Tuft testing a C-Max

[fbov]

 

It's all about air flow. The seminal work on automotive aerodynamics is Hucho, and be prepared to spend $75 for a lot of math. Here's a link to a synopsis, containing about 5% of the book.

 

The story on wings/rear spoilers is they rarely reduce drag, and in some cases (Porsche) intentionally increase drag. If it keeps your (vertical) window clean, you pay for it at the pump. If it looks too small to do anything worthwhile, it's likely beneficial. C-Max has a good one; tufts are aligned to a spreading flow down the rear glass, and don't bounce around a lot until you get to the sheet metal. My back window stays a lot cleaner than the tailgate below it.  

 

To my eye, an HHR's body profile has very little in common with C-Max. Compare the profile with the aero template (Hucho link, Fig. 4). The C-Max is highest above the drivers' head, and drops back down, following the aero template perfectly from the front axel to the tailgate. Extending that profile far enough is the essence of a Kamm's work.

 

But as Hucho notes, there are other requirements, like stability in cross-winds. Very low drag shapes are typically highly unstable shapes. That's why Porsche has a speed-activated spoiler on all their 911-shape cars; they're very sensitive to cross winds without it! No fun driving a car that aims you off the road in cross winds. Add in trailing throttle oversteer and one can see why 911's are hard to drive fast.

 

HAve fun,

Frank

[fbov]

I've been at it again with aluminum angle stock, this time side skirts.

 

The car has a plastic rocker panel cover with removable plastic fasteners on the underside  and fairly standard plug-style panel holders along the upper edge, behind the door. One fastener in the front, to the wheel well panel, and the rest on the underside. They look like this, with center plug pulled, and in total.

 

I made the skirts from 1.5" angle stock, a pair of 8-footers, instead of 2" by mistake, but it's a good idea as it maintains 5" of ground clearance on the sides. The front is bent inward, to shed the wheel turbulence, and upward to match rocker panel contour. The back is bent outward, and the excess angle stock used to create an inner skirt angled inward, forming a wedge in front of the rear tire.

 

To fabricate the skirt, I removed the drivers' side panels by prying the upper plugs out of their holes. I made both from this template, and installed the passengers' side with just the lower plugs removed. It's 1/4" too far forward on that side. The skirt is held on with a combination of 2-sided mounting tape and 9 pop rivets with backing washers.

 

As with the air dam, from a distance, you can't see a thing unless you know where to look. I painted all three with truck bed liner and it's a nice match to the black plastic rockers. I'm quite happy with the result, and hopefully, it'll last!

 

HAve fun,

Frank

[mtb9153]

Interesting idea Frank, wish I had thought of it

[fbov]

I'm of the opinion that we humans don't invent, we discover.

 

Innovative solutions are out there, but obscured by the limits of human perception and conception. The advance of technology that marks humanity as different from the rest of life on this planet, is a result of changes in the way we look at things. A rock is a rock, until we figure out how to break it to form a tool (stone age) or melt it to reveal the metal inside (bronze/iron age), or learn how to make it "just so." (as in the silicon wafers used to make solid state electronics).

 

That's one reason for posting our projects; other people may discover even better ways of doing things. As Sir Isaac Newton paraphrased Bernard of Chartres:

"If I have seen further it is by standing on the shoulders of Giants"

 

HAve fun,

Frank, the philosopher (in the classical sense)

[C-MaxSea]

Very good stuff Frank.  I'd say we are looking at Wunderkind II in the making, i.e. you are doing the very things that Ford will be throwing at the 2016/2017 redesign.  Toss on some more slippery (redesigned) rear flanks, add the new nex-gen battery, and voila Wunderkind II.  Wunderkind I being our current version of course.

 

According to my calcucomputations:

 

In extra good hands and weather, Wunderking I:   50 mpgs       (Our 'test vehicle', no mods, 45 mpg ECO cruising (+-1 mph), could push 50 mpgs with some effort, )

Similary                                          Wunderkind II:  60 mpgs        in extra good hands & subject to weather

 

Not having as much fun as you, yet,

 

Nick

 

(love the Giant's shoulders; we'd still be out there breaking rocks without them)

Edited July 3, 2014 by C-MaxSea

[kostby]

Lowrider C, anyone?

 

Eibach has lowering springs  to drop it another ~30mm (1.18 inches) if you want to lower the car a bit more and make all that side-skirt ground-effects stuff work even better.

http://www.ford-accessories.co.uk/productgrouplisting.htm?marketId=GB&languageId=en&modelId=6&categoryId=39&articleNameId=575

Disclaimer: They're european C-MAX spring kits, but a fully opted British Grand C-MAX Titanium minimum published kerb weight is 1634kg/~3602lbs, almost exactly the North American C-MAX Hybrid published curb weight, at 3607lb.

http://carleasingmadesimple.com/business-car-leasing/ford/grand-c-max/kerb-weight/

http://www.ford.com/cars/cmax/specifications/capacities

Edited June 27, 2014 by kostby

[salsaguy]

Monoprice is the only place to buy any kind of cable where you wont get ripped off royally

 

Frank need a cable?  Go to Radio Shack, there must be one near you in NY.  For that matter go to B&H Superstore in Brooklyn if it is anywhere close to your town.  They are a city block long, massive store from what I've seen on their website.

[fbov]

Boy, this is a moldy oldie! See post 42. I got 10 18-inchers from Monoprice the previous Christmas, dispersed among family members, but not into my briefcase.

[ptjones]

Finally got my act together( sort of LOL) thanks to Frank. Here are pics of new mods. Wheel well covers and hard foam spoiler with Velcro attaching to under cover. 

:)

 

Paul

Edited July 2, 2014 by ptjones

[SnowStorm]

That's one reason for posting our projects; other people may discover even better ways of doing things. As Sir Isaac Newton paraphrased Bernard of Chartres:

"If I have seen further it is by standing on the shoulders of Giants"

Interesting that he didn't say "do more work" or "get somewhere faster" but that he had "seen further".  As you say, "the way we look at things".  The transistor got invented because they were trying to do it, they could "see" that it should work.  Someone didn't stumble across it.  I wonder though, might Sir Isaac have been looking back (history) as well as forward?  Sometimes we need to do that too.  The big issue with highway mileage is drag coefficient.  Cut it from 0.29 to 0.113, power needed goes down, smaller engine, less weight, less rolling resistance, etc, and your 70 mph mileage goes from 40 to 80.  Yes, 0.113 is possible - looking back about 75 years!.

[fbov]

... Yes, 0.113 is possible - looking back about 75 years!.

No question we humans figured out low drag shapes a long time ago, in the age of lighter-than-air craft.

 

Also no question that the car you linked will be highly unstable in crosswinds, rendering it undrivable. Porsches and Zeppelins have tail fins/spoilers for the same reasons... they provide a countering torque in a side wind to prevent the car from turning away from the wind... and usually off the road.We want aeordynamically stable cars...

 

HAve fun,

Frank

[SnowStorm]

No question we humans figured out low drag shapes a long time ago, in the age of lighter-than-air craft.

 

Also no question that the car you linked will be highly unstable in crosswinds, rendering it undrivable. Porsches and Zeppelins have tail fins/spoilers for the same reasons... they provide a countering torque in a side wind to prevent the car from turning away from the wind... and usually off the road.We want aeordynamically stable cars...

 

HAve fun,

Frank

OK, I admit to partly linking that picture just for fun.  But is there a fundamental reason that a low drag shape, suitable for a car, has to be unstable?  Something about center of pressure offset from center of gravity?  Maybe move the battery.  Or stabilize with computer control like a fighter jet.  They are fundamentally unstable as I understand.

[ptjones]

OK, I admit to partly linking that picture just for fun.  But is there a fundamental reason that a low drag shape, suitable for a car, has to be unstable?  Something about center of pressure offset from center of gravity?  Maybe move the battery.  Or stabilize with computer control like a fighter jet.  They are fundamentally unstable as I understand.

I would think the lower the center of gravity, the more stable the car would be. :)

 

Paul

[fbov]

... center of pressure offset from center of gravity?  ...

Correctamundo! Give that man a kewpie doll!

 

There are two approaches described in Hucho.

- add rear side force

- design in high drag in crosswinds

 

The first is the rear fin approach, exemplified by the retractable spoiler on recent Porsche 911 variants. A sidewind hitting the front causes a yawing (spinning) force rotating the car away from the wind. That same wind, hitting the rear fin, causes an opposing force, rotating the car into the wind. Net result is a little side slip, but no change in the car's direction of travel.

 

The second is neat. It calls for a low drag shape in the absence of crosswinds, but one with increasing drag as the crosswind angle increases. As it works out, you don't change drag much until the crossing angle hits a threshold, above which angle the drag increases quickly, and rotating torque drops off. After looking at car designs for the last year or so, I think the crosswind stability feature in the C-Max is the windshield in relation to the A-pillar. Notice that the windshield is recessed a little lower than the A-pillar, which has fairly sharp edges. This clearly collects water off the hood/glass and onto the roof, rather than around the A-pillar and onto the side windows in the absence of crosswind, and in straight ahead wind, does little else.

 

As the wind direction turns increasingly off-axis, this recess starts to disturb air flow off the glass onto the side of the car, eventually causing to to separate from the surface rather than wrap around it. This separation is a transition from laminar flow to turbulent flow, the latter causing the desired drag increase as the wind's angle of attack becomes more severe.

 

I suspect all of us with grill blocks, air dams and other aero improvements owe a great deal to Ford's engineers, for giving us a design that we can safely alter, if we choose. At the same time, I can't see the rear window add-ons of the '14 and later models lowering drag, but I can see them reducing crossswind sensitivity!

 

Have fun,

Frank

 

PS Paul, you're thinking verticalheight... the instability is due to rotation about a vertical axis, in the plane of the road.

Edited July 8, 2014 by fbov

[SnowStorm]

I looked up Hucho's book - expensive and I have too many projects right now to make it worth while.  Cool stuff though.

[fbov]

Latest update, after a month or so of driving with an air dam and side skirts...

 

Clearly a lot of improvement over last Fall's baseline period; mods include (in order applied):

- PCM update

- hood seals above lights and across top of grill area

- VGs on the rear quarter panels, above and below the tail lights

- top grill blocked with plexiglass

- middle grill blocked by an air bladder (bicycle inner tube)

- lower grill blocked by foam pipe insulation (for easy removal in the event of heat issues)

- full width, 2"-deep air dam in front of engine acess panel

- 1.5" side skirts between front and rear wheels.

 

And the biggest mod in my estimation, better driving practices! I gave myself a month to learn to driver a hybrid. It wasn't long enough... and I started with 8K miles on the car, so vehicle break-in isn't an issue.

 

HAVe fun,

Frank

Edited July 31, 2014 by fbov

[ptjones]

Frank I'm guessing you are monitoring your WT's. :)

 

Paul

[fbov]

I have, but only through My View. I haven't seen it above center yet, but this has been a cool Summer, and I haven't done long trips up steep hills, although I will be doing a short trip up one of the steepest local hills... hosting a tour of the Mees Observatory Saturday. I haven't dug into the OBDII/Torque data either... although I did look at a phone mount that would let me see the readouts. Gets in the wife's way...

 

And yes, engine heaters are still in my future... I lose EV control even at 70F ambient; !@#$%^&* normal operation! Thinking of them as a birthday present; we took possession last Aug. 8.  Hoping I cna find a good place for a second one, as you suggest.

 

HAve fun,

Frank

[Jus-A-CMax]

I only have the centre grill covers for now and its been fine even with fast steep climbs. I really don't see any HUGE difference with the lower grill cover on in the mornings. Infact, I have been chalking up mega and PBs last 2 months with the 1 grill cover config. This car is INSANE MPG giver.

[hybridbear]

Latest update, after a month or so of driving with an air dam and side skirts...

Any more updates?