Optimal Tyre Width

[Moosejaw]

Is there such a thing as an optimal tyre width for a car?

I'm aware that you can get to a point when the tyres are too wide, but how is this calculated (if it is calculated) and what effects will start showing on the car?

[Hellisan]

I actually wasn't aware you could get too wide, at least if you're talking about Forza.  I'm not an expert so I'll be watching this one carefully.

Seems to me you can change the width and find that you then need to re-tune to the new size... That if you have the same tune, wider tires can make it more difficult to turn if you keep the same settings... But doesn't seem like a problem inherently, more like a problem in certain instances.> Just my impression of it.

[fndrbndr]

I tend to take the bigger-is-better approach in Forza.  I haven't done too much testing on this, though, and often end up wishing I had a little more hp and a little less tire. 

[Blooze]

I tend toward the "get all ya can get" school myself.

There is a parameter that I track (I got this one from Tonka Crash - we were investigating exactly that, the affect of tire width on handling)  The value is measured in pounds per millimeter (lbs/mm) and we called it Load, and for the most part, all I have determined is that the lower the Load value, the better.  I have found that the lowest Load value is almost always associated with the highest G@120 value.

Load is calculated by dividing the curb weight by the total tire width where...

Total Tire Width =  (Front Width x 2) + (Rear Width x 2)

Note: this is found starting at cell J91 on my build sheet for those that are interested.

Other settings that may be affected in Forza are the Camber and Pressure (i.e., heat) - those are things that may require adjustment.  However, as long as G@120 improves, I doubt that any adverse affect to those telemetry values would slow me.  See what happens to the values when a hired driver is herding the car in the appropriate Endurnce race where you can watch the telemetry at runtime.

In the real world - is there a class specification that restricts the bottom range of that specification, and if so, is it because there is an advantage, or is it because you will not be competitive?

Other than that... hell if I know - good question!

$

[Fit4aking]

I've toyed with tire width a lot in F-class where PI is at a premium.  So far the wider the better.  However..... in higher classes I've found that a compound upgrade will help the handling of the car far better than width.

One of my little projects involved my 2000 GT-R in F and I tried to compromise the tire width to help with overall performance and I was never able to have enough gain in another department to make up for the loss of corner speed. Perhaps as I work up the classes and have more PI to play with a compromise build will prove faster but it takes a lot of time and cr's to work through a whole combination of compounds and widths to find the fine line where the car just won't handle any better.  Not impossible but when I know what has worked in the past I tend to start there and work to the best of known good build strategies.

As far as mandated tire widths in the real world I'm drawn to the drag racers that can build amazing cars but have to run them on 9.5 inch, 10.5 inch or 12 inch tires.  Sure there are limits on the number of power adders (ie superchargers, turbos, NOS) that you can add to the car but basically it boils down to the tire being the major limiting factor.  Another thing I remember from watching the SCCA races was they would limit tire width in higher power cars, like the Corvettes, before they added intake restrictors to limit power.  The Corvettes were still competative in their classes so I think the limited width didn't hold back too much.

[Moosejaw]

Interesting comments. I wasn't just posing the question from a Forza perspective but a real world one too.

My Peugeot 307 has 195's all round but I'd be intrigued to find out why Peugeot decided 195's were suitable over say 185's or 205's. If wider tyres means more potential grip and therefore possible better performance of the car, why wouldn't a manufacturer go down the "wider is better" route? Cost maybe?

I know that if I decided to put wider tyres on my car my insurers would increase the premium I pay. Is this because they see it as an performance improving modification?

There doesn't seem to be much about this type of thing on the internet either, so maybe we shall never get to the proper answer. Maybe the answer is just a simple as "we put 195s on this model as this is all we had in the workshop"!!

[barumba]

It is not so flippant. Wheel and tire specs as determined by OEM (real world) is based on very build specific characteristics. You can really screw up a car's handling and computers if you just throw on any old tire and wheel just because it looks cool. Of course, keeping within the OEM diameter will help, but suspension and wheel/tire relationships such as offset are also important. That's real world talk. Not sure how the game is modelled, but I suspect that the tire and wheel choices in Upgrade wouldn't take you out of the proper correlations. You can also judge whether plus or minus sizing (width) is of any good in game by looking at the Grip COE and Lateral G's before installing.
There is also discussion regarding increased rolling resistance, increased inertial forces in acceleration and breaking with increased tire and wheel mass. 
Some stuff to read here...
http://en.wikipedia.org/wiki/Plus_sizing

And here...
http://www.1010tires.com/tech.asp?type=wheels

"Plus Sizing

Plus sizing your wheel & tire combination was designed to enhance vehicle performance and looks by allowing  fitment of larger diameter rims and lower profile tires. The theory is that while making these changes, you keep the overall tire diameter within 3% of the original equipment tires. This is important because larger variances can cause problems with transmission shift points which can decrease fuel mileage. It can also confuse braking system computers which can even lead to brake failure.

Here's the rule of thumb for "plus sizing":

Plus 1:
Increase section width by 10mm
Decrease aspect ratio by 10 points
Increase rim diameter by 1 inch

Plus 2:
Increase section width by 20mm
Decrease aspect ratio by 20 points
Increase rim diameter by 2 inches."

[Spiny Anteater]

Moose - I'd guess that the reason for the 195's is that Peugeot felt that is the optimum width for best grip versus best fuel consumption (a narrower tyre having less rolling resistance therefore using less fuel).

There does seem to be a trend towards wider tyres as time progresses. For example, excluding the old Pontiac, the tyres on my cars have gone from 145, through 155 to the current 175. I'm now looking at a Focus where the standard width is 205.

The Focus also seems to add to the lower fuel consumption theory. Standard width for the range is 205mm, but the Econetic comes with 195 as standard. And I would say it's because that model is biased very heavily towards fuel consumption as opposed to handling.

[Blooze]

Dealing with the tire width dimension in the real world, I would not discount the input of any number of government bureaucracies and lobbies that are directed by Insurance Companies.  I would not want to even hazard a guess at how much of what we drive today is orchestrated by that bunch of crooks.

$

[barumba]

Here is a similar discussion, somewhat, based on affect of PSI pressure, as it pertains to tires, choices and affect on handling.
http://forza-tuning.net/index.php/topic,1431.msg23667.html#msg23667
Some would think they go hand in hand. Pressure/width/compound. All about contact patch, optimum pressure, optimum temperature, optimum grip.

-wider tire, increased contact patch, good
-wider tire, larger wheels, more unsprung weight, have to deal with that
-wider tire, takes longer to heat (especially rears, may never reach optimal temp), have to deal with that
-wider tire, change aspect ratio, stiffer compliance, affects roll and therefore suspension, have to deal with that
 
This is all good stuff, just part of the tuning challenge.

[Drift2XL]

Dealing with the tire width dimension in the real world, I would not discount the input of any number of government bureaucracies and lobbies that are directed by Insurance Companies.  I would not want to even hazard a guess at how much of what we drive today is orchestrated by that bunch of crooks.

$

Which is why the tire widths have gone wider the later the car model. More tire means more control, to a point. More control equals lower insurance rates. And the fact that every car is FWD.   

The change of tire composition is also huge in this subject too. High silicone tires have much more tread life than the older mostly vulcanized rubber tires. That tread life will help cut down on rolling resistance. As well as the computer modeling trading almost nothing off in a standard tire.

I've said nothing of high performance tires. Because in that realm, there is only one measurement. Lap time.

[feuerdog]

My mental notes on the subject.....

- Wider is better, but usually when building into a specific PI I try to stay around a predetermined class based width(build table).

- If the car has too much grip(rare), I will decrease compound first, and/or width second.

- Light cars don't require as much grip, heavy cars(and cars with downforce) need more.

- Drivetrain plays a big role in determining wear more width is needed(RWD's can sacrifice some front width).

- The single biggest factor I can think of in the relationship with wide tires is how the car transitions from skid-to-grip and how that feeling is related to the driver. Width of tire, weight of the car, and tire compound, can make transitions either smoother or sharper depending on the drivetrain of the car.
Example.....

Light car, wide sticky tires = sharp transitions between grip/slip because the lack of weight

Heavy car, wide sticky tires = smoother transition

medium car, med grip tires = normal transistions

Light car, high grip, narrow tires = smoother transtions

etc.....

I felt the effect very much so with the BTCC spec FWD Citroen C4. It was a difficult car to tune around becuase of the unsettling way it transitioned between grip/slip.

My point is, that there does seem to be a "sweet spot" with tire width when it comes to tuning where too much of several good things can have less than desireable effects.

From my drivers seat(and largely calculator based tuning) I am looking for that generic(read:consistent) feel between cars and builds that suits my driving technique. "Too much tire" is a consideration I have worked around and have compensated for in my builds.

In very rough terms, my minmal tire widths based on class are:
F-205
E-215
D-225
C-235
B-245
A-255
S-265

But, allowances are made up to 20mm bigger and smaller for build specifics related to track(grip/missile) and/or drivetrain(RWD - smaller front/bigger rear)

[barumba]

Hey, FD, nice to see ya!
Perhaps with all this steaming hot summer weather you should consider changing your name from feuerhund, to eis-hund. LOL
Be cool!