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Part of the appeal of the 911, I believe, is that such a unique, peculiar, and arguably outdated design was relentlessly developed to achieve decades of success in motorsport.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

Multilink axles are for comfort and safe understeer when cornering, rather than performance.
In a rece car you don't want soft rubber bushes that alter toe settings.

(I have been reading Competition Car Suspension by Allan Staniforth :) )

The rear suspension on the 911 is very old school because as it cycles the toe changes.

This is why we drive the 911 as we do, not going into corners too hot and keeping the power on. Why? Because power plants the rear of the car and keeps it level, the last thing you want to do is let off the power and have the toe change and whip you around in a circle mid turn.

In all honesty, I like this setup. It allows you to whip the car around tight slow corners very quickly, for example in an autocross. I think its magical. Stable when you want it, and quick and jerky when you dont just by adjusting the throttle.

Wow...I started this thread over a year ago....and still the Pelican community keeps giving! :D

Another factor- Tire technology has changed dramatically since the early 60's when the trailing arm/torsion bar suspension was designed.

The original setup worked great with 185/70/15 tires. Not so with today's much stickier 335/35/18's or whatever.

Can somebody please answer my question?

5axis actually answered your question about a year before you asked it :-) Check a couple of posts before your question, then go to www.kellymoss.com .

the Macpherson strut aka Chapman strut was developed by Colin Chapman(Lotus), he had a mania for lightweight and 1 piece serving multiple functions as a means to that end.

Is is relatively light weight and compact making it easy to use on small cars like the 911. I suspect that is why it was used.

However it's downside is two fold
1) it is difficult to add wider wheels and tires because of scrub radius issues

and

2)it has an undesireable camber pattern which turns positive as the car rolls, thus losing grip

the best setup from a handling perspective is unequal length A arms or their virtual equivalents, which increase negative camber as the car rolls, thus enhancing grip

I've read the back seat was to make car insurance cheaper. A four seater gets classified differently then a two seater for insurance purposes. I doubt insurance companies do that any more, but in the 60's I can believe it.

edevinney,

Thanks for the 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."

I think race cars dont have it, (multi-link suspension) because of they're single purpose machines, that dont need enough travel for comfortable road use, while still performing well in sporty driving; and not due to lack of budget.

For a better understanding of 964 and 993 suspensions get a copy of Fr&#232re, he has a great section outlining the advantages and characteristics of each one.

Are you being tongue in cheek here?

Every time I read that Porsche configured/designed the 911 suspension AROUND a bag of golfclubs, I just cringe.

I have no doubts that the "golfbag" comment was a marketing angle directed at those with more money than understanding about anything 'design.'

IIRC, that what is said in Frère...

Actually Mcpherson strut is a special case of the double wishbone suspension. It is a double wishbone with the length of the top arm equal to zero. 'Tis true. Once you understand the formulas behind one, the other is a logical extension.

Considering that Porsche designed the car over 40 years ago, we can hardly fault them for not using the best of 2005's technology. Mcpherson strut has many good attributes and performs quite well. That these cars are still competitive on the racetrack today is a massive tribute to the design.

What other 60's design continues to be a track weapon of choice on the scale of the 911? None.

I think ya mean, equal to infinity . .. the arc of a an infinite radius is a straight line.

I was absolutely serious about the Golf bag reason. Evidently customers were complaining. Also, the reason for six cylinders was that other companies had developed faster cars, and Porsche then (as now) felt compelled to get in the arms race.

http://forums.pelicanparts.com/uploa...1134861115.jpg

"the Macpherson strut aka Chapman strut was developed by Colin Chapman(Lotus)"
... and...
"Mcpherson strut is a special case of the double wishbone suspension."
_____________________
IIRC, a Chapman strut is the one designed by Colin Chapman -- an improvement based on the earlier McPherson strut. And it is the Chapman strut that is a special case ... the lower arm is a wishbone.

SO, if I recall right, they are a bit different.

THanks JC, that's great. :)

Although, unless that pic is of a technical design spec. (and not simply a marketing push) my point stands.

Often, designs have un-intended consequences. Now, mind you, I'm not saying that the "golf bag spacing" was never on the minds the designers, but, it was not likely -thee- design parameter.

Dr Porsche's consistent design theme, from the bug to 356, to the 914 & 911, has been of compact, low CG suspension.

Brilliant, performance obsessed, Engineers generally do not obsess over things like "space for the golf glubs" (see Arial Atom) . . .although, of course marketing dept's do.

Here's a thought; Porsches insisting on "room for club-bags" could certainly have driven the gas tank design. ;)

I do mean equal to zero. The amount of motion at the top of the strut is zero, which is exactly how much a lever arm of zero length moves.

To further illustrate, the motion of the strut is not a straight line - unless the point of reference is the strut itself. Both the bottom and the top of the strut trace an arc. However the top is displaced a distance of zero, the arc is only an angular change. This is because the top arm is of zero length.

On the Chapman vs Mcpherson thing, I believe that it is normally called a Chapman strut if it is on the rear of the car, Mcpherson if on the front. However the basic design is the same. Not sure why, maybe Chapman was the first to use it on the rear.

That's correct, I shouldn't have relied on my failing memory of things that happened 35yrs ago. In college I had the hots for a Lotus Elan, hence the vague memory of it's suspension design.

Never the less the issue w/ 911 front suspecnsion has always been the way camber goes positive, hence the ceaseless efforts to get more static negative camber on race applications. Porsche has always been hampered by some poor design choices(from a racing perspective) that make our cars pigs, fast pigs but w/ some technical challanges.