Tire Load Range Discussion

[LSMITH]

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Best pics I have of the ol jalopy... ...

 

[Hollywud]

Private Message from Groover to Hollywud

Groover wrote on 02-04-2003 01:04 PM:

I have not seen your question answered yet so I will take a stab. It is the "volume" of air in the tire that enables it to carry a load. Thus a larger (taller or wider) tire contains a larger volume of air and "can" carry more weight.



The other variable is air pressure - the higher the air pressure the more the tire can carry. Tire manufacturers have come up with safe ratings for all tires based on "if the tire is inflated to the max pressure on the sidewall it can safely carry the rated load at highway speeds without blowing out. Heat is the enemy as the tire sidewall flexes it creates heat and the heat can and will seperate the plys of the sidewall and cause a blowout. So the bigger the tire and the more air you can put in it --- the more it can carry.



The load ratings D,E are sidewall ply ratings. An E rated tire has a heavier sidewall which can handle higher air pressure and carry more weight. If you look at the example of a 245-75-16 E vs a 285-75-16 D tire it will make sense.



245-75-16 E holds 80 psi air and can carry 3042#

285-75-16 D holds 65 psi air and can carry 3305#



The 245 E is a heavier ply tire and is a "stronger" more stable tire "Sidewalls flex a lot less due to thickness and higher air pressure.



The 285 D is a lighter ply tire and will exhibit more sidewall flex "not feel as stable" and holds less air pressure. The reason it can haul more weight is the larger "volume" of air it contains.



If the 285 D was rated for E 80 psi it could probably haul near 4000# of weight.



Another thing if you go to a 305 70 16 D it is rated for less # than the 285 and it is a bigger tire "how can this be" the reason is even though the 285 is smaller it is rated at 65psi where the 305 is only rated at 50 psi. The 305 can actually carry more weight but DOT only rates it for 50 psi of air and thus the DOT weight rating is less than the 285.



Hope this is clear as mud



Bottom line either tire will do the job just fine



BTW I run 315-75-16 D Dunlop Mud Rover tires that are rated for 3400# at 50 psi

 

[Hollywud]

Reply: Hollywud to Groover

Thanks, you definitely filled in the blanks to the discussion I started.



As I said in one of my posts, it's hard talking about apples, oranges, and bananas... . they all need to be apples. As I see it, removing the Load Range out of the equation for now, there are (3) variables:



- Volume of air inside the tire

- Pressure of air inside the tire

- Contact patch of the tire with the road (assuming slick tire, for the purposes of discussion)



Volume of air is relative to the size of the tire (height & width). Contact patch is relative to the tire width. Pressure of air is relative to the amount of force to overcome the distributed loads thru the tire via the contact patch area.



Assuming all tires in this example are rated for the same max. single tire load, a tire with the following characteristics will:



- same diameter w/narrow contact patch (and lower volume of air) = high psi to sustain load

- same diameter w/wide contact patch (and higher volume of air) = low psi to sustain load



OK, with that deduction made, the sidewall plies (of same dia. w/narrow contact patch) may need to be increased to carry the same load as a relative tire with a wider contact patch..... because of the pressure increase required to overcome the load forces.





I know I've oversimplified things here (probably dropping off some variables too) but, it's all starting to make sense..... the relationship between tire size (height/width), PSI, max. single tire load, and Load Range (i. e. # of sidewall plies). Oo. Oo. Oo.