3.2 Carrera and SC handling

[burgermeister]

An item worth noting is that the commonly used rake of the car subtracts from the caster. If the car is adjusted to a 1 deg rake angle (nose down), that's -1 deg caster over a level car (I believe factory specs are near level).
(Edited for correctness - thanks for catching that, Flieger!)

And as a point of general reference, lots of caster is not necessarily lots of mechanical trail. It is on a torsion bar 911. But by moving the spindle forward of the steering axis, it is (at least theoretically) possible to have lots of caster without having lots of mechanical trail and associated heavy steering.

[Flieger]

Originally Posted by stlrj

High caster values will contribute to understeer by reducing tire contact patch with additional negative camber at tight slow corners where the steering wheel is turned to it's maximum. At the same time steering effort is increased as the suspension struggles to bring the negative camber back to a more neutral position.

I was speaking of moderate to high cornering loads. The lateral acceleration will cause greater camber change in the opposite direction of the way that caster adds a little (good) camber so the contact patch should be better than with zero caster.

The tilting/lifting of the car due to caster combined with high positive scrub radius will be in the same direction as the lateral inertial reaction force, so there should be little increased effort in the steering wheel due to this aspect in isolation. The contact patch will, however, be made worse in the high scrub + high caster situation. Think of the tilting feedback during cornering on the steering wheel as the effort needed to push a bowling ball off the edge of a hill.

There will be more effort, though because the caster causes trail, and by increasing the slip angle you generate a lateral force at the end of that trail and cause a greater moment arm, which causes negative feedback wanting to decrease the slip angle (so turn the wheel opposite the way you are trying to turn it).

[Flieger]

Originally Posted by burgermeister

An item worth noting is that the rake of the car adds to the caster. If the car is adjusted to a +1 deg rake angle, that's +1 deg caster over a level car (I believe factory specs are near level).

And as a point of general reference, lots of caster is not necessarily lots of mechanical trail. It is on a torsion bar 911. But by moving the spindle forward of the steering axis, it is (at least theoretically) possible to have lots of caster without having lots of mechanical trail and associated heavy steering.

Good point about the caster and trail relationship being dependent on the suspension geometry.

How are you calling your rake angle? Is nose down negative? As I see it, on a 911, the negative rake that many people like is subtracting from caster.

[slw911SC]

I have both an 83 SC Coupe, and an 88 Carrera Coupe. Both are properly set up to stock spec: steering, suspension, alignment, balancing, tires, pressures, etc. etc. No question that the 88 handles better all round than the SC: tracking, cornering, lighter steering feel and better balance. Not to say that the SC is bad by any means, because it's a great feel - it's just not as crisp as the Carrera. That's just my opinion anyway.

[911st]

Wil,

See the camber curve info here: http://forums.pelicanparts.com/porsche-911-technical-forum/454404-effect-negative-camber-lateral-load-capability.html#post4455537

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I love learning new stuf and I think I have learned more about 911 suspension's from this guy than just about anyone else. Thx Burgermeister for the two more new points of thought!!!

Originally Posted by burgermeister

An item worth noting is that the rake of the car adds to the caster. If the car is adjusted to a +1 deg rake angle, that's +1 deg caster over a level car (I believe factory specs are near level).

And as a point of general reference, lots of caster is not necessarily lots of mechanical trail. It is on a torsion bar 911. But by moving the spindle forward of the steering axis, it is (at least theoretically) possible to have lots of caster without having lots of mechanical trail and associated heavy steering.

[911st]

Here is a picture of the outside tire wear I expernced after two days of DE on my 85 with fresh bushings, shocks, cornerballance, alignment, LTD slip, 86 siffer sways and rear torsion bars. Set at about 24.75 rear and 25.25 front and neg camber and caster set to there equal maximums.

We just can not get enough neg camber up front without modifications for track work.

You should be able to see the melted rubber on the outside but none on the inside.

[burgermeister]

Originally Posted by Flieger

Good point about the caster and trail relationship being dependent on the suspension geometry.

How are you calling your rake angle? Is nose down negative? As I see it, on a 911, the negative rake that many people like is subtracting from caster.

Brainfart on my part. The commonly used rake angle indeed subtracts from caster.

[burgermeister]

Originally Posted by 911st

We just can not get enough neg camber up front without modifications for track work.

Ahh yes ... 2 more years of DE days from that camber thread, and I have the identical wear pattern on my fronts (& to a lesser extent on the rears). Depending on the tires, with sufficient static camber, street driving wears the opposite edge, and it ends up semi-even. Non-directional tires are helpful so LH & RH can be swapped.

Still, our camber is sufficient for track work (lots of folks make do with it!). It even provides decent grip. It just isn't optimal for tire wear.

[911-32]

Heavy steering? Try ditching the rubber strut top mount for a monoball type strut top mount. Get rid of the stiction for a dramatic lightening. Worked for me.

[cwitt]

Hi folks, thread starter here again, with some additional details.

I ended up buying the '85 Carrera that I was looking at, and over the winter, I refreshed the suspension. New Elephant Racing rubber bushings all around (A-arms, spring plates, camber plates), new front sway bar bushings (all 4), new Bilstein HDs all around, and turbo tie rods.

I just got it back from the alignment shop and... it still requires a lot of steering effort -- like you need both hands firmly on the wheels for freeway on/offramps.

I'm pretty frustrated, but I still have faith that my Carrera can be dialed in to attain the "fingertip" steering that I've experienced in my buddy's SC, and a number of you Pelicans have attested that these Carreras are capable of.

Please take a look at all the info below, and let me know your thoughts.

Current alignment specs (red numbers are out of spec range):

Front left

• Camber: -0.91 (spec range -0.17 to 0.17)
• Caster: 6.46 (spec range 5.83 to 6.33)
• Toe: 0.16 (spec range 0.08 to 0.17)
• SAI: 11.11
• Included Angle: 10.20

Front right

• Camber: -1.09 (spec range -0.17 to 0.17)
• Caster: 5.76 (spec range 5.83 to 6.33)
• Toe: 0.11 (spec range 0.08 to 0.17)
• SAI: 11.46
• Included Angle: 10.37

Front

• Cross Camber: 0.18 (spec range -0.17 to 0.17)
• Cross Caster: 0.70 (spec range -0.50 to 0.50)
• Total Toe: 0.27 (spec range 0.17 to 0.33)
• Cross SAI: -0.35

Rear left

• Camber: -1.72 (spec range -1.17 to -0.83)
• Toe: 0.31 (spec range 0.00 to 0.33)

Rear right

• Camber: -1.40 (spec range -1.17 to -0.83)
• Toe: 0.28 (spec range 0.00 to 0.33)

Rear

• Cross Camber: -0.32 (spec range -0.33 to 0.33)
• Total Toe: 0.59 (spec range 0.00 to 0.67)
• Thrust Angle: 0.02 (spec range -0.17 to 0.17)

Here are some additional details to consider:

• It was only aligned. The shop had their scales at the track that day, so I have to return for the corner balancing. While I fully intend to get it properly corner balanced, I have a hard time believing that it will alleviate the issue.
• The effort required increases with speed and also with the amount of steering angle required. Lane changes are reasonably easy, because they require minimal steering angle. Freeway on/offramps, or 20mph residential intersection turns are both quite difficult. Don't get me wrong -- it tracks through corners like mad, but it takes a lot of muscle to do get it to turn and hold it there.
• There's absolutely no binding in the steering column bushing, knuckle or rack. When the front end is jacked up, it steers effortlessly.
• The PO installed bump steer spacers on the rack. Not sure of thickness, but I'd guess 10mm or less.
• Wheels are Fuchs 7Jx16 front and 8Jx16 rear.
• Tires are Bridgestone Potenza S-03 Pole Position, size 205/55ZR16 front and 225/50/ZR16 rear. These were mounted by the PO, who knows when, and while they're not bald, but pretty well spent. These are pretty sticky tires, so they might be a contributing factor.
• Tire pressure is at the recommended 29f/36r. I've tried increasing the front to 32, and it helps some, but not nearly enough.
• It's at what I believe is very slightly below euro height. 25.25f/24.75 rear fender heights, and the front A-arms are level at static ride height.
• It's got stock sway bars and torsion bars, all around.
• All A/C has been deleted, and the spacesaver spare, jack and toolkit are out of the car. That's ~100lbs of weight loss, and my guesstimate is that it's about a 60f/40r split in terms of balance.
• I know this is anecdotal, but my buddy's 78 Targa steers so easy that it literally feels like he has power steering, when compared with my Carrera (and of course, he does not have power steering).

Again, your thoughts on this are greatly appreciated.

Cheers,

[stlrj]

Originally Posted by cwitt

[*]I know this is anecdotal, but my buddy's 78 Targa steers so easy that it literally feels like he has power steering, when compared with my Carrera (and of course, he does not have power steering).

Compared to your car, is your buddy's Targa set as low as yours or is it stock?

[cwitt]

Originally Posted by stlrj

Compared to your car, is your buddy's Targa set as low as yours or is it stock?

I believe his SC is at 25.5f/25r fender height, so very close to mine.

[cwitt]

And he's running Fuchs 7Jx16 at all four corners on the SC -- same size fronts as me (different tires though).

[RWebb]

Originally Posted by cwitt

...different tires though...

aha!

I guess you have checked the tire pressures carefully on both cars already?

[Rot 911]

Where are you getting specs showing a positive toe for street use? Street should be -1/8 toe in front and 0 to -1/8 rear. I use the Ray Scruggs specs and have always been very happy with how my '86 steers and handles (except I use a tire pressure of 32 front and 36 rear). Here are his alignment specs and a link to his manual:

Ray’s suggested 911 specs:
Street Competition
Height Front fender lip: (Height varies for
24.5 to 25.5” suspension and
Rocker trim: wheel heights)
½ to 1” lower in
front than rear
Front Camber 0 to –0.5’ -1.5 to –2.25’
Rear Camber -0.5 to –1’ -2 to –3’
(Varies due to rim
width and tire
stiffness)
Castor middle of range forward – wide tires
middle – narrow tires
Front Toe -1/8” +1/16 to +1/8”
Rear Toe 0 to –1/8” 0 to –1/8” (use toe-in
if too much power-on
oversteer)

http://raystrax.com/ha/PDFs/HOME%20ALIGNMENT%20OF%20PORSCHE%20911.pdf

[petevb]

Originally Posted by cwitt

I'm pretty frustrated, but I still have faith that my Carrera can be dialed in to attain the "fingertip" steering that I've experienced in my buddy's SC, and a number of you Pelicans have attested that these Carreras are capable of.

Front left
2. Caster: 6.46 (spec range 5.83 to 6.33)

Front right
2. Caster: 5.76 (spec range 5.83 to 6.33)

Pull caster out of it, shoot for maybe 1 degree less or as much as you can manage.

I recently pulled a bunch of caster out of my car to lighten the steering; it works as advertised. You may need to compensate with a little more static camber if you're competing with the car.

My friend's '87 always had much heavier steering than my earlier SC; a little more caster goes a long way to increasing steering weight.

[cwitt]

Originally Posted by RWebb

aha!

I guess you have checked the tire pressures carefully on both cars already?

Yes, both cars currently running 29-30 psi in the front. But the difference in steering effort remains regardless of whether one of the cars is over/under for front tire pressure.

[crashmy911]

Always wondered why my sc eats 3.2's at the track!!!!!!

[whiz05403]

My steering is very heavy even at speed. How do I reduce the castor?

[Tyson Schmidt]

It's very common to have a bent front suspension crossmember.

This is the piece that the steering rack bolts to. It is fairly flexible fore/aft, and a good curb impact can cause it to bend, moving the steering rack forward due to the angle of the tie-rods.

If the crossmember is at all bent, it places a pre-load on the lower steering shaft bearing. This is the one wrapped in rubber and clamped to the tunnel under the smuggler's box lid.

One of the symptoms of this type of steering stiffness is that there will rather poor return-to-center effect in the steering.

Not saying this is your cause, but it's one more thing to check for.