Jumat, 27 November 2009

tweaking tire pressure for maximum handling

Copy paste dari : http://www.team-integra.net/sections/ar ... icleID=398

DETERMINING YOUR PROPER TIRE PRESSURE FOR HANDLING

Tire pressure has a profound effect on your cars handling ability. Acura's pressure recommendations are geared towards stock tires, and a balance between handling and ride quality. If you want to achieve the maximum handling ability on your tires, you have to deviate from acura's recommendations. With the extra handling comes a sacrifice, often tire wear and ride quality. For performance enthusiasts, and AutoX/Track events, that sacrifice is a small one for the grip and handling that proper pressure provides.



Finding your optimal tire pressure requires:
1) access to air
2) a place where you can abuse your tires
3) An accurate tire gauge
4) a bottle of shoe polish


For air, most gas stations provide air for free, or you can find an electronic air pump that plugs into your cigarette adapter. For shoe polish, pick up a bottle of liquid white shoe polish with a sponge tip. Finding a good place to really work your tires is often the hardest part. Just some suggestions, you could getting permission from an owner of a big parking lot, maybe your boss will let you use the company parking lot after hours, one of the most common places to tweak your tire pressure is an AutoX/Track event.

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What PSI you should start off with depends from tire to tire, but since its easier to remove air than add it, I recommend you start fairly high. I started with 5psi below the rated maximum, in my case that's 40psi.

Start off by driving your car around for awhile, to heat up the tires. As the air inside of the tires heats up, it will expand and the pressure will increase.

Make three-four marks on all of your tires with the shoe polish. Each mark should run from the last block of tread and run down over the shoulder of your tire and onto the sidewall.


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Now you are at the fun part, putting stress on your tires. It is very important that you do this safely, and legally. Tire pressure isn't going to do you any good if you wreck your car or get your license suspended. Start doing some heavy turning, and really your tires squealing. If you don't go fast enough you probably won't rub off any shoe polish. You want to create the maximum load possible on the sidewall of your tires (when the sidewall is going to flex the most). After abusing your tires for awhile, stop and take a look at the shoe polish marks. If the scuff is still on the edge of the tread, you have to much air pressure. If the shoe polish has been rubbed off on the sidewall and past the edge of the tread you need to increase the air pressure.




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Make adjustments in 2 PSI increments, until you find your optimal tire pressure for all four tires. After you have changed the pressure, make the marks again and do another abusive run after a short break. Make sure to let your tires cool down a bit, as street tires get hotter their grip diminishes.

Once you have found the optimal PSI for all the tires, make a note of it for future reference. What you will end up with, is the largest contact point with out driving on the sidewall of your tires. A general rule of thumb, is the larger the contact point the more grip you have. By lowering the PSI the tire has, the more it will sag at the bottom creating a larger contact point. Of course there is a limit, once you start driving on the sidewall your grip will be severely effected because the opposite side of the tread will get lifted into the air.

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ADVANCED TIRE PRESSURE TUNING

Once you have found the optimal pressure for a large contact patch, you can throw the large patch out the window to alter how your car handles. Under heavy steering, there are three possible handling characteristics that you can encounter.

1) Understeer - This is very common for factory cars. Understeer is generally considered safer for normal driving. If your car is understeering you will notice that the front tires will lose their grip and you will slide instead of turning into a corner. Moving the steering wheel even further into the turn will not effect the direction you are going, you will just continue to slide.

2) Oversteer - This is where the rear tires loose their grip and your back end will slide out behind you.

3) Neutural handling - This is where all tires loose their grip at the same time. When this happens, you will end up in a four-wheel drift around the corner. This is often the characteristic most people try and achieve.

Which characteristic you wants depends on what you are doing, but you can tweak how your car handles by adjusting your tire pressure.

Modifying front or rear tire pressure allows you to alter when the front or rear tires loose their grip. If you have the factory understeer (where your front tires lose grip, but your back do not), and you want to move more towards neutral handling, you would alter the pressure in your rear tires so they lose grip sooner. On the other hand, if you had neutral handling and wanted some oversteer you could change the pressure in your rear tires so they lose grip sooner. This would be good for autocross so you could whip your back end around a sharp corner quickly.

Which way to alter your pressure in this case depends on driver preference. Both increasing and decreasing pressure will cause the tires to loose grip sooner. By decreasing the pressure, the tires will get soft and mushy and it will roll over on the sidewall and lift the opposite tread off the ground-decreasing grip. Increasing the pressure you decrease the footprint creating less grip. This also results in a stiffer sidewall, which can increase the steering response. The advantage to decreasing the pressure is it will have a gentle transition and it won't lose grip as quickly as over-inflated tires.

Doing these adjustments should be made very gradually, and should only be done in .5 - 1psi increments.

Once you have found your optimal tire pressure for your goals, you can easily revert back to it whenever you need to focus purely on handling, and can forget about tire wear and ride quality.

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