spinaltoad

Sorry, I don't know how to do subscript in this thing.

Variables

Car weight = W
Weight Distribution Front = Bf
Weight Distribution Rear = Br
maximum lateral acceleration in g’s = G use .85 if you do not know your acceleration
Aspect ratio = Ta
Tire width in millimeters = Tw
if your tires are 235/65 you tire width is 235mm and you rim height to width ratio is 65

Constants(assumptions)

maximum lateral acceleration L= .60
Rim width to tire width ratio = .80 ~ 1.20

Formulas

cornering force: W + G = Fc

cornering force on outside tires: Fc x L = Fc x .60 = Fo

cornering force on outside front tire: Fo x Bf = Ff

cornering force on outside rear tire is: Fo x Br = Fr

front tire cross section: (Tw/25.4mm/") x (Tw5.4mm/"x Ta) = TCSF

rear tire cross section: (Tw/25.4mm/") x (Tw/25.4mm/"x Ta) = TCSR

front tire pressure: (Ff) ) (TCSF) = PSI

rear tire pressure: (Ff) ) (TCSR) = PSI


If there is enough demand, I could make a program to calculate it automatically.

inovashn

nice info. never new all of that. there's science to everything

VSsc400

iTrader: (1)

I would love to see a calculator

spinaltoad

You can even have a drop down list for all the Lexus Models.

mkorsu

You will be my hero if you make a calculator!!!

sc3000

its tyre an im from oz and love vegemite

Un4Scene

iTrader: (1)

I'm gonna take a step back from all the formulae and tables, and cover a couple of important things that I think were skipped. First off, The weight of your vehicle is not being supported by the tire, it's being supported by the pressurized air within the tire - the tire just serves to contain the air, provide traction, and provide suspension. I'll get in to each of those things seperately, but I think there's a couple of other factoids that should be noted first.

As stated before, the manufacturer's recomended tire pressure is based on the original equipment (OE) tire. I don't know of any auto manufacturers that make their own tires, and OE tires are often custom designed and built for a specific manufacturer's specific vehicle, and are usually only available through a dealership. They may appear to be identical on paper, but in actuality there are many cases where you can buy a tire with the same brand, model, size and specs from an aftermarket source (such as Tire Rack) and it will be completely different from the OE tire.

In the real world (as opposed to on-paper comparisons), there are so many variables from one tire to another that you're best bet is to adjust the tire pressure based on actual usage and experience. There are differences in sidewall and crown construction, tread design, rubber compounds, heat dissipation, etc... It's usually ok to use the factory recomended pressure for the starting point, but if you want to get the most out of your tires you'll need to do some experimentation and tweaking - especially if you're not using the original wheels with the original tire sizes.

Containing The Air:
Just like any piece of metal, plastic, or wood, In order to support a given amount of weight, you need a given amount of air. That's simple enough, but here's the kicker:
If you decrease the volume of space within the tire, you must increase the mass of air within that volume to support the same weight!
To clarify, a stock tire and wheel holds a certain amount of air at a certain psi. If you switch from 16" wheels to 19" wheels of the same width, and maintain the same overall diameter, then you are drastically decreasing the amount of space that's inside the tire. In order to compensate for that reduced space, you have to run a higher pressure in order to support the weight of the vehicle. This is part of the reason why a lot of the extremely low-profile tires are rated for a higher max. psi than higher-profile tires are (the other part of the reason is that sportier tires are designed to withstand higher cornering loads, which is also more weight, and therefor require higher pressures). It's not just the tire size either - aftermarket wheels have varying barrell designs that will also change the volume of air that can be contained. Racing wheels are often designed to maximize the available space around the hub so that larger brakes can be mounted - that added space around the hub is taken directly from the air chamber.

The max. psi rating on the sidewall of a tire is not necessarily the maximum pressure that is safe to run - that psi is the maximum pressure that is safe to run when the tire is used at it's maximum weight rating. That being said, I'm not reccomending anybody go out and pump their tires up to 60psi because their vehicle weighs half of the weight rating! Very few cars actually have a perfect 50/50 weight distribution from front-to-back and from side-to-side, so unless you're car is corner-balanced, you don't know how much weight is on each individual tire. Also, every time you turn, accelerate, brake, hit a bump, add or remove anything from the car, or drive on ground that isn't perfectly flat, you're changing the distribution of weight on the tires. I strongly reccomend against trying to test their limits! Even if you don't blow them up, there is absolutely nothing to be gained by it.

Providing Traction:
Whether you're primary goal is high-performance, drivability, or safety, traction is your best friend. Every single movement that your vehicle experiences is transferred through the tires, and affects the traction. For that reason, the tires are arguably the single most important thing you can put on your vehicle. NOTHING has a larger impact on the overall driving experience of a vehicle than the tires. You should always use the best tires that you can afford - there is no substitute for good tires. That being said, the traction that your tires can provide is directly affected by the air pressure within those tires, and there is no magic number that will provide maximum traction. Traction is a factor of friction. Friction is a factor of contact patch, and, no matter what size tires you have, the contact patch will change with the air pressure. This is where the experimenting and tweaking comes in. To start with the tweaking, you have to know what you're trying to accomplish. Tuning air pressure for the fastest 0-60 times and tuning for highest cornering G's will often require significantly different air pressures. For straight-line acceleration you'll want a significant amount of "give" in the sidewall to prevent wheel spin, which means lower pressure. For high-speed cornering you'll want a higher pressure to reduce deflection and sidewall-flex under cornering loads. For street driving, you'll probably want something in the middle, but the longer and harder you drive, the more heat will build up in the tires, which increases pressure. If you've ever been to an auto-X or drag race, you'll notice that many drivers fiddle with their tire pressures between runs - this is the reason why. If you want to get in depth with it, you can jack the car up, use a marker to color all the tread blocks on one side of the tire, and then set it down on a piece of paper to see what you're contact patch looks like. if you want to get really in depth with it, you can drive the car to get the tires up to operating temperature, use a pyrometer to see how hot they are, then go home and use a heat gun to reach that temperature before checking the contact patch. What most people do is just keep an eye on the way that the tread is wearing, though this isn't as definitive. Abnormal wear can also be caused by bad alignment, worn struts, bad wheel bearings, etc... I'm not gonna get into diagnosing abnormal wear here, as it'd be damn hard to explain without being able to touch the tire and feel the difference in person. Maximizing your contact patch will also provide the longest possible life from your tire, as the abuse is distributed across the largest possible area. If you really want to stretch the life of your tires, you could overinflate them till all the tread in the middle is worn down, then underinflate them till they only wear on the outside... it works, but I don't reccomend it.
[Side note: "low rolling resistance" tires that are designed to maximize fuel efficiency are actually designed to provide as little friction, and therefor traction, as possible. Personally, I think LRR tires are one of the most dangerous "developments" in automotive technology that has ever been invented. Unfortunately, many new cars are sold with these to help increase the factory MPG numbers, which helps the auto manufacturers meet federal CAFE regulations.]

Providing Suspension:
As far as tires are concerned, the suspension they provide is primarily just a factor relating to comfort and the durability of other components (not durability of the tires). Generally speaking, the taller the sidewall is, the softer the ride will be. This is due to two factors: a taller sidwall will be more flexible, which allows the tire to absorb larger bumps without transferring them to the suspension; and a taller sidewall increases the volume of the air chamber which means you can run lower pressures. Factory reccomended pressures are usually intended primarily for comfort (they certainly are for any Lexus, aside from an LFA, or one of the special sports models), and they generally DO NOT provide the best contact patch or wear patttern. If you don't care about anything but comfort, run the factory reccomendation, but nothing lower. If you want anything else, you'll probably want a higher pressure.

And with that, I'm gonna call it good. I realize that this doesn't tell you anything about what pressure to run in your tires, but I think it's a decent guide on how to figure it out for yourself. I could go in to a lot more detail, and maybe I will if enough people request it, but, really, I'm just trying to be helpful and get my 30 posts to become a "real" CL member!.

One more bit of usefullness that I've found to be enormously beneficial:
http://www.rimsntires.com/specs.jsp
With that, you can see a fairly accurate comparison between different sizes of tires and wheels. It's not really related to tire pressure, but It's the best tire/wheel size calculator I've ever found.

~Laken

ASTERiSK

I usually try to keep it at 40, pretty consistent. I'm sure some people would disagree, everybody has their own opinion on an ideal PSI.