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 Post subject: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 12:59 pm 
Moar Dew.
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Condensed, in order to avoid a tl;dr:

It seemed like the silver SVX could never get enough tire strapped to it, and even if it did, the scrub radius would have been astronomical. The size envelope was filled with compromise.

The Miata's a different story entirely. Delivered with 195s, the stock wheels are currently wrapped in 205 wide rubber. This is the same width as a stock 2008 WRX at 3/4 its mass and 60% of its power. While a tire's width is hardly the only factor in its performance, this is just another reason that I'm wondering if the Miata actually has too much tire!

Buying what seemed to be the right size for the car, I've since sorted out who the reliable authorities among the online Miata community are, and their general consensus from plenty of research supports using an 8" wide wheel for a 205 tire for optimal grip and feel - two inches wider than the wheels I have on the car! With that 10mm increase, I may have gone up a few bucks and a few pounds for no improved traction and a loss of feel. Well, they look pretty cool, anyway. I wish that was important!

That aside, what's the best wheel and tire combination for any car, and why? More is not always better. I've found an avalanche of information that builds a model for parameters, but it hasn't exactly defined them yet.



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 1:00 pm 
Moar Dew.
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CONTACT PATCH

One of the most debated issues I've found is that of the definition of tires' contact patch. One camp says that the area of the contact patch can be found by the weight supported by that corner and tire pressure (divide for the area) and that, as such, tires with the same diameter and different widths with the same air pressure and supported weight have the same contact patch area. The other is highly offended by this idea, bringing Crayola sketch concepts involving 335 width Corvette tires, bicycle tires, Jeep mudders and snow tires for their evidence.

Unfortunately, neither side will ever admit that the other is partially correct.

For example, in <this pdf>, Boeing uses the weight/pressure=x method, but Chapter 7 in <this pdf> will blow your mind with a much more complicated model. I like to think that the simple formula is good for a simple answer under typical situation parameters, and that the NHTSA information will tell you how that formula is flawed and what will happen outside of normal circumstances without giving the layman an honest answer.

The beginning and end of <this thread> on CorvetteForum.com are full of drivel and namecalling, but parts of the middle have legitimate science that gives and explanation for why a wider tire can offer more grip even though it provides a contact patch with the same area as a narrower tire. The last step is still missing, though - how wide is enough or too wide?

Eventually, you find yourself among dozens of potential improvements or sacrifices - ride, lateral grip, steering feel, weight, fit (suspension rub, fender limits), offset, scrub radius, contact patch shape, weight capacity, optimal air pressure, contact curvature, price... You'll almost want to see what worked well for someone who's experimented with a variety of setups with a car similar to yours and just call it a day.



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 1:50 pm 
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I've put wide tires on narrow(er) rims before. Think the highest i ever experimented with was a 255 on an 8" wheel. Plenty of steady state grip, but the transient response was delayed and I rather disliked it. I currently have 235's on the mini's 7.5" (iirc) wheels. Those seem to fit rather nicely without any of the squishy feeling. Given the choice, I would have gone wider with the wheels. The downside is stretched tires aren't always as forgiving to sudden inputs and require "quiet hands".



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 3:01 pm 
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Finding the right size for any car can be quite a challenge of compromise. Even in the relatively limited world of FMod autocross tires, where we all run the same 18x7.5x10 Hoosier R25B's on roughly the same weight distribution, there's virtually no consensus on what the proper width wheel to run is. Some people run as narrow as a 6" rim, while others are running as wide as an 8.5" rim. In classes where things are more open things get even more complicated as there's more variables in play.

Some of the things to consider are:
- More tire+wheel generally gives more grip, but at the expense of added weight
- More tire on the same rim will give more mid-corner grip, but at the expense of transient response
- More tire increases aero and rolling resistance (less a concern at autocross speeds)
- Too much or too little tire for the rim will cause the contact patch to deform causing lost grip
- Too much tire+wheel may prevent tires from getting up to temperature (less a concern with street tires)
- Too little tire+wheel and you'll likely be overheating the tires
- "Optimum" alignment and tire pressures then will vary depending on the setup you're running

For an ST shod car the general compromise I've heard is between weight and grip - which is somewhat annoying as that's going to be a pretty course dependent compromise. At the most recent shoppingtown event (i.e. 100% corner) you probably wanted as wide a tire as you were allowed - but at a more point+shoot CVMP event you might be better served to give up some grip in exchange for reduced mass.

Given all the compromises there are in play, the only way to know you're at the right spot is to just simply try every combination out there (and the different alignment and pressure arrangements on those combinations), and get data on what the various grip levels, response levels and acceleration levels are for each setup. Then choose the appropriate combination for the course du jour.

Of course doing that is ridiculously expensive and time consuming - so basically you're left with two options:
1) Find someone else who did it and copy them, or
2) Just run the biggest tire you're allowed and can afford on the manufacturer's recommended width rim



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 11:37 pm 
Subie Tuner
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If you look at the specs on tirerack they're recommending a rim width between 5.5 and 7.5 for the 205 section star specs. I would think that 8" would be a bit of a stretch while 7" would be a more square fitment. I guess since the profile is relatively tall people are recommending the stretch to increase the resistance of the sidewall to the lateral forces because the inner and outer sidewalls will be pulling against each other.



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Wed Dec 19, 2012 11:56 pm 
Subie Tuner
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As for tire width vs contact patch, for a uniform sphere like a balloon the contact patch will absolutely be weight/pressure, and I'd imagine Boeing is able to do that because of the shape & construction of the tires they're modeling. We're talking about a very different beast due to the way the car tires are constructed. I also wonder about the effect the shape of the contact patch has on the slip angle of the tire, which is ultimately what determines the limit of grip.



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Thu Dec 20, 2012 11:23 am 
Moar Dew.
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AussieDan wrote:
If you look at the specs on tirerack they're recommending a rim width between 5.5 and 7.5 for the 205 section star specs. I would think that 8" would be a bit of a stretch while 7" would be a more square fitment.
While this makes complete sense...
Scootin159 wrote:
1) Find someone else who did it and copy them

...real life experience is hard to beat.

If I had to guess, I'd say that using the stock wheels, I might be better off with the 195 for less inward sidewall stretch, less patch curvature, less cost, less weight, ... I agree that a 7-7.5" wheel might be best for multipurpose use like mine. If I was doing a proper race-only setup, I'd seriously consider using 205s on 8" wheels per Scott's rule one, above.


AussieDan wrote:
Boeing tire model != car tire model

Well, beyond the difference in the tires, I wouldn't dare guess what the accuracy is of an estimate on a plane's weight. Having put my white SVX on the scales at the Pro Solo a few years ago, I could probably tell you within a quarter of a percent what it would weigh with me in it. A plane, though? Scott, figure this out for us.

Anyway, with P/PSI=SI, you wind up with the most basic (and certainly inaccurate) model. Involve sidewall support, tread, and the effects of pressures on the extreme ends of functional range, and that breaks down rather quickly. Neither group I mentioned is cleanly correct.

AussieDan wrote:
I also wonder about the effect the shape of the contact patch has on the slip angle of the tire, which is ultimately what determines the limit of grip.

The very end of what I've found is this:

Figure that, in a very rough model (with P/PSI=SI being instrumental albeit inaccurate), three tires are identical in diameter but different in width. Tire width is proportionalish to patch width and inversely proportionalish to patch length front-to-back.

Shape and slip angle seem to be quite related! On <this thread> on eng-tips.com, this post is a monster, but worth the time:
Knap, on eng-tips.com wrote:
Knap (Industrial) 22 Jul 06 19:41

...

When you look at lateral grip other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front.

The longer the contact patch is, the more gradually break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak as there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point.

When you fit wider tyres the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit.

If this logic is correct then increasing pressure in the tyre further improves grip, since more pressure = less contact patch area = shorter contact patch = better grip?
However more rubber on the road does help grip due to the hysteresis properties of rubber. As rubber expands to fill a depression in the road, it takes some time to do so. When a tyre is sliding (and due to the slip angle, the rear most portion of the contact patch slides at even low cornering forces), this means that the upward rise of the depression to which the tyre is moving has more rubber acting on it that does the upwards rise on the other side. This allows a pressure differential in the lateral plane, providing frictional resistance over and above that offered by simple friction. As the tyre vertical load increases, the rubber is forced more fully, and more quickly into the depressions, overcoming the hysteresis and reacting on both sides of the upward rise from the depression more evenly – giving less pressure differential and less grip.
Low tyre pressure is better for grip from deformation and hysteresis.
Tuning the pressure is about balancing the contact patch length (which is better as pressure goes up), and the contact patch pressure (which is better as tyre pressure goes down). Even though the optimum grip may be achieved at low pressures higher slip (because the hysteresis element is significant),low pressure increases tyre deflection, which increases heat (less even radius over longer contact patch). It therefore appears that the best way to increase grip is a wide tyre as this gives a shorter contact patch for the same inflation pressure.

However tuning the handling balance using tyre pressures, appears to be a combination of trying to match front and rear slip angles, ultimate grip at the limit, and effect of heat which may effect inflation pressure and tyre compound?

It's rapidly becoming evident that there are no clear conceptual methods for deciding on a proper tire size - only a window - and that there are plenty of factors beyond what will fit and what you can afford.



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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Thu Dec 20, 2012 12:32 pm 
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What I've found, specifically with a 205 width Star Spec, is an 8" wheel (slight stretch) gives slightly better response than the narrower wheel (7"), but it's subtle enough that it's hard to tell. In theory it should, but in practice I suspect most people wouldn't be able to tell the difference, and I'm not convinced that I could under all circumstances.

I have definitely noticed a difference in responsiveness between wide tires and narrow tires on a given car. I used to drive a 1968 BMW 2002 on proper stock width tires (165/80-13 Michelins) and while the ultimate grip was low, it was wonderfully responsive and a delight to drive compared to my '73 2002 with a stiffer suspension and 205 width tires. The '73 was overall faster, but the '68 was fun because it responded so well.

In autocross most people give away feel for higher lateral grip when they can. The practical result of this is you have to "lead" the car more, and allow time for the tire to respond fully before you ask it to turn the car. My 325 is a great example - it has to have time to "take a set" before you can get max lateral grip, so you have to initiate every turn earlier than you otherwise would in, say, a Miata. But, the BMW has a response like lightning compare to the old Corolla on 8" wide bias ply Hoosiers. The first time I hopped out of the Corolla and hopped in my sister's Miata and made an autocross run, I promptly ran right over the first cone in the slalom because the car actually turned when I wanted it to. Some of that was the car, but some was the fat tires with lots of lateral grip and a glacially slow response time (her car was on the original factory street tires at the time).

The post from eng-tips is a pretty good discussion of the interaction of the rough road surface with the tire, which most people seem to forget about.

And airplane weights are probably pretty accurate, because the pig won't fly if you overload it (or get the weight distribution wrong) and the consequences of attempting to fly an airplane overloaded or with improper weight distribution are severe and immediate.)


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 Post subject: Re: Tires - too much, too little, too cloudy?
Unread postPosted: Thu Dec 20, 2012 1:53 pm 
Subie Tech
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LetItSnow wrote:
It's rapidly becoming evident that there are no clear conceptual methods for deciding on a proper tire size - only a window - and that there are plenty of factors beyond what will fit and what you can afford.


This is what it really boils down to. Take all of the above advice and science into consideration, make some educated guesses about what to try, and see what works best for you, your particular car setup, and your driving style/preferences. The only people who do true rigorous scientific testing are extremely well-funded. Not many autocrossers fit that bill.


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