Why doesn't my 911 have double-wishbones?

[patkeefe]

Well, here's the bad news. You can carry two golf bags in the trunk, but if the clubs are in the bag, they don't fit. I used to carry mine in the back seat, but now they don't fit with the roll bar. So, I have to take some of the clubs and put them in the back seat, and the bag in the trunk. I feel like Lamont Sanford when I go to the course.

Finally I said, screw it, and take the 968 to the golf course.

[island911]

Originally posted by patkeefe
Well, here's the bad news. You can carry two golf bags in the trunk, but if the clubs are in the bag, they don't fit. . ..

Wow, so then, clearly the suspension configuration has missed it's most important design constraint.

Chuck; nice obfuscated doublespeak . . . ."zero, which is exactly how much a lever arm of zero length moves.
. . . the motion of the strut is not a straight line "

So, it moves zero, but has motion in a non-straight line?

Dude, stop trying to cover.

[randywebb]

Looked in the R&T dictionary last night -- it confirms the above. Colin Chapman adapted the McPherson strut for use on the rear of a car, the Lotus model given above. So, a modified rear McPherson strut is called a Chapman strut.

The comments about non-optimal suspension geometry are all true -- here is my question: If these compromises had NOT been made, would we like the handling feel of our cars as much? Note the word feel, not just higher handling performance... At least some of the enjoyment - at low speeds anyway - seems to go away with later models that have more competent geometries (i.e. Boxster S; GT-2, even the 964 - those are just some I have driven in a "sporting" fashion). The complicating factor is the great increase in wt. of those models.

[Bill Verburg]

If you want to see a really nice light, compact suspension w/ excellent camber patterns take a look at Honda Accords. I don't know if they still use them but at one point then had a virtual unequal length arm set up that worked really well.

Seems to me I heard a lot of squaking that they were changed but I wasn't really paying to much attention.

[Chuck Moreland]

Originally posted by island911


Chuck; nice obfuscated doublespeak . . . ."zero, which is exactly how much a lever arm of zero length moves.
. . . the motion of the strut is not a straight line "

So, it moves zero, but has motion in a non-straight line?

Dude, stop trying to cover.

Thanks for the mature and insightful reply, you've aided everyones understanding

[jluetjen]

BTW, Everyone seems to be looking down their noses at the MacPherson Strut when in fact it really is an ideal compromise for the "GT" design that the 911 is.

1) Along with the fact that it is the lack of the upper A-arm (or as Chuck correctly describes, an upper link of 0 length), which allows a usable trunk space, To get an idea of what a double-A-arm suspension would do to the 911's front trunk space, just look inside a Ferrari 308's front trunk. You'd be hard pressed to fit anything more a small overnight bag in there.

2) the geometry is really not all that bad.
a) It has good camber charactoristics under braking -- almost better then a double-A-arm design -- because there is relatively little camber gain. Camber gain under braking is bad because it effectively reduces the contact patch by lifting the outer edge of the contact patch.
b) It has ample suspension travel.
c) The camber issue is generally manageable for road use by using a combination of static camber adjustement and caster settings.

3) The weaknesses are not that great.
a) Binding of the strut is not an issue as long as the loads are reasonable and the strut has a reasonable size.
b) Under peak cornering conditions, sure there is camber loss which results in a gentle traction loss. But given the terminal oversteer of the rear-heavy 911, this "weakness" actually results in a car which is "balanced" and forgiving under steady throttle conditions since both ends tend to lose traction somewhat in parallel. (I believe that the 911's drastic TTO condition is more a function of the rear suspension design then the front.) Basically, it results in a car that can be driven at high levels of performance successfully due to it's balance across 95% of the handling spectrum.

[304065]

Originally posted by island911
Although, unless that pic is of a technical design spec. (and not simply a marketing push) my point stands.

Yes, that is the way they designed stuff in the Early 60's, they would bring in one of the advertisements and say, "Butzi! Make it look like zees!"

Just kidding. That's an advertising poster from 1962, with a 356 coupe. The point I am making is that "brilliant engineers" were brilliant enough to realize that they had to sell cars, and couldn't just slap a set of houndstooth seats in a 904 and call it cool.

"Golf bag engineering" is one of the hidden themes of Porsche design. More anecdotal evidence: my cousin went to buy a Boxster S. Upon seeing the rear cargo compartment, she scoffed, "There is no way you can fit two golf bags in there!" At which point the salesmen produced two golf bags in the showroom, removed the woods, and demonstrated that you could, in fact fit two golf bags in the compartment.

It is a little-known fact that the 907 Langheck was designed with this in mind as the below image clearly shows.

[Wil Ferch]

In summary.... the 356 design used the VW-inspired transverse front torsion bars ( to add insult to injury...STACKED one set on top of the other !)...so this took up a considerable amount of trunk space.

The 356 was getting long-in-tooth, and the design schrifft for the 911 was more room ( altogether..people were getting "bigger")...more power ( 6 cyl vs 4)...and very definately room for golf gear in the trunk.

The Macpherson strut ( named after the inventor at Ford-England) had very acceptable camber curve ( vehicle dynamic) qualitites as well as being relatively light and cheap...and mostly space efficient. It took re-packaging the TB's to be longitudinal to gain even more space advantage. Certainly more-so than if the strut had a co-axial spring. Altogether...it was an elegantly simple solution for a number of conflicting needs. As Chuck mentions, it's a testimony to the design that it has been successfully used for so long in competition. It wasn't an after-the-fact marketing ploy.....

Wil

[island911]

Cramer, that's awesome.

As you can see, in a foto of a 904, 917 .. . whatever, the advantages of a compact suspension are significant. That is, a fast race car uses the saved space for better aero . . . bump it up and you've got room for clubs. (or whatever the marketing dept thinks is viable)


jluetjen- A typical wishbone link sweep an arc.
As that link gets smaller and smaller, it sweeps a smaller and smaller radius, right down to "zero-length link" -- would be a fixed point
Whereas, an infinite length link would sweep an infinite radius arc (aka, a straight line.)

Last I checked, the two ends of a strut have relative motion along an infinitely large radius. . . not a point.

[randywebb]

Despite some sniping, this thread has some infromative value.

Many of us, including me, have been mis-spelling the man's name. It is Earl Steele MacPherson (note the Mac, not Mc; also in Scotland there is still a death penalty for not capaitalizing the letter after the 'c').

His design was first used on the French Ford Vedette in 1949.

[CLE]

Lots of interesting information on this thread but, when you go back to the original question of why Porsche didn't use double A-arms, it's my opinion it was purely a cost issue. I believe Porsche was looking for the best solution at the lowest cost and the strut offered that.

When they were designing the 911 in the early 60's it had already been established that a double a-arm type suspension was and an solution for racing and you will notice that all (to my knowledge) of the "purpose" built race cars in that period (this doesn't include converted street cars) were either double a-arm or multi link (like the 917's)

A strut works quite well in certain applications as long as the requirements don't exceed the geometry characteristics. There are many limits to a typical strut, one of which is the scrub radius. As you start putting wider front wheels on the car, this becomes an issue ... of which there are many semi solutions but never the less, issues. The bump steer range is also limited to about a 2.00" to 2.50" range and always has an "S" curve meaning "in to zero to out" with the zero holding for the 2.00". As an example, I recently designed an off road race truck with double A-arms in the front that had 25 inches of total travel of which over 21 inches are at ZERO bump change... that will NEVER happen with a strut. Another issue is the camber curve to desired roll center .

As far as the trunk size goes, the arms actually aren't a factor (unless you make them extremely long). I've built a hand full of double A-arm conversions for 911 fronts (can't find any pics right now) and they didn't need to take any trunk space at all. The a-arm just simply attached to a reinforcement that was attached directly to the side wall of the trunk (btw, once again, I did my first one of these back in 1980 something so, when someone reads this and later comes out with a set up like this saying "another original idea" say what you must but, it's already been done . In fact I've worked on some GT1's and those didn't lose any truck space do to the A-arms either.

A book could be written on all the speculations alone but, the bottom line is .... A strut isn't the worst thing and it's cheaper and good enough for what they wanted to accomplish.
Final note: The 959 was an experiment with no particular cost limits .... it has double a-arms...

[Bill Verburg]

959 front


959 rear

[Wrecked944]

Originally posted by CLE
I've built a hand full of double A-arm conversions for 911 fronts (can't find any pics right now)

Aw man, no pics? You're such a tease.

Did the converted cars actually handle noticeably better or just differently?

[CLE]

Originally posted by JanusCole
Aw man, no pics? You're such a tease.

Did the converted cars actually handle noticeably better or just differently?

They were a bit easier to drive and for sure had a better scrub than a regular 911 which also helped the steering effort and bearing life with 12 inch fronts but, the car wasn't really much faster on the clock. The biggest improvement were from changing the rear to a multi link. I've not done a double a-arm set up on the rear of a 911 .. it would be fun but just haven't done it and I've only seen it done on one car. For all practical purposes the 993 and 996 "multi links are almost an A-arm

One of the ways to over come the camber loss on the front during turn in (someone had mentioned this earlier) is to put a lot of caster in and that will cause the wheel to gain camber as it turns at a high rate than it loses. However, it depends on what your tires want. Non of these are absolute answers... just more options.

[studbike]

remember up near the top of the thread where someone said that the macpherson strut begins losing grip towards the end of its travel? this allows the manufacturer to finely tune the understeer, which is extremely important on the 911 and on bmws (more my background). if these cars didnt reliably understeer, they wouldnt be called "neutral", they would be called tail-happy like corvettes and vipers that have double wishbones. even a civic gets darty near the limit in my opinion, but having so much weight teetering over the front axle, it achieves the hyper-opposite of 911s weight teetered out over the rear axle, but because these are the wheels you control the direction of, a good driver can compensate, thus making the double-wishbones more suitable. If you want a true performance car, you have to pony up the dough and get a spyder, ferrari, etc in my opinion. I love bmws and porsches, but they aren't no-compromise performance vehicles. that's what makes them so livable. and from a visual design standpoint, a double wishbone would have a tough time fitting under the hood of the classic 911, with the extremely low front hood line.

[berettafan]

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...).

Or even better flip the motor/trans!

[73 911E]

When I bought my first Porsche last fall I also wondered why it had back seats, but now I put my two kids in there and drive around town. This serves two purposes
1) increased chick magnetnetism
2) breading 2 future Porsche enthusiasts.

[AlfonsoR]

Originally Posted by studbike

if these cars didnt reliably understeer, they wouldnt be called "neutral", they would be called tail-happy.

Are you sure about that? If the front end is slipping in relation to the rear (understeer) , that is not "neutral" handling. Neutral handling means front and rear slip at the same or almost the same mechanical grip. Maybe you meant "safe handling". A lot of magazine writters associate safe handling with cars that understeer.

[longhornchris04]

Originally Posted by AlfonsoR

Are you sure about that? If the front end is slipping in relation to the rear (understeer) , that is not "neutral" handling. Neutral handling means front and rear slip at the same or almost the same mechanical grip. Maybe you meant "safe handling". A lot of magazine writters associate safe handling with cars that understeer.

Perhaps I misinterpreted his statement, but I thought he meant that they adjusted it to add enough understeer to take an otherwise oversteering car and make it neutral

[AlfonsoR]

Could be. I am no suspension or track expert, but if a car is understeering, it should not be said to have "neutral handling". I guess there are varying degrees of oversteer/understeer to consider, though.