It cant be all that hard - Tire preasure calculation

It cant be all that hard and it cant be all that easy.

In its original incarnation, my car had 165/80R15 tires, probably running 28-32 psi. It now has 235/60 14 front and 265/50 15 rear. This is far from a minor change.

Opinions on tire pressure are every where, just like oil.

There needs to be a way to calculate a starting point. Google come up with some things that seem to be to simple based on the max psi listed on the tire, its load rating, and the gvw, to some stuff that 25 years ago in application of differential equations I would have understood.

Is there a proven way to calculate a baseline pressure ?

Car weights around 2K lbs, maybe a little less, at 30 PSI it feels like you are running on solid tires.

Yep, take some chalk and draw a thick line all the way across the tread. (Imagine coloring in the entire contact patch of the tire).

Then drive around the block. If the outside chalk is worn away but the inside is still there, pressure is too low. If the inside chalk is worn away and the outside is still there, pressure is too high. If all the chalk is worn away evenly, pressure is good.

In reply to Robbie:

Thanks, I read that, I did the IR thermometer, it was pretty even across the tread and not very much over ambient temp. I assumed it was cause the car was so light. I will give the chalk a try as it does not involve temp.

Still hoping for a base line calculation of some sort.

Thanks again

I dont think you will find a calculation because there are too many variables like total weight, weight distribution, suspension type, alignment settings, blah blah. Much easier to just use chalk. If your not getting enough heat above ambient your not driving hard enough. Same problem with the chalk method, if you just put around your not going to see what it will do/what it wants when you arent putting around. If your only putting around stick it to 32 and work from there or call it good.

Yeah you're making too much work for yourself by trying to do it all with math. Use the chalk and temp gun.

lrrs wrote: ...at 30 PSI it feels like you are running on solid tires.

Less than 30.

SVreX wrote:

lrrs wrote: ...at 30 PSI it feels like you are running on solid tires.

Less than 30.

I once had over 30psi in 30x9.5s on my Samurai, and it felt just like having granite cylinders for wheels, like a Flintstones car.

The base line is the stock recommend pressure unless you're going to something a lot stiffer (e.g. giant mud tires). For 165R15 I'd guess VW? You don't want of ton of pressure in the fronts, stock is something like high teens front, high 20s rear.

Note book.

Pick a pressure, drive it for a bit, record the experience.

Repeat a few times at different pressures.

Tire pressure is not an absolute value with only one "correct" answer. It's a tuning variable, so play with it.

In reply to BrokenYugo:

I think you missed the part where the car weighs sub 2000#.

Combined with the 50% increase in tread width, I don't think the factory baseline is very meaningful.

In reply to SVreX:

If I guessed right the factory numbers are a lot lower than the 30 all around he was running.

I have not been successful with an IR gun.

A tire temp probe is what you need if you can find or borrow one.

You need to check temps after driving the car in the manner that you plan to drive the car. In other words, if you are going to be autox or doing a track day, you need to check your temps right after a run.

Also, using a skidpad and finding the quickest time is the best way to determine tire pressures (for AutoX anyway).

As far as the first response: If the chalk is worn in the middle, the tire is overinflated. If it is worn on the outsides, the tire is underinflated.

The inside and outside edges are WAY to dependent on alignment settings.

On very light cars I always heard 1 psi for every 100 pounds is a good starting point. Then adjust from there using tire temp.

Its a bug pan, 70s kit car body, a Kelmark GT. Looking for street pressure to start. I was assuming the bugs recommended tire pressure was around 28-30. So my my first assumption was way off. Looks like the beetle would have been 18/27.

Going to start there and see what happens, hopefully this + the new steering damper on its way solves some of the driving issues I feel it has (just confirmed the current damper is toast). The local bug guy stated it drive great for a bug, and I expect to much, hopefully not.

Thanks again to all for all of the feedback.

lrrs wrote: The local bug guy stated it drive great for a bug, and I expect to much, hopefully not.

You should probably pay closer attention to the local bug guys.

The tires probably weigh more than the rest of the suspension. Its never going to work well.

Keep in mind that the tire does not support the car, the air in the tire does.

So I'll bet you come up close the stock pressure, plus or minus a few psi.

The volume of air is not much different, esp. in the fronts. could even be less.

Just be cautious about the local bug guys being correct, a lot of people think they know a lot about those cars.

The tire manufacturers use the Load and Inflation Tables. Using the recommended pressure on your door jam, you can factor in your new tire size. If the tire is a different dimension the inflation should be different.

https://toyotires2-1524598101.netdna-ssl.com/assets/lib:toyo%20content/Application_of_Load_Inflation_Tables_20151020.pdf

I use that with our daily drivers. With that said, when I autocross I use what the others are doing...adjust and check temperature, stopwatch and document in the log book.

I've never actually tried this, but it might help. You will need a pretty smooth, flat floor to work with. All it requires is a piece of paper to roll over. This doesn't simulate spinning forces, but it should be close.

I have never had a problem starting at the maximum pressure listed on the tire and going down from there. Most people think I am crazy. I like the steering feedback, better fuel mileage and tire life. I have never had a tire wear in the middle doing this. At the last Autocross I attended I had to add air to my tires to get home.

If you know anyone in a paper mill or coating shop see if you can score some "Nip Impression Kit" its like a carbon paper but the ink changes color by pressure ranges.

In reality the vehicle is supported by a column of air. Think air lifts.

The size of the contact patch to the wheel rim.

So a short sidewall can often need more pressure, depends on the width.

Start with the stock pressures and go from there

pjbgravely wrote: I have never had a problem starting at the maximum pressure listed on the tire and going down from there. Most people think I am crazy. I like the steering feedback, better fuel mileage and tire life. I have never had a tire wear in the middle doing this. At the last Autocross I attended I had to add air to my tires to get home.

That's a very good strategy, there's nothing wrong with it. Only reason I don't do that is because from experience I can usually guess a good starting pressure, which just saves a some testing time vs. your approach.

Remember that softer tires run cooler, and often tires will have no meaningful performance difference throughout a wide pressure range...if you can run at the lower end of that range it means you get the same performance with more endurance.

iceracer wrote: In reality the vehicle is supported by a column of air. Think air lifts. The size of the contact patch to the wheel rim. So a short sidewall can often need more pressure, depends on the width. Start with the stock pressures and go from there

It's also supported by the structure of the tire. Can't underestimate that- I've had tires with sidewalls so stiff that they could support the car with no air.

Normal tires won't do that, but higher performing tires will.

GameboyRMH wrote: Remember that softer tires run cooler, and often tires will have no meaningful performance difference throughout a wide pressure range...if you can run at the lower end of that range it means you get the same performance with more endurance.

This seems counterintuitive, unless maybe your talking compound and I'm think inflation pressure?

Can you provide some explanation of why that is?

I'm not saying it's wrong, but my initial thought is the lower pressure lets the tire flex more which will generate heat.