Kammback

Kammback on a 1969 Fiat 850 Coupe

A Kammback — also known as "Kamm tail" or "K-tail" — is an automotive styling feature where the rear of the car slopes downwards before being abruptly cut off with a vertical surface. The purpose of a Kammback is to minimise aerodynamic drag while maintaining a practical shape for a vehicle. The Kammback is named after German aerodynamicist Wunibald Kamm for his work developing the design in the 1930s.

The Kammback was originally used on sports cars and racing cars to improve performance at high speeds, and has been more recently used by hybrid vehicles to reduce fuel consumption.

Origins

1950 Nash Airflyte

1952 Borgward Hansa 2400

As the speed of cars increased during the 1920s and 1930s, designers began to pay more attention to automotive aerodynamics.^[1] In 1922, Paul Jaray patented a car based on a tear-drop profile (i.e. a rounded nose and long tapering tail) to minimise the aerodynamic drag created at higher speeds.^[2][3] The streamliner vehicles of the mid 1930s — such as the Tatra 77, Chrysler Airflow and Lincoln-Zephyr — were designed according to this philosophy.

However, the long tail was not a practical shape for a car, so automotive designers were looking for other solutions. In 1935, the German aircraft designer Georg Hans Madelung showed alternatives to minimise drag without a long tail.^[4] In 1936, a similar theory was applied to cars, when Baron Reinhard Koenig-Fachsenfeld developed a smooth roofline but with an abrupt end at a vertical surface, which was effective in achieving low amounts of drag similar to a fully streamlined body.^[3][5][6] He worked on an aerodynamic design for a bus, and Koenig-Fachsenfeld patented the idea.^[7] Koenig-Fachsenfeld began working under Wunibald Kamm at Stuttgard University, investigating vehicle shapes that would "provide a good compromise between everyday utility (e.g. vehicle length and interior dimensions) and an attractive drag coefficient".^[3][5] In addition to aerodynamic efficiency, Wunibald Kamm also emphasized vehicle stability in his design.^[5] He proved mathematically and empirically the effectiveness of the design.^[8]

In 1938 Kamm produced the first prototype to use a Kammback, based on a BMW 328.^[9] The Kammback, along with other aerodynamic modifications, gave the prototype a drag coefficient of 0.25.^[10]

The earliest mass-produced cars that used Kammback principles were the 1949–1951 Nash Airflyte in the U.S. and the 1952–1955 Borgward Hansa 2400 in Europe.^[5]

Aerodynamic theory

The ideal shape to minimize drag is a teardrop. However researchers including Kamm found that abruptly cutting off the tail resulted in minimal increase in drag.^[3] The reason for this is that a turbulent wake region forms behind the vertical surface at the rear of the car. This wake region mimics the effect of the tapered tail in that air in the free stream does not enter this region (avoiding boundary layer separation), therefore smooth airflow is maintained which minimises drag.^[9]

Kamm's design is based on the tail being truncated at the point where the cross section area is 50% of the car's maximum cross section,^[3][11] which Kamm found represented a good compromise—by that point the turbulence typical of flat-back vehicles had been mostly eliminated at typical speeds.

Usage

1964-1969 Ford GT40

1974-1985 Citroën CX

2000-2006 Honda Insight

2009-2015 Toyota Prius

Initially, the Kamm tail was used on sportscars and racing cars. The design had a resurgence in the early 2000s as a method to reduce fuel consumption in hybrid electric vehicles.

Several cars have been marketed as Kammbacks, despite the profile not adhering to aerodynamic philosophy of a Kammback. These models include the 1971–1977 Chevrolet Vega Kammback wagon,^[12] the 1981–1982 AMC Eagle Kammback,^{[13][14][15][16]} the AMC AMX-GT and the Pontiac Firebird–based "Type K" concept cars.^{[17][18][19][20][21]}

Cars that have a Kammback include:

• 1940 BMW 328 "Mille Miglia" Kamm coupé^[22][23]
• 1952 Cunningham C-4RK
• 1961 Ferrari 250 GT SWB Breadvan
• 1962–1964 Ferrari 250 GTO
• 1963 Aston Martin DP215
• 1963–1974 Bizzarrini Iso Grifo^[24]
• 1965–1971 Aston Martin DB6^[25]
• 1965–1968 Ford GT40^[26]
• 1965–1970 Aston Martin DB6
• 1966-1974 Saab Sonett II and III^[27]
• 1968–1973 Ferrari 365 GTB/4 ("Daytona")
• 1968–1976 Ferrari Dino^[28]
• 1969–1971 Fiat 850 Coupe and Sport Coupe
• 1970–1975 Citroën SM
• 1970–1977 Alfa Romeo Montreal
• 1970–1986 Citroën GS^[29]
• 1970–1978 Datsun 240Z, 260Z, 280Z
• 1971–1989 Alfa Romeo Alfasud^[29]
• 1971–1973 Ford Mustang Fastback^[30]
• 1972–1982 Maserati Khamsin
• 1974–1991 Citroen CX
• 1985-1995 Autobianchi Y10 / Lancia Y10
• 1991–1998 Mazda MX-3
• 1994–1998 Mazda Familia Neo/323C
• 2000–2006 Honda Insight^[31]
• 2004–present Toyota Prius^[32]
• 2010–2014 Honda Insight (2nd generation)^[33]

References