Please Take Caster Setting Seriously!

[Flieger]

Interesting.

911:
6.4+ degrees caster
Yokohama Advan A048 205/55-16 front 225/50-16 rear
7x16 and 8x16 Fuchs
tire pressures front ~25 psi cold, little under 30 psi hot
tire pressures rear ~28 psi cold, ~32 psi hot
treadwear rating: 60

BMW:
~1 degree caster
Kumho tires 185/60-13, IIRC.
tire pressures ~35 psi
treadwear rating:~600

The BMW steering gets lighter once I get moving. The 911 gets heavier with speed.
The 911 has much more self-centering force.
The 911 has a light, sharp turn-in and likes to rotate with a simple flick of the wrist around twisty turn transitions.
The BMW has less of a tactile connection to the front tires. It turns in easily enough but then the steering is mushy and heavy mid-curve compared to the 911, though it is still very good and a fun car.

Steady-state comparisons are hard because I carve curves much faster in the 911.

I would say the 911 steering is excellent (naturally) and is light yet responsive and informing of the road surface/tire interaction.

The BMW is heavier and rolls alot more than the 911 does with the big sway bars. It feels average weight on turn-in then understeer takes over and it feels mushy/heavy.

Not sure if this helps any. :condused:

[Peter Zimmermann]

Originally Posted by Vincent Hill

Peter, Back in 1981 or 82, I got my First National Road Race points in AMA Motorcycle racing at Riverside! The only thing I did not like about being there was the Bad Air! By the end of the day, that mountain would dissapear and after the racing, I would drive through a mountain Pass where I could not see the air BEFORE I would rent a room at a hotel! I liked the Carousel where they had the stands and then the drop to the Back straight! I hated to see that course go!

YIKES! AMA bikes, man, you were a lot braver than me! I insisted on four wheels under me, and a roll cage around me - then I could go fast. And I remember clearly, no pun intended, the non-air-quality at Riverside. It was really bad in those days. My favorite part of Riverside was going through the esses flat in 5th - if you waited until you could see turn 4 you were taking the Richard Petty line through the dirt all the way to 5.

Originally Posted by Vincent Hill

Again, this is scaring me because I have a 1982 - 320I and it steers a LOT lighter than the 1982 - 911 SC. The BMW feels like I have Power Steering by comparrison to the 911. I also have Toyo Tires that are great in the Wet but love to catch small stones in the dry! I have Cheap NTB tires on the BMW but this should not make this huge of a Difference?

I have Bridgestone 750s on my '82, and with caster in the mid-5s it's very comfortable to drive. I run 30 psi in the front tires, rather than the factory recommended 29, and that makes a little difference also. That said, you know the car does not have power steering, but it's not bad, either, even for parallel parking.

[Flieger]

Our E21 BMWs have manual steering and an engine up front. Mine weighs more than 500 pounds more than my 911.

[stlrj]

After reading through all the posts, it got me thinking. So I got out my Alignment Specification book from NAPA and looked up the specs on the later 911s. From 89-94 caster is set at 4.42 degrees and in 95-98 it went up slightly to 5.33 degrees.

Only later with the advent of the 996s are the caster settings way up to 8.0 degrees.

With all the variations of caster specs of these later 911s, I have never heard of instability issues due to insufficient caster.

[Vincent Hill]

Originally Posted by stlrj

After reading through all the posts, it got me thinking. So I got out my Alignment Specification book from NAPA and looked up the specs on the later 911s. From 89-94 caster is set at 4.42 degrees and in 95-98 it went up slightly to 5.33 degrees.

Only later with the advent of the 996s are the caster settings way up to 8.0 degrees.

With all the variations of caster specs of these later 911s, I have never heard of instability issues due to insufficient caster.

8.0?? They have power steering? In the Mid 4's?? Now even I am not going there!

The 911 steers good enough and now it does not wear out my wife to drive it.

Flieger, No Arguments from me! The BMW does not have and cannot have the feel of the 911. Wierd is the fact that they tie rods are Exactly what comes out of the turbo Porsche. I added a roll Bar on back and it now corners OK but not to where I would fight over a turn with any Porsche. Also at 70 ot 80 MPH the BMW feels like I could lose it at any time and the Porsche feels like driving a truck in Stability!

Peter, that really put a smile on my face when you stated that you "Clearly" remember! That back straight was serious and the turn on to the front straight was not for Sissies!

[Tyson Schmidt]

The caster settings are not the only things that change from model to model. The SAI changed dramatically from the 911 series to the 964/993 series. This was to be able to achieve a zero scrub radius on the 964, and later, a negative scrub radius on the 993.

This is the primary reason for the caster change, not the advent of power steering.

(SAI= steering axis inclination)

[Evan Fullerton]

Also remember that caster does a lot more than affect steering weight. The two reason that Racers like a lot of caster is that it provides a strong self centering feel which is very handy when the car gets loose well into the triple digits. The other affect caster has is that it is effectively dynamic camber in relation to steering angle. If you are driving a camber limited car on tires that like large amounts of camber, you can use large amounts of caster to make it a little better. The only caution I have is with lots of caster has the result of very limiting your contact patch under large steering angles if your car does not have a lot of roll which can manifest its self in sudden onset of terminal understeer. It is all dependent on the use and set up of the vehicle vehicle though so there really isn't one right caster setting for everyone.