Tire Pressures, Tire Widths, Wheel Widths, and Contact Patch Area

[Whiskey11]

Soooo... help me wrap my head around this tire calculator nonsense: http://bndtechsource.ucoz.com/index/tire_data_calculator/0-20

If I put in my current wheels/tires (18x9's, 265/40/18 RS3V1's (97W)) and my desired wheels/tires (18x11's, 285/35/18 RS3V2 (101W)) I get less contact patch area with the wider tire. In fact I'd have to run 4 less PSI to get the SAME contact patch area as my 265's give me. I get more width to the contact patch, just less length...

If I run 285/30/18 ZII Star Specs (93W), at the same pressure as the 265/40/18 RS3V1 I get essentially the same contact patch area at the same pressures.

How is it that the shorter and by all accounts stiffer sidewalled ZII's providing more contact patch area than a taller, softer sidewalled RS3V2? Am I missing something here? I see the load index rating for the RS3V2 is higher (101W XL vs 93W) but would that be an indicator for sidewall stiffness?

I mean, ok, I get 30mm more width in the contact patch but I lose 20mm in length. That would seem to imply to me that the 285's will launch worse and brake worse than the 265's will but provide more lateral grip at the same tire pressures?

With 18x10's (a narrower wheel) I get more contact patch with both 285's but the RS3V2 just barely matches the area of the 265/40/18's... the ZII Star Specs get higher than that still somehow?

The only thing I could think of is that the 285's support the load over a wider area so with the same load and same pressure it will see less length to the contact patch. How does this translate into grip then? If we go by these numbers wouldn't the ZII Star Spec technically have better lateral + longitudinal grip abilities (multitasks better) than the RS3V2? It should also grip more overall due to the larger contact patch area no? Given that the ZII is often described as peaky I don't see how it could possibly multi-task better?

On the upside to the shorter ZII SS would also help bring my current 77 mph top speed in 2nd gear down to 72 mph which is at least halfway reasonable.

Help me figure out where to go for tires next year in STU. Also, Dunlop doesn't do contingency, Hankook does. Hankooks are cheaper too.

 

[5.ohmygod]

The only experience I have with this sort of thing is in rock crawlers. When it comes to off roaders, when tire size is increased the air pressure has to be dropped to get the tire to have the desired footprint. With a bigger tire, the weight of the vehicle is spread across a lot more area. For example, I run a 43x14.5" tire on my crawler and I have to run 4 psi in the rear and 5 psi in front. Any more pressure than this and the truck just isn't heavy enough to sufficiently 'flatten' the tire out. Even with the tires at these pressures, they don't give an appearance of being flat.

 

[dontlifttoshift]

I run 285/30/18 ZII Star Specs (93W), at the same pressure as the 265/40/18 RS3V1 I get essentially the same contact patch area at the same pressures.

If you have 35psi in your tires and you have to hold up 3500 pounds then you have a 100 square inches of _contact_ patch. It doesn't matter if you are on 265s, 285s, or 335s.

 

[Vorshlag-Fair]

That tire calculator you posted looks like a bunch of junk data. I've used dozens of these and all they are good for is relative differences, nothing as specific as tire pressure calculations and contact patches. I assure you - the car WILL BE FASTER on the 285mm tire on an 18x11 vs the 265mm tire on the 18x9. It will also be easier to drive. I know because we've done this upgrade on many S197 Mustangs, including in Solo competition, including our own cars.

http://www.rimsntires.com/specspro.jsp <- Bookmark that one.

I like that tire calculator, as it has a nice graphic side-by-side comparison. I've used it many hundreds of times. Note: These "calculated" tire widths and heights almost never perfectly match the real measured widths and heights. Always look at the manufacturers' published tire data for a given model and size.



The image above is using the 265/35/18 on the left, but I like the 265/40/18 you were using better (we tested with both Hankook sizes in STX and the 40 series had taller gearing, of course). These calculators are great for figuring out offset from backspacing measurements, tire height differences (for speed changes), and "Seeing" relative changes from one size to the next.

One other thing - ignore the contact patch calcs. Just literally don't think about that because you are over analyzing this, in my humble opinion. We cannot change "contact patch length" without massive tire diameter changes, and even the "tread width on the ground" numbers on the 265mm squeezed onto a 9" aren't realistic compared to a 285mm stretched onto an 11" wheel. Adding tire width on an S197 on street tires is more about adding "heat capacity", as these cars (yes, even 3V) can overheat the rear tires painfully quickly.

Also, autocrossers ALWAYS want to see a tire stretched rather than squeezed, as it makes for A) more usable tire on the ground (no, you cannot easily calculate this) and B) it makes the tire react more responsively. I look at the "recommended wheel width range" for a given tire and go to the widest width listed, and sometimes still add 1/2" to the wheel width, especially when I am "tire width limited" like in STU.

For instance, we ran an EVO X in STU. This car is as heavy as an S197 (3600 pounds) and makes GREAT power, but is limited to a 245mm max width tire because it is AWD. So we used the widest 245mm we could find (Yokohama AD08) stretched it onto an 18x10" wheel (see images above). It helped... but it was still too heavy to be competitive with 2950-3000 pound EVO 8/9 cars. Tires would overheat halfway through a run. That same car on 265mm to 275mm street tires (mounted to 18x10.5" wheels) was a BEAST in an autocross, though (see below). It regularly crushed STU on 265s, whereas it was a hot mess on 245s.

Again, I wish ST classes had a sliding scale of max tire widths that scaled up with weight. That is the fatal flaw of Street Touring... and what ham strings the heavier cars in each class. What wins STC? The lightest sh!tbox that is legal. What wins STX? The lightest car. STU? STR? The lightest car wins. Virtually every autocross class is determined at the scales. It is ALL about weight, and more specifically tire-to-weight ratios, more so than anything else. When you have a fixed tire limit but variable weight limits, like in all ST classes, it focuses your car choices.

Oh yea, last thing - the car above has our 18x11" wheels and is currently sitting on 285/35/18 RS3 Hankooks. If you want any specific measurements, just ask.

Cheers,

 

[Whiskey11]

Thanks for the responses. Seems to make sense.

I will say one thing about the RS3V1, either my car hasn't been set up correctly or it utilizes the rear tires way better than a 5.0L does or it takes waaay too much to overheat it! I have had these RS3's overheated a grand total of ONCE. I've gotten close recently as the tread depth gets closer and closer to the wear bars though. How do I know the RS3's were overheating? Times slowed down. This occurred during the Nebraskhana event last year where myself and a Co-Driver did basically 10 back to back runs with long enough stops to change drivers quickly since we were the last two competing against each other. By my last run the car would push and then oversteer horribly within one run where it normally was pretty tame with a hint of oversteer.

Recently, the fronts have been getting hotter and hotter but the car is a lot looser than it has been in the past. Rear diff is damn near shot (carbon fiber clutch packs did not last as long as the organics did on Lincoln's surface) though so who knows if that is contributing to it any. Generally I wouldn't think so but I know I'm not the only one who noticed this. I'd come in after a run and the rear tires would be hot, but the fronts would be way hotter and yet the car is still pretty loose. It takes some pretty sweeper intensive courses to really get a lot of heat into an RS3 to make them sloppy. I just hope the RS3V2 doesn't overheat too easily on these cars...

 

[Norm Peterson]

If you have 35psi in your tires and you have to hold up 3500 pounds then you have a 100 square inches of _contact_ patch. It doesn't matter if you are on 265s, 285s, or 335s.

Regardless of whether knowing what the actual contact patch area might be useful, that's at best only a crude approximation. Unit loading over the tires' contact patch areas can easily vary over a 2:1 range within the same tire's CP, due to the stiffness of the tire carcass (and void ratio, and . . . etc.).


I can't plot things as nicely as the Rims & Tires site does, but I can overlay two tire & wheel plots for direct comparison and this little Excel app can use actual tire dimensions as well as defaulting to the nominal size numbers that everybody else's tire calculator uses . . . and there are a couple other things I can do with it. Eventually I may get rid of the gridlines on the presentation sheet.

Fictitious example.

Norm

 

[dontlifttoshift]

Regardless of whether knowing what the actual contact patch area might be useful, that's at best only a crude approximation.
Norm

I realize that it isn't an exact formula. Your spreadsheet is neat but it is based on advertised information, yes? Garbage in, garbage out. Certainly a Rival will put more rubber to the ground than a PSS, and certainly the nearly solid blocks on the outside shoulder of the Rival will help in the 2:1 loading scenario.

I don't understand the point of the figures and calculations. More tire = faster, also a crude approximation. On a 140 hp civic, there is a point of diminishing returns but on a 3500 pound car with enough power and brakes I don't think you could go too wide with todays available tire sizes.

 

[Norm Peterson]

I realize that it isn't an exact formula. Your spreadsheet is neat but it is based on advertised information, yes? Garbage in, garbage out.

Quoting myself . . . with a little emphasis. If the actual tread width is available, I will always use that value, otherwise I have to make some sort of assumption. Not garbage when the measurements are available, and it isn't garbage when they aren't.

and this little Excel app can use actual tire dimensions as well as defaulting to the nominal size numbers that everybody else's tire calculator uses . . . and there are a couple other things

Certainly a Rival will put more rubber to the ground than a PSS, and certainly the nearly solid blocks on the outside shoulder of the Rival will help in the 2:1 loading scenario.

Of course that will help the situation and may help make the unit loading more uniform (at least from a %void space point of view).

I don't understand the point of the figures and calculations. More tire = faster, also a crude approximation. On a 140 hp civic, there is a point of diminishing returns but on a 3500 pound car with enough power and brakes I don't think you could go too wide with todays available tire sizes.

When you don't have access to the proprietary tire data (as the car mfrs must), all you have left are a few simplified calculations.

Figures get kind of important when you're using up virtually all of the available room, such as with 18x11's on S197s and the 15x8.5's that I fit to a 1979 Malibu (with about 0.03" wheel to tierod clearance). Nobody else was venturing past 15x8 at the time.

FWIW, tire vertical stiffness should be figured in with suspension stiffness as a second spring in series. As your suspension gets stiffer, the greater the importance of the tire stiffness term, and at the F1 level more of the overall "suspension travel" may be in the tire than in the suspension itself. Even in modified production cars like yours and mine, there is over half a degree of apparent roll caused by differential tire deflection (transferred load). Something to consider when you're choosing alignment settings to start from or considering a decambered stick axle.


Norm