wheel/tire debate.

I agree that a wider set of wheels/tires in the rear of the Z LOOKS better. But what about performance. Straight line performance seems like it would be better because of more traction at launch. However, I talked with a wheel manufacturer about this and seem to believe I will be sacrificing cornering performance if I do this. They recommended same size wheels/tires at all 4 corners. They mentioned that unmatched widths would create understeer (which some think the z has already).

I am more concerned about performance than overall looks, so should i do same wheels/tires all the way around?
If not please convince me otherwise.

 

This is all true until you make supsension mods (shocks and/or sways)...then everything changes.

The theory is that rear wheel drive cars need wider tires in back because the rear tires need to be able to sustain cornering forces in addition to acceleration forces (throttling during or out of a turn). Of course there are many other factors involved in this.

The best method is to choose a balanced setup (suspension/wheels/tires). This balance varies based on the components chosen.

 

One of the big factors for tire/wheel sizing is weight distribution. This can play into it as much as the fact the car is rwd. For a car that has ~50% or more of the static weight on the front wheels, equally sized tires all around will probably be best for roadcourse performance. When you start getting more weight on the rear, the wheel and tire width should start to increase because they will need to carry more load in the turns.

You can try running wider tires on the rear of a car with characteristics like the 350z, but you'll be overworking the fronts and/or underworking the rears. You can correct for this somewhat with suspension tuning, but in the end you will probably have to get rid of some rear bite which kind of negates the advantage to running a wide tire on the rear.

That's a quick and dirty explanation...I can go into more detail if you want.

 

GaryK,

So what you are saying is I should run even tires all around, UNLESS i do suspension work? What kind of suspension tuning would be needed? I plan on doing coilovers, so would the TEINS with electric controller be the best suited for this? I had originally planned on the HKS setup.

How much performance difference would you see when running the 245/35 and 275/35 tires that seem to be popular here. I want to do some autox / roadcourse as soon as i can get involved.

I guess what i am saying is would **I** be able to tell the difference, or would it take a professional driver to feel it?

Of course there is the added bonus of being able to rotate your tires, but im not sure if that would extend the life of the tires by that much anyways.

Im just skeptical of the setups some people are running, and if I am going to drop 2-3k on wheels/tires, I want to get it right the first time.

 

Great Topic
Why is the manufacturer not putting same size wheel and tires all the way around.

 

Someone tell me if these overly simplistic scenarios are wrong and why...

- If you run 8.5 all around your not car will have equal grip at all 4 corners (theoreticaly, not counting f/r weight ratio).

- If you run 8.5f/9.5r your not decreasing front grip, only adding rear grip...the car will understeer but no sooner than it would have lost traction in the previous scenario. Gained rear traction but lost nothing?

- If you run 9.5 all around, you increase grip all around. Overall gain?

- If you run 9.5f/10.5r, same scenario as above but with yet more rear grip.

- 10.5 all around would be better yet but I don't think you can run 10.5 in the front. This is the limitation.


Doesn't this all mean that the "weakest link" on this car in stock form is the front grip? If so, "weakening" the rear of the car by lessening tire width only accomplishes more balance so that all four wheels break loose and we 4wheel drift instead of understeering. Is this better that we 4wheel drift at x G's instead of understeer at the same G's? We can get the car to oversteer by putting 7" wide tires on the back but we won't get around the track any faster.

The point is that we put the widest possible tires on the front of the car and the rear should be the same or larger.

...unless we mod the suspension, then this may change.

I currently wear 8.5f/9.5r and plan to move to 9.5f&r only because I don't want to roll the fenders to get 10.5r.

 

Probably because it's much safer for a car to understeer than oversteer.

Also, rear wheel drive cars are much easier to intentionally induce oversteer with clutch/throttle.

 

Before going any further, let me give you a disclaimer: I have not done any testing on a 350z, so what I say is not backed by direct experience on a 350z. I do have experience with other cars with varying characteristics, and a solid understanding of suspension and handling.

With stock suspension, I would say definitely to use the same size wheel and tire front and rear. My only concern with this is the tolerance for tire size variance in the ABS system. From what I understand, it does not cause a problem with the ABS though. From the factory, the 350z is balanced toward slight understeer. By running the same tire front and rear, the car should be more balanced and should not tend to understeer anymore.

If you are going to modify the suspension, it gets complicated very quickly. I haven't really looked at the spring rates being used in aftermarket kits as compared to the stock spring rates. Same is true for the aftermarket sway bars. If the guys that design these kits try to remove the tendancy to understeer while assuming wider tires in the rear, then you could have a problem when you run the same tires front and rear. Since you are going to modify the suspension, the tires are a tough call. Sorry to be vague, but my recommendation is to run either the same size tire on both ends, or keep the difference small (20mm or less). In your situation, I'd personally try the same size. Then, if it oversteers I'd either add some front roll stiffness or take away some roll stiffness in the rear. When you are deciding what size wheels to buy, get the same size front and rear no matter what you want to do with the tire sizes. Althought the wheel widths are the same, you could still run a slightly narrower tire on the front if you want.

If the car is balanced toward understeer, I think you will be able to tell. If the understeer is bad enough, it can be quite frustrating. Personally, I would rather have a car that is loose (oversteer), but for John Q Public that is by far the worst situation. A loose car is capable of faster lap times, but is harder to drive at the limit. And when you get into trouble, the car will tend to go off the track/road pointed the wrong way. With a car that pushes (understeer), it is harder to get into big trouble and end up sliding off the track backwards. This is probably why Nissan has outfitted the car with wider tires in the rear. It tends to understeer, which is "safer".

 

yeah, marketing and safety help drive the wider tires in the rear.

those scenarios are pretty much correct. go with the widest tires possible on every corner and tune suspension for cornering behavior. i think too wide tires in front make steering heavy though, and of course wider tires have more drag, track grooves, more road noise. change tire widths, psi, compounds, etc if you can't get the suspension tuning right.

 

This isn't exactly true. If you have a car that is well balanced with a given tire combination, then put wider rubber on the rear relative to that combination, it will be very hard to get through the turns as fast. It will most likely start pushing at a speed less than the speed at which it previously started to "drift". I have seen this first hand on a car that was running 275's all around, then was fitted with wider wheels and 315's in the rear. Granted that is a pretty big difference in tread width, but it really made the results obvious.

There are lots of other factors involved, but this is the general effect of putting more rubber on than you need. When working at their potential, tires have to operate at a "slip angle". This means that the tires are actually sliding, but it is very controlled and is a very small amount of slip. With the wider tires on the rear in the above scenario, the front tires get to the optimum slip angle before the rears do. Its kind of hard to understand, but the point of optimum slip angle for the fronts is at a lower speed now because the rear is not operating as close to its optimum slip angle. The rear is not starting to "step out", so it takes more steering input to point the car in the desired direction. For what its worth, this actually isn't coming from a book, but its what I visuallize happening based on various bits and pieces that do come from books on suspension design.

 

If one doesn't have any suspension mods, the only changes that can be made with handling are 1. size of the tires, 2. tire pressures, 3. front toe in, and 4. driving technique.

Is the above statement true? and maybe someone could comment on how one might approach making improvements for autocross. I just purchased a tire pyrometer and hope to see what I can learn by measuring changes in tire pressure and temperature.

The new tires I recently purchased are SO3s 235/40/18 and 265/35/18. I tried to get 255/40/18 for the rears but they were backordered from Bridgestone.

 

i was unaware that the factory tires were not the same size

 

225/50WR17 and 235/50WR17
or
225/45/WR18 and 245/45WR18

 

It does not matter what the width of the wheel is, it is the width of the tire on the ground and camber. Changing camber will change the effective contact patch of the tire. Want to fix the understeer on a stock 350z just remove a little of the rear camber. Yes I know this sounds backwards but when you are cornering the tire will role over on its sidewall and having more camber sets the tire to have the most contact patch during corning, so by running less camber (standing the tire up) when it is loaded you will be reducing the cornering contact patch.

BTW, I'm running with 245x45x18 on all four corners.

Most car manufactures tune the suspension for some understeer, that way the average driver can handle a car that is about to or is sliding without looping it.

For suspension tuning you have 3 major points of interest. Turn in, Apex and Exit. For a race car you want a little understeer for the initial turn in, that way under trail braking the backend does not come around. Some where before apex you want the car to start to oversteer, that way you can balance the slip angle with throttle and steering and get the car to rotate at the apex. Some where after the apex but before the exit you want the car to be well balanced so that you can put down as much throttle as possible after the apex.

 

What about this setup?

Front - 255/40/18
Rear - 275/40/18

The heights are very close to stock, wider, and the front and rear heights will be about one inch closer to each other compared to stock. Wouldn't this be ideal?

 

I agree with GaryK

 

I can't remember the strategy for tuning tire pressures with a pyrometer but I remember reading to tune camber you go to a skid pad and do laps in one direction, measure the tire temps across the width of the tire. If it's higher on the outboard side, you need more negative camber and vice versa. repeat in the other direction

 

Julian already answered part of this, but here's a quick rundown on tuning tire pressures:

In general, you want equal temps across the tread. There are situations where you have to accept some temp gradient. You probably will have some problems getting equal temp across the tread because of the limited camber adjustement on the 350z.

-If the outside is hotter, more neg camber

-If the inside is hotter, less neg camber. However, you have to be careful because you can sometimes run hotter on the inner part of the tire because of toe and/or ackerman problems on the front. I don't know if this will be an issue on the 350z.

-If the middle is too hot, run a lower tire pressure.

-If the middle is cooler than the inside and outside, run more tire pressure.