Next steps to tighten up suspension
10-12-2012, 03:15 AM
#31
I would also add that if part of your objective of suspension mods is to tighten up the overall "feel" of the car, an often overlooked mod is seats. If you haven't already done so, switching from standard or sport seat to fixed back or high side-bolstered seats (especially if mounted directly to the floor) significantly increases your connection to the car and the sense of what the car is doing, in particular the rear. Swapping to aftermarket bushings, tie-rods, mono-*****, etc while keeping the factory seats is inconsistent (IMHO) 
10-12-2012, 10:54 AM
#32
^^ the later cars don't have the provision to adjust the angle (castor) of the instant center (blue line) but still have some amount of rear toe-in due to lateral loading. This is tuned-in via different relative bushing stiffness. Presumably, a longer wheelbase & electronics in the newer cars allows a reduction in 'compliance steer' while still being sufficiently spin-resistant. The terminology is somewhat misleading since Porsche refers to alignment changes due to suspension motion as 'kinematics' in the later tech manuals.
For the same reason, replacing bushing #5 with a monoball would be bad idea without also installing a monoball in the toe link (bushing #2) - the softer rear bushing would result in toe out.
I think the loads through the various arms are determined by their relative locations, meaning that the toe links share the majority of cornering loads w/ the a-arms in a 993. This in unfortunate as any slippage back there results in toe-out. The other downside is that compressing bushing #5 draws the bottom of the wheel inward, reducing camber. This is reduced somewhat by using the same, softer bushing in position #4 but not fully.
The 991 was cleverly redesigned to move the toe link above & ahead of the axle - this still allows for tuning of toe in due to cornering loads, but reduces the influence on camber.
For the same reason, replacing bushing #5 with a monoball would be bad idea without also installing a monoball in the toe link (bushing #2) - the softer rear bushing would result in toe out.
I think the loads through the various arms are determined by their relative locations, meaning that the toe links share the majority of cornering loads w/ the a-arms in a 993. This in unfortunate as any slippage back there results in toe-out. The other downside is that compressing bushing #5 draws the bottom of the wheel inward, reducing camber. This is reduced somewhat by using the same, softer bushing in position #4 but not fully.
The 991 was cleverly redesigned to move the toe link above & ahead of the axle - this still allows for tuning of toe in due to cornering loads, but reduces the influence on camber.
the instant center is found by projecting the horizontal arms to infinity, the point where they would theoretically meet is the instant center, this would then be used to find the roll center
I never heard of anyone leaving a stock rear toe arm w/ monoballs, the RS however did use the same rear toe arm as the others.
When I did the switch from a relatively new toe arm to Tarret mono-***** the change in ride characteristic was huge
10-12-2012, 05:46 PM
#33
Three Wheelin'
The blue line is the instant axis not instant center, they are 2 different things
the instant center is found by projecting the horizontal arms to infinity, the point where they would theoretically meet is the instant center, this would then be used to find the roll center
I never heard of anyone leaving a stock rear toe arm w/ monoballs, the RS however did use the same rear toe arm as the others.
When I did the switch from a relatively new toe arm to Tarret mono-***** the change in ride characteristic was huge
the instant center is found by projecting the horizontal arms to infinity, the point where they would theoretically meet is the instant center, this would then be used to find the roll center
I never heard of anyone leaving a stock rear toe arm w/ monoballs, the RS however did use the same rear toe arm as the others.
When I did the switch from a relatively new toe arm to Tarret mono-***** the change in ride characteristic was huge
10-12-2012, 05:55 PM
#34
So, where does this leave a car like mine as I have changed out my rear toe links with adjustable Rennline versions, I was under the assumption that these would help maintain my toe during hard cornering as well as add a more precise method of setting toe while eliminating the cam bolt adjustment. After the new arms where installed I felt the car had a much better feel in the corners. Should I now replace the rear arm #5 and #1 with an RS model?
The cam adjustment on the toe arm isn't the most secure, that was the another impetus for me to go to the Tarett toe links w/ locks and booties
10-12-2012, 06:08 PM
#35
The blue line is the instant axis not instant center, they are 2 different things
the instant center is found by projecting the horizontal arms to infinity, the point where they would theoretically meet is the instant center, this would then be used to find the roll center
I never heard of anyone leaving a stock rear toe arm w/ monoballs, the RS however did use the same rear toe arm as the others.
When I did the switch from a relatively new toe arm to Tarret mono-***** the change in ride characteristic was huge
the instant center is found by projecting the horizontal arms to infinity, the point where they would theoretically meet is the instant center, this would then be used to find the roll center
I never heard of anyone leaving a stock rear toe arm w/ monoballs, the RS however did use the same rear toe arm as the others.
When I did the switch from a relatively new toe arm to Tarret mono-***** the change in ride characteristic was huge
By using a mono-ball toe link, you reduce (greatly!) the compliance of that link vs. the rear a-arm bushing. The result will be an increase the amount of toe-in vs. lateral loading, stabilizing the rear of the car - assuming that's what you mean by 'ride characteristic'? Of course the stiffness will also increase & change the 'feel' of the car.. dunno.
iron, if you're happy w/ the feel of the car, I'd leave it alone & enjoy until something wears out.
10-12-2012, 06:57 PM
#36
Rennlist Member
Interesting the noise and ride seems so affected by control arm pivot monoballs in a 993.
Other cars I have had with monoballs in the pivots haven't been all that different in noise and ride.
Other cars I have had with monoballs in the pivots haven't been all that different in noise and ride.
