An interesting article on air pressure & load index

[Aabidah]

great that was really an interesting 

[SGCarPrices]

i also not aware too.

Anyway thanks for sharing

[Orablue]

Part 3: Reinforced Tyres 

 

Its been a while since I posted the article in this thread.  Back then, I deliberately framed the discussion around tyre upsizing and avoided discussing reinforced (extra load) tyres because I couldn’t find any reliable information.  Here's part 3 for completeness. 

 

‘Reinforced’ (‘Extra Load’, XL) tyres are those that are designed to carry additional load capacity at a higher inflation pressure compared to standard load tyres. The sidewall of the tyre is marked with either “REINFORCED” or “EXTRA LOAD” or XL. If these markings are absent on the sidewall, then it is inferred that it a standard load tyre.  Standard tyres and XL tyres require different inflation. If you are using a load inflation table, make sure it is meant for the type of tyres you are using. 

 

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DISCLAIMER: I am not a certified tyre or a rubber specialist. The information here is what I have learnt and I may not be 100% correct. For your driving safety, please consult your tyre specialist if you have any questions regarding the proper tyre inflation.  

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It is important to understand the basic concept, that is, tyres need to properly specified and inflated to support the gross weight of the vehicle. Otherwise, it may lead to premature failure which is not good for you and other road users. 

 

 

So where do I start? 

Your base information is usually from the original OEM tyre and inflation table that you can find on your car.  Take a 2016 Mazda 3 Skyactiv 2.5 SP for example, 

1. OEM type: Dunlop Sport Maxx TT 215/45R18 89W (I am assuming this is a standard tyre, ie. non XL)
2. Manufacturer recommended tyre inflation (regular load) (front/back): 35 / 33 psi

 

 

Using the table, this translates to (front /back) : 1235lb / 1202lb = 560g / 545kg per tyre

 

 

If you do not know the original OEM tyre specifications, which is unlikely these days, it is possible to estimate the tyre load requirement for your car.  Take a Mazda 3 Skyactiv-D 2.2 (I assume is about the same weight as Skyactiv-G 2.5 variant) for example.  

1. The maximum permitted gross weight (front/rear): 1050 / 880 kg
2. Implies that the minimum weight each tyre needs to support is 1050/2 = 525kg and 440kg for the back. Of course, you want to add some buffer when you calculate it this way. 

 

 

This is in line with the manufacturer’s tyre specifications and inflation.

 

 

 

What if I upgrade from standard to reinforced (XL) tyres? What should be the correct inflation?

 

Example: 

OEM: 215/45R18 89W inflated at 35/33 psi

New: 235/40R18 95Y XL ( note: there is a -0.8% circumference difference but it will not affect the inflation)

 

As the load rating of the tyres have been changed, the manufacturer’s tyre inflation table may no longer be applicable.  Fortunately, it is easy to find out the appropriate pressure. It is kind of Toyo Tires to make the tyre inflation tables available to everyone. You can download a copy from https://www.toyotires.com/tires-101/tire-load-and-inflation-tables and without their permission, I have included a truncated copy here for archive.  

 

The idea is to maintain the same load carrying ability the tyres. Based on the Toyo load and inflation tables (see below), we can easily cross reference them and find the appropriate inflation. Thus, the inflation for the new should be about 32.5/31.5 psi front and back. 

 

 

Conclusion

In conclusion, know what you are doing, don’t follow blindly, don’t tailgate the car ahead, always plan to make a safe stop and keep your tires inflated correctly. 

 

 

 

 

I'm not a guru in this subject, just that it affected me. So, here's the story in layman terms for the benefit of everyone... maybe one day it may save someone ... haha.. good luck, be safe!



DISCLAIMER: The information here is what I have learnt and I may not be 100% correct. I am not a certified tire specialist or a rubber specialist . I'm not responsible for any accident, injuries or loss of life as a result of using the information provided here. Take my entry here with a pinch of salt. It is recommend that you understand the concept behind it so that you know what you are applying.

DISCLAIMER: This article is not 100% applicable to SUV/commercial vehicle


1. Background
- when designing a car, the manufacturer will choose an appropriate tire to support its weight
- some possible factors
. - vehicle weight
. - potential load
. - handling characteristic
. - vehicle performance (remember that at high speed, the down force will add some substantial weight)

Take it that there are good reasons for the manufacturer to select a certain load rating


2. Industrial standards
- to make it easy for everyone, a standard is created
- unfortunately, there are 2 types of standards for passenger cars
. - p-metric & euro metric (it must be the Americans!)
. - p-metric tires are marked as P215/45R17 91V (note the P)
. - euro metric tires are marked as 215/45/R17 91V (without the P)
- we are using euro metric tires
- why we need to take note of p-metric and euro-metric? because the max load is measured at different inflation
- the standards also defined extended load tire which is measured at a higher inflation

- if you visit U.S. websites, the example may be using p-metric tires

so, max load is at 2.5 bar (250kpa) regardless of max inflation which the tire can handle



3. Tire - Manufacturer's Spec

- for simplicity,
. - we will assume that our vehicle will not exceed 160km/h
. - we will not consider extended load (XL) tire where the max load is at higher inflation

- tire size: 205/50R17 89V
. - max load is 580kg @ 2.5bar (see table below)

- manufacturer recommended inflation, normal@ 2.2bar F/R (32psi) = 525kg
. - total = 525kg x 4 = 2100kg

- manufacturer recommended inflation, full load@ 2.3bar/3.1bar = 545/580kg
. - total = 545kg x 2 + 580kg x 2 = 2250kg



4. Tire - Upgrade, Plus+2
- say you are currently using the original 15" wheel and decided to upgrade to 17"
- still 17" but deviate from the manufacturer's recommendation
- tire selected : 215/45R17 87W
. - max load is 545kg @ 2.5bar (see table below)

- manufacturer recommended inflation, normal@ 2.2bar (32psi) = 525kg
. - total = 490kg x 4 = 1960kg
. - so with this tire, we are short of 2100-1960 = 140kg (=> overload!)
. - at normal load, to support the same amount of weight (525kg), we need to increase the inflation to 2.4bar (240kpa)

- manufacturer recommended inflation, full load@ 2.3bar/3.1bar = 510/545kg
. - total = 510kg x 2 + 545kg x 2 = 2110kg
. - so with this tire, we are short of 2250-2110 = 140kg (=> overload!)
. - at full load, to support the same amount of weight (545/580kg), we need to increase the inflation to 2.5bar (250kpa) BUT the rear max out at 545kg regardless of inflation in excess of 2.5bar (250kpa)
. - so after final adjustment, we have
. 545kg x 2 + 545kg x 2 = 2180kg
. 2250 - 2180 kg = 70kg short (=> ie the rear tires will be overloaded!)


5. Summary
never choose a tire of lower Load Index
if your replacement tire has the same Load Index, then same inflation pressure
if your replacement tire has higher Load Index, you may use the same inflation pressure or adjust base on the inflation pressure load capacity chart
never go below 2bar
Load Index has nothing to do with the speed rating (ie. H,V,W,Y,Z,ZR...)
bear in mind that with the adjustment, the tire with lower load rating may experience high load stress than one that is correctly specified.

calculation may not be application to tire with puncture repair
calculation may not be application to worn out tire

CODE

Load Index (LI) vs Inflation (bar, 1bar=100kpa), table values in kg

  2.0 2.1 2.2 2.3 2.4 2.5 (bar)

80 375 390 405 420 435 450

81 385 400 415 430 445 462

82 395 415 430 445 460 475

83 405 425 440 455 470 487

84 420 435 450 470 485 500

85 430 450 465 480 500 515

86 445 460 480 495 515 530

87 455 475 490 510 525 545

88 470 485 505 525 540 560

89 485 505 525 545 560 580

90 500 520 540 560 580 600

91 515 535 555 575 595 615

92 525 550 570 590 610 630

93 545 565 585 610 630 650

94 560 585 605 625 650 670

95 575 600 625 645 670 690

96 595 620 640 665 685 710

97 610 635 660 685 705 730

98 625 650 675 700 725 750

99 650 675 700 725 750 775

Table obtained from http://www.generaltire.com/generator/www/d...res_2005_en.pdf

 

 

toyo tyres load-inflation tables 20170203.pdf

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