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Old 08-03-2004, 10:55 AM   #1
lakatu
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Default Is the Rear Engine Design Fundementally Flawed? Part 2

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.
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Old 08-03-2004, 12:16 PM   #2
sameerrao
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Nice analysis there. This is the first time I looked at your series of posts. Nice thought provoking stuff.

I have some comments to make :

1. You mentioned the 917 as one the best handling examples of a rear engined car. Having looked at the engine bay of my Autoart 1:18 model it is clear that the 917 is mid-engined and not rear engined - then engine lies ahead of the rear axle. The 934 and 935 are of course true rear engined cars

2. On the rear engined issue

My take on this is that the rear engined approach was what Ferdinand did on the Beetle (and the Auto Unions before it) and carried it forward to his road cars for reasons of practicality as you rightly pointed out.

Today, the location of the engine is not as critical as it was 20 years back. Suspension design has progressed to such an extent that design flaws as it were have been tweaked out of the car. Thus great handling cars are there from all spectrums - Front engine/rear drive, Mid-engine rear drive, and so on....

Placing an engine in the rear was successful in racing because:
- Very good traction on the exit of corners as the weight was directly on the rear tyres
- Porsche had really skilled drivers to compensate for tricky wight transfer issues in wet conditions and sudden manouevres. The Auto Unions particularly were a nightmare to drive and required the skills of a Rosemayer, Stuck or Nuvolari to exploit the maximum from it.

From a hillclimb prospect there is no better design as you need max traction in the rear.

But today apart from the GT3-Cup and GT3-RSRs, rear engined is absent from racing. Pretty much everything is mid-engined - Formula One, IRL, CART, LMP or front engined - NASCAR, BTCC, etc.

So from a racing standpoint, yes rear-engines were successful in its time but so was mid-engined cars... so I cannot conclude that one is instrinsically better than the other.

From a road car perspective,
Rear engined Porsches were pretty lethal up to the late 80s - many an amatuer found himself going backward through the hedges. Todays 996 is nothing compared to a 911 SC or equivalent model. Wider tyres, more downforce in the rear, and aero work done to reduce lift in the rear have made the new 911 a pussycat.

The older cars were tricky at the limit and difficult to drive quickly when faced with the ordinary obstacles one gets on normal roads
- bumps and odd cambers on the road
- obstacles in the mid corners forcing you to brake while cornering
- wet roads

You needed to re-learn how to drive when you had the older 911s. This is a good thing in my book

Is today's 911 the best handling sports car? Its up in the air, I mean there are so many good cars that would qualify - Ferrari Modena, Lotus Elise, etc. The current 911 is good (effective in my book) but to me I think the older cars are better (more character).

In conclusion, rear engined is flawed from a weight distribution side of it but I love it because of its flaws. Conquering a 911 will make you a better driver.

Also to explore all sides of this issue, I would like to paste a review of the 996 GT2 by Damon Hill.

You know that feeling you get just before your plane touches down on the runway? Like a floating, hovering, drifting feeling, heart in mouth, breath held and nobody saying anything. A sort of stressful collective meditation. As long as the plane dangles in the air, the unspoken question permeates the fuselage: “Has he got this right?” and we sweat the clammy sweat of cowards. Strangely, that is exactly what you don’t get in a Porsche GT2.

What you feel instead is a kick up the backside when you accelerate, and a feeling that if you don’t lift off the pedal, pretty soon you could actually be taking off.

But there’s also an inexplicable feeling of being stuck to the bitumen at all times — the very opposite of floating — so you know there is no danger of becoming airborne. Porsche seems to have exclusive rights to this sense of utter solidity within a motor car. Everything is tight, like a body builder’s buttocks (I’m sorry about that simile, but just take a look next time you get the chance). The seat is tight, the upholstery is tight, the ride is tight. If I had to find a word that typified all Porsches it would be “tight”.

And if I needed a term to typify the GT2 in particular, it would be “extra tight”. It is just as well it is tight, because this is a seriously fast car with such an ample spread of torque it makes you feel you’re on the rebound from a bungee jump. With its 462bhp, this is the most powerful 911 ever produced for road use.

When you drive a GT2 people in other Porsches pull alongside and stare. The silver metallic paintwork must have something to do with it — it makes it look solid, like an ingot of silver in the shape of a Porsche. I know it also looks like a toad, but it’s a sexy toad, one with an engine and a place to sit. In fact two places to sit. You could have seats in the back too, if you wanted, but that would mean taking out the roll cage, and you really need one of those — probably more than you need passengers.

So practicality is compromised, but how practical are you going to be in this car anyway? Do you honestly need to get where you’re going that quickly? Do you really need to corner with that amount of confidence?

That is not to say this Porsche is without fault. The 911 is famous for its rear-mounted engine, and as any student of automotive design will tell you, the best way to make a car handle well is to keep its area of greatest mass as close as possible to the centre of gravity (ie, the middle of the car). The Porsche’s engine, by contrast, almost protrudes from its rear, like a portly child sitting alone on a seesaw with nobody to balance the other end.

So why don’t they put the engine near the centre of gravity? Because that is where we sit, and this flaw means the car wants to swap ends under braking like a bolas. Bizarrely, it would probably become almost undriveable without someone sitting inside it partly to counter the weight of the engine — but since it won’t be happening I’ll excuse Porsche that obscure criticism.

And considering this monumental conceptual error, they do an amazing job of mitigating the self-induced problems. But no matter how well this car handles, which it does fantastically, it will always have to get around the fact that there is not enough weight on the front wheels to help stop it really quickly. Because the engine is over the rear wheels all the setting-up has to trade off front-end grip for stability. I know I’m a former champion, so I’m a little too sensitive to these drawbacks for the needs of normal life on the roads, but I can’t help thinking to myself, “What if?”

What if Porsche moved the engine forward? There would be nowhere for the rear-seat passengers then, but considering that the rest of the car makes Ferraris look a little like Airfix kits (and they only have two seats), why not just do it for a car like the GT2? They could still do their four-seater versions, yet have a flagship model that totally flattens every other supercar on earth.

Until I get the job of design director at Porsche we’ll just have to make do with the GT2 as it is — a 196mph seesaw. But even the most beautiful jewels have their flaws, and those flaws are what makes them distinctive. There is no other car like a Porsche, and the GT2 is the epitome of that original 911 concept, a sports car built around the sacred ideas of Ferdinand Porsche.

So accepting that, we are left with a truly gorgeous car that will satisfy every need of ordinary road-going mortals (into which category I humbly submit myself since retiring from competitive driving). Weight distribution excepted, this car excels in every other area of engineering. But the pièce de résistance is — as always with Porsche — the engine.
A superb 3.6 litre flat-six (you may remember Hill’s Cylinder Equation from my last Sunday Times article, which stated that engines with a multiple of three cylinders sound the best) engages drive through six cogs selected by a great cable-operated gearlever that helps eliminate vibration. All clever stuff.

Not that you need to shift particularly quickly or very often, as the vast torque is spread so widely. I must admit that I can’t stand racy engines that need to be buzzed all the time to make them do anything at all.

(As an aside, did you know that they use Porsche engines in helicopters? It helps them stay up longer apparently, and that doesn’t surprise me in the least.) The added plus to such engine characteristics is that on a long trip through Europe it will hum along at particularly low revs, allowing you to listen to music at speeds in excess of the UK limit and still have enough muscle left to pull out and pass without even changing gear. A word of advice, though: be very polite to the police when they stop you, for they will be justifiably unamused.

You know, there really is no ounce of doubt about this car. We include it in our P1 fleet because of its pedigree, its looks and, of course, its performance. But the ultimate reason we love it so much is that we know it will deal with all varieties of driver with as much nonchalance as Kimi Raikkonen exhibits upon winning a grand prix.

And, of course, it is a truly great driving experience, blessed with sufficient acceleration to provide you with a momentary face-lift.
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Old 08-03-2004, 03:24 PM   #3
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Thanks sameerrao. Excellent thoughts and comments 8) and I enjoyed reading the article by Damon Hill. I agree with your comment that the advances in suspensions have reduced the disadvantages of the rear engine design. In part, that is where I think I am heading with the series. That the design wasn't so fundamentally flawed but that there were limitations that where included in the original design which made the 911 rear engine design difficult to drive at times :roll: .

You’re right about he 917 being mid-engine but it has 70% of its weight on the rear axle which is unusual for mid-engine designs. I guess the way I wrote about the 917 was a little confusing ops: . I didn’t make a clear distinction that my point with the 917 was about the amount of weight on the rear axle and not the location of the engine. Where I was going with the 917 was that having a large disproportionate amount of weight on the rear axle doesn’t mean the car won’t be able to handle well.

Of course, there is a difference between having 70% of the weight on the rear axle and locating the mass farther away from the center of gravity than a mid-engine design with 70% of the weight on the rear with the center of gravity and the mass being closer together. Locating mass from the center of gravity causes a rotational force or torque that rotates around the point of the center of gravity. The amount of the torque is a function of the distance of the mass from the center of gravity. In the case of a mid-engine the distance between the mass and the center of gravity is smaller than locating the mass way out on the rear. Therefore the rotational forces are smaller in a mid-engine than in a rear engine as Damon hit on.

I plan on talking about these things in the final post. Great comments . Keep them coming.
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Old 08-03-2004, 04:21 PM   #4
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nice reading
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Old 08-03-2004, 04:41 PM   #5
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Thanks ...

Actually looking at Porsche themselves most of their racing efforts have been mid-engined
- GT1
- 917
- 908
- 956
etc.

I think only the 911 derivatives were rear engined race cars. And if you look at the mid-engined list above they were unbeatable in their time

With aerodynamic forces being highly important nowadays, it is importamt that he cars have a pinched rear and two much mass behind the rear wheels will not allow this.
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Old 08-03-2004, 08:13 PM   #6
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Thanks for the comments, very interesting reading.
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Old 08-03-2004, 10:29 PM   #7
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Well I can think of at least one other rear engine car Porsche raced, the 356 . Then of course there is the 961 (959) but that might be consided a 911 derivative by some. But you're right other than the 911 derivatives which were so successful there hasn't been much else.
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Old 10-03-2004, 09:01 AM   #8
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excellent post guys keep it going a great read
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