Personally, I had the car setup with the Hotchkis bars at full stiff front and full soft med (I think it was like 63% stiffer front and 58% stiffer rear). I also had 275 tires all around to help reduce understeer (that is why I went with the settings I had). The car understeered like crazy with this setup. I then just tightened up the rear bar one more hole and it seeme dto have solved the problem (with some air pressure changes which I will say did most of the difference in understeer). I would consider loosening up the front mar to its middle setting to give me M/M setup. I would think that would help a little more, but it might become too oversteer prone with the 275's all around. With the stock 225/245 combo, it would probably work fine....

http://usastreetracing.com/images/vi...aos2teaser.wmv



DOWNLOAD THE TEASER VIDEO BY CLICKING ON THE LINK

 

Q45tech

This is great info. Could you explain how you're arriving at the numbers posted? For me to fully understand this, I need to see equations / calculations and such. Thanks!

04.5 G35C TB/Graphite 6 speed, XM Radio, Premo Package, Navi, OEM Chin Spoiler, Hard wired V1

 

This shouldn't be so hard. The vast majority of us simply drive (hard) on the streets and want to know what combination of suspension mods would give us a noticeable increase in performance for a car that's already a great handler. My unmodded (suspension) coupe can stay with modded coupes (in our club) on the mountain twisties if I drive well. But, I still want a better handling car. We should know plenty about how to do that, yet this discussion leaves me with only two options:
1. Do nothing for fear of going backwards.
2. Enlist Q45Tech to set up my car (not going to happen)

So, other than dissuading me from installing a swaybar (although the suggestion to add a rear swaybar seems to make the most bang for the buck at this point), this discussion seems a bit pointless. Help me with some practical answers to what should be a fairly simple problem--independent from appearance, how do you find a cost-effective way to improve G35 street handling? For those who want a track response and want to consult their pit crew after each lap, perhaps we should start another thread for those boys . . .

Thanks.

 

This isn't really that hard. Our cars do understeer from the factory for obvious safety reasons but compared to many other cars, the G35 understeers a lot, lot less. Yes, in stock form the G35 already handles great. Can it be made better? Without a doubt. Any of the swaybars out there right now for the G35/350Z will make the handling better. It just depends on how much better and what the end goal is. I personally like to track my car whenever I can and with the advice of a professional suspension guru I know, I was explainded to that in order to get rid of the understeer as much as possible, getting only a rear sway bar is the way to go. Adding a thicker front sway bar may make the car corner flatter but will give a false sense of better handling. In a hard cornering situation the thicker front bar may lift the inside front tire and will lose precious traction at the time when you need it most. And that makes sense since getting a thicker front sway bar will only add more understeer as well. Obviously this is in a context of a "track" like situation so it may not apply to the person who just wants a better handling car for the streets. So to make your car handle more towards the neutral side there are two things you can do regarding sway bars from what I can see. Just depends on how much understeer you want to dial out and what you like to do with your car.

1. Buy stiffer rear sway bar only.
or
2. Buy stiffer front and rear sway bars. Set rear bar stiffer than front.

04' G35S 6MT Ivory Pearl, Willow, Premium
Z-Tube / NFC Y-Pipe / Custom Mid-Pipe / 350 EVO Rear Sway/ '03 350Z Shocks and Springs

 

When you know weight distribution and real weight, you start like this. 4300 pounds with 54/46 ratio therefore the springs should be at that ratio. The front stiffness should be equal to the desired ride frequency in Hertz.
http://www.rqriley.com/suspensn.html

NF= 188 / the square root of SD all divided by 60 to convert to Hertz.

188/SQRT 8" body compression of spring= 1.108 Hertz then what spring stiffness will compress the spring 8" with what body weight on it. 4300 x 0.54=2322/2=1161 less unsprung weight = say 1000 pound/8"=125 pound/inch spring.

Shock absorbers have a 7" stroke so to fully use them and correct for mount angle of them and spring ~~1000/7"= 143 pounds per inch for springs with and 8" compression under static load of 1000 pounds with a desired 1.1 Hertz natural frequency.
The springs and their rubber mounts and the tires side walls are in series mathematically.
http://64.233.161.104/search?q=cache...tiffness&hl=en

http://www.informs-cs.org/wsc00papers/135.PDF

To limit body roll you use a sway bar instead of a stiffer spring which would require stiffer shocks to control to make the street ride tolerable over bumps.......not necessary on a race course that has no bumps or minor bumps.

The rule of thumb is a sway bar should never be more than equal to the springs and the mechanical couple [bushing compliance] should net out as <50% of the spring stiffness.

Nissans set the springs correctly [54/46] for weight distribution and a stiffness that will not bottom/stretch the shocks and cause them to explode via bump stops and spring bind.

They must consider both a single light weight driver [90 pounds- 400 pounds] and a fully and somewhat over loaded car full of fat people with luggage.........this may be a 1200 pound difference. Luckily you can only put so much weight in the front seats and it is equally divided with the rear springs.
Most cars set a much lower max load [~~900 pounds including 100 pounds of gasoline]

So the static front 1080 [nothing in a4000 pound car] to 2 x 300 pounders = 600 x.5=300/2= 150 extra per front spring ......sags an inch plus [146/inch springs].

FRONT Sway bars are losely on almost non coupled in the 1st inch [of body sway] to allow a passenger or not.

Nissan uses a motion ratio on some non McPearson strut cars [old Q/G, others] that means the bar moves 1/4 of the wheel rate [the wheel moves up an inch the bar couple point moves 1/4" and then there are rubber bushings to compress.

This allows the bar to be invisible in 1" bumps and then progressively start restricting body roll as the bushings compress. So from 1-2" only maybe half the bar stiffness is used.......actually gets coupled to the wheel, then from 2-3" the bar is extering its actual rated value [0+ 0.5 +1]/3"=0.5 of stated stiffness over the 3".

See why I say the bar is really half its stated stiffness in installed state!

This fixed [springs at stated rate] and progressive stiffness from front bar] acts to go from a neutral steering to a progress understeering as the body roll increases.......self protective.
You feel the under steer and know the tires are getting overloaded.
146> 220>292 lb/in or 146+220+292=658 lb in full 3" of body roll instead 146 x 3= 438 lb without the front sway bar.

Typically the front body doesnt roll the full 3" because the combo is stiffer than the roll force at least when the bushings are new and not cracked or deformed.

The rear with softer springs and weak bar or no bar [in early Q45 without active or t option] rolled like a mother especially with a rear passenger/trunk load........great warning that you were driving too agressively for the tires.

On cars with McPearson struts a softer [longer right angled bar arm] is used and the progression is less pronounced and they use a isolated front subframe to limit bump hashness in the first inch [and a few rubber bar bushings]. As the McPearson has little or no camber gain the tires roll under under lateral forces [camber goes positive in roll] which creates the understeer! instead of the nonprogressive front bar.

Calculate the roll stiffness and how much weight is transfered to the wheels in side to side transistions.

 

Hopefully you see it is not that complicated to calculate the correct rear roll stiffness as I did with the front earlier especially without a complicated spring change.

The hard part is deciding where you set it [just you as driver or with passengers and and what fuel level and what tire temperature].

Always err on the weakest REAR bar that will do the job to protect against rain.

I think where some are missing the point is that a 50%stiffer bar sounds big but remember it is the total rear stiffness [springs plus bar]. If the rear bar starts out at 60 and the springs are 120, a 50% stiffer 60 bar is only 90 and added to 120 is 180> 210 or just 16% totally stiffer
The ratio of the front to rear will only shift rearward by alot less since the front is greater than the rear to begin with.

For ease - think - half or less [usually a lot less] than the 16% above.........so a 50% stiffer rear bar is not that dangerous and shifting ratio 8% more like 7%- 6% rearward is still in the fine tuning range.

I am guessing at the OEM rear bars stiffness from looking at pictures and measuring lots of other Nissan bars on a jig, But it is no more than 80 and probably 60-70 lb/in. So I could be off a few % say worse case 8% going down to 5% so the understeer/oversteer ratio might have a variance of 3%.

Progressive rear springs are much more dangerous as their ultimate rate may increase 50-65% [without considering the bar progressiveness as the rubber bushings compress].

Progressive springs may change the rear double what a very stiff bar does.

This amount may be beyond the normal drivers ability to compensate........luckily the stiffer rear bar limits the body roll so the the ultimate progressions rarely occur.

Progressives [Eibach and H&R] springs should always be used with a stiffer than oem rear bar to limit the progressiveness of the springs.....limit body roll to 2.25" rear from oem 3.5".........somehow the combo of the rear springs and bar should be 50% stiffer than oem and the front bar made stiffer the 5-10% to help cancel the oversteer probablity.

See why aftermarket front bar makes the first hole 5-10% stiff because they guess buyers of sway bars might also be buyers of lowering progressive rear springs. The 10-15% stiffer front lowering springs with the 5-10% stiffer front bar position nets out as a 10% stiffer front which reduces the rear stiffness ratio gain from the progressive springs.

All this gets confusing when dealing with percentages without actual PRECISE start numbers.

 

Q45tech, great stuff!

It's been a while since I've cracked open a physics/statics/dynamics book (for the theory behind this) so I may have to crunch some numbers to get a good grasp on this. Great explanation though.

04.5 G35C TB/Graphite 6 speed, XM Radio, Premo Package, Navi, OEM Chin Spoiler, Hard wired V1

 

A problem we need to be concerned about is improper adjustment causing a pre-load on the bar. The pre-load is caused by not having the bars ends neutrally adjusted when making the change. Pre-load in sway bars generates extremely differing handling characteristics between left or right turns (especially at corner entry phase). Depending on which way the bar is out of adjustment, it (the bar AND the car) will think it is already leaning into the turn so it will twist the bar more, or it will be starting with a "twist", then go to neutral bar load, then go to twist. You can see how the amount of load transfer for a left vs. right turn of the same speed, radius, degree of lean, conditions, etc. would be very different. The solution is simply to be sure the bar(s) have no pre-load on level ground, with the driver

(or someone/something of same weight) in the seat.

As when you [the driver sits in the car ] the left springs compress more than the right, the body leans, and the sway bar gets torqued...............1/2" [body drop] might mean 40 pounds of preload on the rear and more on the front.

You find washers to shim the endlinks [calculate 1/4 of the body drop ~~ 0.05- 0.125" of slack to adjust on the rear bar link..........critical when you use something non giving like urethane bushings............rubber is somewhat auto adjusting due to slop in the first part of compression.......why oem uses rubber!


It's all the little fine adjustments that can make the difference compared with the typical lowered [slammed] car where things are just slapped on without any thought.

 

Unfortunately we don't have a rear sway bar complex adjuster to correct for the use of gasoline and the rear weight reduction that occurs [20 gallons ~~ 125 pounds] as Nascar and other racers do. 125/1700= 7.4% pretty significant in the rear tire friction from weight reduction on the tires, that is roughly 1/3 of the transfer weight in a side to side slalom [rearCG/ rear track width multiplied by rear sprung weight corrected for G load [0.8-0.9G].

Do you set the rear bar for an empty , half , or full tank of gasoline.........HALF: understeer, going to neutral, going to oversteer as the rear end gets lighter.

Hopefully you see why factory way under rear bars their cars and leaves it up to owners to set the ratio to please themselves.

Less than 1 in 50,000 cars [educated guess] on the road have modified rear sway bars.

Has the aftermarket industry as a whole sold 50,000 rear sway bars in the last 8 years for all brands of cars.

I know that the entire production run for a 24 mm rear bar for use on a 90-96 Q45 was 50 units [back in 1993] and they took 5 years to sell out. 80,000 Q sold and 50 bars made.
We see multi hundreds of Q annually and I've seen 1 bar plus my own.

Obviously the G35 has attracted more attention and when 100 sets are sold the ratio will be 1 in 1000.
Every sway bar manufacturer dreams of creating a "craze" and selling a 1000 units of a single design.........wow $125,000 of gross wholesale revenue, you can go broke in the mean time.

Buy the least expensive adjustable you can find!