The Saab 900 SuperAero & aerodynamics issues

When designing a car there are dozens of conflicting issues to negotiate. Some wants a super sporty sleek car other a more family practical oriented. For a manufacturer this is a tough issue to deal with especially when You are a small player with a limited number of lines of models and nothing is then allowed to go wrong the cars just have to sell like cupcakes.

Saab started out of an airplane maker with the range of B-17 Dive-bomber, J21 and the jet powered J21R interceptors, so aerodynamics was nothing new. The 92 model was slick for its time but low speed up to its modest top speed of 110km/h was all-fine. The issue is when going faster, when the airstream over some areas of a car body creates lifting power just like what a wing does on a plane and the 92 was more or less across section of a wing. At high speeds the rear part of the body on the 92-93-96 models create lift, this force has a negative impact in the performance on the road. A tuned 96 V4 with an Sonett gear box (high 4 gear) would make that car go 190 km/h, that is not a pleasant experience, rear wheels hardly touches the ground.

Aerodynamic issues emerged with the 99 model as well the notchback body had terrible wake turbulence due to two conflicting airstreams one coming from the roofline down over an touching the end of the trunk lid, the other from under the car coming up following the rounded sheet metal under and up the trunk floor in-between the body and the bumper. The design was of Sixten Sason and the young Björn Envall. The wanted to add a little metal plate in that gap put that would have added a dollar or two and was scrubbed. Interesting is to see that this plate came as a rubber-metal member on the 1979 99 model 13 years later.

The 95 had quite early an invention named the “Cheese-Slicer” at the end of the roof line was a slot that enabled the air to flow down over the rear window and keeping it clean counteracting the regular wake that brings up dirt and particles that make the rear window of most station wagons dirty.

The next development step for Saab was the CombiCoupé (Wagon Back) of the 99 that was launched in 1974, with a super long rear hatch making a giant opening, and thus becoming a very popular family car. The lifting powers where partly taken care of with the after sales spoiler that was all the way down on the hatch near the opening handle. Later on the 99 Turbo a rubber spoiler was positioned just below the rear window. This spoiler later got predecessors from Saabs aftermarket department with the “whale tail”, and some German makers like the gigantic Schussler wing.

Well when making a car for the public they might not always accept not so pleasing wings and arrangements, well some aficionados might. Saab was among the first manufacturers to offer a line of spoiler both front and rear in the mid 1970 ies. This continued with the AirFlow kits in the beginning of 1980ies for both the 900 and the 99 lines. Body moldings created a better flow over the wheels and alongside the lower parts of the car. Of great importance was the rear section positioned underneath the rear bumper that helped on the wake issues. For the 99 Notchback a rare wing/spoiler arrangement was introduced in the AirFlow kits, a rare piece today.

As seen in the picture of the prototype 900 SuperAero the design department went on extensive work on controlling the airflow over the hatch. On the roof sat an array of 7 turbulence generators. This was nothing new for Saab on some of the last versions of the Saab J37 Viggen Fighter jet that similar turbulence generator where positioned on the rear of the front canard wings to create a better airflow over the main delta wing configuration. Also a complete air-box surrounding the rear window and the top the rear of with a AirFlow inspired air skirt under the bumper. But this was not all on this prototype when it comes to aerodynamics. The front lights where built in like on Citroen SM creating a smoother airflow and the wheel design was new to minimize turbulence.

The car could not be driven just a metal skeleton with body panels and doors that could not be opened. Aerodynamics later was upgraded on the Viggen version in 1999 but not to the extent of the prototype vehicle.

PS. If anyone is interested in a 99 CC AirFlow kit I have that NOS planned for a secret project, but as things have evolved I will stick to the classic styling of this specific car, will unfold the project soon here on SU. DS.



12 thoughts on “The Saab 900 SuperAero & aerodynamics issues”

  1. Well well, the aerodynmic issues…
    The aerodynmics of the 900/9-3 was no highlight of the car, I must say, at a cd value of 0.34. That was outdated even back in 1993.
    And the 9-3 II SportKombi (legitimate successor to a hatchback) was even worse. While it had a slight improvement of the cd value, the taller and wider body easily compensated for that. Actually, in order to reach the same top speed, you had to buy a stronger engine.

    • Thyl,
      Cd is only one of many aspects of car aerodynamics. It is important for high speed driving to have a low Drag coefficient, but who is driving fast (outside Germany)????

      The 92,93, 9 6had a good drag coefficient, but the rear axle lift was too high as Trued says. Maybe a Kamm-back (like the Sonnet II, III) would have a higher Cd value but a much better Lift stability. And we all know the case of the Audi TT Mk. I where the rear axle Lift instability made the car un-drivable even at low speed and Audi had to mount a really ugly spoiler to correct this (Improving the Drag).

      BTW, if you are interested of the common drag properties of the different car shapes, I recommend you to search for the SAE Paper 2012-01-0168. In this paper the aerodynamic performance of a generic car body with 3 different back-ends is analysed in the wind tunnel.

  2. Regarding lift stability, remember how the first Porsche 911 turbo looks. It has an absolutely huge wing. I think there must be at least one Saab 96 somewhere with a wing like that, as they were quite popular as an aftermarket wing in the 80’s.
    And I think that it is because of this lift instability that the two-engined Saab 93 “Monster” only has a recorded top speed of 196 km/h. I think I have read somewhere that it actually is a lot faster, but a little nasty at high speeds.
    I have seen the Monster at the museum, but I really want to hear it also. 🙂

    • AFAIK, the wing of the 911 turbo has a thermal origin.
      The wing has a grill on the top, and behind that grill you can find the charge air cooler.

      The current model has kept the wing (as many people identify the 911 Turbo with the rear wing) but uses air intakes in front of the two rear wheel-arches for the charge air cooler.

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