Rear stabilizer bar question

[Jeff Williams]

Does anyone know if the 300ZX RSB is the same dimenions as the Q45?

I was at Z1 Motorsports, and they had about 25 Z's laying around, and I noticed, on a rear suspension, that the stabilizer bar looks exactly the same as my Q's, except it was much thicker.

If it is compatible, I might be trying one out.

[Q45tech]

No! The only bars that will fit the 90-96 Q are for the Q* due to track width and exhaust differences........the J and Z bars are over 1 inch narrower and the convolutions are different.

* oem Rear 15.9mm and 20mm plus any customs that were created in past 24 mm solid Stillen.Front oem 28 and 29mm.

Not to say that you couldn't heat and bend one to fit [the 2 arms might be angled out enough] ??????? Probably easier to built the correct one from scratch.

[Jeff Williams]

Thanks Denis. I got the end links off the bar. I was skeptical of the size, and I did not have my Q with me (Greg's old bar is in the trunk). I did not want to spend money on something I might not use.

I am getting the new bushings and new bolts (didn't think about them, when I got the end links) from Joe.

I hope the ned links are the same height. If they are, I saved about $80, in parts.

[oxs1]

I am also interested in the rear sway bar. How much will it cost and where can i find it?

[Q45tech]

The forum sponsors have the oem 20 mm rear bar at a good price or find a junk Q45active car. Be careful the Q45t has the 15.9mm rear bar which is unfeelable.......in that it changes things by less than 10% [12 pounds per inch] compared to the 20mm [40 pounds per inch].

[oxs1]

How much $ do they go for?

[rsiwicki]

Joe sells a kit for about $250 that includes everything. I have the "t" model and I just bought the "a" model bar. If you want my old bar you can have it, but I still think that you will need the brackets and other items as I just bought the replacement bar and rubber things.

[AZhitman]

Rob - That's very cool of you. I just did the same - I got Fred's "a" bar, and sent Jeff my "t" bar.

[elwesso]

So basically the T bar on a standard Q will do nothing???

[rsiwicki]

As per Q45tech....yes that is correct and you will need to get the front sway bar as well for the rear sway bar to work. Q45tech did a good write up on this a couple of weeks ago that explains what you need to notice any benefit.

[VimyJ]

As per Q45tech....yes that is correct and you will need to get the front sway bar as well for the rear sway bar to work. Q45tech did a good write up on this a couple of weeks ago that explains what you need to notice any benefit.

As I understand sway bars, they don't have to be matched per se. Often, race cars will have different thicknesses of sway bar from stiffer up front to softer on the rear. The purpose of this type of set up is to reduce vehicle sway maximally on the front while keeping traction maximized on the rear for acceleration and to limit oversteer (to keep the back end coming from coming loose due to too much vibration transfered from one rear tire to another). Of course, I'm talking race set ups in this case not passenger cars.

[rsiwicki]

vimmj.....you are correct about the front versus rear sway bar being of different sizes. I don't know right off the top of my head what size the front is on the Q....I just remember that Q45tech saying you need both both bars (different sizes each) to make it work properly and efficiently.

[Jeff Williams]

All suspension modifications are subject to the "seat of the pants" feeling of the driver.

Some people like a loose car, while others like a car that pushes a bit.

I happen to like my car a little on the loose side.

What this means to me, is that I want the rear to get a little more bite, and a little less roll.

This will induce understeer, on occasion.

With the new struts, and links, and the front stabilizer bar in place, I want a little more stiffness, in the rear. Not a lot, just a little. I feel the car is near perfect, for my style of driving. A 6% or 10% increas in rear stiffness, or decrease in bldy roll might just be the ticket for me.

My fist step, is the smaller "t" bar, on the rear. If I don't like it, I will upgrade to the "a" bar, front & rear.

I bought the end links, for $10, the bar cost me $13 for AZhitman to ship it to me, and the bolts cost me about $5. It took a little over 30 minutes to install, and that included removing the rear tires, and all the exhaust brackets, rear of the front doors, to get it on.

I will do some tests, on my roads that I drive on, this week, and let you guys know what I think.

[Rex]

As per Q45tech....yes that is correct and you will need to get the front sway bar as well for the rear sway bar to work. Q45tech did a good write up on this a couple of weeks ago that explains what you need to notice any benefit.

Here's some of Q45Tech's post about the bars, sway bars that is:

Springs and Bars

More Bars

Best one for last

Happy reading

[maxnix]

Q45tech from my archives. The links are still valid.

True a bar's torsional stiffness is a 4th power function of diameter [assuming the MODULUS is the same] [ NOT THE SQUARE AS YOU MENTIONED} but in the Q/J case the bar attachments at the midpoint so it just happens to be a 1/4 reduction from bar stiffness to wheel stiffness. The sway bar if calculated in isolation is 4 times stiffer [the front bar is 664 lb/in but for every 1" of wheel movement the bar only twist 1/4"] so we say the bar's effective wheel rate is 166 lb/in. People skip over this lots of times and novices hear 600 pound springs/bar and wonder why their car has only 150 pound springs/bar. A 10-20% change can be dramatic depending on how close the designers got in the first place see below: http://www.rqriley.com/suspensn.html http://www.eibach.com/ERS_Worksheet.htm

Another thing missed is the angle from vertical must be corrected by the Cosine x spring rate.

One of the things that owners miss is that the way the sway bars are coupled to the lower arm [in the middle] results in a 1/4 coupling ratio. Slack in the bushings is created by limiting the ability to tighten the assembly nuts so the bars essentially are not there for the first 1-1.5" of wheel travel while the 1/4-3/8" of total slack is taken up.Placing a metal washer [or two] between the bottom nut [on the front] and bottom and top nut [on the rear] and the bushing retainer allows the bushing to be preloaded.This makes the bar act immediately vs the OEM design......effectively doubling [or any amount of adjustment in between] the bar stiffness at the full 4" of wheel travel.Obviously getting the bar too closely coupled will affect harshness so you must make FINE FINE adjustments in 1/4 turn of the nut increments [you will feel the resistance to tightening]! Always set the bar preload with you or your weight in the drivers seat. It will only be correct at one weight load but we are speaking of very fine PRECISION adjustments.When everything is correct [all new bushings and parts] the system can be set so that you can feel a major difference in real roll couple between a full and 1/4 tank of gasoline [this is a 90 pound change]. The Q like magic gets transformed.The stock front 29 mm bar is roughly equal to the 166 pound in springs.The A [20 mm] rear bar adds roughly 40 pounds of additional stiffness to the rear 120 pound springs. IRS systems have a bit of extra rear roll due to the higher roll center and you should not attempt to remove all of it as the toe compensation design [under deceleration/braking] will be compromised.

When you have a 53/47 weight balance [on a 4350 lb car] the correct term is the larger rear bar would make the transient handling slightly less understeering.Oversteering only occurs when the front wheel C of F exceeds the rear.Only when the rear stiffness exceeds the front or you exceed the C of F of the rear tires by accelerating in water [while trying to turn at the same time] can oversteer occur.The roll stiffness of the front is roughly 250 in/lb vs rear Q45t 135 in/lb or 165 in/lb with a 20 mm bar quite a ways from 235 lb/in which would be the neutral balance point.Even the solid 24 mm Stillen Bar [no longer available] just [exceeds]/equals this neutral point [by 15 lbs]. It was assumed that the bar would be used as part of a redesign which included 20 pound stiffer rear springs so the neutral point wouldn't be exceeded!The front of a Q has the ability to reach 0.78G while the rear can get to 0.92 G [due to less weight on the rear tires [slope of C of F vs slip angle vs weight on tire].....the 120 pounds lighter load on each rear static plus the almost 100 pound lower corning load ~~~~ 220 lbs.

The safe [dry] rule of thumb is never install a rear sway bar equal to or > the rear spring stiffness: and a 20 mm bar is only 34% of spring if perfectly coupled with solid metal bushings...the more typical rubber OEM bushings slack will reduce the 1st inch [of wheel/body travel] to almost no added stiffness.....then the bar will kick in progressively. So under a fully side to side swing [-3 to zero to +3] the 20 mm might get close to 85/366 or 23% total stiffness increase. {ON A GOOD DAY}

Story of interest: http://www.autospeed.com/A_0477/page1.html One important point to note, one which is often misunderstood, is that regardless of the amount of roll allowed by the suspension design, the actual degree of weight transfer remains unchanged. This is only affected by the track, C of G height and cornering acceleration. So, as with most design features in anything mechanical, the selection of roll axis position is a compromise: too low and we get excessive roll, too high and other undesirable handling traits surface. In practice, the com- promise varies with different types of car but always such that some roll occurs. Lowering the C of G is another technically possible way of reducing the roll couple, but this can only be done to a certain extent, due to the boring necessity of leaving comfortable space for the occupants.The roll couple that such compromise leaves must be resisted by the car's springs, which leads us to roll stiffness. This term is defined such that the degree of roll is equal to the roll couple divided by the roll stiffness. Stiff springing obviously reduces roll and hence increases the roll stiffness, but if this is the criterion for selecting spring rates we will usually end up with an uncomfortable ride over normal road irregularities, so the anti-roll bar was developed to ease the situation. The anti-roll bar is a torsion bar (torsion spring) connecting the suspension systems on each side of the vehicle in such a way as to allow both wheels to respond unhindered to two-wheel bumps, such as a ridge across the road. But if the wheels try to move independently, as with a single-wheel bump, or in opposite directions when the car rolls then the anti-roll bar resists this tendency. Roll is reduced as intended but comfort suffers as the effective spring rate of each wheel is increased in the individual single-wheel bumps, although the combined spring rate of the two wheels is unchanged over joint disturbances. Again a compromise must be reached between the requirements of minimum roll and good response to road shocks. Roll bars, unlike the springs, are undamped (theoretically damping could be incorporated, although the manufacturers have generally concluded that it is not worthwhile). This is another reason for limiting the influence of the anti-roll bar, as oscillations might occur if the undamped bar is too stiff. It is well known that the under/over steering characteristics of a car can be substantially modified by tuning the springing and anti-roll bar stiffnesses, altering the roll stiffnesses of each end. While the vehicle as a whole has a certain roll stiffness, this is made up of the separate roll stiffnesses at the front and back, which may be quite different. For example, let's consider the case of a beam axle pivoted on the chassis at its mid point, and devoid of any form of springing. As unlikely as this layout seems, you may see it fitted to the front of some tractors, because it has good terrain-following properties. Now, because the chassis is completely free to rotate about the pivot point of this beam axle, then no roll stiffness is provided at this end of the machine and so all the stiffness needed must be available from the other end. This lack of any roll stiffness means that body roll cannot cause any weight transfer to the outside wheel, and hence as the total weight transfer must be the same anyway, the other end must obviously be subjected to proportionally more. Tyres have the interesting property that although they are capable of supporting higher cornering forces when subject to higher vertical loading, this does not go up in proportion. In other words, the co-efficient of friction is reduced as more weight is placed on them. In practice this means that weight transfer reduces the combined cornering force capable of being developed by the pair of tyres at one end of the car. Now as we have seen, the weight transfer at either end of the vehicle can be controlled to some extent by altering the roll stiffnesses of one or other, or both ends. Therefore, the tyre slip angles needed to produce the required cornering forces can be adjusted by modifications to the wheel springing, thus giving us the means to alter the under/over steering properties. Dampers, too, have their part to play in the extremely complex interrelations ....."" http://www.ctv.es/USERS/softte...g.htm

Since the midpoint of lower arm where the bars attachometer only moves [1/4] the total vertical is a max of about 1.75" on a lift with the wheels drooping about 1".

[Jeff Williams]

Iget from this, that a 10 - 20% change in roll stiffness can be dramatic.

I also understand, from other posts, that the "t" bar only makes 6 - 10% difference. And, adding washers will increase the "quickness" the bar comes into effect.

So, it seems, that the "t" bar might be just what I wanted, after all.

Right now, I have the bar, with old bushings, and no spacers. I have the new busings fro the front, but will be getting the new bushings for the rear, soon, from Joe.

I will replace the bushings, then do some testing, then try the washer/spacer idea.

Thanks!

[VimyJ]

Essentially, adding sway bars to a suspension is, in effect, adding an additional set of springs that can amplify road irregularities by transfering shock to the unaffected wheel. The greatest suspension asset of a J or Q is the independence of thier four wheels to react individually to road "noise" which translates to maximum traction or, put another way, optimizes tire contact with the road for increased control compared to a beam type axel.

I was very pleasently surprised the first time I encountered a "washboard" road section in my J. In my previous cars with a beam axel type of suspension, you are at the mercy of "an object in motion tends to stay in motion" because the tires are bouncing all over the place as the solid axel is transferring vibration from one side to the other. With the four wheel independent suspension, control is much greater because the vibrations are confined to the individual wheels. Too stiff a sway bar and this advantage is compromised. This is, no doubt, why there must be some "slack" provided through the sway bar bushing and/or the preload so the wheels retain their independence.

Tire sidewall strength, spring rate, center of gravity and anti roll bars not to mention strut damping and rebound charateristics makes me hestitant to mess with the Nissan boys solutions as my car is driven "normally" 96.5% of the time.

[maxnix]

Tire sidewall strength, spring rate, center of gravity and anti roll bars not to mention strut damping and rebound charateristics makes me hestitant to mess with the Nissan boys solutions as my car is driven "normally" 96.5% of the time.

This is something Q45tech discusses also, but which I omitted for brevity, or the quotes would have gone on forever (thank you Dennis).

VimyJ's statement and conclusion emphasizes that suspensions are engineered as a system, and the consequences of component changes should be carefully considered, and better yet tested, before they are implemented. We are fortunate in that Dennis has done a lot of testing with various spring rates, swaybar combinations, and bushings. We are much more knowledgeable due to his expenditure of personal funds and time.__________________Brian1995 Q45 & Q45t & 2000 Q45

Discover the power of the button!

[VimyJ]

We are fortunate in that Dennis has done a lot of testing with various spring rates, swaybar combinations, and bushings. We are much more knowledgeable due to his expenditure of personal funds and time.

I concur absolutely and without reservation! Not to mention all the other contributions to "Zen and the Art of Infiniti Maintenance" he has made to NICO.

[Q45tech]

Well Nissan's compromises CANNOT make everyone happy.Why they make Eibach springs and some sway bar choices.

Just be careful about stiffening the rear suspension more than 50% TOTAL: Eibachs and 20mm bar [rigidly coupled with washers compressing the bushings or urethane].........new blue Tokicos can barely keep up with this combo.