Tire pressures

[StarPD]

Newer and younger members may not be familiar with how and/or how much tire pressures affect all-around performance, especially ride and handling, so here are some of my observations from years of playing with cars.

As I've noted in other threads, I'm now learning what works best on my new to me 2005 Q45, on which I recently installed TSW 18x8"-40 "Kyalami" chrome wheels and 245/45ZR18-100W Bridgestone Potenza RE050A "Pole Position" tires.

Mfr fecommended tire pressure is 33 psi.Tires were new and not broken in yet.

Initially, I started with 36 psi. Ride was initially a little harsh, but the car had good tracking both straight line and in curves. Turn in was good, but not excellent. Stability was good. Noise level was reasonable, but impacts with expansion joints and potholes was loud. There was no tramlining.

I tried 34 psi, and ride was improved, tracking remained good, noise remained the same (low), and impact noise was better. Turn in remained good and there was stil no tramlining.

I then went to 33 psi according to my tire pressure gauge. (The TPS indicated 32 psi) Ride was improved, but tracking deteriorated and tramlining appeared. Turn in wasn't quite as good, and stability was down.

By now, the tires were broken in.

I set pressure to back up to exactly 34.5 indicated on my electronic gauge, 34 psi on the TPS. Excellent tracking returned, and tramlining disappeared. Turn-in improved, and ride remained good. Impact noise was increased slightly, but is still acceptable. Traction and stability are excellent.

I really like these tires. They have mitigated a lot of my steering and handling issues, and the car is a lot closer to what I think it should be. I still need to find a thicker rear sway bar to reduce understeer even further, and find a way to improve steering feel, but the steering/handling/ride is at least acceptable. I think when I go to Michelin PS2s next year, it will be even better, but for now, especially considering the price difference, the Pole Positions work well for me.

I believe I've found the "sweet spot" for these tires on this car. 34.5 psi seems to be where it likes to be at for best all-around performance. Every car/tire combination has its own "sweet spot", where ride, noise and handling are at optimum. Owners should try different tire pressure settings on their car, leaving each change for at least a few days, driving over different roads in different conditions and at different speeds and with different driving techniques.

My experience is that nothing under mfr recommended ever works well. Usually, depending on vehicle and tires, anywhere from 1 to 4 psi over recommended turns out to be best. Tire pressures on cars that call for equal pressures front and read CAN be adjusted with different pressures front and rear to achieve different handling characteristics. Higher rear pressures can mitigate understeer, but frequently result in harsh ride. Higher front pressures can provide sharper steering response, but frequently results in increased understeer. Regardless, experimenting can tell you what works best for you. Remember, if you replace your tires with different ones, you will have to go through the process all over again. What worked with one kind or size of tire may not work with another.

While experimenting with tire pressures, they must be set absolutely cold early in the morning before driving, in the lowest ambient temps for the area and season. It's usually easier to inflate the tires above what is desired at night before parking, usually around 5 to 6 psi above hot pressure. then bleed them down first thing in the AM to the desired setting. I am getting my master tire pressure gauge (one that is used to determine accuracy on other gauges, cost me around $125 in 1974) back from my son, and will use it to monitor pressure and check my current electronic gauge.

FWIW, as I've stated in other threads, I am unusually sensitive to ride and handling characteristics, and can feel/sense variations most people cannot. When I get a chance to try the car out in some extreme handling rides, like fast rides on curves in the mountains, I'll report back again.

[szh]

Good info!

I like to set my tires to 36 psi for a number of reasons:

1. They tend to drop about 1 to 2 psi in a matter of a week of two. Gives me the time to measure and correct occasionally, rather than continually!

2. Slightly higher load capacity. The load indexes are measured at 36 psi standards, so this is a convenient setting point.

3. I use an external digital gauge, and I figure setting it a bit high to compensate for measurement errors (in the gauge) is better than being on the low side.

Z

[szh]

While experimenting with tire pressures, they must be set absolutely cold early in the morning before driving, in the lowest ambient temps for the area and season.

An absolutely essential point!

Dennis (Q45tech) has often mentioned here that the pressure should be measured and adjusted "before the sun has had a chance to warm up the tire".!

Z

[maxnix]

My experience is that nothing under mfr recommended ever works well.

Not only that, it is usually unsafe at sustained interstate speeds with only the driver and hafl a tank of gas on board.

Look at the pressure vs. load carrying capacity charts Z once published.

[StarPD]

Not only that, it is usually unsafe at sustained interstate speeds with only the driver and hafl a tank of gas on board.

Look at the pressure vs. load carrying capacity charts Z once published.

Absolutely. Unfortunately, there are folks who aren't aware of how serious this can be, and will deliberately underinflate their tires for the softer ride it provides.

As a matter of fact, just like szhosain says, there is usually inaccuracy in gauges, and it's far better to err on the side of safety. And like he states further, tires DO lose pressure over time. I suspect that if any 100 cars were checked for tire pressure, the majority of them, not our members, of course, would be considerably underinflated.

Tires should be checked at least weekly, and regardless of base settings each owner prefers, they should be set a little high to allow for this loss.

Too much pressure can be almost as bad as too little though, and care must be taken with higher than recommended settings. It's one of the few exceptions to the rule "Too much is almost enough". Well, that works for money, horsepower, and chocolate though.

[szh]

Look at the pressure vs. load carrying capacity charts Z once published.

I will dig those out (somewhere on my computer, I think!) and repost them ...

Z

[StarPD]

I will dig those out (somewhere on my computer, I think!) and repost them ...

Z

Those tables are certainly good for starters, but as I've stated, each tire/vehicle combination can be adjusted (above mfr recommended of course) for optimum ride/handling characteristics, depending on the owner/driver's preferences. It takes a little experimenting to find that "sweet spot" for each combination and each individual.

Please do post the tables though. They are an excellent template, and each one of us should know them.

Thanks.

[szh]

Those tables are certainly good for starters, but as I've stated, each tire/vehicle combination can be adjusted (above mfr recommended of course) for optimum ride/handling characteristics, depending on the owner/driver's preferences. It takes a little experimenting to find that "sweet spot" for each combination and each individual.

Oh, definitely. My tables are just generalized pressure vs. load-index for the various tire sizes from the Tire and Rubber Manufacturers Association - you still have to look at the actual car-tire combination to decide what to do.

I still have to look for them though.

Z