Quote:
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.
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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?
Quote:
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.
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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.