[ChiragM]

Some cars come with a very high PSI rating. This affects the ride comfort of the car, and in some cases, reducing the tyre pressure can make a significant improvement to the ride quality. On the other hand, high tyre pressure means that there is less rolling resistance, which in-turn, improves fuel economy.

Some cars like the Volkswagen Passat came with really high recommended tyre pressures and hardly any owners paid heed, running lower than the recommended pressures. What are the technical reasons why some cars have high recommended tyre pressures?



Here are a few cars with high recommended tyre pressures:

Passat: 40+ PSI
Polo GT TSI: 36 PSI
Tata Indica Dicor: 36 PSI
Nissan Magnite: 36 PSI
Toyota Camry Hybrid: 39 PSI
Mercedes C-Class: 36 - 38 PSI
Hyundai Venue: 36 - 38 PSI for full load
Volkswagen Tiguan: 38 - 45 PSI
Skoda Octavia: 36 - 46 PSI for full load
Skoda Kodiaq: 39 PSI (ECO)

[Aditya]

Thread moved out from the Assembly Line. Thanks for sharing!

[srini1785]

Simple fact: Normal atmospheric pressure, that which is all around us is 14.5Psi (1bar). Industrial air pressure settings are of the order of 6-7 bars. A screw compressor can produce upto 12 bar. So, in relation, 40psi (approx 3 bar) is not that high.

The tire pressure setting depends on the following factors that i can think of ;

1. The weight distribution between front and rear.
2. Type of drive , 4W/2W.
3. CG of the car.
4. Suspension settings.

So just follow what is recommended.

[gkveda]

The resultant damping effect on the car is algebraic sum of Individual Damping created by

1. Tire side wall flexibility
2. Shocks Damping
3. Seat sponge damping

And the load factor on the dampers are
1. Car weight distribution
2. Human, luggage Load on the car
3. Bad road's push back of the vehicle

Based on both - load and overall Damping figure, the rider gets the required ride quality.

In a car where the shocks has less damping, it needs to be compensated by other two.

So, if car manufacturer specify high PSI for a car, it means, the shock is designed with low Damping figure and vice versa

The only variable here to alter ride quality is, air pressure inside tires that in turn alters side wall flexibility. So after designing the car completely, during testing, if the required Damping is not achieved as per the design, the manufacturers try to alter the tire pressure to some extent in order to compensate for the other two.

[greenhorn]

At least for the Indica dicor I had asked this question earlier as the recommended pressure is much higher than that if my turbo which is suspension and body wise an identical car

https://www.team-bhp.com/forum/tyre-...-pressure.html (Rationale behind the 'recommended' Tyre Pressure?)

[saikarthik]

Originally Posted by gkveda

The resultant damping effect on the car is algebraic sum of Individual Damping created by

So, if car manufacturer specify high PSI for a car, it means, the shock is designed with low Damping figure and vice versa

The only variable here to alter ride quality is, air pressure inside tires that in turn alters side wall flexibility. So after designing the car completely, during testing, if the required Damping is not achieved as per the design, the manufacturers try to alter the tire pressure to some extent in order to compensate for the other two.

Few good pointers, but I am not sure of the point related to algebraic sum. The suspension performance is neither linear nor a direct sum in my humble opinion.

There are differences in design on how to absorb different frequencies and amplitude of the road undulations and manufacturer will cannot leave it to increasing or decreasing tire pressure with respect to a soft or hard suspension.

Also, this needs to be agreed with the tire suppliers or they won't honor the warranty terms.

Also one key pointer is the load and rolling resistance as mentioned in above posts. The contact patch variation and sidewall flex affects greatly the acceleration/braking performance and FE. So it has to be maintained at optimum level for balance, so if load increases you need more pressure to keep similar contact patch (braking distance will increase due to inertia due to mass increase - different phenomenon).

Less tire pressure = more prone to punctures as well.

[gkveda]

Originally Posted by saikarthik

but I am not sure of the point related to algebraic sum. The suspension performance is neither linear nor a direct sum in my humble opinion.

I agree. In real time scenario where there are multiple forces influencing from all directions due to varying velocity, horizontal movement due to road design, varying air temperature, etc, complexity just increases the nonlinear response.

But, for sake of understanding, simplifying the scenario like smooth and uniform road, constant velocity, considering, its electical equivalent is 3 resistors in series

So, equivalent resistance (Equivalent resultant damping} is R=R1+R2+R3

This is what I meant by Algebraic sum

[Kosfactor]

Vehicles that have a relatively taller sidewalls may have tire pressure in the range of 29 -34, vehicles with wide low profile tyres may have higher tire pressure, this is to keep the entire width of the tire in contact with the road for a specific load.

Tyres have to support the weight, provide traction for acceleration, braking and steering. They also have to make sure the tyre lasts for a reasonable distance.

If the air pressure recommended is high for a specific loading - driver only, passengers + luggage etc it means that is how much is required for the provided suspension, type of tyre etc to maintain optimum traction, handling and longevity of the tire. Those who mess with tire pressure outside OE recommendations are taking unnecessary risk.

[CarBot]

Might be off topic, My new POLO TSI had 50 PSI from factory, I was worried as the TD car suspension was smooth and new car's was very hard. Later upon checking found out to be on higher side. I have kept it at 32psi now.

[NaXal]

Personally I don't care about PSI value as long as the car suspension is properly tuned to give me a comfortable ride.

If the car maker has selected a specific value, that means they have tuned their car & suspension for optimal performance & ride quality.

This is what they wish their customer to experience. Their understanding & offering on their developed product.

When talking about lower PSI value for ride quality, why stopping there? One can go ahead with suspension change to further soften the ride !! Modification options are endless & different people have their different needs.

What really bothers me & makes me frustrated is the different PSI value for different load.

I am commuting to office with single driver so the recommendation is to set my tyre pressure at 32 PSI all around. Fine, no problem. There is no work load & three of us in office decide to go out for a meal, duh, now my car needs 33 PSI front & rear if one person is seating in rear bench but 34 PSI rear if both are seating behind?

So I need to find an air pump to first change the PSI & then go out?

Ok, what happens when I am returning home again? Lower the PSI for single driver?

I mean what are they thinking when writing their manual? They didn't even provide an air pressure pump & they recommend their customers to set different PSI values for different load?

Lastly what's the logic behind sending a new car with 45/50 odd PSI in all tyres? Every single new car that I have done PDI, regardless of the manufacturer, came with such high PSI in all 4 tyres. Dealership won't decrease those values rather if there is an old stock, they will increase the PSI to such levels before delivery !!

[puneetakhouri]

Originally Posted by NaXal

What really bothers me & makes me frustrated is the different PSI value for different load.

Totally agree. I have a Baleno and a Ford Ikon. On one hand the Baleno recommends 29psi irrespective of the load and also you can actually feel the difference in ride quality when using 29psi vs the roadside tyre wala default 32.
But on the Ford has such a weird recommendation 32(F) 30(R) in normal load and 36(F) 41(R) under full load with luggage. I mean these variable recommendations are so stupid, am I supposed to find a tyre pressure guy to adjust the pressure every single time? Also these pressures are cold tyre pressure, so if I go to the airport to drop somebody should I go to a tyre wala to set it 36-41 first, then reach airport, drop people, wait 30 mins for tyres to cool down and then find another tyre wala to bring pressure down to 32-30 and then drive back!

Ridiculous

[NPV]

Originally Posted by CarBot

Might be off topic, My new POLO TSI had 50 PSI from factory

This was most likely caused by the dealership.
It's a known issue that dealerships practice over inflation of tyres. I think there was an older thread discussing this very topic.

[nick.cs]

Wouldn't then the factory setting be more attuned to the test and track conditions which will be different than the actual usage of the car. European cars could be a culprit here and might actually go by global standards than country specific standards as they seem to be the common factor with high PSI.

Even still 50 PSI is insane.

Most of the time i am the single driver so i do 33 front and 32 back. Also i think 1-2 psi here and there is something which happens over the course of a few days eventually. Unless one has a TPMS, OCD, tyre inflator and time to keep the pressure tuned to the exact specs each time. I just make it a point to get my tyre refill done every 20 days or whenever i don't get the right vibes and just try to enjoy the drive.

[SmartCat]

I'm shooting from the hip here, but I think high tyre pressures help improve fuel economy numbers significantly under standard test conditions (on rolling road)



Low fuel consumption under standard test conditions is not just for marketing purposes. Many countries have tax structure that encourages low fuel consumption.

[landcruiser123]

I think FE is the main reason. I'm willing to bet many of the car tire pressures in (premium) European cars are rated for good roads in their home country and aren't changed for India.

Also, the budget cars may recommend higher pressure for maximum tire life as they are designed for the cheapest possible OEM tires.

Originally Posted by NaXal

What really bothers me & makes me frustrated is the different PSI value for different load.

I am commuting to office with single driver so the recommendation is to set my tyre pressure at 32 PSI all around. Fine, no problem. There is no work load & three of us in office decide to go out for a meal, duh, now my car needs 33 PSI front & rear if one person is seating in rear bench but 34 PSI rear if both are seating behind?

No need to change pressure for short trips like your group lunch or airport runs. Won't harm anything.

However, if you are planning a long road trip with multiple people(+ luggage), please adjust the pressure for optimum performance & economy. You might consider doing it while preparing & inspecting your car before the trip.

Lastly what's the logic behind sending a new car with 45/50 odd PSI in all tyres?

Possibly to avoid flat spots in the tire while the car is sitting in the manufacturer & dealer yard. Dealerships are lazy to change the tire pressure.