[lakatu]

In part two I am going to briefly touch on the design compromises that contributed to snap oversteer in the 911. To begin with I wanted to provide evidence that the rear engine design was a successful racing design. I believe that the competitive nature of auto racing would prevent a design that was fundamentally flawed, as so many people say, from being moderately successful not to mention the extreme success that 911 racing derivatives had. I think there has not been a more dominate car in auto racing than the 934 and 935 .

Some of domination of the 934 & 935 can be explained by better reliability and acceleration verses their competitors. It is hard to win a race especially an endurance race if you can’t finish. However, this doesn’t account for the majority of these cars success.

As I see it, there are four major aspects to a successful race. The first I already touch on which is a cars reliability. The next three have to deal with the forces that a car transitions through in completing a lap around any track. These forces are acceleration, deceleration (braking) and lateral acceleration (turning forces). The key is to turn in the fastest average speed around the track this will also equate to the fastest time. Deficiencies in one area can be made up for in others. So that if cornering speeds are your weak point you can potentially compensate for that by another advantage that you may have let’s say in braking or acceleration.

A little background about racing in this era might be helpful. I will touch on it really briefly but I think there are a lot of people here who could add more color and specifics to these cars along with 911 derivative racing models . So I will leave that issue to be discussed by others. Additionally, there is a nice historical summary of the 935 racing history at http://www.911handbook.com/articles/...retson935.html and another article about the 934 http://www.911handbook.com/articles/...wdavem934.html . Porsche also has historical information of their website under motorsports.

The model numbers were based on the groups they raced in, Porsche raced 934s in Group 4 and 935s in Group 5. Both Group 4 & 5 racing were based on homologated cars. Group 4 allowed a few but very few modifications. For instance tire width was extremely restricted and no additional aerodynamic modifications were allowed. Weight restriction in the Group 4 was 1120 kg (2469 lbs) and the 934s produced between 500-600 hp. Group 5 on the other hand allowed more extensive modifications. Weight limits were also lower with 1025 kg (2260 lbs) and produced between 600-900 hp.

The 935 was raced for 9 years after it was developed. That is pretty amazing when you think of it that a race car design was successful for nine years after its conception. The reason that it stop racing wasn’t because it was no longer effective but because of rule changes. During that nine year winning history the 935 became the most successful GT racing car.

I have gathered some quotes from drivers of the 934 and 935 because I think they provide insight into the cars they drove and the rear engine design. I highlighted points that I think are interesting and will further discuss. Beside their names are some of their accomplishments in these cars.
Here are some quotes from drivers of the 935 and 934.

David Hobbs: (Back to back IMSA GT wins in 1982) “I drove for BMW against the Porsches for a long time. When I finally got into a 935, my first impression was, ‘No wonder it used to beat us.’ It wasn’t the dog I thought it was going to be. After all, it was derived from the 911, which was a design disaster. But the 935 was quite nice, and of course it had massive power. Even before the compressor came on, it had plenty of grunt.”

John Paul, Jr.: (1982 IMSA GT champion) “The power of the 935 was awesome, but what blew me away were the brakes. When I first tested one at Road Alanta, my braking points were exactly the same as the ones I’d been using in my Formula Ford-except that I was going twice as fast. I firmly believe to this day that, if they’d let them run, the 935s would still be winning races. I have a lot of good memories of those cars. Beating Redman at Daytona on the final turn of the final lap. Setting the all-time lap record at Sebring, faster than the 917. Winning Daytona and Sebring back to back in ’82. at Daytona, we led twenty-three straight hours and set an all-time mileage record for a twenty-four hour race.”

Hurley Haywood: (1970 Daytona 24 Hours and 1981 Sebring 12 Hours winner) “Of all the cars I’ve driven, the 935 was the most spectacular to watch. And it was probably the most difficult to drive quickly. You had a lot of power in a short wheelbase car with all the weight hanging off the ass end. There weren’t a lot of people who were really able to get all there was out of it.”
“One of my most memorable races in a 935 was Sebring in 1981. I was racing with Al Holbert and Bruce Leven , and the car was diabolical. We weren’t ever able to get comfortable in the car, even in practice. And a race car is only good for the first lap; everything goes down hill from there. Well, we won the race. And it wasn’t that all the fast cars broke. We had to really fight, and that’s not much fun when you’re not comfortable in the car. After the race, Al looked at me and said, ‘You can drive a bad car faster than anybody I know.’”

George Follmer: 1976 Trans-AM champion “The 934 was a typical 911-a lot of oversteer, very twitchy, especially in high speed corners. The tires weren’t big enough, and it had a lot of throttle lag. You had to tippy-toe around until you were pointed in the right direction.”

Tony Dron: 1982 Le Mans class winner in 934 “The basic quality of that 934 was magnificent. It did all those miles and kept running like a train. By the time we stopped racing it, it had done six years of endurance racing. The floor pan was gone. The next time in the shop, we should have stripped out the floor and welded in a new one! It had gotten to the point where there was no ‘wood’ left. You know I actually did work out once in an idle moment of insomnia, that I’d spent about four hours out of my life on opposite lock at over 150 mph. Most all of it with that 934.” He goes on to say that critics of the 934 had either a lack of experience with the car or with one as well sorted out as his.

Some of these comments seem to be contradictory. You have to remember that different cars were massaged to handle and perform differently by their racing teams. Additionally, over the nine years the cars as a whole evolved and became better suited to their racing niches.

Problem Areas that Contributed to Snap Oversteer in 911 Design
Some of the aspects of early 911s exacerbated it’s tendency for snap oversteer. I highlighted these issues because they are separate factors that were addressed by Porsche, and yet they were in part responsible for creating the 911's reputation for treacherous handling :roll: . In my opinion people fail to recognize and separate these aspects contributions to the 911’s oversteering characteristics but instead negatively attribute them to the rear engine design . All of these issues have been changed or addressed as the 911 model has progressed to the current 997 version. As a result of these changes the 911 has lost much of its reputation for being a difficult handling car and has instead gain a reputation as being very neutral and driver friendly. Some have suggested the 911 is one of, if not the best handling sports car currently available 8) .

Wheelbase: The 911 because of its compact size has a small wheelbase. This has several affects on the dynamics of the car. A smaller wheelbase results in quicker turn-in response. This can be a problem if the quick rotation into a corner upsets the weight transfer so that it is sudden and upsetting to wheel grip. A short wheelbase also results in greater transfer of weight during acceleration and deceleration. Finally, if or when the 911 transitions into oversteer a shorter wheelbase means that the rear end rotates faster around the car and therefore requires quick response in order to catch the slide with opposite lock. The wheelbase was lengthened in 1969 and again with the development of the 996.

Aerodynamics result in rear lift: The early 911 design was created during a time where very little if anything was known about the aerodynamic affects of lift at high speeds. As a result, the design resulted in lift to the rear end of the car and therefore lower levels of grip as speed increases. This was especially noticeable in 911s with out aerodynamic spoilers at high speeds on long sweeping turns. This has latter been reduced through the changes in the rear portion of the 911 as it has transitioned into the latest version of the 997. It is obvious that the rear end has been raised and has a sharper drop off compared to the original design. This along with the extendable rear spoiler has eliminated rear lift.

Rear semi trailing arms: Semi trailing arms have a tendency to toe out under lateral braking or drop throttle conditions. This toeing out of the rear suspension creates oversteering properties. The issue was partially addressed with the 964 when new coil springs allowed changes to the rear suspension so that they weren’t completely fixed by the torsion bar. The new design allowed movement that actually creates toe in under trailing throttle and decreased tendency to oversteer. This issue was completely addressed by the multilink suspension of the 993.

Turbo lag: For turbo charged cars the effect of turbo lag and then the sudden explosive nature of acceleration was also a potential problem that worked to disturb the weight transfer between wheels. A lack of power due to turbo lag could prevent the driver from transferring sufficient weight to the rear wheels. The sudden explosiveness of the turbo charged engine could also cause too much transfer to the rear wheels or exceed the adhesive ability of the tires. The result causes the rear tires to break free in power oversteer when the driver did not want it.

Small Tires: My personal belief is that Porsche for a reason that is not completely clear to me, has used tires that were inadequate in size . I believe that the width of the tires presented tradeoffs and that Porsche maximized the tires size to achieve a combination of goals. However, I believe that strictly viewed as it relates to handling, the tires are too small. Over the years, as Porsche has moved to the 997, the tires have become wider. I believe this has contributed to the handling performance improvements. Certainly, this might be an area of debate that others may wish to comment on . Obviously, there are other negative affects of wider tires that factor into the decision. A partial list includes increased rolling resistance, increased unsprung weight and decreased aerodynamic efficiencies.

In my next, and I think final post, I will examine in more detail the 3 of the 4 factors that I discussed at the beginning of this post required to win a race. The three that I will discuss in more detail are acceleration, braking and handling as they relate to the dynamics of rear engine designs. I plan on throwing in a little physics to provide a more detailed insight into the advantages and disadvantages of rear engine cars.