Vintage thing No.70 - Cooper Type 39 Bobtail

It was the sight of this car that persuaded me to swot up on me Coopers. I knew it was a Cooper Bobtail because I could read the programme at the VSCC Hillclimb at Wiscombe. But what was a Cooper Bobtail when it was at home? Or racing at a hillclimb? At first I dismissed it as a single seater. These seem anti-social to me. But then I realised you could seat another person in it at a pinch and came to view it as a kind of 1950s McLaren F1.

There are several reasons for this layout. One is that the Cooper Type 39 was based on the 500cc record breaker, which was itself based on the good old 500cc that Cooper pioneered. These little "D wagens" are the only single seaters that I really like. Another reason was that arch rivals Kieft had just done something similar.


A passenger was technically possible but in practice had to be very small.

But if you don't want any passengers, you just pull up the draw bridge.

Many years ago I found a copy of Racing and Sports Car Chassis Design by Mike Costin and Dave Phipps in Truro Library and according to this the Cooper chassis broke all sorts of rules. It had curved tubes and load bearing brackets were often mounted in the middle of their unsupported lengths.

They follow the contours of the swoopy bodywork and I wonder which came first, the bodyline or the chassis design.

The Cooper chassis was designed by Owen Maddox who partially got around this faux pas by using thick think walled tubes. He justified this approach as realising his ideal of "a series of brackets joined together with tubes."

But there was some other force at work - possibly witchcraft - becasue Maddox was something of a genius to get everything so "wrong" working so well.

This car was being driven at Wiscombe Park by Reg Broome and carried Stirling Moss to victory in the 1956 British empire trophy at Oulton Park. Reg kindly opened up the tail for me in the paddock. Inside was an engine of very familiar lines to me, a 1260cc Coventry Climax in line four and great granddad of the Imp engine. This car now runs in Class 6 for Sports & Racing Cars up to 1300cc. The FWB type engine is normally of 1460cc (76.2 x 80mm) and puts out 72bhp at 6100 rpm on an 8.8:1 compression ratio.

In a car weighing 896 lb, this would give a kg/kW ratio (you know how I like power to weight ratios) of 7.25, slightly worse than a Kent-powered Caterham 7.

The gear linkage interest me and connects to a right hand gear lever.

I had to ask what the transaxle was, though, and Reg explained how this was a Citroen Light 15 box turned through 180 degrees. It isn't entirely standard, of course. Conversion to allow a mid-engined installation was by the Parisian firm of ERSA (what does this stand for?) but as power outputs went up with subsequent versions of mid engined coopers, this gearbox became something of an Achilles heel until Owen Maddox designed the C5S unit for Cooper. This was assembled for Cooper's by a name that subsequently became world famous in Imp racing circles - that of Jack Knight.

Nowadays these gearboxes are catered for by Crossthwaite & Gardiner while Hoole Racing provide 5 and 6 speed versions. I imagine that most if not all of the foibles of these gearboxes have been engineered out, although some people replace them with Hewland boxes.

The design of the rear suspension is interesting and is another Owen Maddox invention. There's an extra leaf spring that's clamped in the middle but has its outer ends wrapped around through bolts that attach to the chassis tubes. On the same brackets are rollers that bear upon the lower side of the longer transverse leaf springs. When one wheel rises, the shorter "curly" leaf pushes the centre down, emphasising the transverse leaf-spring's anti-roll properties. The curly leaf also located the transverse leaves more positively in the fore and aft plane, preventing them from twisting whilst still allowing some free vertical movement.

I really like the wheels. I think these eight spoke, bolt on wheels anticipate the Minilite alloy wheel design. They were also the work of Owen Maddox and were made in magnesium alloy to replace the earlier wheel design by Colin Darby, which I, too, find very pleasing to the eye. Coopers always had interesting wheels and their design set the cars apart. They made them sit on the road aesthetically, as well as contributing to the road holding by reducing unsprung weight. So that's a double win.

But it's the bodywork that makes the Bobtail special. This, too, was designed by Owen Maddox and crafted by father and son team Fred and Pete Bedding, who did most of the Cooper alloy panel work. Maddox had been reading about Professor Kamm's theory about cutting off the rear of an aerofoil and making the airflow mimic its shape and decided that it was worth exploring. According to my copy of Scibor-Rylski's Road Vehicle Aerodynamics, if you terminate the bodywork not far behind the rear wheel, the airflow really does behave in this way.

I also have a recollection from those far off days in the Lanchester Polytechnic wind tunnel that the Kamm (or Camm) tail aided stability. If the rear body section resembles a wing, it naturally enough derives lift at high speeds. And I also seem to recall that kinking the flat ended tail down the middle helps things further. But Scibor-Rylski remains silent on this subject and still intimidates me with the impressive algebra. Or are they formula? I did Art at A level, not Maths.

In Dog Nye's excellent tome Cooper Cars, he mentions that John Cooper had a friend called Vyvyan Stanbury who worked with Sir Sydney Camm at the Hawker aircraft factory. Stanbury often offered aerodynamic advice to the Cooper team so as far as John was concerned it was a Camm tail.

A fringe benefit of the Kamm or Camm tail was that the car took up less space on the transporter. According to Nye, John Cooper never really approved of the Bobtail look, taking after his father Charles in the "if it looks right it is right" school of thought. He claimed that it was done to fit the cars on the racing car transporter.

This was just the kind of legend that Coopers liked to perpetuate. They saw themselves as welder/fabricators who got lucky, working from a shed full of tubes and tube benders. No wonder Ferrari called the British challengers "Garagiste!" Or "Assembliatore!"

Note the exclamation marks, which indicate much Italian arm waving in disgust at people who don't even make their own engines(!)

Was Enzo calling our brave boys Engine Punks?

Anyway, the rear bodywork also rested on the ground nicely when the back was opened up to gain access to the engine so maybe John was right with his workmanlike, oily-fingered claims. Then again that Owen Maddox sounded like he knew what he was talking about.

Well, who do you believe?