Tire load rating / index

[Treeboy]

I would like to understand better how the tire load rating requirements for F15, or any car in general work. I decided to post here, just in case some members have experience in this field, or perhaps someone from the industry could provide insight.

F15 owners manual lists specs for different size wheels, summer, winter, A/S, square and staggered setups. Recommended tire pressure varies based on the speeds (below/above 100 mph) and third row options.

As for the tire load ratings, for the front tires the load and speed rating go (from 18" to 21") 109H, 107H, 107W, 106X, 106V to 105Y. Same for the rear tires: from 109H, 107H, 111W, 110W, 110V to 108Y.
I couldn't find any solid info online about whether the larger rim tires require lower load rating to support the same vehicle. It looks that way in the manual. But it does not make sense to me. If the load rating is really defined as the load index number that corresponds to the tire's load-carrying capacity. That would mean that two tires with the same load index can support the same maximum load.
Why it is then ok to use 21" staggered setup with tires that have load rating F:105 and F:108, but in case of 19" staggered setup these numbers has to be at least F:107W and R:111W?
I would appreciate if someone could explain this to me.

PS: I should point out that I am trying to buy tires for 19" staggered setup and when looking for Conti DWS06 the setup is suggesting tires with loading ratings: F:105 (245/50) and F:108 (275/45). I have contacted Continental and various tire dealers, but it is really hard to obtain response that would explain the above. The advise I am getting is to contact BMW and ask them if I can use 19" tires with lower load rating .
Please lets not make this about 20"/21" looking so much better on the X5...

[CatalinP]

The difference is because of the contact area between the tire and the road surface. 21" rims means much wider tires, therefore less weight per square inch on the contact surface.

[Treeboy]

Originally Posted by CatalinP

The difference is because of the contact area between the tire and the road surface. 21" rims means much wider tires, therefore less weight per square inch on the contact surface.

Sorry, I still don't get it. I would, if the tire load rating index was defined as an assigned number that corresponds to the tire's load-carrying capacity per linear inch of width, or per square inch of contact surface.
If the tire with 275 width can support the same vehicle load as 255 with load rating of 108 (both are recommended for the same X5), why the wider tire still has lower load rating index of 105, and not 108 as well?

[F15 X5 Silversurfer]

If you go to the website of any tyre manufacturer you will find the load / speed index charts which give the rated tyre load PER TYRE for a given pressure at a given speed.
Speed / pressure / load are all interlinked and are very important when a vehicle manufacturer specifies a particular tyre specification when a vehicle is being developed. There is a lot of confidential co-operation between vehicle and tyre manufacturers at this stage.
The final decision on which specification tyres are to be fitted, including recommend pressures, is a compromise between these two parties to provide the best handling characteristics etc. for the vehicle concerned and often it is the position that a particular tyre may only be available with a particular Load Speed Index (LSI) from the manufactures involved.
Drivers are at liberty to alter their tyre pressures for their individual vehicles and I do this after I have observed how my tyres are wearing with regards to over / under inflation, but I have many years technical experience within the tyre industry. The adjustments should only be small, only a matter of 4-5 PSI maximum up or down from the current recommended pressure, but great care must be taken as only a small adjustment can have a significant effect on a vehicles handling, especially regarding under / oversteer when cornering. If you are not confident with this I suggest that you keep to the recommended pressures.
From a drivers point of view it is therefore important that they should only fit tyres that are equal to, or exceed the O.E. specifications. This is even more pertinent on higher specification vehicles where performance and handling will be compromised by using under specified tyres.
Also, at least here in the UK and Europe, insurance companies will always check tyre fitment specs should you be involved in an accident and will look to void the insurance if incorrectly under specified tyres are fitted even if they had no direct influence on the accident. All this can lead to a minefield for those involved.
Apologies for the length of my blog but I felt that an explanation would be of help.

[CatalinP]

Originally Posted by Treeboy

Sorry, I still don't get it. I would, if the tire load rating index was defined as an assigned number that corresponds to the tire's load-carrying capacity per linear inch of width, or per square inch of contact surface.
If the tire with 275 width can support the same vehicle load as 255 with load rating of 108 (both are recommended for the same X5), why the wider tire still has lower load rating index of 105, and not 108 as well?

Because the structure of the tire is different, made to certain specifications.

[Treeboy]

Originally Posted by F15 X5 Silversurfer

Speed / pressure / load are all interlinked and are very important when a vehicle manufacturer specifies a particular tyre specification when a vehicle is being developed. There is a lot of confidential co-operation between vehicle and tyre manufacturers at this stage.
The final decision on which specification tyres are to be fitted, including recommend pressures, is a compromise between these two parties to provide the best handling characteristics etc. for the vehicle concerned and often it is the position that a particular tyre may only be available with a particular Load Speed Index (LSI) from the manufactures involved.

I do not disagree with you. The automakers are tweaking the car in development in cooperation with tire manufacturers. One could argue how much new development was done for F15, given the fact the tire specs changed only in regards to speed rating and recommended tire pressure. But let’s assume they got it right for E70, so why to fix what isn’t broken. I admit they test the car extensively and provide customers with what they believe is the best setup for safety, handling and performance purposes. They perhaps even test different wheels offsets to ensure the best handling and cornering. And of course, they recommend RFT for the best performance. I am not disputing this. Except perhaps the RFT part.

My question was specific to tire load rating. Although car manufacturer could offer two or three makes of tires as a factory option, I think it is safe to assume that parameters like load or speed ratings values should be binding for any reputable tire producers across the industry.

My question was not tied to performance, more to safety. If tire with a load rating of 105 can support 2039 lbs in 21” staggered setup, why in case of 19” staggered setup the front tire needs to have at least 107 load rating (and therefore support 2149 lbs). It is not like by switching the tires from 19” to 21” the car becomes over 600 lbs lighter (when factoring front and rear tires load rating change). Or does it?

Originally Posted by F15 X5 Silversurfer

From a drivers point of view it is therefore important that they should only fit tyres that are equal to, or exceed the O.E. specifications. This is even more pertinent on higher specification vehicles where performance and handling will be compromised by using under specified tyres.

I agree. It would be huge risk to go to lower than OEM specifications for the speed and load rating.
Many BMW drivers here ditched RFTs (myself included) and it is arguable how much the handling suffered because of that. Use of RFT is not mandatory in Canada (by government or insurance companies).

Drivers adjust tire sizes sometimes. For whatever reasons, aftermarket wheels that might be 1” wider, preferred tire not to be available in specific size etc. I am not an expert to estimate how much poorly E70 or F15 handles in corners if let’s say square 19” setup is changed from OEM recommended 255/50 R 19 107 H M+S XL RFT to 275/45 R 19 108 W A/S XL non-RFT in all four corners. I would appreciate to hear what others think about such change, but I would not think going from 255/50 RFT to highly reputable UHP all-season tire 275/45 (like Continental ExtremeContact DWS 06) with higher load and speed ratings would be excessively negative change. But others may disagree.

[Treeboy]

Originally Posted by CatalinP

Because the structure of the tire is different, made to certain specifications.

There might be improved structure on the wider tire of the same model. But if that is the case and tire therefore increases the maximum load it is able to support (carry), why this change is not shown in higher load rating for this tire? It just does not make sense to be.

[CatalinP]

You are assuming the tire structure is the same, no matter how wide it is. That is wrong.
For the same tire model, even the profile is changing as the tire gets wider.

[Treeboy]

Originally Posted by CatalinP

You are assuming the tire structure is the same, no matter how wide it is. That is wrong.
For the same tire model, even the profile is changing as the tire gets wider.

Sorry, but in contrary...

We can assume they are entirely different tires.
19” tire is made by Brand 1, all season non-RFT front tire, 255/50R19 107 W XL. So it has to be able to support 2149 lbs.
21” is made by Brand 2, summer UHP RFT front tire 285/35R21 105 Y XL. This tire has to be able safely carry and support 2039 lbs (not more as per the tire manufacturer's specs). If the 21” tire was able to carry 110 lbs more, there is no reason why the manufacturer would not index it as 107 Y.

[CatalinP]

You're comparing beans with apples.
Every tire is rated to carry the load it was designed to carry, plus a safety margin.

[Treeboy]

I see, I thought I was comparing load rating of two different tires :