tire pressure?

Kfoote
really you give a general number for tire pressure dropping on a huge amount of different cars out here, and you shouldnt do that. if this person has low profile tires on there car and they have a min psi rating of 65, you telling them its safe to run down to 25 pounds is out of control.

then you say it will help the initial launch and might comprimise cornering. it wont help your launch and it will hurt your cornering.
most street tires have soft sidewalls compared to a performance tire. but nowhere near soft enough to flex and help with a luanch if you drop the tire pressure.

then you go on to say "total grip of a tire is a function of the surface area of contact. Lower tire pressure = larger contact area = more grip = better launch."
that is very true, if you own a non radial tire. i dont know about every car made but i know that you would have to look hard and far to find someone without radial tires.
you made a comment about my speed and tire pressure rating saying you had to go over 130 to make a difference. all tires have different heat ratings and tire wear ratings and its a specific number for a general thing. if your tires are rated at a high enough speed they will also tell you if you need to lower pressure at what speed you need to lower it to.

whats this about having diff tire pressure on diff wheels of your car? most cars have the same tires all the way around, why would you want them to be diff tire pressures? thier is no point to ever do that unless the owners manual in your car says too. ive owned plenty of cars with all diff types of drivetrains and all diff purposes, none of them have ever had diff tire pressures for diff wheels.

i got kind of lost there trying to read everything and go back and forth with it all, so if i missed something or messed something all up forgive me and question it, i will respond

Kfoote
really you give a general number for tire pressure dropping on a huge amount of different cars out here, and you shouldnt do that. if this person has low profile tires on there car and they have a min psi rating of 65, you telling them its safe to run down to 25 pounds is out of control.

I realize this is an exaggeration, but I'm generalizing based on the vehicle is a car between 2200 and 4500 lbs, running on tires running 35-60 series, and are somewhat performance oriented. In my experience, 25 PSI is where the bead starts to pull off a standard street tire in very hard cornering, which is why I have stressed that this is for straight line racing only.

then you say it will help the initial launch and might comprimise cornering. it wont help your launch and it will hurt your cornering.
most street tires have soft sidewalls compared to a performance tire. but nowhere near soft enough to flex and help with a luanch if you drop the tire pressure.

The softer the sidewall, the more reducing the tire pressure will help, but even if the sidewall is very stiff (run-flat tires are a perfect example), it will help some, just not as much.

then you go on to say "total grip of a tire is a function of the surface area of contact. Lower tire pressure = larger contact area = more grip = better launch."
that is very true, if you own a non radial tire. i dont know about every car made but i know that you would have to look hard and far to find someone without radial tires.

This works for radial tires too, just not as well as for bias ply tires. The contact patch is not increased as much as with an identical bias-ply tire, but it does increase. Radial tires still expand and flex with tire pressure, just not as much. You may only see a 5% increase in surface area, but a 5% increase in surface area is better then no increase in surface area.

you made a comment about my speed and tire pressure rating saying you had to go over 130 to make a difference. all tires have different heat ratings and tire wear ratings and its a specific number for a general thing. if your tires are rated at a high enough speed they will also tell you if you need to lower pressure at what speed you need to lower it to.

...and for most moderate to high performance street tires currently on the market, the number is generally about 1 PSI per 10 MPH over 120 MPH.

whats this about having diff tire pressure on diff wheels of your car? most cars have the same tires all the way around, why would you want them to be diff tire pressures? thier is no point to ever do that unless the owners manual in your car says too. ive owned plenty of cars with all diff types of drivetrains and all diff purposes, none of them have ever had diff tire pressures for diff wheels.

This statement shows that you have probably never played with tire pressures, never been to a race track or auto-X as anything more than a spectator, and that you probably do not have the scientific or mathematical background to make the statements you have been making. First off, none of the cars I own have the same manufacturer's reccomended tire pressures front and rear even though they all run the same size tires and wheels front and rear. Manufacturers reccomended tire pressures are for the OEM tires on the car, and are generally set up so that they have understeer while cornering for safety for the general public. This usually has very little to do with what the actual working range of the tire is, aside from that it's safely within that range. The vehicle manufacturers reccommended tire pressures are a good starting point for general reference, but are not an absolute that must be followed. Changing tire pressure affects the effective spring rate of the sidewall of the tire, and can have a significant impact on the cornering and balance of the car, and can be used to tune the car to specific conditions. As an example of this, in my Porsche 944 on track, a 1 PSI decrease in the right rear tire from what I had been running improved my lap time at NHIS's North Oval configuration by over a second because it allowed me to get the power down a bit sooner coming out of the slow, tight right hand corner leading on to the oval, and able to carry that extra bit of speed for about 1/2 mile. A lot of maximizing the speed of the car usually comes down to tuning the tire pressures for the very specific conditions that the car is going to be exposed to. For straight line acceleration from a stop, the idea is to maximize the grip in the drive wheels to get as much power down as soon as you can, and minimize the grip of the non-drive wheels to reduce drag. Since we are only dealing with straight line acceleration, cornering really isn't an issue. As long as you remember that you can't corner very well, you're fine. I would not reccommend 25 PSI for normal daily driving, but unless you are conering very hard (faster than you really should be on the street), with a reasonable set of somewhat modern radial tires, it's not dangerous.

i got kind of lost there trying to read everything and go back and forth with it all, so if i missed something or messed something all up forgive me and question it, i will respond

For a little background, I have 80+ days driving on track over the last 10 years, hold an SCCA National license, have done suspension setup work on several race winning cars, was at race tracks working on or driving 55 days in 2004, and have a BS in Mathematics. Tire pressures are the basic tuning tool that is used on most race cars.

sorry it took me so long to get back to you....i tried your drop tire pressure theory. i did a burn out on the street at full psi on my street tires. thier were two marks of almost equal strength, both were full width, and very consistant with color. i dropped my tire pressure from 44psi to 25psi, and tried the same thing. again two tire marks, only this time i noticed that on both tires the middle of the tire was less dark than the rest of the tire mark. my guess is that the tire isnt inflated enough and the sidewalls are too strong. leaveing all the weight, and tracktion on the outside edge of my tires. giving me less grip then when i was at full tire pressure
i run goodyear eagle f1's gs-d3's 245 50 16

You can't tell anything from burnout marks. It could be that the tire pressures are too low and there's less contact, or the lower tire pressures give you more grip, and spin less. Aside from that, a burnout is definitely not the fastest way to launch a car. The only good data would be to go to a track and get 60 ft times.

i was just using the burnout as a way to show you that less tire is hitting the ground. and i know a burnout is not the best way to launch a car

I just realized that you said you're at 44 PSI cold. That's WAY too high for that tire. You should be somewhere in the mid-30's in that car cold with those tires for normal street use. Assuming this is in the Trans-Am, I would start the pressures at 34 F and 32 rear. You will be amazed at how much better the car handles.

also i dont know if its been mentioned, but assymetrical tread tires have an opposite effect with pressure variance.

Asymmetrical tread patterns may affect the necessarry tire pressure in different ways, but the optimim tire pressure for straight line accleeration is usually affected more by sidewall stiffness than anything else. There are some tires that have an asymmetrical tread pattern and an asymmetrical sidewall, and the asymmetrical sidewall can do very strange things to the optimum alignment and tire pressure setup.