I'm about to buy new tires for my Z, and I also have a hotchkiss swaybar set on the way. The stock Z is setup with some understeer, which I'd like to reduce a bit. Should I buy wider front tires, or narrower rear tires, or should I keep my tires the same and adjust this with a swaybar?

The tires I'm planning on buying are BFGoodrich KD's, 245/40/18 front, 275/40/18 rear. Unfortunately, they don't make a 255 size in 18-inch, but they do have 265/40 and 265/35, which I could put on the front, and keep 275 on the rear. OR I could keep 245 on the front, and put 265 on the rear. Any suggestions? I drive my Z on the street maybe half the time (when it's dry), and I track it occasionally too. Thanks for your input...


Edit: One secondary question: Will 265-width tires (in 18 inch) fit on the front of the 350Z with no rubbing issues?

 

I'm curious to hear averybody's opinion on that myself as I'm about to get my tires for next season and wonder if I should go 275 all around.

One thing for sure, 245 front and 275 rear is bad bad bad. I doubt the swaybars could correct that.

 

A wise man once said...

You should always increase grip whevever possible... not take it away

For example:

Understeering car: better to add grip to the front to balance it, then to take it away from the rear to balance it (which generally mean softening the front sway). Still doesn't really answer your question other than elliminating the thought of reducing the rear tire size.

Another thing to consider... depending on the tracks where you drive, and your tire wear... choosing the same size tire all-around has the added benefit of enabling you to rotate them when needed.

Since you already have the sways on the way... I'd start there if they are adjustable. Save the money on tires for right now (unless the ones you have are gone). Find out which Hotchkis setting is most like the stock setting, then set it a bit softer. If you're putting a new rear bar on, leave it in the closest setting to stock until you find out what the front change will do. For that matter, I'd leave the rear bar alone and just try the front.

My whole point is this... change ONE thing at a time, be it the tire size or the sway bar adjustment.

 

Yes, 265's fit up front. I run them with a 23mm offset. A 275 will also fit with a 25 mm offset.

I ran 265/35/18's up front and 275/35/18's in back last year (RT-615's) with Cusco sways, and the car was very well balanced for autocross with that setup. Like Stack said, one thing at a time though.

 

Stacy,
I agree with not taking away grip and only changing one thing at a time and test.
Yet, I don't believe that changing the sway bar will take away grip. I always thought that your spring/dampner/sway bars determined how fast your weight was transfered. And taking that he will not change the spring/dampners than by softening the sways bars he will slow the weight transfer.
And a common myth is that stiffer sway bars are better, as a general rule of thub, the stiffer the sway bar setting - the smoother you have to be as a driver to use all the grip of the tire. Plus, when the track conditions are less than ideal (ie rain) you want to slow the wieght transfer so that you don't put an instantanous high load on an outside tire and start to slide.
The Hotchkis sway at the softest setting are still stiffer than stock. Myself, I would install them both on the car, set them per Hotchkis recommendations and then test and tune, until the set up matches the driver.
As for the 265 fitting in front - should not be a problem as long as you have the right offset wheel. I run with 275 in front and I could fit a 285 in there without a rubbing problem (and many people do.)

 

The basic problem with aftermarket sway bars is that they get stiffer than stock, not softer. The new Hotchkis for example:

Hole 1 = 1525 lbs/in (22% Stiffer than stock)
Hole 2 = 1845 lbs/in (48% Stiffer than stock)
Hole 3 = 2270 lbs/in (82% Stiffer than stock)
Hole 4 = 2855 lbs/in (129% Stiffer than stock)

The same was true of all the major brands.

 

In a steady state cornering situation, sway bars will affect the weight distribution on all four wheels, thus affect the under/oversteer behaviour of the car.

As a general rule, a stiffer bar will unload the outside wheel on the opposite axle (front to rear, rear to front) as the car "squats down" and the stiff sway bar equiped axle sees an increase in it's virtual spring rate (ie it uses some of the inside wheel spring to assist).

You are right in saying it takes care to drive such a car. Hit a curb at the apex, and the outside wheels on that axle will have a tendency to lift and possibly breaking traction.

Me don't like swaybars...

 

Thanks for the feedback everyone. It makes sense that I shouldn't give up traction by making my rear tires smaller just to correct understeer.

As for the front tire size, will 265-width tires be okay on 8.5-inch wheels? I'm pretty sure they'll "fit", but will the tire be too "pudgy" or something?

My wheels are 18x8.5" in front, and 18x9.5" in back, so I think I'm down to the following tire sizes:

• 245F/275R, and deal with understeer using swaybars, or
• 265F/275R, deal with understeer with tires (and swaybars if necessary)

 

Running 245/275 tires, i oversteer with stiff sway bars. 265/275 would be better bed if you have stiff sway bars.

 

go to Sport Compact Car magazines website. with their project 350z they have done a WHOLE lot of suspension work getting to the elusive 1g mark.

they had tire sizing take up one issue alone, so youll find some useful information there.

 

Very informative post, tuning in for further comments.

I hope the OP doesn't think I'm "hijacking" the thread because I think we are in identical situations.

I will be running Nismo wheels next year which are 8.5 and 9.5. Am planning on purchasing tires (Nitto NT01) but am torn whether to go with 245/275 or 275/275. For reference, I have Hotchkis sway bars set to recommended medium/medium setting at the moment. Thoughts? Suggestions?

Again, I normally wouldn't have posted but since my situation is very similar to the OP's, I hope he doesn't mind...

 

Welcome to the thread, I don't mind at all!

We do have the same size wheels. I don't have the swaybars on yet, and in the interest of changing one thing at a time, I might do the tires first, then the swaybars.

One constraint I have is that I want to keep the VDC/TCS happy, so my front/rear tire circumference ratio needs to be close to the stock setup. I guess I hadn't considered running 275F/275R, one reason being the VDC issue. Also I don't want to overdo it on "fixing the understeer" -- I don't want a tail-slinger! Anyway... I'm waiting for more advice from the more experienced track folks

 

I'm also instersted in the point you brought up regarding non-staggered tires on a staggered rim setup.

The "pudged out" look you refer to is actually directly related to sidewall stability, which is a critical component of high speed cornering...

 

Some guys that track with a "square" tire setup have had ABS issues. I can't remember how it was though.

 

I have run 275/18 all around on Hoosiers and Gram Lights wheels, and found it to be very responsive and balanced. It definitely dialed out a lot of the understeer and with a 3.5 negative front camber, bit into the turn in crisply. It does promote the ability to yaw the direction of the car with the throttle and brake, to "steer with your feet", and if you are not ready to mess with that, be careful about such a set up. Great advice up above is "one thing at a time". That applies not only to the mod that you make mechanically, but the impact it is going to have on your driving approach. With FI power I needed more hook up, so I have returned to some stagger with a 275F/315R set up, on Enkei 9.5 inch front and 10.5 inch rear wheels and NT-01 tires (very nice).

Why do we have so much understeer on our cars when it comes stock? Because it is safer. When the car starts to push the natural inclination for most people is to lift throttle, which puts inertial force forward on to the tires for more grip, and with the combination of the lift on throttle lowering speeds allows some recovery (even though the inexperienced driver is probably also dialing in more turn with a non gripping front tire, a truly bad thing to do but reflexively typical).

When you take out the understeer, it is so that you can "go faster" in a turn.
But part of that is to facilitate rotation. So now you bring to bear a need for a new library of driver skills, counter steering with a car that is losing rear traction, and feel for how to induce, control and recover from slide at both the front and the rear of the car. And by the way......vdc is out the window for this stuff on the track as it interferes with your controls and actually makes it more dangerous. On the street you should not be pushing the limits of the car like this anyway, so leave it on for safety and surprise events.

This car is heavy, so when the tail wags it can be very easy to catch the first part of the developing spin, then hook it back the other way a full 180 degrees with the euphemistically called "overcorrection". It really is "under recovery", because the input is late...timing is the key part to countersteer...but semantics aside, it is dangerous. the slides one inputs deliberately are not television style drifts through an intersection chasing bad guys, but a matter of a few inches or feet very quickly managed with smooth hand inputs....Jackie Chan hands. All of these mechanical adjustments with tire size stagger and sway bar stiffness settings are indeed achieving returns towards a potentially better performing car....IF you have the skills to handle it. If not...you are just putting yourself and others closer to harm.

I think you are putting too much tire on the 8.5 inch rim with a 265 treadwidth. Consult the tire manufacturers recommended rim sizes, but I think a 9.5 inch rim is what you will want to consider for that wide a tread.

 

Nice post Eagle1!

 

When I first started tracking, I ran 255/40 fr and 275/40 rr on 8.5/9.5 LMGT4s. With my koni/rsr shock/springs & stillen sways, I had major front end push. So I got two more 9.5 LMGT4s for the front, went to 275/35/18 RT-615s on all corners and the push went away and now have slight oversteer. I haven't had any abs issues on the track nor vdc issues driving to the track (30 mi. one way). So I'd recommend running 275/35 on all corners if you're NA.

I'm changing my wheel setup again (need 315/30s in the rear for FI torque) so I have a pair of 9.5 LMGT4s for sale

 

That's interesting that you're not having VDC issues with the same size tires all around. I just bought a set of 275/35/18 RT-615s to autocross on and was concerned that I'd have to turn VDC off for the trip to and from the event. I'll have to give it a try.

 

Eagle, thank you so much for taking the time to write that up, great post!

I am really now leaning to 275 all around on a 9.5 rim.

So much to learn, which is why it is great having folks like yourself and the others that posted responses around

 

What offset do you guys run up front with 9.5" wide wheels and 275's? Are there any fender contact issues or suspension contact issues?