Sunbeam's 'drag' racers

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Sir,

I was pleased to see Bill Boddy’s article on the 1000 HP Sunbeam. It is a much underrated car and, I believe, historically and technically more important than the hugely charismatic Golden Arrow.

In the mid 1920 s, there was little established knowledge concerning the aerodynamics of cars (the oldest relevant wind tunnel report I’ve been able to find was a 1924 NPL report, but I’d welcome evidence of earlier research). Brooklands Museum has a wind tunnel report on a model of the 1000 HP Sunbeam dated August 12, 1926. It established that minimising the frontal area of the car was crucial for drag reduction. This may seem obvious now, but was less so at the time. The tests gave the Sunbeam a drag coefficient of Cd 0.34 for a frontal area of 18.7 sq

The ‘minimise frontal area’ lesson was absorbed by the Golden Arrow designers. They tightly sculpted the bodywork around the engine components, generating an eye-catching appearance while keeping the frontal area down to an amazing 11.7 sq ft. To reduce wheel drag, fairings doubling as radiators filled the space between front and rear wheels, and more fairings were fitted behind the rears.

Despite this careful design effort, the drag coefficient was 0.46, much higher than the apparently less sophisticated Sunbeam, which must have disappointed and baffled the Golden Arrow team. There are two reasons for this: first, there are no fairings in front of their car’s front wheels. These are large wheels, occupying about 25% of the frontal area, and could account for over SO% of total drag. The second reason was skin friction drag, which was unknown to designers in the mid-20 s, as the viscous effects of air were only just being researched. In trying so hard to minimise frontal area they had inadvertently produced a car with a very large surface area and hence a high skin friction drag.

In summary:

S(sq.ft) Cd S x Cd

1000 HP Sunbeam 18.7 0.34 6.36

Golden Arrow 11.7 0.46 5.39

Drag is proportional to S x Cd, so the last column of the table shows Golden Arrow still achieved a lower drag than the Sunbeam, but probably not as low as hoped for.

The early LSR cars all had wheels mounted outside of the body, sometimes with their own fairings. The Golden Arrow and the ill-fated Silver Bullet were among the last of that configuration.

The Sunbeam, with its compact shape and wheel profile shrouded in the bodywork, was the first of a new generation, the forerunner of the teardrop shape that culminated in such beautiful cars as the Railton Special and Goldenrod.

I am, Yours etc,

Ron Ayers, Claygate, Surrey