Handling

[cygnusx1]

Last weekend I raced on a four man team 6 hour enduro Kart-race. I became spoiled by the handling of those Birel carts. Compared to the karts, my Z pushes. Many folks have told me that my Z's front bar is too large and I now feel the same way. Is there a way to "loosen" the bar by grinding a flat along it's length until I acheive the desired effect? or is it easier to just buy and try? Is there another way to reduce push asides from sway bars and spring rates? Tire pressures maybe. I don't have camber adjustment. My tires are wearing PERFECTLY even all around right now in mixed curvy road and track day driving.

 

BTW we placed 5th on 12 teams in the enduro. It was our first time out. What a blast!!!!

[johnc]

Karts are a hoot! Recently did a shifter kart day at WSIR's kart track. 105 degrees F and the 40 lap main just about killed me. When I came by start finish and was shown the crossed flags (signifying halfway) I just about pulled off right there. But I kept going and finished mid-pack in the "Master's" (old fart) class. I couldn't get out of the kart for 5 minutes after the main - I was whipped.

 

Regarding handling, don't mess with your anti-roll bar. It will just fail. Front camber is critical to reducing understeer on the 240Z. I also helps turn-in. Tire sizes are also import (equal size on all 4 corners) along with driving style.

[katman]

What you do also depends on when and where it pushes. Turn in, mid corner, high speed versus low speed corners, after power application, etc.? JohnC is right though, if you have even wear for street driving then you're probably about 2+ degrees short on negative camber (and a smidge of toe out) to get the front to bite for real racing.

[Guest Z Draci]

Why not get adjustable endlinks?

 

I've driven many cars and I think the front end can be improved a lot on a 240Z. It mostly has to do with weight distribution--the long bonnet and long inline engine makes the front feel heavier than it really is. I actually tried to load up my rear hatch with heavy items to see if it make a difference. It actually did "lighten up" the front and made turn-in less understeery. Like all cars with a long front end and a short rear hatch, it likes to understeer on turn-in but oversteer like a so-and-so on exit.

 

Another thing I've noticed is that the front end flexes a LOT. Putting in some chassis braces should help a great deal for a crisper turn-in. I think the PDK Fabrication front strut bar setup would do wonders for the 240Z. (I haven't ordered one yet so can't say for sure)

 

I think dampers are more important than springs or sway bars. They react first before your springs. I have the Tokico Illuminas on my car and I'm a bit disappointed. I think they were designed more for smooth track surfaces than the bumpy public roads. Their bound is way too stiff and rebound rate is not high enough for street use. If I wanted perfection, I'd have a custom setup made so that these things can be adjusted for your car's usage. I should also note that an old 240Z chassis doesn't like the added stress from stiffer dampers. I can already imagine stress fractures occuring . . .

 

Toeing in on the front will cure some turn-in understeer problems. Toe in too much and it'll snap oversteer. You can compensate for this by setting for understeer in the rear to do power-on exits. (like the Porsche 911's)

[cygnusx1]

It turns in crisp but in mid corner it wants to push either on or off the throttle. Of course snapping off the throttle brings the nose back in a bit but it seems a little pushy otherwise. I have no camber adjusters but I have the car fairly stiffly sprung, sway bar'd, and bushing'd so it stays real flat in the corners. I tweaked the Illuminas to 2-front and 3-rear to help but it wasnt enough to rid the mid corner push. How does tire pressure and toe play into this...which way should I go with at least those settings for track days?

 

Not specific #'s, but more tire pressure front or rear does what?

 

Thanks for the input.

[JMortensen]

Lower pressure = more grip.

Lower pressure = higher tire temps

Lower pressure = more tire rollover

 

Toe out front = better turn in

Toe in front = more resistance to following ruts in the road

 

Toe out rear = scary

Toe in rear = more stability straight line and trail braking

 

Ideally what you want is not a lot of tire pressure so that the tire can get hot and sticky (without overheating). The problem is that when you don't have enough camber you have to run higher pressure. If you run the pressure too low for the camber then you'll roll the tire onto the sidewall. You really HAVE to have the camber in order to lower the pressure.

 

The sucky thing about Z's is that you can only adjust front toe from the factory. I guess you can shim in a tiny bit of caster, but that's pretty much the extent of it.

 

I agree with John and Keith that you probably need another 2º of neg camber. A couple more degrees caster would definitely help too. Toe out the front, then if it's tail happy toe in the rear. Of course you're looking at some pretty major mods to do any of that...

[260DET]

Its a bit strange if your tyres are wearing evenly yet you are getting significant understeer, usually the outer edge of the tyre will suffer if its a camber or caster problem.

 

Anyway, radial R tyres need a fair bit of camber to work properly, 3.5 degrees front 1.5 rear works for me, see clicky pic.

 

[johnc]

It mostly has to do with weight distribution--the long bonnet and long inline engine makes the front feel heavier than it really is. I actually tried to load up my rear hatch with heavy items to see if it make a difference. It actually did "lighten up" the front and made turn-in less understeery.

 

You're joking right? Perceived weight distribution (not actual weight distribution) is the cause of understeer and adding weight to the rear of the car to correct the "perceived weight distribution" fixes it?

[datsunlover]

Hmm.. you, that sounds right. Didn't someone here say that the Zs , along with most vehicles, are built to intentionaly 'push' when driven to the limit? (Understeer instead of oversteer) Really, that fact saved my arse once.. Back when I had my first car (81 280zx) I came around a left hand 'curve' on a backroad, driving hard having fun. Suddenly, the car wasnt turning anymore.. I turned the wheel more, and nothng changed.. wtf!? So I lept off the throttle but before I could even get on the brakes the car got back in line. (I actually had to turn the wheel back a bit, to correct for the amount I had turned while sliding)

 

IMO adding weight to the rear would only make the situation WORSE though.. I overloaded my truck one day and coming up the highway was a real rolercoaster.. Front end was light as a feather and very twitchy. Heck, I drove the Z the other night basicaly on fumes, and after puting a little over half a tank of fuel in it I noticed a big change in the way it drove.. not as nice..

[cygnusx1]

Jon,

Thanks for all that very useful info regarding pressures, and angles. That is what I was looking for. Eventually I may go to camber plates and/or adjustable cotrol arms but for now I may try shimming some caster in and toeing out the front a smidge.

 

260DET,

On the track I definately heat the outside edges faster than the rest of the tire but for the most part (99.9%) I am on country backroads roads, so they wear evenly overall.

 

 

It's tough for me to justify a fully adjustable suspension setup for a street driven car but I've obviously reached the limit of the stock geometry. I have enjoyed my track times so much, and have learned so much about the huge difference between a street car and a race car that I am now considering buying and/or building a dedicated track car....someday...maybe a go-kart will quench the thirst...

[ZR8ED]

Lots of responses here,

 

Here is a low buck idea (I like the low buck stuff, as doing things the proper way is usually expensive enough that most people just live it and don't make any changes that may help)

 

I learned this fact from some of my autoX friends over the years..

 

In a direct answer to the original question

 

"Many folks have told me that my Z's front bar is too large and I now feel the same way. Is there a way to "loosen" the bar by grinding a flat along it's length until I acheive the desired effect? or is it easier to just buy and try? Is there another way to reduce push asides from sway bars and spring rates? Tire pressures maybe. I don't have camber adjustment. My tires are wearing PERFECTLY even all around right now in mixed curvy road and track day driving."

 

You can loosen the effects of the bar by switching back from the urethane endlink bushings to rubber endlink bushings.

 

This is the route I took with my 1.25" front bar, after installing my more rearward positioned, lighter weight VG motor. The car pushed badly when driven hard. (it was fairly neutral before on the same track) I switched the bushings back to rubber, and it made a noticeable difference. Tires, springs and shocks were untouched.

 

Quick method to check, cheap too!

[johnc]

This is the route I took with my 1.25" front bar, after installing my more rearward positioned, lighter weight VG motor. The car pushed badly when driven hard. (it was fairly neutral before on the same track) I switched the bushings back to rubber, and it made a noticeable difference. Tires, springs and shocks were untouched.

 

FYI... this will only affect the initial response of the anti-roll bar and will really only affect understeer at turn-in or in the very first part of the corner. Once the end link bushings are compressed the anti-roll bar rate is based on its OD and the combined length of the lever arms.

[gwheeler]

FYI... this will only affect the initial response of the anti-roll bar and will really only affect understeer at turn-in or in the very first part of the corner. Once the end link bushings are compressed the anti-roll bar rate is based on its OD and the combined length of the lever arms.

 

John, I respectfully disagree. I believe the end link bushings do indeed add to the roll stiffness (even statically). The bushings act like springs in series with the ARB. And springs in series always reduce the overall rate.

 

Picture the car rolling (right wheel up, left wheel down). The right end link bushing under the ARB will be compressed, the left bushing above the ARB will be compressed. The amount of bushing deflection will depend on the spring rate of the bushings. Urethane bushings having a higher spring rate, they would deflect less, transferring more motion to the ARB, which would increase understeer, even during steady-state cornering.

 

To take the illustration to the extreme, imagine what would happen if you did not install end link bushings, so the studs are free to move relative to the ARB arms. In effect your end link "bushings" have zero stiffness. Now the overall roll stiffness from the ARB system is zero. On the other hand, if you were to install rod ends instead of bushings, the overall roll stiffness would be due to the ARB only.

[tube80z]

John, I respectfully disagree. I believe the end link bushings do indeed add to the roll stiffness (even statically). The bushings act like springs in series with the ARB. And springs in series always reduce the overall rate.

 

John is right. The end links stack up and your spring analogy goes out the window, just like tender springs. This only changes the rate of change, not the rate.

 

Cary

[johnc]

The bushings act like springs in series with the ARB. And springs in series always reduce the overall rate.

 

Although that's true, its misleading in this example. If you were citing two linear rate coil springs stacked on top of each other I wouldn't argue at all, but in this instance we are talking about 1" tall rubber bushings that in my press take 500 psi of hydraulic pressure to compress 1/4" and 2,200 psi of pressure to compress 1/2" (just checked it). They act as extremely short progressive springs.

 

Lateral load transfer is based on weight, CG, and track width. Chassis roll resistance is provided by the width of the spring base (the distance between the springs on each axle), spring rate, and by the ARBs which transfer load laterally.

 

The chassis doesn't care if the roll resistance is from bushings, the ARB, spring rate, or spring base - it just feels a rate. Softer bushings just slow the initial rate the ARB adds to roll resistance and may allow an additional fraction of a degree of roll in the chassis but the bushings themselves are a very small contributor to roll resistance.

 

On the track you'll feel softer ARB bushings as a little delay in the front end taking a set. If you've had turn in understeer problems then it will delay the onset of understeer enough that the front tires may be able to finish turning the car before the full ARB load is applied. This might keep the front tires from overloading and make the understeer appear to go away. Its not, its just been put off a bit.

[gwheeler]

Softer bushings just slow the initial rate the ARB adds to roll resistance and may allow an additional fraction of a degree of roll in the chassis

 

You and Cary are correct of course. I was only thinking about the initial bit of roll.

 

Sorry!

 

:-/

[Guest Z Draci]

You're joking right? Perceived weight distribution (not actual weight distribution) is the cause of understeer and adding weight to the rear of the car to correct the "perceived weight distribution" fixes it?

 

I fully understand what you are saying. However, I still stand by what I said.

I probably wasn't driving my 240Z properly when I tested this out. I was braking late and lifting off on turn in. For this style of driving, making the rear heavier should work out fine.

 

For a car like the 240Z, I should probably brake early and accelerate through the turn.

 

Sometimes, things learned from textbooks doesn't work in real racing situations. I had my share of arguements with car techs before races . . .

[260DET]

Lots of responses here' date='

 

...................................................................................

 

You can loosen the effects of the bar by switching back from the urethane endlink bushings to rubber endlink bushings.

 

This is the route I took with my 1.25" front bar, after installing my more rearward positioned, lighter weight VG motor. The car pushed badly when driven hard. (it was fairly neutral before on the same track) I switched the bushings back to rubber, and it made a noticeable difference. Tires, springs and shocks were untouched.

 

Quick method to check, cheap too![/quote']

 

Some loosen the link bush nuts right off, so that there is a gap between the bush and the bar. This of course delays the operation of the bar in roll. Plus, comparing roll more than sufficient to take up the slack, the effect of the bar is reduced because its travel is reduced.

[cygnusx1]

Update, the front tires did not wear evenly. As everyone suspected, without adding negative camber to the front, the outside edges wore out faster. Two track days and lots of twisty street driving took its toll on the outside edges. That just proves that the Z definitely needs more negative front camber the harder you drive it. It also, sort of explains, the push feeling in the car. Since removing the big heavy 280Z bumpers it seems to turn in much snappier and stick more neutral.

[mom'sZ]

cygnusx1: I remember this thread and just reread the whole thing. No one ever mentioned, so I thought I would just throw this out there, instead of the cost of camber plates, maybe you could install adjustable control arm bushings. They are eccentric and allow some camber to be adjusted in. Since camber plates change the ride height, you really have to do all four, but with these adjustable bushing maybe you could do just the front. I believe Richard posted a link to a company that makes them in another thread and I think MSA has them too.