The standard FULL formula wont work on Nissan bars [or any bar which has convoluted bends].In General you just have to go with the 4th power part of the equation and assume that Nissan wouldn't exceed the rule of thumb that the bar should not be stiffer than the springs.

A bar attached dirtectly to a front strut can be a 22mm and still provide the same amount of wheel rate stiffness as a 29 mm bar attached to the midpoint of a lower arm.

On cars like the Mustang [Camaro, etc] where the spring sits midway on the lower A arm a 600 pound spring may only exert 150 pounds/inch at the wheel.....softer than the Q 166 #! Because the Mustang weighs less.

A 20 mm rear bar with a 6" 90 bend arm may be twice as stiff as a 20 mm bar with a 12" arm.

All one can do is assume the engineers got it right then softened it up a little.....why Eibach springs are only 10-17% stiffer.....there is a narrow range where the ideal is found. Floaty to harsh is only 25% depending on vehicle weight!

Assume that a 3200 # FWD car has 61% or 30.5% on each front tire thats 976 pounds and a Q has 52% or 4100x0.52/2= 1066 pounds gee a whole 90# difference STATICALLY so the wheel rates of all springs for all PASSENGER cars will vary very little!

Same with Front sway bar Wheel rate....the ratios are tied together!

When you add a rear bar you can soften the front bar!

Now rear bars are different some cars have then and some don't.My 78,81,83 Chev Wagon [police package had them] My Moms old 1952 Studebaker Land Cruiser had them. The 90-96 Q was designed to have them but they were deleted for the American market to help the ride and make the car UNDERSTEER more so buyers moving from Caddy or Lincoln wouldn't get in trouble

I put a HICAS rear bar on my 90 Q and was pleased with the results. Would a Q45A bar have worked differently and still bolted on a standard Q? I have the Eibach springs and Tokico blues.

Remember, the Q45 front bar is one mm less in diameter than standard front bar. Hence, Q45 tech recommends (and runs) the Q45a rear bar (in his case, with Eibach and Blue tokicos[rear only]).

20mm/15.9mm = 1.2578 raised to the 4th power or 2.50 times what ever the 15.9 mm bar povides.

160 pound inches vs 64 pound inches .....but both bar couple to the midpoint so divide by 4 for the weheel rate = 40 vs 16.

The formula never quite worked out so I measured the 15.9 mm bar in place with scales and rulers and screw jacks say within a 10% absolute accuracy.

With stock springs 122 pounds per inch [yielding 90% to the wheel trig angle and offset] =110 pounds per inch at the wheel.

So a t bar adds 16 to 110 or 14.5% more roll resistance.An A bar adds 40 to 110 or 36% more roll resistance.

The rear Eibach are progressive softer than normal in the first half inch, normal, then up to 20% stiffer at 2.5-4". So one has to visualize the curve.

The variable toe on the HICAS system did 85% of the work and the small bar did the rest.

The smaller [28 vs 29mm] front bar on HICAS and A cars was to reduce the impact harshness and acted like the rear bar was larger since the front to rear stiffness ratio can be changed by making the rear bigger or the front smaller or both they chose both. Make the front [13.2% bar alone - 6% total] softer and make the rear [14.5% bar plus springs] stiffer- Yields a 20% improvement in the front vs rear roll couple ratio.

You never allow the rear to be as stiff as the front in fact you never allow the ratio to exceed the weight ratio 52/48.A stock standard Q with no rear bar is 72% stiffer on the front than the rear, the addition of a 20mm rear bar bring this down to 67%, a decrease in the front bar brings it down to 63%........still along way from the 52% limit.

When you add Eibach springs the ratio goes back up since the springs are stiffer than the bar but the progressive nature of the rear springs makes it non linear in that after 2" of rear roll the stiffness starts to increase.

If you think just about springs [leave sway bars out of the equation] the stock ratio is 58% front biased so the front bar increases it as the springs compress and the bar bushings are compressed from 58% > 62%> 65%>68% >72%.......progressive increasing under steer.With a rear bar [and smaller front bar] it still understeers but 55% >57%> 59%> 61%> 63% is the new curve.

With HICAS a 20 mm rear bar is just below the safe limit without HICAS a larger rear bar could be used. On an Active car a larger rear bar could be used.

Ideally a 22 mm rear bar would be optimum for an medium experienced driver assumming the rear tires were sticky enough to hold in a side power slide.

They [Stillen subcontractor] made a 24 mm solid rear bar which required SOFT 255/275 rear tires and Eibachs/Blue Tokicos and all the adj upper links/ tension rods and some very fine alignment -in the hands of an experieced top notch driver it was very very good in the DRY!

AGAIN it all depends on the tightness of the sway bar bushings as 1/8" [total give] on each side is like the bar wasn't there !

{{{{All the above applies to dry conditions were the road tire interface exceeds 0.8 C. of F.! As when you introduce power [acceleration] to the equation, the rear tires friction is decreased [shifted to] acceleration which takes away from handling.

If the rear is spinning a slight pressure on the side will push the rear of a Q sideways as there is no lateral traction left.....same with all RWD with LSD......without LSD one tire is always not spinning [much] so it takes 10x more lateral force but only in rain can you spin both Q rear tires for any time in normal driving.}}}

The Q rear has much more handling ability [forgetting acceleration] due to the lighter weight on each tire and the fact that the tires are not trying to turn [steer] the vehicle.{{this is true because tires are not linear as you approach 66% of the rated load [1000 pounds] one pound of extra weight does not equal 1 pound of extra traction and the more weight the worse it gets....the rears start out with 80 pounds less weight per tire}}.....obvoiusly a loaded rear seat or trunk changes things...the loaded weight could be 46/54 not the unloaded 52/48 and the rears could have 50 more pounds than the front

The trick with suspension redesign is to start forcing the rear to handle more force/work, this takes load away from the front making it feel lighter and less ponderous.

With the factory pieces you can't go wrong as there still is significant understeer reserve remaining for all but the worst of drivers.

Also the 20 mm rear bar was used with the 30% softer springs of the active car so it was 33% of the total rear stiffness vs 26% with standard springs vs 27-23% with Eibachs.

For a passenger car 25-35% is ideal giving a good balance between handling improvement and ride.....the 300zx had [16/20/ and 26mm TT Racing version rear bars] up to about 52% % of its rear stiffness from the sway bar [26 mm]

In fact the similiar 97+ Q use a 21 mm rear bar with 10% softer springs [F/R] plus a rear/front strut tower brace and a softer front bar.....the shocks are softer [than even the 94 shocks] also to match the springs......in part due to decrease weight of the CIMA.

No the parts are not interchangable due to differences in rear track width [2" difference in width] and exhaust pipe routing.

In general we are not trying to limit the body sway since the suspension camber curve is built to compensate for the roll keeping the tire nearly flat out to 4" of roll what we are doing is balancing the roll couple ratio so the tires are doing equal work.

Learn to never use the brakes in a curve and always power out from the middle! But never down shift in a curve always just before it! :ylsuper

With 29 psi cold all around and a single driver the rear tires are 1-2 psi overinflated [ratiowise] but on a fully loaded Q [900 pounds of passengers and luggage] they would be 5-6 psi uninflated COLD [need 38-40 psi hot] to handle the weight just going straight. The contact patch area has to change by 30% for with 1000/30 psi=33.3 square inches vs 1250/30psi= 41.6 square inches to make the patch size identical you need 38 psi.....25% more weight 25% more psi.

Failure to correct the air pressure will change the slip angles of the tires and of course the traction margin at each end.