CD.. no not the model

[Keith Clements]

TECHNICAL RAMBLINGS
Bob Culver
A Matter of the Moment.
In the technical automotive journals the topic of the moment has varied over the years from multiple carbs, super high compression and pursuit of brute power in the early 1970s, through emission control to economy. The latter has been accompanied by emphasis on electronic engine control systems (for efficiency in the face of lead free petrol and lowered compression ratios ) , and by an obsession with weight saving. None of this has much in common with Jowetts but another favourite current topic is streamlining, and this is closer to home.
As vehicle speed rises a large and rapidly increasing proportion of the engine power output is utilised to move the car through the air . (Typically 1/2 total output is used to overcome air resistance at 40 mph, and 4/5 at l00 mph) .
At any particular speed air resistance is proportional to the cross section area of the vehicle multiplied by a streamlining factor.
This factor varies from 1.35 for a parachute, through 1.17 for a flat plate, 0.41 for a hemisphere, 0.34 for a 30 degree cone, to a mere 0.05 for the ideal teardrop shape. In other words a perfectly shaped object offers less than 1/20 the resistance of a flat plate of the same ''full frontal'' area.
The streamlining factor for cars has improved from about 0.9 for London to Brighton veterans, to 0.4 to 0.5 in the 1950's , worsened to 0.45 to 0.55 in the 1960's when hooded headlights and sunken grilles became fashionable, then improved again as rain gutters and separate bumpers have disappeared and as windscreens have become horizontal.
With weight, area etc. the same, a car with a streamlining factor of 0.3 requires, for 100 mph, less than 2/3 the engine power that would be required with a factor of 0.6 (a situation similar to
comparing a 1972 Citroen with a 1950 model ) .
The following factors may be of interest
early Saab 0.35
Javelin 0.41 FF64p5
A40 Farina 0.4 FF64p5
1962 VW 1200 0.47
1972 Citroen SM 0.34
1975 VW Golf 0.42
1977 Audi 100 0.42
late Capri 0.414
late Escort x3 3 i 0.39
1982 Audi 100 0.3
1982 Sierra 0.34
1984 Renault 2 5 0.28
1984 VW Golf 0.34


The Javelin has one of the better streamlining factors of it's time , and the A 40 Farina factor is unusually low for it's period .
As evidenced by the VW Golf , it is not necessary for a vehicle to be of rounded shape to have a low streamlining factor. And seemingly minor modifications often improve the factor markedly. For example the 1972 Citroen and the 1984 Golf were greatly superior to their immediate and very similar predecessors.
In the 1950's and 1960's road testers often found that the estate versions of cars were mysteriously faster than the basic cars. An abrupt rear is superior to the boot arrangement of the average (notchback) car. In general , the typical modern full width fastback shape is an improvement on a van back only i f the fastback is very shallow . (In the case of the Javelin, the taper may confer some
additional advantage) .

Van shapes with abrupt rear are surprisingly slippery. A square edged bus shape has a factor of 0.85 and a slightly rounded bus shape a factor of 0.46 (about the same as a VW Kombi ) . Many modern vans with penetrative fronts have particularly low streamlining factors (before the bull bars are fitted ! ) although the large cross section area reduces the overall advantage compared with a car.
It may be of interest to those ardent Jupiter enthusiasts searching for absolute maximum speed to note that the streamlining factor for a sports car is usually improved with the hood erect,
compared with the full bare screen and open cockpit .

Taken from the Flat Four 1990-04 kindly scanned in by Bryan Walker and put into the Gallery. Subsequently OCRed by me with text uploaded to Notepad to correct any errors and get rid of special characters that upset the database.

[Keith Clements]

Now I always thought the Jav had a CD of .35? Could that have been in prototype form before all the bumpers and chrome were attached?
I used many other tricks when racing including filling in the grill, no windscreen, tonneau fitted, and some chassis tidying with sump guard and underskirt. Richard Gane also removes spare wheel door to let the air out and reduces width of wings. Some of this was gained from designing and racing the Gravity racers down the Goodwood hill climb.

As yet the 1977 August edition (issue 64) referenced in this snippet is not yet in the Gallery. Can anyone find it and scan it in please?

[Chris Gibson]

The information referred to by Bob Culver would appear to be based on an article published by Autocar on 22 May 1982 entitled 'Dragging out the facts'. The Javelin tested was the well known 'NOC 11' The following week there was a second part on modifying a Ford Capri to reduce drag and a bit about the effects of fitting a roof rack to a Peugeot.

Whilst the Javelin Cd at 0.41 was good for its day there was a suggestion that the sloping rear was too steep and imparted too much lift to the rear. Spoiler?

I have a copy of these pages and would be able to scan them, though the Autocar pages are a bit wider than my scanner so some of the photo/sketch captions can get missed. Photos may be an option. If you want one or both of these to be added to this topic let me know.

[PJGD]

Chris; Whatever you can scan or photo and post would be appreciated.

[Chris Gibson]

I have managed to scan the Autocar wind tunnel test report with the odd letter missing at the edges, it is still intelligible. It is attached as a pdf document

[Keith Clements]

Thanks very much Chris.
What a great article, nowadays they would do studies on bluetooth capability or computer control of engine, suspension and steering!

It looks like NOC11 could have been improved for the drag test. The fog light does not help and I think it had indicators on stalks at front and back. Putting the radiator muff on would help considerably, as would removing bumpers. Perhaps then it would get down to the .35 figure. Many Jowett exhausts hang too low, so careful re-bending to take them out of the air flow would help. I presume NOC 11 had the enclosed spare wheel tray which is better than the lattice.
the gutters around the doors, trafficator pods and door gaps increase drag. Some can can be filled in with rubber to create a smoother surface.

Such modifications will improve fuel consumption considerably at motorway speeds.

At the Goodwood members' meeting at the weekend I looked at all the Edwardian cars and could see some attempts at streamlining. Whether this was because it looked better to an engineer's eye or was based on aerodynamic knowledge is another matter. I did pose the question to Gerald Palmer and he maintains that it was the 'Silhouette' of the car that was important at the time so the cd figure is just a benefit from it looking good.

My gravity racer had its wheels almost fully enclosed in an aerodynamic cover, just the bottom inch of the tyre protruded. The whole of the undertray was enclosed with a fibreglass mat. The suspension arms had polystyrene added to give them a teardrop shape. The frontal area was a pure wedge and the rear a long taper. The cockpit was fully enclosed by my cape on Velcro with just my head sticking up. It raced against many of the Formula 1 apprentices teams. Great fun.
Gravity race video
The crash
Testing prior to wheel enclosure