloor trays, sculpted wing mirrors and lots of small aero details across the body) to get the drag coefficient down low as it helps lower the CO2 or NOx ratings in official tests that are done on rolling roads with the drag coefficient used via a formula to make it 'real world'. Fuel efficiency has moved towards actual driving with WTLP changes for electric cars.
The biggest effect on efficiency now is weight. Accelerating 2 tonnes up to 40/50/60mph is a lot harder than 1 tonne but that will only be solved with improvement in battery tech.
Yep, as matt says, bigger gains to be had by fitting 50mph speed limiters to every vehicle.
Imagine what would happen to a political party who seriously suggested that?
Because they're mostly sat in traffic at less than 12mph on a 2 mile return trip to Morrisons. So aerodynamics wouldn't make a jot of difference.
And because SUV's are fashionable, bigger is better etc. Just look at any model car and compare 80s/90s size to now.
Modern cars have quite a few things to make them more aerodynamic than 20 odd years ago. Some small things make a significant difference. Modern mirrors are usually tear drop shape, Vectra was one of the first, windscreen wipers now park further down in the scuttle below the windscreen when not in use, aerials don't poke out of the front, windows are flush etc.
One of the biggest gains is smoothing out the underneath with a sump tray and sometimes exhausts are a bit recessed. This let's the air flow smoothly under the car. Lowering the car isn't really practical for most cars as they'd hit speed bumps. A guy at work had the eco version of the Skoda Citigo and that was lowered to create less drag but even though it was short could catch aggressive speed bumps and some lanes or tracks needed to be driven with one wheel on the grass in the middle.
Formula one cars have huge wings that create masses of downforce to hold the car on the track bit also create lots of drag so not a benefit for normal driving.
Some cars are already very aerodynamic and are probably already pushing what is possible within the confines of seating 4 people and so on.
This.
Because they’re mostly sat in traffic at less than 12mph on a 2 mile return trip to Morrisons. So aerodynamics wouldn’t make a jot of difference.
A quick look at the M4 suggests there are plenty of people travelling further than that.
The problem is that many people can afford the extra fuel to push around a big un-aero car so they do.
Some cars are already very aerodynamic and are probably already pushing what is possible within the confines of seating 4 people and so on.
This.
But I think it's also interesting that these are a small minority of cars sold, and that SUVs account for such a large proportion when their aerodynamics are so poor for the space inside (and all the rest of it) that they offer. The majority of people, I think, don't understand the point about how big a difference aerodynamics makes. But I think it's also true that a lot of people don't really care, or have not been educated to understand why they should care.
We can't replace aerodynamics with slowing down, because we can (and should) slow down a more aerodynamic car too. And then the big box is relatively bad in just the same way as before. OK the numbers are different but until you get down to impractically low speeds the qualitative point still holds. It's not either/or.
Hatches with a CX of .29 (Zoé and BMW i3), and mid-size cars down to .23 for the Tesla Model 3 and .26 for the VW Id 3 seem pretty good to me.
The frontal area counts too so using the SCx (frontal area x cx) gives a good comparison within a vehicle class. A Peugeot 3008 has an SCx of 0.76 whilst the Model 3 is 0.5. 50% more drag for the SUV.
There's probably not much appetite for things like this, which is probably the sort of thing you're looking at for any significant improvement.
Interesting reading.
I have also just remembered that my thinking for this was also for trucks and busses. Which are typically just huge rectangles pushing a hole through the air. Surely even just changing our typical UK/EU flat fronted design to the type you see in the USA more typically would be making a big difference.
People saying gains are marginal but when you add them together over however million cars and however million miles they're doing it would add up to a lot. Improve a car's efficiency by 2-3% x by several million cars x by several million miles.
They are and some rather clever techniques used to make them so while trying to remain practical.
There are gains to be made on several fronts, (aero, weight, speed etc)
But all of them pale into insignificance compared to the gains we could make if we stopped rushing around the planet as if we owned it.
This might be related, but why do some electric cars have wheels that look like dustbin lids?
I think you answered your own question.
Drag coefficient (Cd) was a marketing thing on cars from late eighties and early nineties. I suppose it’s been trumped by more objective fuel economy and emissions stats.
I think there are restrictions on the total size of HGV's in Europe, to maximise load space they minimise the size of the cab. This isn't an issue in America hence them having the big nose rather than flat front
The VW xl1 has a drag coefficient of 0.19 which is probably as aero as you can reasonably get. The shape would be massive if it had to hold 4 people and some luggage, so that's probably part of the reason why
Cars have got massively more aerodynamic since the 80s. Smooth underbodies, hidden wipers, faired in headlights.
Unfortunately frontal area has gone up and wiped out most of those gains.
Trucks have got more aero by adding fairings on top of the cabs but they still have big frontal area. Putting lots of them very close to each other would be a good solution - one punches through the air, the rest follow through the hole. I'm going to patent it and call is "a train".
Whilst I don’t expect to see F1 style cars on the road, I would imagine there’s a huge amount we could do to improve the way modern cars are punching holes in the air to make it more efficient?
Most (I'd image all of them probably) road cars have got massively lower drag coefficients than F1 cars.
Cars have got massively more aerodynamic since the 80s
But they haven't, the SUV has inverted the trend.
I give you the Jaguar F-Pace: cx .35 Scx .96 (yes that's roughly double the drag of a Model 3) but the 80s audi 100 was already below .3.
Unfortunately frontal area has gone up and wiped out most of those gains.
Aye: https://www.newscientist.com/article/dn4462-suvs-double-pedestrians-risk-of-death/
It's terrible. SUVs are the worst thing that's happened to recent motoring. The 'seating position' argument that most of the population hark on about is trash. They are jacked up hatchbacks. If you need a more upright, natural seating position you use an MPV because that has the same floor height but much higher roof, and higher seats vs floor as a result. Rather than the whole thing just being on stilts.
Things like the VW XL1 were on the right track.
Imagine if we were commuting in electric pedal powered two or three seated enclosed, aerodynamic things shaped like kidney beans.
An average velomobile can cruise easily at 50kmh.
Trucks and (most) vans serve a purpose transporting goods, tools and materials about the place. The average commuter sitting in his 2.0l petrol exec wagen doesn't need to be sitting there.
I have a Ford SMax, which I’ve always thought looked pretty aero*
It's not though, it's just nicely styled. Look at the depth of the front above and below the number plate, the open spoked alloy wheels, the blunt rear tailgate with just a nod to efficiency by sloping the roof at the back.
My van is completely flat at the back, no effort to smooth airflow at the rear as volume and load length sell vans. 20 years ago van sides all sloped in as they got taller, sometimes the roof width was narrower than the internal wheel arches. This reduced the frontal area, especially the high top above the windscreen. Vans are all square now, wide at the top and all that internal volume results in more wind resistance.
There is talk about making a length allowance for lorries so that they can be more aerodynamic without affecting load capacity. Imagine what you could do at the front and rear of a box truck if you had an extra 150cm to play with that could not be used for load space.
But they haven’t, the SUV has inverted the trend.
But they have. You’re comparing an SUV with a hatchback instead you should be comparing them to 4x4 such as a Range Rover as they’re more similar.
The people who used to buy hatchbacks now buy SUVs and that the problem, but they aren't buying range rovers.
Just taking people around me:
Peugeot 208 replaced with Renault Capture
Renault Megane replced with Renault Capture
Renult Laguna replaced with renault Capture
Citroën C4 replaced with Citroën C5 Aircross
You used to drive a Golf and A3 IIRC Drac but now drive an SUV. Your SUV rpelaced hatches, it's elctric and won't be poisoning people but it's definitely an SUV with its inherent ineffciencies.
You’re discussing a different subject. Cars have become more aerodynamic but you’re arguing people are buying different cars now.
You used to drive a Golf and A3 IIRC Drac but now drive an SUV. Your SUV rpelaced hatches, it’s elctric and won’t be poisoning people but it’s definitely an SUV with its inherent
Well remembered.
Golf VI 0.31
E-Tron Sportback 0.26
https://en.wikipedia.org/wiki/Automobile_drag_coefficient
Yes, less aerodynamic ones dubbed SUVs. Because the manufacturers are replacing the aero-efficient models in their ranges with SUVs. Citroen are typical:
(edit: sorry that links the wrong page you have to click "véhicules neufs" to see the complete range)
They don't make lightwieght aero-efficent cars anymore. They call them all SUVs and most of them are a CO2 disaster compared with an 80s Citoën AX.
Cars have become more aerodynamic but you’re arguing people are buying different cars now.
This. Back in the 80s just getting close to a Cd (drag coefficient) of 0.3 was revolutionary. Now many cars are below 0.3. But resistance is CdA, Drag coefficient x frontal Area, and cars are a lot bigger so A is a lot bigger and overall drag is higher. Some of the reason for the increase in size is down to passive safety - crumple zones, side impact protection and so on. But a lot of it is because people want SUVs "because I feel safer higher up".
And the Scx of the two vehicles Drac?
the E Golf is Scx .62 I can't find Audi admitting the Scx of the E-Tron anywhere.
This might be of interest to people:
Have a look at some of the measures hyper-milers take to make their motors more aero.
People buying SUVs is not the same as cars being more aerodynamic.
They don’t make lightwieght aero-efficent cars anymore.
Not aero, but have you seen the weight of a new model C4 Cactus? Under a tonne for some models, and they are really remarkably spacious inside (a mate has one). I was highly impressed and it's not a brand I've ever cared for.
EDIT is his the C4 or C3? Can't remember, but the C4 is even lighter than the C3.
Whilst I don’t expect to see F1 style cars on the road,
Well F1 cars are not very aerodynamic at all. They're all about generating as much downforce as possible and downforce generates drag - and lots of it. They're all about cornering. Look at an Indy car in oval track mode and you'll see the difference between that and an F1 car in terms of how clean the exterior of the car is. OK they're playing by different rules and alot of F1 silly bits of appendages are not allowed, but they have alot more of that stuff when in normal circuit configuration or street circuit configuration so in oval track configuration they downforce generating features are significantly dialled down to reduce drag.
Modern cars are aerodynamic. Their coefficient of drag are very good. You're limited somewhat because you have to have holes in the front for radiators and stuff that you don't need on EV's, but compared to CofD's of cars from the '70's and early '80's they're much more aerodynamic despite their larger size. Modern cars have things like panels on the underside of the car to smooth the airflow under the cars. They have much better suspension that keeps the body under control alot better and therefore in a more aerodynamic position. Windscreens are raked back due to transversely mounted engines shortening the front of the car. Much tighter panel gaps and constructed in a way to minimise the number of panels on the car and therefore panel gaps. Flush window seals, windscreen wipers hidden behind the bonnet etc. Your car is brisling with features that are aero-optimised and far more aerodynamic than an F1 car. Also alot of the latest cars have moving louvres to block out the air intakes at the front of the car too so when cooling is not needed they can close up.
SUV's are not that less aerodynamically efficient than normal cars. They are higher up but they are have more ground clearance, so a bigger gap under the car and alot more air under the car so the frontal area is not actually that much greater than a similarly sized non-SUV. My wife X3 has a similar CofD to a normal similarly sized car. Can't remember the numbers but did look it up a while back and was surprised.
The thing that has driven the physical size of cars and their weight to increase is not interior space but crash regulations. But despite that thanks to more efficient engines and aero cars are much more efficient than those of 20 or more years ago.
Way back when in the 80s Ford brought out the Sierra - possibly one of the first cars designed with aero in mind (dustbin wheel trims included).
What I don’t understand yet though is the focus on electric/hybrid cars with front ends designed to look like ICE cars (with the associated cooling required). Going forwards car designers need to properly rethink car design to better suit requirements rather than just emulating ‘conventional’ expectations of what a car should look like. Tesla is the biggest culprit with their front ends #gopping
The whole thing being discussed by Edukator and Drac. Isn't there a name for this phenomenon? Like you improve something or create more capacity for something to run better but actually it just runs less efficiently or there's more volume to fill the capacity.
The best example I can think of this is - too much traffic, build more roads to alleviate traffic, instead of less busy roads just more people travel.
Isn't it the same with you create more efficient cars so now we can have bigger cars and that 'slack' created by the better aerodynamics is just taken up or cancelled out by the bigger cars.
As we move to Electrification, vehicle mass becomes less of an issue (because EVs have bi-directional powertrains) and aero and rolling drag hold the key to decent range with an affordable sized battery.
Unfortunately, we are not yet, as a society, ready to forgo our large, square, practical, easy to park, safe to crash, comfortable to sit in passenger cars for "aero pods"
As many people have tried, like the Aptera or the Riversimple Rasa, and failed and will keep on failing.
What could change is a sea change in the cost of energy. We have grown lazy and greedy on the glut of very cheap hydrocarbon fuels we have spent the last 80 odd years pulling out of the ground. Fuels that allow the average user access to MW levels of power and energy consumption (remember, a 250kW output engine actually burns around 1MW of fuel!)
But, if the energy we use becomes expensive, either due to taxation, or due to scarcity, then this will change. Suddenly, a car that can do 300mpg (already easily possibly for an aero EV) will have a market.
When is that day? no idea, sorry.......
Not sure the buying public really care about it unfortunately, style wins currently. I believe the automotive aero departments only really get involved quite late on in the development to tidy the flows up around wing mirrors for noise reduction, underbody and cooling flows etc. They do the best they can to improve efficiency with the limited freedoms they have. Unfortunately, the SUV style being popular removes any benefit from any improved aero efficiency due to the large frontal area. The marketing departments love to talk about CdA, but the fuel & planet cares about drag in Newtons not coefficients. Rolling resistance from wide tires is also a backward step in fuel efficiency.
EVs do need radiators btw - the batteries and motor inverters are liquid cooled in most cases.
Cars are more aerodynamic but there's all the other things that need to be factored in, the shell is only a wrapper for an engine, chassis, 4 seats and a boot after all.
EV's can mix it up a bit as batteries and smaller motors allow a bit more flexibility in where stuff is put, although the ultimate end goal would be self-driving cars that would allow vehicles to run much closer together. Running closer would give significant aero advantages but get it wrong and you're in a big accident - see any cycling race for a view on how that ends.
In the EV space, cars like the Ioniq are way more efficient than most - the problem is that it doesn't matter much. The "fuel" is super cheap compared to petrol or diesel so no substantial saving. The benefits for range can be offset with bigger batteries - and the SUV/crossover shape (high riding but not a lot more ground clearance) gives that extra space to fit them.
As others say, so much work has already gone into improving aero efficiency with smooth undertrays and other details. Most mainstream cars have more efficient tyres than ever too. It's all been in the cause of dropping ICE car CO2 figures down a little to squeak in under a lower tax band but it means there's not much to gain unless you get really low or narrow (to cut down frontal area) and you can't do much of that without compromising on practicality.
Going forwards car designers need to properly rethink car design to better suit requirements rather than just emulating ‘conventional’ expectations of what a car should look like. Tesla is the biggest culprit with their front ends #gopping
what requirements? most teslas appear to be designed to keep the front as smooth as possible, whilst maintaining legal heights for lights etc. What else should be changed?
And because SUV’s are fashionable, bigger is better etc. Just look at any model car and compare 80s/90s size to now.
As pointed out, modern cars have to be bigger to accommodate all the mandatory crash protection, like airbags along the sides and for the passenger, instead of one for the driver in the steering wheel, greater crash protection in the doors, crumple zones, etc.
Look at the construction of an original Mini, compared to a base model BMW Mini, whatever that is, a Countryman is almost the size of a Berlingo!
But a lot of it is because people want SUVs “because I feel safer higher up”.
Citation needed, because I don’t believe it. I have a compact crossover, an EcoSport, which replaced the Octavia I drove for fifteen years, and I bought that after driving hundreds of different cars over roughly five years, and ‘feeling safer’ had nothing whatsoever to do with my decision; getting in and out more easily had everything to do with it. I really struggle to get out of BMW’s, often because the damn seats are set too low, and you have to get back out to allow the seat to go back to its full height before getting in again, but I was getting to the point with the Octavia where hauling myself out of it was becoming a chore, the Ford I more or less slide in and out, in fact I had a quick drive around our big storage area in a year old MX-5 recently, and I could get out of that easier than my Octavia, mainly due to the flaired rear arches letting me lever myself out with my upper arm. Getting my feet around the pedals with steelies on, however…
Anyway, I honestly believe it’s an aging population wanting vehicles that are comfortable and easy to get in and out of, while not needing loads of space for kids and stuff is what’s driving the sales.
If you need a more upright, natural seating position you use an MPV because that has the same floor height but much higher roof, and higher seats vs floor as a result. Rather than the whole thing just being on stilts.
Have you actually looked at how an MPV is put together? The seats aren’t much higher than a regular saloon/hatchback, but the roofline is higher, and they’re longer, in fact if you put a Zafira next to something like a Jag F-Pace, height is almost the same, but the Zaffy’s longer. And Berlingos and Partners are much bigger, but the driving position is truly horrible, I’ve driven enough of the bloody things multiple hundred-mile distances in exceptional discomfort to never want to drive one ever again, but they’re bigger than my Ford, which uses the Fiesta platform.
And the point you’re ignoring is the one I’ve just made above - MPV’s are designed for families and need to accommodate same, along with all the stuff that entails, SUV’s/crossovers are being bought by people who don’t have those needs and obligations.
I absolutely love my EcoSport, it ticks every box on my list of wants/needs in a car at my time of life, and I’ve only found one significant flaw - the reversing camera is too low, being just above the number plate where it picks up lots of spray; it should have been put up with the high-level brake light.
I’ve never bothered to check, but I do sometimes wonder just how many who criticise people who buy Kugas, EcoSports, Kadjars, etc, then get excited at the thought of owning a T4/5 or a Transit Custom, the seemingly essential lifestyle vehicle for a significant number of the STW membership…
"Cars have got massively more aerodynamic since the 80s. Smooth underbodies, hidden wipers, faired in headlights."
Far too many haven't. If anything it's gone in the opposite direction. Frankly, it's pathetic.
A Vauxhall Carlton saloon from 1986 had a Cd of 0.28 whilst the estate had a Cd 0.32. Both of these had vast interiors, huge boots, tons of legroom. Yet despite that the frontal area was tiny compared to so many cars nowadays.
The equivalent large family car nowadays is a some stupid (faux) off-roader with a worse Cd, disastrous frontal area and less room inside. And it weighs 50-100% more.
My Dad had a series of those Carltons as company cars. I remember it was easy to spot the estate one in a supermarket car park because it was taller than 99% of the cars in there, being a big estate with roof bars. Nowadays you'd never spot it amongst the fashion victim roly-poly/spine-destroying SUVs towering over it.
Modern engines and gearboxes are pretty amazing in their efficiency. It's a shame that allows them to be used in such inefficient overweight oversized vehicles and still get good performance and economy. It could be so much better...
SUV’s are not that less aerodynamically efficient than normal cars. They are higher up but they are have more ground clearance, so a bigger gap under the car and alot more air under the car so the frontal area is not actually that much greater than a similarly sized non-SUV.
The frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
A Vauxhall Carlton saloon from 1986 had a Cd of 0.28 whilst the estate had a Cd 0.32.
Audi and the Ford Sierra were pioneers in the low drag thing. After that, it quickly became standard. In 1980, CDs of .40 or higher were normal, by the 90s, below .30 became normal. That makes a huge difference if you are cruising on an autobahn because drag increases by the square of the speed (and power by the cube). At 60 mph, drag is four times the figure for 30 mph. Driving around town at low speed, with lots of stop-start driving, aero won't make much difference, but weight will. Cars now are much safer, but also much heavier. Hybrids with regenerative braking reduce the penalty of weight because they recycle some of the energy that would normally be dissipated by the brakes as heat.
I would argue that efficiency/mpg is easily beaten by not actually driving a car? During lockdown i have driven virtually no business miles yet we still do the same amount of business?
If i factor in trains, underground and the odd plane journey i didn't do then it becomes an impressive reduction.
16000 less miles
50 North East to London less Train rides
10 less intrnal flights
The above for my little business can be multiplied by 7 regular travellers.
Look at the construction of an original Mini, compared to a base model BMW Mini, whatever that is, a Countryman is almost the size of a Berlingo!
I read somewhere that the Mini Countryman is the same size as an Austin Maxi, which was a pretty big car in it's day (one for the older people!) and it still had a curb weight under 1,000kg.
But as already covered, I would rather crash in a new Mini than an Austin Maxi so getting weights back to 1970's weights is not going to happen.
Because they’re mostly sat in traffic at less than 12mph on a 2 mile return trip to Morrisons. So aerodynamics wouldn’t make a jot of difference.
+1. Came to say basically this.
A quick look at the M4 suggests there are plenty of people travelling further than that.
There may be. Yet...
More than half of car trips nationally are less than five miles. In urban areas such as inner London, a third of car trips are less than two miles
The combination of the pollution burst that is being created as cars warm up in the first five minutes of journeys, together with the large proportion of journeys being short ones, is making a significant contribution to the UK’s air pollution challenge.
Licence Bureau say the vast majority of journeys between 1 and 5 miles are covered via car.
SUV’s/crossovers are being bought by people who don’t have those needs and obligations.
Go past any school round here and the MPVs are significantly outnumbered by SUV and Crossover cars. Audi Q's Peugeot's with two 00's, Range Rover products, BMW X's, Kugas, Sportages etc.
Most two child families don't need an MPV for space. I don't need mine although it brings some significant advantages for carrying bikes and sailing gear. It suits us now with current lower mileages we are doing but it's a barn door aerodynamically. We only "needed" the space for camping gear but since we got a trailer it's mostly wasted.
If you didn't have kit heavy hobbies and only had two kids you'd easily get everything in a crossover or normal mid size estate (which I'll be going back to in a year or so).
Drac is kind of right about it comparing SUV platforms to older hatchbacks BUT the SUV is increasingly stepping into the ubiquitous family car/daily driver for the masses role and so in my view it's a valid reference if we define cars by use case not body styling.
Just look at all the Tesla’s that are being made. Not one of them is about being eco friendly with even there smallest car being way bigger than it needs to be
Maybe not more aerodynamic but they are making cars more efficient. My last 3 or 4 cars have all had better MPG than the one before it despite the cars getting bigger to fit a growing family.
On paper or in real use, Hooli? Is that the manufactureres figures or brim to brim and GPS distance?
As cars get more powerful/faster I've noted that if you use that performance the real world economy is dreadful anywhere except a motorway where you are at a constant speed and match the manufacturer's figures no trouble. It's only if you drive as if you were in the slowest (lowest power : weight ratio) car capable of being run through the test cycle that you'll achieve the manufacturers figures. Use the extra performance when pulling out of junctions etc. and the consumption will be lousy.
The test cycle flatters powerful cars.
Same with electrics, our new Zoé is less economical than the old one despite being only 20kg heavier. It's got a more powerful motor so my default style of using the power that's available in eco mode means I waste a bit more leccy out of every roundabout. It takes an effort to take things easy to achieve the figures we got with 20 bhp less.
If you drive faster and accelerate+ brake harder you will use more fuel/energy. You can't get away from that. Even with an ev regen isn't 100% efficient.
As well as all the other good points made above about space and usability, looks etc, there is another issue:
I studied a bit of aero as a hobby when I was into model planes and stuff, and important thing to remember is that below about 40mph it does not make a huge difference on a car.
what requirements? most teslas appear to be designed to keep the front as smooth as possible, whilst maintaining legal heights for lights etc. What else should be changed?
All of this....
It's almost like they said 'right, we don't need a large grille to cool the engine so let's just shrink it a bit. I have never liked Teslas because of the bonnets.
On paper or in real use, Hooli? Is that the manufactureres figures or brim to brim and GPS distance?
As cars get more powerful/faster I’ve noted that if you use that performance the real world economy is dreadful anywhere except a motorway where you are at a constant speed and match the manufacturer’s figures no trouble. It’s only if you drive as if you were in the slowest (lowest power : weight ratio) car capable of being run through the test cycle that you’ll achieve the manufacturers figures. Use the extra performance when pulling out of junctions etc. and the consumption will be lousy.
Real world figures rather than manufacturers. I've never really paid a lot of attention to the brochures MPG figures for exactly that reason.
and important thing to remember is that below about 40mph it does not make a huge difference on a car.
Small bet on all of those average journeys all being at sub 40mph, too.
The frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
Forntal area is a specific thing. The CofD is a coefficient that describes how aerodynamic the shape of the car is.
The frontal area describes the size of the thing - the hole it has to punch in the air. The frontal area of an aircraft for example isn't just the area of the machine plus the area of the 35,000 feet of altitude beneath it.
The C of D of an X3 is 0.29...pretty good and up there compared to any other car out there. The CofD of a 5 series is 0.28 so near as damn it the same despite the far more bulky and boxy appearance of the X3 - so hard to Judge and difficult to distill complex aerodynamics to one number. I'm struggling to find the numbers for frontal areas, but I'd hazard a guess the frontal area of the X3 and 5 series are similar (comparing those two as they are similar width). Though I'm sure ether 5 series is more aerodynamic overall it will be alot closer to an X3 than many would assume.
The wife's X3 is easily capable of achieving 50+ mpg if we were to drive it more sensibly. Right now its 40mpg but it does the bulk of short journeys so not the most efficient way of driving. And its permanent 4wd so alot of mechanical inertia to overcome compared to a 2wd car that saps away at efficiency. But on a long journey we're well into the 50's mpg and that's cruising at 'decent' motorway speeds. Could be more efficient if we cranked the cruise control back a bit.
The biggest influence on efficiency is the way you drive the car as Edukator describes. You can't have performance AND efficiency.
More than half of car trips nationally are less than five miles. In urban areas such as inner London, a third of car trips are less than two miles
Yeah of course but there are still plenty of people going further. The thing to remember is that the half of trips less than 5 miles is responsible for much less pollution overall because they are not going as far (even though the car is less efficient per mile).
Just look at all the Tesla’s that are being made. Not one of them is about being eco friendly with even there smallest car being way bigger than it needs to be
I don't think so - they need to be that big to fit all the batteries in - they don't make one with a 100 mile range do they?
In the EV space, cars like the Ioniq are way more efficient than most – the problem is that it doesn’t matter much.
I disagree, it's hugely important. The two main problems that people see with EVs are range and cost. The more efficient a car is, the greater the range will be for less battery and hence lower cost.
The cheaper Nissan Leaf has an on-paper range of 160 miles or thereabouts and has a similar sized battery (actually slightly larger) and cost about the same as a Hyundai Ioniq, but the Ioniq has 191 mile range. Anecdotally the gap increases further if you are driving faster e.g. on motorway trips.
The frontal area of an aircraft for example isn’t just the area of the machine plus the area of the 35,000 feet of altitude beneath it.
The aerodynamics of a car interact very strongly with the ground. Air doesn't flow freely under the car, you get a high-pressure region under the car and very turbulent flow.
The wife’s X3 is easily capable of achieving 50+ mpg if we were to drive it more sensibly.
People say 'ooh, 50mpg that's decent' but really, these days it's not. You can do much better than that in a similar sized car.
I'm looking at BMW's site, and they are quoting WTLP figures (as they have to) which are apparently much more representative of real world driving than NEDC was.
X3 (2.0d): 42-45 mpg
520d: 51-59 mpg
That's a dramatic difference. Now, you can say that manufacturers figures are bollocks, whatever, but as said this is the new cycle which is much better, and these figures are both from BMW and one would expect if they are fiddling it they would do the same for both cars, no?
I think it's easy to pick on a single element of car design, size, weight, height, aero or whatever and poke a finger as where these huge teams of talented people and billions of pounds of design went wrong, but you can't argue cars are more efficient now then ever, plus they're safer, more reliable, more recyclable etc.
As for the original question of more aerodynamic. Yes we can all remember the cars that out parents drove that were both huge on the inside and small on the outside but Governments, on our behalf, have decided that it's not acceptable to sell a car that won't protect all it occupants in a 50 mph accident, which means that a car from 2020 with the same interior space as one from the 80s will be much bigger. They could get around this 'easily' by say, using a lot of carbon fibre, which would also make them a lot lighter, but who wants to pay £70k for a Vauxhall Astra. Car manufacturers do work very hard on aero, they have to, emissions regs see to that.
Yes, the current trend for taller cars does make them less efficient, they're not really SUVs and they're certainly not 4x4s, they just have taller roofs so you can sit passengers a little higher and make it a bit shorter. For example the Skoda Kamiq (my Wife's car actually) has a efficiency rating of 45mpg combined, the Skoda Scala (hatch back version) is 46mpg.
Surely even just changing our typical UK/EU flat fronted design to the type you see in the USA more typically would be making a big difference.
#1. They have even worse visibility of their surroundings than euro style tracker units.
#2. Increases the train length.
A few people have mentioned slowing down. I made a reasonable comparison on a trip down the A1 from Durham to Leeds and back. Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg. I was surprised at the difference.
My old R reg 1.9d Seat Ibiza used to return over 55mpg without any trouble.
Went on holiday with the GF up to Scotland (from Essex) sticking to A abs B roads. Remember calculating the mpg when we got home and it was over 60mpg. The car was well loaded, but speeds were never high.
Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg.
Yeah but Durham to Leeds is downhill.
A few people have mentioned slowing down. I made a reasonable comparison on a trip down the A1 from Durham to Leeds and back. Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg. I was surprised at the difference
Makes a huge difference in my Car Averaging 50mpg on A roads, I'll beat 50mpg easy, even with the roof racks on I'm too lazy to remove. Averaging 70 on the UK motorway and it's low-40s.
It's not the be all and end all though, driving down through France at 80, even with the car loaded up and bikes on the roof, I'll see just shy of 50mpg, the difference is France is a low flatter than Wales and the roads are quieter, you almost never have to brake / accelerate once you're up to speed.
I've mentioned this before, compare the Mokka-e to the Corsa-e
Both have a 45kWh usable battery, and 100kW power, 192 lbft torque.
Therefore imagining the exact same powertrain.
Weight unladen is 1530kg for the corsa, 1550kg for the mokka
Corsa is quicker to 60 too.
But the corsa has EVDB rated range of 170 and consumption 260 Wh/mi
the mokka 155 miles, 290 Wh/mi. (Theres no WLTP for the mokka on ev-database.uk)
Isn’t it the same with you create more efficient cars so now we can have bigger cars and that ‘slack’ created by the better aerodynamics is just taken up or cancelled out by the bigger cars.
Dunno what it's called, but it's certainly true. IIRC when you take an average the USA commutes twice as far as Europe, drives vehicles half as efficient, and pays 1/4 for the petrol. So the net financial cost is the same.
[strong]thols2[/strong] wrote:
QuoteThe frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
Neither of those things is true.
The area is just the frontal area (unless car manufacturers are fudging it in a way that no one else does?)
And Cd is derived thus:
It's still a bit of a fudge as it's dependant on Reynolds number, which will vary across the car (the Reynolds number of the wing mirror will be different to the Reynolds number of the bonnet).
I think your confusion that it relates to a flat plate is because it's dimensionless, so CdA is then expressed in m2 (and is generally <1). But a flat plate has a Cd, it's about 1.2 to 1.9 depeding on the Reynolds number.
The other bit you misrepresented the science on is "highly turbulent", flow is either laminar, transient, or turbulent. This is represented by the Reynolds number which is a way of expressing the ratio of inertial to viscous forces (velocity, density and a dimension divided by viscosity). It can be thought of as measuring the diameter of the smallest turbulent cell which can form in a system. So in a pipe if the dimension (the diameter) is smaller than the cell, the flow remains laminar. So on an object like a car which is big, traveling quickly, through a very inviscid fluid, the flow is almost always turbulent.
The diagram below shows how flow develops over a surface, note that as there isn't a constraint the flow always becomes turbulent as the turbulent cells can form.
This is actually a very good thing. Without turbulence you would end up with a huge area of negative pressure (it's that tumbling motion that allows it to rapidly change direction over the boot of the car) so you get a wake, rather than a stagnant vacuum.
But relating that back to drag coefficients, the amount of turbulence doesn't really matter. There's a Cd for an object within the laminar flow region, and a Cd in the turbulent flow region. It doesn't change further with more turbulence (once outside the transition region).
As a guide Reynolds number <10 is laminar, >10,000 it's fully turbulent. Typically for a car most of the components on it will have Re >100,000.
Isn’t it the same with you create more efficient cars so now we can have bigger cars and that ‘slack’ created by the better aerodynamics is just taken up or cancelled out by the bigger cars.
Not entirely. I learned to drive in a 950cc Fiesta Mk2 and it did about 35mpg, despite being visibly very flimsy and having no safety features beyond front seatbelts. First decent car I bought was a Seat Ibiza TDI which was far quicker, had way more tech, was infinitely better to drive, similar in size and could get 60mpg. The 2006 Prius we just replaced was way bigger than the Ibiza, similar MPG, much cleaner, packed with tech and safety features. A 2021 Prius is faster, bigger, more efficent and has more tech again.
Even an SUV like a VW Tiguan is much bigger, faster and more efficient than that Fiesta was. So yes, we're losing some efficiency gains by buying bigger vehicles, but not all of them.
I had a fluid mechanics lecturer in University who was very respected in the aero dynamic world. McLaren F1 team offered him a contract but he refused it, which I think he regretted. He kept the contract on the wall in class.
He asked us why we though cars weren't more aerodynamic especially compared to the 1950s. The sloped backs created a lot less drag and would make a massive difference in fuel efficiency... He believed the oil and gas industry had a huge influence on the decisions made to make cars less efficient.
Here is a nice old article about a manufacturer that paid attention to aerodynamics
http://www.citroenet.org.uk/miscellaneous/aero/aero01.html
There is another element of people are idiots to this.
Ask the general population what orientation of a "traditional teardrop" shape is more aerodynamic.
A soft nosed sportsback is about what we can expect so get out of a traditional "car" shape.
Pretty much a Tesla 3. (just why didn't they make it a hatch/5 door?)
I had a CX 🙂
Yep it doesn't matter how good the CD and engineering is if they can't sell them.
Pretty much a Tesla 3. (just why didn’t they make it a hatch/5 door?)
It would have broken the lines/look of the back window if they had a join.
I actually like the look of the model 3.