Why doesn't my 911 have double-wishbones?

[safe]

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 )

[AvonGil]

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.

[Wrecked944]

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

[Dantilla]

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.

[blue72s]

Can somebody please answer my question?

[edevinney]

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 .

[Bill Verburg]

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

[EdT82SC]

Originally posted by safe
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...).

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.

[blue72s]

edevinney,

Thanks for the link.

[Nine9six]

"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.

[Bill Verburg]

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.

[island911]

Originally posted by john_cramer
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. . . .

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.'

[randywebb]

IIRC, that what is said in Frère...

[Chuck Moreland]

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.

[island911]

Originally posted by Chuck Moreland
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. ' . ..

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

[304065]

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.

[randywebb]

"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.

[island911]

Originally posted by john_cramer
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.

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.

[Chuck Moreland]

Originally posted by island911
I think ya mean, equal to infinity . .. the arc of a an infinite radius is a straight line.

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.

[Bill Verburg]

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.