Tire Inflation Pressure

[mslerner2002]

I have been on a quest to improve the ride quality of my BRZ. Recently I downsized to Enkei 16" x 7" rims with 225/50r16 General Altimax HP. The extra 1" of sidewall has smoothed the ride noticeably over the 225/45r17 Michelin Super Sports that I previously had on the car. (Sure, the car doesn't corner nearly as well as before, but someone can start another thread if they want to compare and contrast the merits of Grand Touring All-season tires vs. Ultra High Performance Summer Tires). But I digress....

So, I am now obsessing on tire pressure and I'd like to enlist all the engineers out there to warm up their calculators and join in!

The sticker on my BRZ door says GVWR of 3748lbs with a maximum cargo capacity of 699lbs.

My 225/50r16 Altimax HP have a Maximum Load capacity of 1389lbs/tire and a maximum inflation of 51psi.

Assuming (1) BRZ + (1) occupant = 3194lbs (yes I am probably lighter than you)

Assuming 3194lbs for car + driver / 5556 lbs max load capacity for the four combined tires = 58% of tire capacity utilized.

Assuming 51 psi is the inflation required at the tires 1389 lb maximum load

Can I inflate my tires to 58% of the 51 psi maximum to arrive at the ideal tire pressure?

Here is another little tidbit: At the vehicle's maximum load of 699lbs yielding the 3748 GVWR, the tire capacity utilized jumps to 68%. At 68% of the 51 psi maximum tire pressure, you get 34.4 psi which is pretty close to the manufacturer's recommended 35psi.

Thoughts???

[phrosty]

There's no special number for PSI -- you can go up to the maximum and you'll get better mileage and handling, but it'll be a less comfortable ride. So long as you've got enough to handle the load you're good. I would give some wiggle room, and not go lower than 30.

[7thgear]

too much math

grab a pyrometer and set your psi as low as you can before the tire starts heating too much on the outsides compared to the middle...

[OICU812]

Originally Posted by 7thgear

too much math

grab a pyrometer and set your psi as low as you can before the tire starts heating too much on the outsides compared to the middle...

+1.

Put 33lbs in em try em go from there no need for a novel lol.

[Rayme]

Ideal tire pressure for what? Fuel economy? Comfort? Grip?

That question has no answer.

[wparsons]

Originally Posted by 7thgear

too much math

grab a pyrometer and set your psi as low as you can before the tire starts heating too much on the outsides compared to the middle...

It really all depends on what you're trying to get out of the tires too, you'll get longer wear and better mileage with slightly higher pressures.

[Muskokan]

i go 32 on my fat winters and 36 for stock summers. i think you are over thinking this waaaay to much. smoke a blunt friend

[ZDan]

Originally Posted by mslerner2002

Assuming (1) BRZ + (1) occupant = 3194lbs (yes I am probably lighter than you)

If the car weighs 3194 lb with a full tank + you, you must weigh about 430 lb or so!

As far as tire pressure, it isn't only a function of load. You'll be fine anywhere in the 30-36psi range, adjust to your liking.

[jadatis]

Registered to this forum to meddle in this topic.
Live in Holland ( Europe) and got hold of the formula that the European tyre makers use to detemine needed pressure for a sertain load, and went running with it.
Learned myself Excell to make spreadsheets for it.
Gathered much info on tire-pressure and tires in time.
Wont introduce myself in a seperate topic, because I dont have one of the cars of this forum and mosly only answer to tire-related questions.

Now about your topic.
You are on the right way with determining the loads , but the total weight is mostly not equally devided over front and back.
Normal with only driver front is more load then back.
Fully loaded rear more then front.
R/L equall pressure is adviced thoug weight can be different too, even crossed weight differences R/L between the axles.

Then about the tires: The 51 psi is the maximum pressure ( Pmax) of the tire for P-tires and XL/reinforced/extraload, only important here .
The needed pressure for the maximum load/ maxloadpressure/reference-pressure (Pr) is lower, and the difference between Pr and Pmax is used for higher speed then 160km/99m/h and alighnment ( camber angle above 2 degr) to highen up Pr with a system depending on speedcode of tire.

So if you have P-tires and speed is below 99m/h you can use Pr of 35 psi for the maximum load. For XL// this Pr is 41 psi for American tires.

so enaugh for my introduction.

[mslerner2002]

Just to clarify,
1) I chose to ignore the 53/47 weight distribution
2) I realize everyone says we all believe our fr-s/brz weigh ~2800lbs but the GVWR sticker on the door is what it is. Is it possible the car weighs 2800lbs dry and then when you add all the fluids, it gains 200+ lbs???
3) The comment about using a tire pyrometer is spot on. On my next long road trip, I will measure the inside, center and outside temperatures of all the tires. The only caveat is that I am running -1.0 front neg camber and -1.7 rear neg camber, so the insides should be a little hotter.
4) Ideal pressure can be for both handling AND comfort. I have done my share of track days and SCCA racing and most times the starting cold temp is ~30psi and hot runs around 35-36 for most production cars. Sure there are instances when a hot temp of 38 psi can be the target, but when I ran my Michelin Pilot Sports on this car, the ideal hot temp was 35-36psi. Obviously, if I set my cold temp to 31psi and drive around, they will likely warm up to 34psi. Why should I run 35 psi cold and have a tire at 38psi hot when it won't likely improve my grip AND feel like I am riding on marbles?

[jadatis]

Mind that the inside tire temperature wich determines the pressure in it , is something else then the temperature of the treath surface you measure with a pirometer.
The inside tire temperature warm is courced , 1 by the bending of the rubber wich produces heat and transports to the tire inside.
But 2 is the heat of the brakes transported trough the rims wich can even highen up the tire inside to boiling point of water so 100dgrC/200dgr F.
So in races where a lot of braking the tire inside temp is higher then when strait road with only bending of the rubber that heatens the tire inside.
In any case temp highening highens the pressure .
I once made a spreadsheet for it , in wich you can see what pressure to fill cold in special conditions to get the same warm pressure as when normal conditions, so same deflection and same warming of the treath surface.

Placed it on my one drive that belongs to my hotmail adress with same username as in this forum.
https://onedrive.live.com/?cid=A526E...E092E6DC%21793

first download the pressurecalculationwithtemp spreadsheet to your computer,from the motorhome map, by RIGHTCKING and then choose Downloadfrom the dropdown box. After download and eventual virus-check , open it in Excell programm on your computer, but open office CALC can handle it too.

To be continued if wanted!

[Kostamojen]

You forgot to calculate for loss of tire pressure over time

[diss7]

I run 40psi on street.

I aim around 30psi on the track for grip, so 26psi cold.

For drift it depends on the course. I run 28-30psi front (they don't really heat up much) and the rear varies. I've tried upto 70psi, but durability was super affected, tyres got too hot, chunked up, exploded after only a few high speed runs (still had 95% of the tread)
I've found 55psi in the rear to be a good spot. Tyres durability good. Can still hold 3rd gear slides. Definitely loose as a goose, but with 150whp thats what you need. They will still explode when they're toast though, but they only do that (assuming youre using decent tyres) once they are at steel belt stage. So once you hear them 'chatting' from the steel belts, button off.

My problem is, when the tyres are that low, it feels like the car has 50 more hp. Steel belt turbo!

But this is what it does

[Juvenile]