CFECO's X-Car Project Re-Do

[Xist]

Originally Posted by freebeard

With the stock seating I have to drive with my left foot tucked under my right knee half the time. You might consider a centered driving position.

That reminds me of Brian Regan trying to do yoga: "Place your right foot on the small of your back..."

[freebeard]

I plan to widen the front track, use smaller dia. tires

I hope it doesn't look like a lowrider. Maybe stagger the seats? My car has stock 165-15 on the back and 165/50-15 in front. 26" vs 22".

BTW I think that is a Ghia pan, it doesn't have the Type III front suspension. That is mounted on four rubber bushings and they ride a lot nicer than the Type I.

[CFECO]

freebeard....What are you driving, if I may ask?

[freebeard]

It's like deja vu all over again. My 'weekly driver' is pictured in Permalink #59, next to the trolls under the bridge. It's a 1302—the flat screen Superbeetle. Two years of production; mine's a '71.



How about what have I driven?

[aerohead]

Hucho credits the 1600 Notchback with Cd 0.38,same as the Ghia convertible with top up,and New Beetle.

[CFECO]

Originally Posted by aerohead

I printed a copy of your side elevation photo and enlarged it to fit the AST to analyze it both as a normal saloon body,and a Jaray combination form.
*The roofline descends as steep as 36-degrees,which might work if it was a very narrow single-seat,or tandem layout (think P-38 Lightning cockpit).
*With a shoulder-to-shoulder side by side cabin you'd be looking at Paul Jaray's narrowest 'Kombination' car of Cd 0.245 potential (about half the drag of the original Beetle).
*With a standard width cabin,you'd have separation right as your contour hit 22-23-degrees down-slope,with horrific attached vortices and higher drag.
*Your lower body,minus the greenhouse is fine.Although I'd recommend dropping the leading edge of the nose down even with the belly of the chassis.You don't want any high pressure ram-air striking under the nose or you'll have the same front lift of the Beetle.
*How you do the greenhouse is going to determine your outcome.
*If you can find a photo of the upcoming 2016 VW Golf,you'll see that it's roofline is right on the 'Template',with Cd 0.24,same as today's Tesla S.This more gradual contour is a sure thing for flow attachment on a standard width cabin.

How does the Pillbug do so well, as it is considerably "under the Template"? My theory is that the air coming around the sides of the greenhouse add to the air coming over the roof and is able to more easily follow the sharper angle, same with the EV-1. Here is my unfinished X-Car, "unscientifically" compared to the Pillbug, but it's a close representation. The Elevation lines, also done spur of the moment, show a similar pattern to the Pillbug. I will do that better in the future!

[gone-ot]

TIP -- Use a carpenter's laser (or cheap hand-held laser) to illuminate each "layer" you want to mark and "follow" it's pointing around the model using a hand-held Sharpie.

[aerohead]

Originally Posted by CFECO

How does the Pillbug do so well, as it is considerably "under the Template"? My theory is that the air coming around the sides of the greenhouse add to the air coming over the roof and is able to more easily follow the sharper angle, same with the EV-1. Here is my unfinished X-Car, "unscientifically" compared to the Pillbug, but it's a close representation. The Elevation lines, also done spur of the moment, show a similar pattern to the Pillbug. I will do that better in the future!

The Schl'o'rwagen is Cd 0.186,same as the 2014 VW XL1,both very low drag by contemporary standards.
The GM Impact/EV1 is rated by GM (as of 1999) at Cd 0.197.Also a good number.
Hucho gave us half a dozen ways to get to Cd 0.15 in his second edition book.
He said that we could go below 0.15 with extra elongation of the body,and with further integration of the wheels into the body,that we could approach Cd 0.07,to 0.09.
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The reason the Pillbug and EV1 don't have lower drag is because of their small fineness ratios.
By adding 16-inches of boat tail and lowering 1.5-inches,GM was able to get the Impact down to around Cd 0.153 for their world speed record at Ft.Stockton.
Had the Pillbug had more aft-body length,and a more gentle roofline,it would also have had lower drag like the SolarWorld GT and Cambridge University CUER eco racer.
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According to Hucho's elliptical drag table,your car,as drawn,will have the potential for Cd 0.21,due to its effective fineness ratio of 1.3:1.
*If you basjoos both the front and rear wheels as on AeroCivic,you should come in at Cd 0.185 if the Beetle's cooling system adds no more than 0.01,and all your cut lines,wipers,
mirrors' are no worse than the 1999 GM Precept PNGV car.
Cd 0.185 is nothing to sneeze at.Certainly.
What Kamm might have had you think about is, that if you'd vault the roofline up to the 'Template',you could truncate the length for in-town 'practicality',while reserving the potential to add an extensible tail,only for the highway,which could get you down around Cd 0.13-0.12.
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As your design sits,I predict that you'll lose your boundary layer on the roof,even with the point,25% the length aft of the face of your rear tire.
The Pseudo-Jaray fastback contour will have too much pressure rise to sustain attachment,and the flow will roll up into vigorous,power-robbing, attached longitudinal vortices.
"...the shape of a body in front of the largest cross-section has only minor influence on the total drag.The main contributions to the drag force originate from the rear part of the body.It is not important to find a proper shape to divide the oncoming flow but it is very important to design a rear body surface which brings the divided streamlines smoothly together.Optimum shapes are 'streamlined' bodies having a very slender rear part." Hucho,1987

[CFECO]

My goal is .2 for the old style bug, which is the easiest one to start with and build. Plus if a good result comes of it, there is a ready market for bodies for the old VW Chassis. The Kamm idea is a great thought too, add a receiver hitch and the long tail for the highway.

[freebeard]

there is a ready market for bodies for the old VW Chassis

Your up against the dwindling supply problem and you're still on your prototype! Your competiton includes people who bring a rust-free Mexican boneyard body up and put it on a domestic chassis with a valid VIN number.

You might consider also offering a replacement floopan for interested parties that can't source their own. It was done back in the day for mid-engine corvair power, ferinstance. the thread I really should start would show a one-piece aluminum monocoque tub underslung under the center tunnel by the thickness of a Prius battery.

You could have dwindling base plates on an hydraulic ram [scissor arm].