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Old 03-30-2008, 12:58 PM   #1
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Cd of a Magic Wagon, and improvements thereof...

Hi folks,

A couple of weeks ago I started investigating the aerodynamics of minivans, and how I might apply any knowledge gained to my '88 Voyager. Initially I was inclined to treat it pretty much as a "brick" and thought that techniques such as those applied to improving the aero of semi-trailers might apply. Well, they do and they don't it seems...

Firstly what am I starting out with?... I got some weird figures from here...
and here...

0.30! for a '90 mini??? or 0.37?? according to C-PATH report. Now that last number was a bit of an enigma, since it is meant to describe the characteristics of the Uniq M90 and Chrysler TEvan... but report was published early '91, and the production TEvan didn't come out until '93.. the Uniq M90 was announced in '90 as a conversion of a Chrysler TEvan... hmmm ... it seemed that for Uniq to develop it for '90 it must have been based on a '90 or earlier model year minivan, 1st gen, same shape from '84 to '90. Now I wouldn't have thunk they'd be getting their numbers wrong here. Numbers for the 2nd gen minivans seem widely available at such places as internetautoguide etc, and are all saying 0.39 for those. Now the shape and angles didn't change a whole hell of a lot for the second gen, the nose got more refined, the mirrors and rack got more refined, but it got a little wider.

So after more digging, I seemed to get some sources that agreed with each other...'s_a_t-115.htm

At 0.43... but that would appear to be for the '84 model at release. In the pictures in those links above you will notice the quad sealed beam headlamps and chrome bumper do not give it a particularly clean front end. My '88 has the flush mount single lamps and clusters that look rather cleaner, and also what's known as the "premium" bumper, a moulded plastic piece front and rear that look a lot cleaner than the chrome bumpers. Now interestingly, after the intro of the premium bumper it was found that in certain climates the V6 motors would tend to overheat when towing up grades etc, due to reduced airflow into the grille. A TSB was issued and a retrofit kit with templates for cutting extra holes in the bumper was sent to dealers... ergo it would seem that due to better shaping and less grille inflow, later 1st gens with premium bumpers would have a slightly better than 0.43 Cd... So one wonders if the Uniq model was based on a premium bumper model and whether it had any extra aero tricks or not. However I suspect that the weight of the batteries would have cause these to squat quite low on even upgraded springs, so that might have something to do with it as well... But then on the 2nd gen one might assume they redesigned the plastic bumper to let more air in, so it potentially lost Cd due to that.

Confusing also was the assertion in a '92 aerodynamics text that automakers used to use height x width for frontal area and Cd calculations and as of '92 they used the actual projected frontal area... so don't know when that changed. I think that means that using a higher area in the calc makes the Cd seem smaller. Actually wondering if the increase in width for the same profile brought down the Cd much for the 2nd gen, I'm guesstimating as much as .02 in the Cd reduction came from gaining width, I know it comes back in the CdA calcs but it makes it sound better.

So anyway, I'm figuring I'm starting somewhere just either side of .40 which is considerably less bricklike than I first imagined. (neener neener neener to the Pontiac Solstice owners at .45) I guess I should try actually measuring Cd as described by iwilltry here...
But I'm having trouble thinking of somewhere around here that I can do it.

While digging out all this I was also studying recent minivan models and boning up on my aerodynamic theory. I came to realise that induced drag may be more important than vaccum/base drag. Looking at many recent car designs I began to see the reason for the more wedgelike shape with the trunk higher than the hood. This was to throw the lift vector due to cabin shape forward such that the lift acted as a force with a forward component instead of a rearward component, holding back the vehicle. This is why the numbers for the new beetle don't look so impressive, it would do better running backwards and is reportedly much improved with a spoiler. Also there seemed to be some suggestions in design of encouraging upflow of the air along the vehicle, instead of downwash... for much the same reason, to ensure the lift vector pulled forwards. In particular looking at 2008 minivans, practically all of them are "wedged" forward and also have hood shapes that appear to encourage upflow along the sides of the vehicle somewhat.

So I began to think of some somewhat more radical improvements for my Voyager than just wheel fairings and minor tweaks. I was thinking that I would find some kind of body moulding to put on a line between the bottom of the front arch and the top of the rear arch to make something of a flow directing strake cum fence, to encourage upward flow along the sides of the vehicle. I had also been looking at the shape of the Audi A2 and was wondering if some tweaking of the roofline, to be a little fatter forward would help.

I was still pondering the wisdom of this approach when I stumbled across a concept car that gave me something of a "Eureka!" moment.. (scroll down)
The Mercedes/DaimlerChrysler Bionic 0.19 Cd. Now what's particularly amazing to me about this, is that firstly they call it a design for a compact car, the damn thing has dimensions nearly the same as my Voyager, a scant 2 inches shorter, 0.5 of an inch narrower and 6 inches shorter (which appears to be in the nose not the cabin length) so it kind of boggles my mind why they didn't take this up as a next generation small minivan to take on the likes of the Mazda5 instead of the lackluster Dodge Journey which looks like having the gas mileage of a minivan with none of the benefits.

What seems particularly encouraging for me is that it seems that it is not so very far off the proportions of my '88 Voyager and also seems to confirm the thoughts I was having about encouraging upflow along the sides. I notice an exaggerated version of what we are seeing on the hoods of '08 minivans also. The general cross-section seems not too dissimilar from my voyager either, the main difference being the thinning of the cabin and the tumblehome towards the rear.

So I am now considering some rather more radical changes to the voyager than when I first thought about aero improvements. I am thinking for instance that I want to make a bullnose hood and nose treatment such as on the Bionic, it could also double as a reverse ram hood scoop. Then above the windshield I may use a 1 or 2 inch strip to virtually fatten the forebody at that point, making the air take a more curved path over the roof. On the sides I would take much of the lip off the back of the front arches and use a deflector/trim strip up the side as seen on the Bionic there. May also make a fattened "eyebrow" spat for over the rear arch to give the effect of the rear cabin thinning seen on the Bionic. If you look carefully, the rear wheel does appear to be half covered by clear perspex.

The rear underbody will also need some work. I am thinking it also will have to be tuned to aim air upward into the wake. I would like to see how this is done on the Bionic, couldn't find anything on that yet myself.

These things along with wheel fairings and an airdam treatment look like being really positive on vehicles of this shape. I've gone from feeling sorry for myself for having such a sucky shape to work with to feeling rather positive about it. I can't see myself rigging rear-view camera/monitors, so will be looking for mirrors that look like they will do better for helping the flow coming off the sides of the hood. Will possibly use 2nd gen mirrors, don't know yet.

I guess the goal here would be to beat the 0.30 Cd of recent minivans, and I would consider it a major victory if I could get as low as 0.25 on it, which doesn't really seem too outlandish when you see how close in shape and dimensions to the Bionic it is. I'd also like that 83mpg figure... but I'll settle for 50ish It is about 66% percent of the weight of my van, but it's not so ridiculous a disparity as current minivans that are getting towards double that, and I do know people who have got these vans stripped down to that weight if I really wanna go for it. I guess the engine efficiency has to be up around somewhere near 40% or so to be getting figures like that too.

Well, that aboot wraps it up for the moment, gonna see what else I can find out before I start messing anything up.

Road Warrior

Ah, tagging on a better site for pix...
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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