Why doesn't my 911 have double-wishbones?
 

I hope this isn't a painfully ignorant question. I ask because I am trying to learn about performance suspensions and all of the books I've been reading have suggested the double wishbone design is superior to most or all others. At first I figured maybe this didn't apply to 911's for some reason. But then I read that the GT1 had a 911 front end with double wishbones. So then I really started to wonder. Our cars typically benefit from Porsche racing technology. So why not the double wishbone suspension? I don't know nearly enough about this topic to have a clue. But I figure you folks will know !! SmileWavy

Good question: Basically it comes down to packaging, and the 911 needed lots of trunk room since it had no conventional rear trunk, so the front struts and torsion bars were the best compromise. Most new cars including the 911 use multilink set ups in the rear, which is as good or better than double wishbone. The front end is still a challenge to package and most cars use Macpherson struts for this reason....evn Honda went this way on latest Civics which have always been double wishbone.

The original 911 was far from a "clean sheet" design. It was evolved from Ferry Porsche's prior cars, which didn't start life as performance machines. The claim that a 911 is just an overgrown Beetle has a lot of truth.

Dr. Porsche designed the Beetle, which had torsion bar springs and struts in front, swingarms in rear. I don't know why he chose those designs but I'd guess that low cost and packaging were primary and performance very secondary. The Porsche 356 was derived from the Beetle, as Dr. Porsche used many Beetle mechanicals and the basic Beetle layout. Then the Porsche 911 was derived from the 356, at least the same basic layout was retained including the basic suspension.

Considering the peculiar layout of the 911, it would be hard to squeeze double-wishbone suspension into the package. In front you would sacrifice precious trunk space, in the rear you might make the drivetrain area too cramped.

Part of the appeal of the 911, I believe, is that such a unique, peculiar, and arguably outdated design was relentlessly developed to achieve decades of success in motorsport. A triumph of will, I suppose.

Later 911s, the 964/993/996/997 models, weren't really clean-sheet designs either. Certain aspects of the original 911 design couldn't be changed (like the engine location) and perhaps tradition influenced the suspension too. Neverthless, in the rear - which I imagine is the more critical end, suspension-wise - Porsche eventually went to something even more sophisticated than double-wishbone, namely the multi-link suspension of the 993. I actually don't know what the 996/997 use but I assume it is some form of multi-link.

I call multi-link more sophisticated than double-wishbone, in my opinion, because it can be designed to have the geometry change as the suspension is placed under different loads and moves through its travel, so the suspension can be multi-purpose: soft with enough travel for comfortable road use, while still performing well in sporty driving. I am going to guess that race-cars don't use it because they don't have the budget to design a multi-link suspension (I seem to recall reading that it cost Porsche millions to design the 993 rear suspension), their suspensions don't actually have much travel, and they have only one purpose.

Anyway, that's my answer - with the caveat that it is only based on some reading, various TV documentaries, and some logical inference. I never had a chance to ask Ferry :-)

Isn't one advantage to the "primitive" rear suspension on the 911 is that it is incredibly tough? A good thing for endurance racing...

Trunk? TRUNK? <i>Was ist trunk</i>? Am I to believe that I'm being forced to drive with a <i>minderwertig</i> suspension because of the need for a bigger trunk? Isn't a <i>trunk</i> just a giant <i>cupholder</i> stuck between the front wheels? I say anything beyond the space needed for a battery, tool kit, spare tire and gas tank is just so much excess luxury for the latte drinking, cellular telephonegentsia. Someone fetch me my sawzall !!:D

If I remember correctly one of parameter when designing the 911 was that a golfbag should fit under the hood.
What I dont understan is why they bothered with a useless backseat, when they could have used the space for luggage (maybe they didn't want people to add more rear weight...).

Janus;
Remember, the 911 wasn't designed as a racing car and then accomodations made for people. It was designed specifically to transport 2 + people and their belongings from point A to point B swiftly, efficiently and reliably, and at a cost (and thus price and margins) which would provide a large enough market to support the company.

Now if you were to further optomise it so that you're only carrying 1 person with no belongings, from point A to point B as swiftly as possible -- you're talking about a race car. The 904 and the 906 were just about the last Porsches which were designed as race cars and then retrofitted to accomodate real people in the real world.

Getting back to your original question, the MacPhereson strut is a remarkably clever suspension design that allows room for the belongings of the two people that I mentioned earlier, while still providing geometry charactoristics which are compatable with the semi-trailing arm rear suspension. So I guess the question I have for you is "Why not use a MacPhereson strut???". What problems does it provide? It was good enough for the 935!

Another detail about the front suspension...

A MacPherson strut suspension has really good geometry. It has a low roll center and a good camber curve. It is simple, reliable and tough. It is possible to create a better suspension with unequal length a-arms, but not by much. Also, so much of the weight is on the rear of these cars, so that further reduces the importance of the front suspension. Finally, the banana arm arrangement has a lot more geometry problems so messing with the front is kind of pointless until the rear is sorted out.

The real advantages of the unequal length a-arms are a wider variety of possibilities in suspension geometry, adjustability and unstrung weight. If the MacPherson strut has been designed with your needs in mind then the possible gain is marginal. The adjustability would be nice as most of us are doing non-standard things with our cars. (lower it and give me grip; comfort be damned!) The last bit about unsprung weight would be nice but people are putting on 30lb turbo twists and larger rotors (that they don’t really need or use), so it probably isn’t important.


Wayne

Does anyone have any links to more info on the 993s rear multi-link suspension and how it works? Is it possible to retro fit it to a 964?

Probably cheaper (and better) to just sell the 964 and buy a 993 . . .

Not true. The 911 suspension is different than a 356, and early beetle. The early beetle and 356 have an upper and lower trailing arm front suspension with transverse torsion bars (no strut), with a swing axle rear suspension (no semi-trailing arm). The 911 has a McPherson strut front with longitudinal torsion bars, and semi-trailing arm rear suspension with transverse torsion bars.

Now, I do not deny that the 911's design was strongly influenced by the 356, but the suspension configuration was new.

If the 911 rear suspension design was so bad, why did they go with the same basic design in the rear for the clean sheet 924/944 as well? If you look at one it is very similar to the 911 until the 964 came out with coil over suspension and some suspension arm changes. There are plus and minus to every design.....that's what engineering is all about. There is what you know you can do, and then there is what you are allowed to do, either via cost or time constraints. The point is A-arm is simplest and best at doing the thing it is seen used for in top levels of racing...performance for a race car. Now a passenger car has different requirements in terms of packaging, meaning trunk space and interior room and cost which forces engineers to develop alternatives like the MacPherson strut, torsion bar springs, etc. The 911 racing history speaks for itself, the suspension worked great for its day, new designs have evolved, ie multilink rear designs which are improvements. I would blame myself for not being fast enuff before the car, spend time improving my skills before finding blame with design.

I don't think anyone here said that the 911's suspension was "so bad". The MacPherson strut arrangement is so close to optimal that it would be a hard sell to switch production to it.

The rear suspension would never be considered as a possibility in any modern car but that doesn't mean it was an embarrassment. Look at the year it came out. Vehicle dynamics was in its infancy and this was one tough suspension. What is truly amazing is how far they could take it by continually refining what they had.

It really was a dinosaur by the time they gave up on it though. The camber and toe problems make the 993 and 996 suspension seem light-years ahead.


Wayne

Another thing that has changed with the newer (after 89) was the coil overs. Most real track cars have been converted to these.

Here is your conversion. It looks very cool, and expensive. http://www.kellymoss.com/pages/RaceDept/993suspconv.htm

:mad: that did not workout the way I had hoped. Just go to Kellymoss and click on services then scroll down to the suspension section.


http://www.kellymoss.com/Images/cars4sale/993_4_sale/993conv1640.jpg

If money is no object, get an ERP front end and swap in the aforementioned 993 multi-link rear.

I would personally rather not have toe correcting rear suspension doing things behind my back.

I believe that most of this is taken out of 993 and 996 race cars by using solid pivots instead of rubber in the multi link wishbones.

Jack has it right. BTW, I wonder how mt911's car is coming along....

Is it possible to retrofit 993 rear mult-link suspension to a 965?

Whoohah! Get ya' a Weissach axle!

Janus, the reason we don't have double wishbones is so we can carry double golf bags. It is a well-established fact that the ability to carry two golf bags was one of the design criteria on Butzi's short list. The 901 was a sports car before it was a race car.

When I had my '88 and used to DE and AX her, I had one of those cambermeister strut braces. I don't think it helped, and it got in the way of carrying more than two beach chairs, a cooler and a radio. The propane barbeque had to ride in the back seat, which was made more difficult by the rollbar cross tube. That's the thing about going fast. . .it forces you to make inconvenient compromises. . . :)