A discussion of aerodynamics and the C-Max

[Recumpence]

Hey Guys,

 

This thread (as titled) is a discussion of the basics of aerodynamics and how they relate to our beloved Mighty-Max.

 

My apologies to anyone reading this who is an engineer trying to read my jibberish.   :)

 

Disclaimer (you have to have a disclaimer, you know);

I am not an aerodynamic engineer. The principals I am sharing with you were gleaned from years of RC helicopter experience, top speed RC car building (120mph is my highest speed attained), years of recumbent E-bike manufacturing, and research into hypermiling over the last couple years. Some of the terms I use may be incorrect or incorrectly applied. But, I will do my best to communicate my thoughts as clearly as possible.

 

Here are some general principals. Later I will apply these to the C-Max and point out some highlights of the car's design and some "Problems" with the design that can be improved upon. Bear in mind, this car was not originally designed as an efficient hybrid. It was designed in Europe as a minivan/people mover with a standard gas engine or diesel, not a hybrid. So, Ford was not looking at absolute perfection in the aero department. This is what leads me to the "Problems" with the C-Max's aerodynamics. I will discuss the good points of the design as well.

 

Here are the general principals;

 

#1 Drag is your enemy. Generating airflow around a moving body is not necessarily a bad thing, but drag is very bad.

 

#2 Smooth (laminar) airflow is ideal. Turbulence is not as good as laminar flow. But, a certain amount of turbulence can be a good thing (I will discuss this later). The Vortex is your enemy. It is far better to generate turbulence than vortices. This is an important principal I will discuss shortly.

 

#3 The front of any body moving through any fliud (air is, essentially, a fluid) should be rounded, while the trailing edge should be sharp. This sharp trailing edge helps the laminar flow rejoin at the rear of the moving body. A rounded rear section creates vortices, while a sharp edged rear section creates turbulence. Remember, turbulence is not always a bad thing.

 

#4 Laminar (smooth) airflow over a surface is not always the best. A smooth surface with air flowing over it generates minor vortices between the surface and the moving sheet of air. This vortex generation can be mitigated in many circumstances by generating a small amount of turbulence at the leading edge of the body. This added turbulence eliminates or reduces the vortices and allows the airflow to move more smoothly over the body surface. This is the "Golf Ball" affect. The small dimples in the golfball reduce the votices over the surface of the ball and increase the ball flight range due to the reduced drag (back spin also adds lift, but that is another story).

 

________________________________________________________________________________

 

Now, I will apply these principals to the C-Max and we can discuss ways to take advantage of the positives and discuss ways mitigate the negatives of the design.

________________________________________________________________________________

 

 

The front of any body moving through the air should be rounded, while the rear should be sharp. The nose of the C-Max is wonderfully rounded. However, the rear of the body is also somewhat rounded especially at the side edges. If you look at the tail-lights of the C-Max, there is a flared section molded into the lense. If you picture that flare continuing all the way down the side of the car. This would give a sharp edge for the air to cleanly separate from the body of the car. The Volt and the Prius have this edge down their sides. A body style that tapers to the rear, then is sharply cut off is generally referred to as a "Kammback" design. This name is derived from an aerodynamic engineer whos last name was Kamm, who did extensive research into this phenominon back in the 1930s. The C-Max has mostly rounded edges at the rear. These rounded edges encourage vortices (remember, vortices are a bad thing. They increase drag substancially). So, this is one major drawback to the C-Max body design.

 

The nose of the C-Max is rounded. This is generally a good thing. There are a few issues with the design, however.

 

First, there are 3 grille openings. This looks cool, but those openings increase drag by taking in air like a small parachute, rather than directing that air around the body of the car smoothly. That is why grille blocks tend to increase MPG. They reduce drag by redirecting air around the body, rather than through the engine compartment.

 

Second, the nose of the car is 3 inches higher than the lowest portion of the belly pan just behind the front wheels. This forces air under the car and compresses it at the lowest area behind the front wheels. This compression increases the airspeed under the car and icnreases drag. Lowering the leading edge of the chin of the car that 3 inches and lowering the front pan to match will reduce the compression of air under the nose as well as drastically reduce the air speed under the car, thus reducing drag.

 

Third, though the nose is overall rounded, it does have a number of areas that trap air and disrupt the otherwise smooth flow around the nose. These are the fog light wells (if equipped), and the previously mentioned grille openings. Ideally, the fog light wells should have lenses molded around them to direct the air around the body, rather than piling it up inside whose wells. The grilles should have partial blocks made as I already mentioned. However, those blocks would be best made to be flush with the edge of the grille openings and rounded to match the curvature of the nose of the car. This will help the air flow more smoothly around the body than if the grilles merely had blocking panels stuck in front of them.

 

Another area of drag on the C-Max is the open section under the cargo area around the muffler. This acts as a loarg parachute. The rest of the car has a basic belly pan, but nothing under that rear section. This section should be covered (the best way to do this is another entire discussion).

 

The wheelwells of the C-Max are well designed in a couple ways. First, they are close to the size of the tires. This improves airflow around the edges of the tires to rejoin the airflow to the sides of the car better than if the wheelwell to tire clearance were larger. Second, they are somewhat rounded. This gives the air a chance to sort of "Jump the gap" between the wheelwell edge and the wheel.

 

One other area that could be improved is the wheels. Smooth wheel covers increase aerodynamics by reducing drag both from the air flowing past the wheels in forward motion as well as eliminating the affect of the rotating wheel acting as a fan. This fan action creates drag also. So, there are at least two areas of aerodynamic improvement available by adding smooth wheel covers.

 

 

 

 

Now, bear in mind, this is just a very elementary 30,000 foot overview of these principals and how they relate to the C-Max. There is FAR more to say. But, this will give everyone a basic understanding of aerodynamics and, hopefully, increase our knowledge of how to obtain the highest possible mileage from our cars.   :)

 

Matt

Edited June 15, 2013 by Recumpence

[nsteblay]

Great write up Matt! I do think the aerodynamics of the CMax is one reason it gets poorer mileage than its main competitor the Prius. On the other hand aesthetically I can't stand the looks of the Prius and much prefer my CMax. Are aesthetics more important than gas mileage? Of course the answer is highly subjective. I like the balance Ford has obtained between efficiency, aesthetics and performance.

[joe]

Recumpence, good discussion - the C-Max design might be a compromise in order to get air flow to cool down electrical parts.  E.g., the upper grill may be providing cooler air over the electric motor and components in the engine compartment.  Open wheels also helps to keep brakes cool. 

 

I agree that the fog lights could have been shaped more arodynamically.

[Recumpence]

I can say in a hybrid, brake cooling is a non-issue. :)

 

Matt

[SnitGTS]

Could the open area under the cargo compartment be to help with cooling the battery pack?

[salsaguy]

since the cmax is not a corvette and since it uses regenerative brakes the real brakes don't heat up that much or create brake dust, so the larger air openings are not need in this car.no one will try to get 0-60 in 4secs in the Max.

 

Open wheels also helps to keep brakes cool. 

[salsaguy]

great post recump, yes if you ever looked at solar racing cars like those made in colleges, they have to take advantage of every little bit of aerodynamic tips as smaller solar panels on those cars just don't build up that much energy so they don't want to waste any power that is created, by losing it to drag.

the Prius V does has a slightly lower Cg (Coefficient of drag) than the CMax (.29 vs .30, but the regular Prius has a lot lower) so that is one reason the Regular Prius gets better mpg at fwy speeds (above 50s mph) as aerodynamics affect drag as speed gets higher.

that's one thing we don't know, but someone might, as what is the speed where aerodynamics really affects drag on the car.

i also agree that the C-Max looks better than the Ugly Prius, and will continue to look good in 5-7 years, which is the typical life span of most cars. the Prius looks bad now, just think how bad the Prius will look then when "future" cars will be driving around the roads, some driving by themselves no doubt. great article with pix here where about about our what recump said above is matched in this pdf.

 

http://www.dimec.unisa.it/Leonardo_New/doc/Development%20of%20aerodynamics%20for%20a%20solar%20race%20car.pdf

 

:)

Edited June 15, 2013 by salsaguy

[Recumpence]

Could the open area under the cargo compartment be to help with cooling the battery pack?

The pack is fan cooled through grilles inside the cargo area in the car, not under it. :)

 

Matt

[salsaguy]

so recump, you didn't say what mods you planned to incorporate after reading this.and you didn't mention the side view mirrors on the side of Max which is a big no no for low drag.

[Jus-A-CMax]

Great write up Matt...now I don't need to call yer ;)

 

One thing I'll throw out to the mob here - Gas Pods. Been thinking of using it to cut down on the grime and dust that accumulates on the rear. It suppose to improve the aero by reducing the vortex in the rear by the creation of turbulence. Deturbulators as they call it (hope I got it right as I am no aero eng myself). So, anyone used or experienced this? One of the reasons why I don't think the rear aerial will be replaced soon as it too will create turbulence for the rear (as versus the sharkfin).

[salsaguy]

never heard of a gas pod

this site has lots of info and vids on them

http://www.gaspods.com/

[Jus-A-CMax]

never heard of a gas pod

this site has lots of info and vids on them

http://www.gaspods.com/

Those are the ones. There are other deturbulators. Side note -  I was washing the car this evening and noted in the rear lights some aero shaping in the plastic molding, interesting. Yes, those side mirrors are the least efficient I have seen in any car, even my Jaguar is more streamline and its a gas engine car - what was Ford thinking (great aero, slick body and then a drunken designer after a Friday pub crawl must headed back to the office and whipped up that slab wall design. Ugh).

Edited June 16, 2013 by Jus-A-CMax

[SnitGTS]

The gas pods look like something the mythbusters should be testing!  

 

Does anyone have any real world experience with them?  The concept seems logical but does it actually work?

[mtb9153]

Great write up Matt...now I don't need to call yer ;)

 

One thing I'll throw out to the mob here - Gas Pods. Been thinking of using it to cut down on the grime and dust that accumulates on the rear. It suppose to improve the aero by reducing the vortex in the rear by the creation of turbulence. Deturbulators as they call it (hope I got it right as I am no aero eng myself). So, anyone used or experienced this? One of the reasons why I don't think the rear aerial will be replaced soon as it too will create turbulence for the rear (as versus the sharkfin).

Jus, what are Gas Pods :headscratch: not familiar with that term.  I've been toying with the idea of trying to fabricate a rear spoiler across the hatch below the width of the window.  But wondered how effective it would be to help breakup that dirt you mentioned which shows so badly on our Platinum paint.  If I were to build a prototype spoiler I wish I could get permission to take Max inside the 40' x 80' wind tunnel at NASA Ames Research near by me here.  Really want to see a stream of air passing over the surfaces.

[mtb9153]

The gas pods look like something the mythbusters should be testing!  

 

Does anyone have any real world experience with them?  The concept seems logical but does it actually work?

Never mind Jus, I did a google search and found them.  I've seen these on the Mitsubishi turbo sedan, I've been thinking about fabricating them, but these are available?  Why bother.  Would look nice across the roof line behind the moon roof on my SEL

[salsaguy]

the thing about the gas pods are that unless you know exactly where to position then with a flow study map by the company or others it's hit or miss where you can achieve the biggest bang for your buck. better to get the magnetic  ones and experiment with different placements than using the ones that are applied with sticky backed tape, and you can't change them around.

best to try and find others with your same car and get their real world experience before trying it.

you can save 40% on the cost of them (20 up front and 20 later) if you sign up to be a test member on their team and keep track of your mileage before and after installing them; something all users here are very good at doing with fuelly.com or just pen and paper and spreadsheets.

 

 

i would like someone here to try 'em on the Max and see if they can get the 5% claimed increase.

Edited June 16, 2013 by salsaguy

[mtb9153]

The gas pods look like something the mythbusters should be testing!  

 

Does anyone have any real world experience with them?  The concept seems logical but does it actually work?

Yah but they look cool, I just ordered a set of 9 in black for myself as a Fathers Day present. :rockon:

[salsaguy]

great mtb, you will be the first here. be sure to open a new thread to track your review and results of these. did you sign up to be an official gaspod team tester to save 40%? if so hopefully they let you use your mpg data already collected on Fuelly so you don't have to start all over for 2000 miles before you can start to use 'em.

 

hmm black pods on a white car...

why? don't you think they will stand out too much? (with 9 all lined up on the back of the car.)

did you order magnetic or sticky ones?

 

are you going to replace that fishing pole antenna  on the road Max with a shark fin type? to me that antenna alone is the reason for the bad drag issues ;)

[salsaguy]

found this guy who put the GasPods on his VW

here is his Fuelly numbers before and after installing them. see notes:

http://www.fuelly.com/driver/GaryTDI/golf

and this is his discussion of his experiences with them:

http://forums.tdiclub.com/showthread.php?t=354594&highlight=gas+pods up

here  is a whole thread on them at eco modder site. lots of controversy about these for sure:

 

 

http://ecomodder.com/forum/showthread.php/gaspods-stick-airfoils-23114.html

[StoBro2]

One thing to keep in mind about the design of the C-Max- It has existed since 2010 ( I think) in Europe and was designed as a multi-purpose vehicle. It was originally configured for a range of conventional internal combustion engines. Ford later decided to "borrow" the design for North America and sell it here strictly as a hybrid to compete with the Prius. The design is, at best, a compromise. For all we know, Ford may already be working on a next generation C-Max design strictly for North America that will be a from-the-ground-up hybrid with better integrated battery packaging and improved aerodynamics.

 

I wonder how many years away that will be?

[Recumpence]

One thing to keep in mind about the design of the C-Max- It has existed since 2010 ( I think) in Europe and was designed as a multi-purpose vehicle. It was originally configured for a range of conventional internal combustion engines. Ford later decided to "borrow" the design for North America and sell it here strictly as a hybrid to compete with the Prius. The design is, at best, a compromise. For all we know, Ford may already be working on a next generation C-Max design strictly for North America that will be a from-the-ground-up hybrid with better integrated battery packaging and improved aerodynamics.

 

I wonder how many years away that will be?

Yup, I mention this in my opening post.

 

The design is a compromise which is what nearly all auto designs are. :)

 

Matt

[StoBro2]

I imagine the level of success of the current C-Max will determine whether Ford will decide to invest in a ground-up hybrid design or continue the borrow-and-adapt strategy.

 

I've been working as a design engineer for the last 30 years and I know all too well that ALL design is a compromise! :)

[fotomoto]

One thing to keep in mind about the design of the C-Max- It has existed since 2010 ( I think) in Europe and was designed as a multi-purpose vehicle. 

 

 

I imagine the level of success of the current C-Max will determine whether Ford will decide to invest in a ground-up hybrid design or continue the borrow-and-adapt strategy.

 

I've been working as a design engineer for the last 30 years and I know all too well that ALL design is a compromise! :)

 

I believe the days of designing and building one car for only one market are over.  World car platforms with model variations are now the norm.  

 

What I'm hoping we'll see is a hybrid specific platform with sedan, hatchback, CUV and AWD SUV variations where an un-intrusive battery location is the #1 design goal.

[SnitGTS]

ne thing to keep in mind about the design of the C-Max- It has existed since 2010 ( I think) in Europe and was designed as a multi-purpose vehicle. It was originally configured for a range of conventional internal combustion engines. Ford later decided to "borrow" the design for North America and sell it here strictly as a hybrid to compete with the Prius. The design is, at best, a compromise. For all we know, Ford may already be working on a next generation C-Max design strictly for North America that will be a from-the-ground-up hybrid with better integrated battery packaging and improved aerodynamics.

 

I wonder how many years away that will be?

 

If they do then that's my next car! (I'll give my current C-Max to my girlfriend, she loves it just as much as I do!)

[salsaguy]

stobro, if the production schedule of the 2014 model is really as projected and discussed here some on the forums, (a very short run) then the redesign of the Max might be coming for the 2015 model year. if thre is interest and i know there is, they might bring the 3 row grand cmax they have in Europe but that wouldn't get as good mpg but would fill a void many seek in a hybrid car to compete with the Hond CRV and Mazda offering and other midsize SUV types.