5 years later, I'm here to put the final nail in the beam suffering coffin, I am no longer beam suffering and no I didn't buy a new car! I finally built the lowered roll center panhard bar setup, and it's been glorious! I noticed an improvement on the street, but it really didn't shine until I went autocrossing last weekend. The rear finally would come around without hesitation, and the car had no issues rotating, the rear felt less on rails and more like my 05 Legacy GT (which has multi-link IRS).
Simply put, Nissan engineered way too much grip into the rear end, which is why these cars are so bad in racing applications.
Before I get to panhard bar, here is some back story and mods I've tried: I bought all the parts for this built in 2013, but when I decided to sell this car I gave up trying to install it. After selling my autox B15 in 2015, I started racing the G20 again and beam suffering came back (for what ever reason B15 was never this bad, it's beam design is slightly different though).
First attempt at beam fixing was
Bending the beam: seemed to work at, but overtime it sprung back, I think my wheel bearings are shot and are wobbling (even though they pass all the inspections and movement tests).
Heim SRL Control Rod
This little mod helped reduce bushing binding found in stock control rod, and the difference was noticable on the street at low speeds but while autocrossing didn't help much at all. Like stock, it would still bind at full droop and jounce (causing the infamous tripodding).
Since it's adjustable you might be temped to use it to reduce the beam shift, but you quickly run out of adjustment room and the squishy bushing in the end starts binding. Which means almost infinite wheel rates on one side.
SRL Panhard Bar
I replaced the squishy bushing with a solid one and got rid of the control rod. This effectively made it into a 23.5" panhard bar.
Some will say that's too short and will cause beam shift, while true Nissan's SRL fails at being a straight line mechanism anyway because we get beam shift when we lower the car still. I built little models and tried to draw straight lines, and Nissan's SRL design could never draw a straight line.
Actually making it have a solid bushing with control rod drew the straightest line, but it would bind halfway through travel, which was expected!
This made the car handle a little better, it made it a lot more consistent between left handed and right handed turns at least. Rear end would still rise up on hard turns, and it still had issues rotating, but this was the best setup so far.
ES B15 Beam trailing Arm Bushings
Up until this point I've been running B14 ES beam trailing arm bushings. I was always concerned with them me being able to push them in by hand (aka they were too loose for P11's bushing sleeves). Going to B15 bushings tightened up the rear end slightly, mostly noticeable in slaloms, probably didn't help with reducing under steer though
Rear Swaybar
At this point I started making the rear end even stiffer, I added back the Addco Rear Swaybar (which has been off since before coilovers). With stiff rear springs (10ks), I didn't expect it to help at all, but to my surprise the rear end started walking out on me but only on lift off oversteer on the street. Unfortunately it didn't help that much auto-crossing.
At this point my Cusco -1* camber UCA bearings took a crap, so I installed the stock upper control arms giving me -0.5* static front camber. The car basically went back to square one,
the Gulstrand mod on UCA brackets did help a good bit, it made the camber curve be more aggressive.
5K Front Springs, 10k Rear Springs
With the car back to being an understeering pig I decided to make the front even softer, I had 5k SWIFT springs laying around the garage, so went ahead and installed them. This helped increase front grip (mostly in straight line), and on my 15" all seasons I could get into a 4 wheel slide, that's a first! I guess that's to be expected with 56% stiffer rear!
Installing my 225/45/17 summer tires did give the rear end too much grip, 4 wheel drifts were no more. To make up for that I tried much stiffer tire pressures in the back, I did 34psi front 40psi rear. During my first auto-x lap I almost spun the car with lift off oversteer on a long high speed turn. Reducing the rear tire pressure to 38psi solved that, but the front grip was still lacking due to tires being overloaded.
Lowered Roll Center Panhard bar and no rear swaybar
Finally, getting to the star of the show... After that autocross event, while I was happy the car finally got a little tail happy, rotation was still not there when you wanted it, it was only there when you don't want it. I finally forced myself to spend a week welding the rear panhard bar from 2013.
I utilized the stock body mount location to lower the panhard bar attachment
Then reinforced it:
Beam mounting point is fully adjustable:
Final product looks OEM
This lowered the rear roll center to be about 7" off the ground on my 225/45/17 tires, where before it was close to 12". Lowering the roll center helps reduce the roll stiffness and thus rear grip. This is why we have to resort to drastic measures like 5k stiffer rear springs AND fully stiff 24mm swaybars just to get the rear end to give up a little grip. It also doesn't help that we have non adjustable -1.5* rear camber.
As you can see the rear swaybar had to be removed, but even with it off I could still feel the difference on the street. The rear end no longer wanted to rise up like it would before, and while the front would still slide at the limits, it was a different feeling slide (like maybe the front is not getting load with enough weight right now). No autocrossing was done on this precise setup yet.
8K Rear Springs
10k rear springs were getting a bit too stiff for daily driving. I had the 8k springs left over from the front still, so while I was replacing the FA500's lower mounts with an updated version (with slits to prevent beam contact), I installed the 8k springs. This made the rear wheel rates be 45% stiffer then the front (instead of 56%). There might have been a slight shift to less tail happiness on the street.
After adjusting the shock rebound/compression stiffness (had to increased it way up for front and rear) the car was actually really stable in side to side movements. This is the setup I went autocrossing on last weekend, what surprised me the most is, even without a rear swaybar, and with 2k softer rear springs the car had major rotation! It was a little too tail happy still on lift off with 32f/38r psi's.
Conclusion
I'm finally to the point where the rear end is no longer fighting me, and it's getting to the point where I might need to make it be softer then the front, especially once I gain some front camber back.
I hope you enjoyed my adventure, I plan on doing a "How to make your P11 not suck at autocrossing" guide soon where I'll include my fun adventures with front suspension modifications too but for now Beam suffering is no more!
Originally Posted by Vadim 
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I found the same thing when I did my panhard bar. No longer was I trying to setup the rear, as the rear is doing its job better than the front. I am getting some new tires soon which I think is one of the last key components to setting up my car. car is turning flat, and I mean flat... just the tires are spent.
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