ajmacdon wrote:
There's a mix of art and science here that is pretty interesting. On the science side keep in mind that you get zero extra rubber on the road with a wider tire. As much as your brain wants to ignore the math it's a fact that your contact patch is very simply vehicle mass divided by tire pressure, that's it. That's IT. What you are changing is the shape of the contact patch. A wider tire puts a short, wide rectangle of rubber on the road vs a narrow tire with a long, narrow rectangle of rubber. Trust me, in the vehicle dynamics world we've done the math and made these measurements 100 different ways, you don't put any more rubber down until you lower the air pressure (which you really don't do substantially until you're way oversize on your tire and there's a lot of other problems when you get to that point).
SO, given the amount of rubber is the same the extra grip comes from the shape of the patch. If you're building a deck you don't lay the joists on their side and the same is true of a tire contact patch. For a lot of interesting reasons a short, wide contact patch improves your cornering capability. A great, readable book on this subject is "the racing and high performance tire" by Paul Haney. He's not an engineer but a motorsports journalist with a lot of engineer friends so it's quite a good read. The most explainable aspect is that when providing any amount of side force a tire is operating at some slip angle, its never rolling straight along when you're cornering above about 0.3G. With a long, narrow contact patch a large part of the contact patch is already sliding geometrically at lower cornering forces (think about the double rear axles of an 18 wheeler and how they slide around intersections with their combined 4 foot long contact patch) A slipping piece of rubber is not as grippy as a static piece so you loose a lot just due to most of your contact patch sliding before the actual car is sliding (which happens when the last little bit of the contact patch finally lets go). On the short, wide patch of a wider tire much less of it is slipping as the tire operates with any slip angle so it can all grab statically right up until the bitter end when the whole tire lets go. Again, great drawings, etc in Pauls book.
The art comes in balancing the extra grip you get with a wider tire with the extra feel you get with a nice, long contact patch that you can feel transitioning to slip much easier (since parts of the contact patch start sliding much sooner in the cornering process). There's also a point where you can't put heat in the wider tire at all since it does't slide enough. There's a lot more interesting reasons why you reach a point of diminishing returns with added width that are probably too much to type here.
On our FSAE car we found identical lap times for our top drivers with 6" wide vs 8" wide tires but the car drove completely differently with each setup. Novice drivers were much faster on the narrower tire. Like FSAE cars, our cars are exceedingly light relative to the typical design considerations for pass car tires so you have other tire design aspects working against you when you go to really wide pass car tires designed for 3-4k lb cars.
Another consideration is wheel width. I don't advocate for stretched tires like you see on stancenation but you generally want to go with the widest rim on the tirerack rim width column for your specific tire. (they usually have about 1.5" variation, it's just data from the tire manufacturer that they are kind enough to pass along in the tire specs page) At GM we tuned the balance of the Camaro SS 1LE by using the same 305 tire front and rear but a half inch wider wheel at the rear. That gave the rear a bit more grip and much better precision. We did the opposite on the ZR1, we used the same tire as the Z06 but widened the front wheel to handle the extra weight of the bigger engine. You certainly don't want to do what we all probably did in high school and cram the widest tire you can on whatever wheels you have, it will drive sloppy and lack grip relative to the tire width. Either figure out how to go wider on the rim or check the ego and drop tire width.
So, in the end, that's how I'm choosing tires for the stalker. I can fit a 9" wheel on all 4 corners. A typical 40 series 245 tire has a wheel width range from 7.5" to 9" so 245 is the answer. Again, most people would say you can cram up to a 275 on a 9" wheel but that would drive like garbage and likely not go any faster.
Hope that's interesting, there's a lot more to talk about but get Paul's book, he does a much better job.
Alex
Great post, with experience based comments. Thanks.
The most interesting part to me is the comment about different drivers liking different tire sizes, even though the lap times were the same. This type of experience is why I suggested the OP try different combinations. Getting something you are comfortable with, is the most important part for me. If I like the way the car responds, it increases my confidence and allows me to drive harder, for longer periods of time with less mental fatigue.
Ken
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