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It seems now with my recent attempts tuning the 09 Solstice and 77 Firebird that cars with live axles behave differently than cars with independent suspension. I'm used to tuning old Pontiac and Chevy muscle cars with live axles cuz these are the cars I like the most. I can whip out a good tune for these cars in 10 to 20 laps but when I hop into a newer car with a fully- or semi-independent set up its totally different. While tuning the Solstice GXP tonight and the 77 T/A a couple of nights ago, around the Circuit Catalunya Full, I couldn't figure out why it was handling oddly through the corners. At times it seemed sluggish and slow and that was because spring offset is kinda high with being up to .8 or over on the outside springs. However, other times it seemed like it was riding stiff because there wasn't enough offset (.65) and the car, therefore, didn't have enough grip yet to lay down the power. I've tuned several cars at this track so I know that at that point in the turn is when to start gassing but I know If I do, the car will spin. The dampers are kinda messed up too, I think I had the resistance about right but then I drove over the rumble strips and the car will spin. It was crazy, I'd never had a problem like this that stays around.

Is there another approach for independent suspension tuning or knowledge about them I should know to make the right tune? Why do the spring rates act different through certain corners? And dampers? Do the roll bars play a different part? I know there are plenty of guys here that know about tuning these so please leave a reply. Its greatly appreciated for any thoughts or observations. What can I do to solve this guys?
Do Independent suspensions handle better when loose? and what offset should they have ?

Here are some stats for the 77 Firebird:
IFS, Live Rear
Stock Pontiac 6.6
A-Class, 700 PI
Race Tire Compounds
Rollcage
3,158 lbs
476 HP
54% FWD
Spring rates (lb/in)(F/R): 665.4/615.3
Ride Height (F/R): 5.2/5.2
Anti-roll bar setting(F/R): 18.54/13.01
Rebound (F/R): 6.4/5.7
Bump (F/R): 5.4/4.6
Downforce (F/R): 90/160

and 09 Solstice:
Fully-Independent suspension
50% FWD
A Class, 700 PI
Stock Engine
Race Tire Compounds
No rollcage
2,955 lbs
436 HP
Spring Rates (lb/in)(F/R): 600.2/705 (I know they should be the same and they were but loosening the front and stiffening the rear seemed to be a good idea. Previously they were both 675 lb/in)
Ride Height F/R): 4.0/4.0
Anti-roll bar setting(F/R) : 17.97/6.49
Rebound (F/R): 7.1/7.1
Bump (F/R): 5.0/5.0
Downforce (F/R): 85/155

Huge thanks to everyone that will really help me with this, I love Pontiacs and these are some of the coolest that FM7 has.

Edited by user Tuesday, July 16, 2019 11:53:16 PM(UTC)  | Reason: Finish up post

What tire widths are you using on the Firebird and Solstice?

Around 315

You need to give me the exact tire widths for both cars.

Maybe a little more background info why I request tire widths: I'm almost certain Forza doesn't differentiate between independent suspension and solid axles at least with regard to tuning. In fact Forza does not simulate cars on an individual level but rather on car type level which means for example all classic muscle cars using the same simulated car model.

As a proof I'm gonna give you two calculated setups (thats why I need the tire widths) for the Firebird and the Solstice. The Firebird is a classic muscle car and the setup is calculated the same way as for all other muscle cars from that era. The Solstice is not a muscle car but a normal street car that shares the same car type like for example a BMW M4.

Edited by user Wednesday, July 17, 2019 2:56:44 PM(UTC)  | Reason: Not specified

The Solstice has 285 on the front and 305 on the rear, those are the max option and the Firebird has 275 mm in the front and 325 on the rear.

Does Forza use these setups for all cars in a division? Also it seems really odd that the mechanics Forza uses do not take into account what suspension type a car has. I have to test that for myself. Im going to go see right now if that's true. The camber should change for both wheels when one hits a rumble strip if it's simulating a live axle.

1977 Pontiac Firebird Trans Am: (476hp, 3158lbs, 54% wd, rollcage, race compound, 275/325 tires, Forza aero):
Tire pressure 29.0 29.0
Camber -0.8 0.0
Toe 0.0 -0.1
Caster 6.0
Anti-roll bar 22.932 17.732
Springs 774.751 435.461
Ride height 5.2 5.2
Rebound 7.3 6.7
Bump 4.9 4.5
Aero 100 200
Brakes 48% 125%
Diff acceleration 66%
Diff deceleration 35%

2009 Pontiac Solstice GXP (436hp, 2955lbs, 50% wd, race compound, 285/305 tires, Forza aero):
Tire pressure 29.0 29.0
Camber -2.3 -1.8
Toe 0.0 0.0
Caster 5.0
Anti-roll bar 20.075 20.075
Springs 699.240 599.493
Ride height 4.0 4.0
Rebound 7.7 7.4
Bump 4.7 4.5
Aero 100 200
Brakes 48% 125%
Diff acceleration 68%
Diff deceleration 35%

I haven't tested the setups myself but pretty sure they're running well provided you gave me the right data.

Thanks and ill go ahead and try it now.

Is your answer to why independent suspensions are driving so differently because of the mechanic you mentioned in your previous post?

They’re driving differently because they are completely different car types / body types.

How does that play into tuning?

The body type defines the basic characteristics of a car like chassis and suspension stiffness and suspension geometry so it affects pretty much all aspects of tuning.

Forza does not differentiate based on suspension type.