Tire pressure delima

[VeeDubb]

The placard shows 33psi and it is not contingent on load. The only statement about load is that total cargo (including passengers) should not except 975lbs.

On a side note: Passats with upgrades wheels running 235/45/17, the LI is 94. I don't know much about tires so can you explain why they would drop the LI for bigger wheels?

 

[CapriRacer]

VeeDub,

Just to be clear, it only lists one inflation pressure?

If so, then I am confused as to why they would specify an XL.

 

[Tempest]

Glad to have engineers on this board.

Learning a lot here.

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It is a geometry problem and the false assumption is that the inflation pressure is holding up the vehicle - the inflation pressure is a stiffening agent for the tire's structure.

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Still trying to wrap my brain around that one.

 

[VeeDubb]

CapriRacer,
Yes it is only one pressure listed. I also looked in the owners manual and it's still one pressure, 33psi

 

[Shannow]

CapriRacer,
I was overly simplifying things by basically stating a tyre with zero stiffness. You are quite correct

 

[CapriRacer]

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CapriRacer,
Yes it is only one pressure listed. I also looked in the owners manual and it's still one pressure, 33psi

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OK!!

A number of possibilities come to mind:

1) That the XL tire was already specified early on in the design process - either because the weight was expected to to greater than it turned out to be, or it was an exisiting tire on another vehicle.

2) The XL is needed because you have the Turbo - slightly stiffer sidewalls.

In either case, it doesn't appear that you need the XL just from a load carrying point of view.

 

[CapriRacer]

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.....Still trying to wrap my brain around that one.

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I have been trying to find a simple explanation to illustrate this and here's my latest!

Imagine a RunFlat tire with no inflation pressure in it. The tire holds up the vehicle and functions just like a tire with inflation pressure. Clearly, there will be a footprint and clearly, its size would not be infinite (which goes to the average ground pressure vs inflation pressure argument).

The fundamental difference between a RunFlat and a non- RunFlat is the stiffness of the sidewalls. So the inflation pressure adds stiffness to the sidewalls, but isn't holding the tire up - the tire itself is.

For those former engineering students, if you carefully construct a free body diagram, you will find that the force generated by the upward inflation pressure is cancelled by the force generated by the downward inflation pressure.

 

[VeeDubb]

Thanks Capriracer. I also appreciate your approach to answering questions. Awesome info.

 

[Tempest]

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For those former engineering students, if you carefully construct a free body diagram, you will find that the force generated by the upward inflation pressure is cancelled by the force generated by the downward inflation pressure.

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So the useful part of the air pressure is pushing out to the sides, not up or down! Very interesting!

 

[1sttruck]

A truck often has different recommended tire pressures for different loads and speeds, one would expect something for a minivan as they can potentially carry a fair number of people, so I'll guess the one tire pressure is just a way to keep it simple for minivan owners.

 

[Shannow]

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For those former engineering students, if you carefully construct a free body diagram, you will find that the force generated by the upward inflation pressure is cancelled by the force generated by the downward inflation pressure.

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Yes I largely agree, but even if you look at a roller bearing (steel on steel) deformation of the roller/race is required to carry the load. There is no true point/line load.