Load rating calculations for Tires?

[IndyIan]

I keep reading in posts here not to get a lower load rated tire for your car. So I was wondering how are load ratings calculated for tires? Is it just the static weight of the car on the tires?
So for my Neon:
Car weight = 2550lbs Maximum load = 800lbs for a total load of 3350lbs
The published weight distribution is 63/37 so max load on the front tires is .63 x 3350 = 2111 lbs (using the false but safer assumption that the weight distribution would stay the same at max load, in reality the rears would get loaded more)
So I’d really should to get tires with a minimum load rating of 83 or 1074lbs?
The car came with optional 14” tires with a load rating of 86. The base model tires were 165/80R13’s.

 

[Corvette Owner]

From TireRack.com:

Maximum Load

A tire's maximum load is the most weight the tire is designed to carry. Since a tire's load carrying capacity is related to the tire's size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry assigned inflation pressure.

Additionally, load ranges are used to separate tires that share the same physical size, but differ in strength due to their internal construction. "Higher" load ranges are used to identify tires that have a stronger internal construction, and therefore can hold more air pressure and carry more weight.

Each load range has a assigned air pressure identified in pounds per square inch (psi) at which the tire's maximum load is rated. Listed below are the air pressures at which maximum load is rated for popular P-metric and LT tires:
Tire Load Ranges

Inflation Pressure Assigned
For "Maximum Load" Ratings
P-metric
Standard Load (SL) 35 psi
Extra Load (XL) 41 psi
Light Truck
Load Range C (LRC) 50 psi
Load Range D (LRD) 65 psi
Load Range E (LRE) 80 psi


P-metric tires used on passenger cars and station wagons are rated to carry 100% of the load indicated on the tire's sidewall (or listed for the tire in industry load/inflation charts). However, if the same P-metric tires are used on light trucks, (pickup trucks and sport utility vehicles for example), their carrying capacity is reduced to 91% of the load indicated on the tire's sidewall. This reduction in load results in causing light truck vehicle manufacturers to select proportionately larger P-metric sized tires for their vehicles to help offset the forces and loads resulting from a light truck's higher center of gravity and increased possibility of being occasionally "overloaded."

For example, P235/75R15 P-metric sized, standard load tires used on cars and light trucks would be rated to carry the following maximum loads at 35 psi:
Cars Full Value 2028 lbs.
Light Trucks 9% Reduced Value 1845 lbs.


Additionally, while a tire's maximum load is the most weight the tire is designed to carry, its load carrying capacity at lower inflation pressures is proportional to how much inflation pressure is used. For example, P235/75R15 P-metric sized, standard load (SL) and extra load (XL) tires used on cars would be rated to carry the following loads at the inflation pressures indicated:
Air Pressure (psi) 20 23 26 29 32 35 38 41
P235/75R15 SL 1543 1635 1753 1852 1940 2028

P235/75R15 XL 1543 1635 1753 1852 1940 2028 2105 2183


Note: 35 psi is the assigned "maximum load" pressure for standard load tires and 41 psi is the assigned "maximum load" pressure for extra load tires.

The above chart correctly shows that an extra load tire is not rated to carry any more load than a standard load tire when both are inflated to the same pressure (up to the standard load tire's "maximum load" pressure of 35 psi). This is because a tire's load capacity is a function of its size (which determines the size of the "air chamber"), its construction (which determines how much pressure can be held) and the actual air pressure used (which determines how many air molecules are forced inside the chamber). All tires with equivalent physical dimensions carry equivalent loads (until they reach their maximum load pressure).

The tire's maximum load is indicated in relatively small sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum load capacity is branded on the tire in kilograms (kg) and pounds (lb.). These values can also be found in the industry's tire load & inflation charts.

NOTE: P-metric and Euro-metric sized tires' "maximum load" inflation pressure may be, and often are, different that the tire's "maximum inflation pressure".

 

[CapriRacer]

Read this:

http://www.barrystiretech.com/loadtables.html

What you should get out is that the load rating for tires - what's printed on the sidewall - is for ideal operating conditions! And we all know there is no such thing as ideal operating conditions.

So tires are oversized. How much is a trade secret for each vehicle manufacturer and seems to vary a bit.

I had a professor who once said "Overdesign / Underutilize!", which is a short way of saying it is the design engineer's resposnsibility to overdesign whatever he is designing and that when selecting components, he should make sure he oversizes them.

I have calculated that you could hold up a car with a single 1/2" SAE grade 5 bolt. So if someone were designing a short bridge, would it make sense to use a dozen of these bolts for the entire bridge? Obviously not.

Same thing with tires. This was the point of the inflation pressure discussion (arguement) between Ford and Firestone - load carrying capacity and how much is enough.

 

[Rand]

XL Load tires are also a heavier tire. they weigh more.

so I think I'd be happier with an XL load rated tire at 32psi than a SL at 32psi even if they theoretically can carry the same weight.. the heavier XL tire would be more resistant to damage from potholes etc.

 

[crinkles]

so from what i read on this thread
a P tyre achieves it's load rating at 35 PSI.

Using the tables in the link above i have worked out that Toyota used exactly a factor of safety of 1.2 (20%) with respect to the carrying capacity of the tyres. this is with the recommended pressures of 30 front and rear.

I have 94 load rated tyres (670kg) at 35 PSI. max pressure stamped on the sidewall is 44 PSI. so this 44 is not the PSI where the load rating occurs but the max allowable pressure. Am i right?


GVM (from owners manual) is 2085 kg (i.e. loaded). at 30 PSI the 94 rated tyres can carry 625 kg looking at those tables. 4 * 625 = 2500 kg, and 2500/2085 = 20% safety factor!!

they do however make the distinction that when loaded the rears at 34 PSI to account (i guess) for load distribution to the reat.

EDIT - i may add to this an anecdote. one day i was pumping my tyres my wife asked "are you pumping them to 35??" this was weird because we deal in kPa down here. i said no i di 210 kPa which is recommended, 30 PSI. she said her dad (my FIL) always said 35 PSI for cars end of story. now he is a crankyk [censored] but knows a lot. so i think he got is 35 PSI from this classification system as indicated on the links above, and just carried through with the 35 PSI for the rest of his life. I guess it's not a bad number to pump passenger car tyres to! i think 30 PSI recommended for my camry is quite low and it makes the tyre look like it only has 15 PSI in it, but hey, it's what's recommended???

 

[mechanicx]

The way I understand it is the tire sticker on the car list the maximum weight that can be loaded to including car weight (GVWR), and the recommended minimum tire pressure (usually 30 or 32 for cars, more for trucks) for the OEM tires size and OEM tire load rating number (with speed rating) to cary that weight.

Now the maximum tire pressure on the tire is for loading the tire to that specific tire's maximum load which would be more than the GVWR per tire if that makes sense how I explained it. Edit: but i see now in this thread that maximum tire pressure is just max pressure and not where max load capacity is reached.

In other words, you should never load the car higher than the GVWR and so provided you are using the same tire size and load rating as OEM (or higher), you should not have to inflate above the car's tire placard spec.

 

[mechanicx]

Originally Posted By: crinkles
so from what i read on this thread
a P tyre achieves it's load rating at 35 PSI.


Yes, except I think the post said there is also "XL" P tires that achieve an even higher rating at 41 psi

.

Quote:
Using the tables in the link above i have worked out that Toyota used exactly a factor of safety of 1.2 (20%) with respect to the carrying capacity of the tyres. this is with the recommended pressures of 30 front and rear.

I have 94 load rated tyres (670kg) at 35 PSI. max pressure stamped on the sidewall is 44 PSI. so this 44 is not the PSI where the load rating occurs but the max allowable pressure. Am i right?


Yes I think 44 psi is max pressure for the tire and 35 psi is
the minimum pressure to reach maximum tire load capacity. However your Toyota's maximum load rating only requires 30 psi. So even at maximum recommend load for the car,inflating to 35 may cause more wear in the tire's center, harder ride, and less grip.

Quote:
GVM (from owners manual) is 2085 kg (i.e. loaded). at 30 PSI the 94 rated tyres can carry 625 kg looking at those tables. 4 * 625 = 2500 kg, and 2500/2085 = 20% safety factor!!
they do however make the distinction that when loaded the rears at 34 PSI to account (i guess) for load distribution to the reat.


Yes the rear axle on this application apparently has a higher load rating than the front. Like you mentioned there is a safety factor for the tires, but like wise there's a safety factor for the chassis so you should only load to the car's maximum recommend load (and pressure) and not to the tire's higher maximum recommended load (and pressure).

Quote:
EDIT - i may add to this an anecdote. one day i was pumping my tyres my wife asked "are you pumping them to 35??" this was weird because we deal in kPa down here. i said no i di 210 kPa which is recommended, 30 PSI. she said her dad (my FIL) always said 35 PSI for cars end of story. now he is a crankyk [censored] but knows a lot. so i think he got is 35 PSI from this classification system as indicated on the links above, and just carried through with the 35 PSI for the rest of his life. I guess it's not a bad number to pump passenger car tyres to! i think 30 PSI recommended for my camry is quite low and it makes the tyre look like it only has 15 PSI in it, but hey, it's what's recommended???


Yeah the recommended pressure of 30 psi will carry the car's max recomended loaded and you won't be overinflating the tires especially when you aren't loading the car to max load.

 

[crinkles]

thanks. this thread has taught me a lot.

FWIW the regular spec on my camry is 30 PSI all round (up to 4 people with no luggage) but when the cargo and towbal download is on the rears need to be 34 PSI.