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a successful learner…… loves learning…… seeks challenges…… values effort…… persists in the face of obstacles

All content copyright Willem Winch or otherwise credited.

…more 4x4 Tech Articles here:

Tires Part 1: …the disadvantage of LT tires

It is beyond dispute that adjusting the tire pressure is a crucial recipe to improve traction, to minimize the risk of punctures and the impact of vibrations caused by corrugations. While it is relatively easy to reduce the pressure, it takes a little bit more effort and equipment to increase the pressure.

Now, before I want to get into the details about suitable equipment, compressors etc., a few comments on the impact the correct or incorrect tire pressure has on safety.

Most of the readers will know that a LT (Light Truck) tire will be more suitable and able to cope with harsh off road tracks than this would be the case with normal passenger tires. Beside that a tire with a LT construction will have a higher load rating - obviously required to cope with normally higher loads a truck is built for - due to the thicker walls for an off-road vehicle it also has the advantage that such a tire is less prone to get pierced by a sharp rock or root sticking out of the ground.

BTW: a tire is classified as a LT tire via the load range or the ply rating; the ply rating is somehow misleading as a LT tire with a ply rating of 8 or even 10 doesn't have 8 respectively 10 layers. Likely it will have significant fewer layers / plies. (More interesting reading about tire codes here on Wikipedia.)

A common (but correct ?) conclusion is that a LT tire is superior compared to a passenger tire in every realm (except wrt comfort of course).

Before I get to the realm where I think a passenger tire outperforms a LT tire here a question that kept me pondering for a while:

When counting the carcasses of blown up tires left and right of the road, it seems that there are more truck tires than passenger tires, although there are more passenger cars on the road than trucks - how come?...of course mileage and maintenance will play a major part, but IMO there is also another reason.

A wheel / tire will always require some work to force it to turn; let's take a wheel barrow for example: when pushing a wheel barrow with the same load, but with different tire pressures, which one will be easier to push? …of course, it will be the one with the higher pressure.

So obviously the lower the pressure, the more energy is required getting the wheel turning. Reason is the work required to deform the tire:

if the pressure is high, the tire forms nearly a perfect circle, hence not much work is required to push the wheel barrow. If the pressure is low, the load of the wheel barrow will deform the tire and consequently trying to force the tire to turn requires continues deformation work to flex side walls and tread.

Now, this deformation work for flexing is affected by the pressure and the load, but also by the wall thickness itself. You could do a simple test with a thicker and a thinner rubber strip and bend the strips with the same frequency and amplitude and obviously it will require a higher effort to do this with the thicker rubber strip. And if you could do it long enough you would find out that the thicker rubber strip will heat up faster than the thinner one due to the deformation work that is partly stored as heat energy in the material.

And this is exactly the reason why LT tires are more prone to heat up and get destroyed if run with a too low pressure!

Someone might comment that a LT tire with a stronger wall will deform less than a passenger tire when the pressure drops, hence counterbalancing the pressure loss better than it would be the case for a passenger tyre.  

While this is correct, with respect to the required deformation work it only means that the material for the thinner passenger tire will flex a few percentages further compared to a LT tire with the same pressure. Think about the footprint of both tires: without advanced measurement the increased footprint of the passenger tyre will be hard to recognize and the deformation work for this few "percentage further flex" is still little compared to the increase in deformation work required for the thicker material.

Referring to the gas equation seems to evolve another objection: air expands when heating up - so again, at a first glance it seems that the heat will increase the pressure when causing the air to expand, resulting in a kind of self healing effect. Now, to verify that this “self healing effect” is minor is best done by entering some real world values into the equation.

The gas equation is                         p x V = n x R x T                    [1]

where

p = pressure in Pa

V = volume in cubic meter

R = gas constant

T = temperature in Kelvin

n = amount of gas in moles (or mass)

Now looking at a scenario where a tire heats up from condition “1” to “2” the equation can be written

                                          p1 / T1 = p2 / T2  = n x R / V           [2]

or

                                                 p2  = p1 x T2 / T1                      [3]


Due to the fact that the temperature has to be entered in Kelvin the pressure increase is only minor. Lets assume I had my pressure reduced to 18 psi for off roading and wouldn’t inflate them before driving home. Driving home on bitumen with high speed would cause the temperature to raise from 40 deg C to 150 deg C - I exaggerate here a little bit, that would be definitely a very hot tyre, already damaged beyond repair. However, when entering these figures in Kelvin into equation 3, the pressure increases just by factor 423/313 respectively 1.35; e.g. my tire pressure, reduced to 18 psi, would increase only to 24.3 psi…not enough to counterbalance the flexing that causes the tire to overheat.

I remember a discussion on a forum where a member monitored and recorded the temperatures of the tires of his 4WD. He used one of those tire monitor systems that shows the temperature and pressure on a cabin display and compared the data he recorded for his new, 8 ply rated tires with the data of the 10 ply rated tires he had before. Running the tires with approx. the same load, same velocity, and the same pressure, the temperature of the 10 ply rated tires were significant higher! No surprise - it has to be like this due to the higher flexing work required for the 10 ply rated tires.

Back to the carcasses found left and right - and sometimes in the middle - of the road: so another reason, beside the maintenance and mileage, for finding more truck tire carcasses is that they heat up much more if run below the correct pressure due to the thicker walls. A creeping air loss, unrecognised, will cause them very fast to overheat where a passenger tire would be more forgivable. You can run a passenger tire with a too low pressure, and while it will wear the tire out, the thinner wall requires little continuous deformation work, so not much heat energy is stored in the material that could destroy the tire. Of course, driving fast enough, thus increasing the deformation frequency will also blow up a passenger tire, but they definitely will outperform a LT tire with respect to higher tolerance for incorrect low pressure.

What makes the LT tire strong - the thicker wall - is also its weakness when it comes to incorrect respectively too low pressure!

Now, this physic has some consequences for the 4W-driver who wants to equip his 4WD with tough LT tires: a 10 ply rated tire will have a somehow ridiculous high load rating for a 4WD, and will require a relative high tire pressure to avoid overheating. My Pirelli Scorpions LT for example advice on the sidewall to keep the pressure between 40 - 80 psi. I can tell you they are not very comfortable to drive with 40 psi (that’s the minimum threshold!) or even more.

Hence a lot of drivers drop the pressure below the recommended threshold to gain some comfort, but considering the above this bares the risk that the tire heats up. Most of the time people get away with it - one of the reasons is the speed limits here in Australia together with the higher velocity ratings. E.g., my Pirellis are "S" rated, cleared for up to 180 km/h: with 40 psi they should allow 180 km/h without overheating and with lower velocities the risk of overheating obviously mitigates.

Note, that when switching from passenger tires to LT tires the car specific tire tag will lose its value as reference due to the reasons outlined above! The Terracan’s tire tag says 29 psi (32 psi for the rear when loaded), obviously far below the minimum threshold for the Pirellis.

Low speed is also the reason why slow off-road work with a significant reduced tire pressure won't do any harm with respect to overheating, conversely it will protect the tire from getting punctured by reducing the tension and allowing more contact surface with an obstacle. However, when back on bitumen it is highly recommended to get the pressure back to the recommended threshold for a safe trip home.

Well, that was a quite long introduction to get to the second part of my tire article about compressors and hand / foot pumps (yes, hand / foot pumps!), but here we are now.

Load Range

Ply Rating

A

2

B

4

C

6

D

8

E

10

F

12

G

16

H

20

J

22


…one does not discover new lands without consenting to lose sight of the shore…


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p. issued: 18-01-2013   -   last update: 23-02-2013

LT specific features / source: Wikipedia

source: Wikipedia

Part 1

…it’s opening a can of worms, and all the questions seem to be discussed to death, however, some questions kept me still pondering…


…and if you are in doubt how reliable your cheap air compressor is, here I show you an inexpensive backup for the piece of mind…

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