Load rating 100 tires meet spec, why are 104 "required"?

[GreaseMonkey]

Dude, remember that it’s not only your life on the line here. It’s other people’s too.

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[tuminatr]

I agree that the 100 tire is rated less than 104. But both exceed GAWR so both appear sufficiently rated to support the weight. The 104 is more sufficient. I'm open to the idea that there is a reason for the 104 rating - but from published specs that reason isn't just "weight".



At 4G the dynamic load on a fully laden car would briefly load the axle to 4 times the GAWR. If the tire must be speced for the dynamic load I would need tires rated 6510 lbs or more. Can I get dump truck tires in 225/60R18?

I believe DOT regulations for trucks and trailers are based on static weight and GAWR. Tucks and trailers hit potholes and bumps too. Perhaps Ford over-specs the tires on the assumption that a statistically normal Mach-E drivers will drive more exuberantly and hit potholes more exuberantly than a statistically normal truck driver. But this is just another speculative rationalization on my part.

Maybe it has something to do with regenerative braking? Regen brakes likely didn't exist when these tire standards and specs were made.

I get what you are saying about gross vehicle weight but that's not how tire load ratings work or what they are designed to do.

To understand that we need to talk about how and why the load rating is important how it changes the contact patch. Lets start with the load rating chart. To use a metaphor a tire is like a suspension bridge it will support a certain amount of weight. The belt package is the internal structure that supports the weight much like the cables on a suspension bridge. The air pressure adjustment is set by the car manufacturers to support a determined load. on a suspension bridge they tighten or loosen the cables to support load properly and be flat. With a tire the load rating shows not only how much weight it will support but at that weight it has a flat contact patch. That's why if you overinflate a tire you wear out the middle and under inflation causes the shoulders to wear.

There is a calculator that the manufacturer does to determine the correct contract patch for the load. In the case of the MME it needs to support around 1820-1850 lbs per tire to have the correct contact patch.

A load range 100 tire will support a maximum of 1764 lbs even at 50psi. You are essentially under inflating the Tire. A load rating 100 tire supports the same load a the 104 rated tire does at 36 psi. This will cause the tire to run hotter than it should and cause shoulder wear.

 

[Teslaeata]

Static loading requirements are a different kettle of shrimps to dynamic loading so we must just do as we’re told, always follow Manufacturer’s recommendations and don’t bother questioning trying to be smart??‍

 

[geeklifer]

I don't think he wants to know "why". I think he just wants someone to tell him he is right.

 

[Teslaeata]

I don't think he wants to know "why". I think he just wants someone to tell him he is right.

The internet forums are full of ‘em?

 

[andycunn]

I don't think he wants to know "why". I think he just wants someone to tell him he is right.

I do have the position that there must be some reason other than weight thresholds for the XL rating. I haven't expressed any opinion beyond that to be right about.

This is by far the newest car I've ever owned. My 2005 and 1992 vehicles have door stickers calling for a tire size but they do not state a load rating. For those vehicles, particularly trucks, I've always checked that the tire exceeds half the GAWR as the rule. Since I've gotten interested in this subject I've noticed that the Mach-E and my 2016 Edge post a load rating. Both cars spec the load rating a few grades over the GAWR. Maybe I've been doing it wrong all these years on the older vehicles. Should the tires be spec'd to, say 120% over GAWR as a general rule? I know, for example that building codes by convention require circuits to be rated 20% over the expected electrical load of the appliances they serve. It's possible that tires are similar. I don't know. If a rule or convention like that exists probably somebody more into the details of the industry would have added that to the thread by now.

 

[jefftuck]

I do have the position that there must be some reason other than weight thresholds for the XL rating. I haven't expressed any opinion beyond that to be right about.

This is by far the newest car I've ever owned. My 2005 and 1992 vehicles have door stickers calling for a tire size but they do not state a load rating. For those vehicles, particularly trucks, I've always checked that the tire exceeds half the GAWR as the rule. Since I've gotten interested in this subject I've noticed that the Mach-E and my 2016 Edge post a load rating. Both cars spec the load rating a few grades over the GAWR. Maybe I've been doing it wrong all these years on the older vehicles. Should the tires be spec'd to, say 120% over GAWR as a general rule? I know, for example that building codes by convention require circuits to be rated 20% over the expected electrical load of the appliances they serve. It's possible that tires are similar. I don't know. If a rule or convention like that exists probably somebody more into the details of the industry would have added that to the thread by now.

Or, it’s possible that engineers who are subject matter experts in very specific disciplines have run the numbers and come to this conclusion, outside of some simple rule of thumb.

Remember, these tires are more expensive for Ford to buy and nobody is buying Mach-e because of their heavy duty tires, so all the bean counters couldn’t convince the engineers to put on cheaper, lighter tires.

 

[highland58]

If you need to cut corners, the tires are not the best place to do that. Shortcuts are not always the best choice...

 

[andycunn]

If you need to cut corners, the tires are not the best place to do that. Shortcuts are not always the best choice...

Sure. At this point it seems there is no confident answer other than to just use the better tires.

Just for more speculation if everybody isn't already sick of this topic. - Maybe it's just Ford policy to over spec tires a little bit on non-commercial vehicles just for extra margin to avoid a repeat of the Firestone / Ford Exploder fiasco 25 years ago? Adequately spec'd Firestones were not adequate when paired to a late 90's explorer. Those cars needed more adequatley spec'd tires.

https://en.wikipedia.org/wiki/Firestone_and_Ford_tire_controversy

 

[jefftuck]

Sure. At this point it seems there is no confident answer other than to just use the better tires.

Just for more speculation if everybody isn't already sick of this topic. - Maybe it's just Ford policy to over spec tires a little bit on non-commercial vehicles just for extra margin to avoid a repeat of the Firestone / Ford Exploder fiasco 25 years ago? Adequately spec'd Firestones were not adequate when paired to a late 90's explorer. Those cars needed more adequatley spec'd tires.

https://en.wikipedia.org/wiki/Firestone_and_Ford_tire_controversy

I mean, sure, the answer is that the engineers who make these decisions picked the more expensive tires. As did, apparently, the engineers who work for all the other EV manufacturers.

But, you do you

 

[OH2AZ2OH]

There are 4 points of contact between your vehicle and the road. Those 4 points limit deceleration, acceleration, and lateral load limits. They are basically one of the biggest safety devices you have available. Would you buy the cheapest/barely good enough seat belts? Air bags? Parachute?

 

[andycunn]

I think I've found the answer in the following article. Indeed, SL tires are insufficiently rated for the weight of the vehicle, but for a very subtle reason.

https://www.tirerack.com/upgrade-ga...FZRtzN9g07ncvmH1uk3yPQjr5DH8hBglM4dzpQDU8CYJ4

Quoting the article: "However, there is an idiosyncrasy governing the use of Euro-metric and P-metric sized tires on vans, pickup trucks, crossovers and SUVs because these vehicles have a higher center of gravity and greater probability of being overloaded than passenger cars. In order to accommodate this, vehicle engineers are required to specify Euro- or P-metric sized tires rated to carry 10% more weight than would be required if they were used on a passenger car."

For pickups, vans, suvs, crossovers federal regulations require that the tire be rated to GAWR/2*1.1
For sedans it is GAWR/2

My rear GAWR is 3255 lbs. If this were a sedan the required tire capability is 1623lbs. But, because the Mach-E is a crossover the minimum is 10% higher: 1785 lbs. A 100 rated tire at 1764 lbs max does not meet this standard. Close but not enough.

On my older non-sedan cars without a load rating in the door jam, I have been shopping tires incorrectly. I never knew about the 10% derating rule for passenger car tires. Though the 10% derating doesn't apply to LT tires, I gather but that's a different tire category all together that are designed for higher center of gravity from the onset.

 

[p-zilla]

For pickups, vans, suvs, crossovers federal regulations require that the tire be rated to GAWR/2*1.1
For sedans it is GAWR/2

My rear GAWR is 3255 lbs. If this were a sedan the required tire capability is 1623lbs. But, because the Mach-E is a crossover the minimum is 10% higher: 1785 lbs. A 100 rated tire at 1764 lbs max does not meet this standard. Close but not enough.

On my older non-sedan cars without a load rating in the door jam, I have been shopping tires incorrectly. I never knew about the 10% derating rule for passenger car tires. Though the 10% derating doesn't apply to LT tires, I gather but that's a different tire category all together that are designed for higher center of gravity from the onset.

That's interesting, because EVs have a much lower CG than the average crossover/SUV/van/Truck because all the weight is in the battery pack on the bottom of the car. Ford was saying a Mach-E's CG is only 4" higher than a regular Mustang, which puts it in normal car territory. My guess is the propensity for crossovers to be overloaded combined with possibly outdated regulations that don't take EV low CG into account is the reason.

But even that being said, I'd still give myself more margin and buy XL tires instead of SL. Tires are quite literally the most important safety device on your car, don't skimp.

 

[andycunn]

That's interesting, because EVs have a much lower CG than the average crossover/SUV/van/Truck because all the weight is in the battery pack on the bottom of the car. Ford was saying a Mach-E's CG is only 4" higher than a regular Mustang, which puts it in normal car territory. My guess is the propensity for crossovers to be overloaded combined with possibly outdated regulations that don't take EV low CG into account is the reason.

True. With that heavy battery being so low and the Mach-E not being very high even by crossover standards to begin with, the center of gravity is likely not much higher than most sedans if at all. But, if the federal regulations say a crossover has to derate the tires by 10% then that's that. It is easier and cheaper on the part of the OEMs to spec for XL tires than to lobby for an act of congress or regulatory agency.

The "propensity" to be overloaded seems a dubious motivation to me as well. I see overloaded passenger vehicles on the highway all the time and when I do they are usually so grossly overloaded that they're still grossly overloaded even if the tires are up-spec'd by 10%.

But even that being said, I'd still give myself more margin and buy XL tires instead of SL. Tires are quite literally the most important safety device on your car, don't skimp.

I agree. This inquiry became mostly academic about 20 posts ago.

To rationalize my persistent, now bordering on obsessive interest: I maintain several vehicles, several trailers and tow varying sized loads with varying types of vehicles (though not with the Mach-E). When towing in particular I'm deliberately careful with load ratings (GAWR, GVWR, GCWR) and tire inflation appropriate to the load at hand. In my small sample size only the relatively newer vehicles call out a tire load rating explicitly. If the GAWR/2 tire rule commonly used in these applications has exceptions I'd like to know what they are. Afterall, if I'm going to be deliberatley careful I'd like to understand the subject I'm trying to be careful about enough to be confident that I'm doing it correctly!

 

[tuminatr]

Here is another one for you to ponder. Load ratings on aftermarket wheels are something most people don't pay any attention to. So, how many of the aftermarket wheels you see on a MME don't correctly support the weight.

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