Today I decided to do some research on the whole 21" Tyre issue, as I'm interested in the Hartge 21" Wheels. First issue was, that there are not many 21" Tires available, and some questions have been raised as to whether the Continental ones, supplied by Hartge are within spec, this is important for insurances reasons in some jurisdictions. I decided to try and find out what the story is. So I contacted Hartge,Tirerack, a local supplier and get some specs from BMW's own publications. Please note all weights are in Kgs
So first I got BMW's specs (from published Specs and brochures)
Front Axle Max Load = 1090Kgs
Rear Axle Max Load = 1270Kgs
I asked Hartge what is the relationship between these numbers and the tyre spec they chose and they explained it
as : BMW's Axle Max Load/2 (divide by 2 for 2 wheels per Axel)
so using that formula we get
Load Index /Tyre load Front = 1090 /2 (divide by number of Tyres) = 545Kgs per Tyre
Load Index/ Tyre load Rear = 1270 /2 (2 Tyres) = 635Kgs per Tyre
So bases specs are Front 545Kgs, Rears 635Kgs
Now I was told (local source) that some "extra" is factored in for increased load on front wheels when breaking hard and on Rear Wheels when accelerating hard. But I cannot find any source, info , test or published explanation of what that "extra" is and how you figure out what it is ?
That leads to a grey area - of well how do you know a tyre is within spec ? I'll give you an example based on figures we know
Base Load Front = 545Kgs
BMW 19" Original Equipment Front Tyres Load =710Kgs
Hartge supplied Conti in 21" Tyres Load is 650Kgs
So without knowing how the "extra" is arrived at, just looking at base specs, then both tyres look more then up to the job , as the both are capable of dealing with more then 545Kgs. The question is , whether there is a pertinent/important reason t for BMW's choice of a 710 Kgs capable tyre ?
So faced with a Tyre and Wheel choice how can you verify that your chosen setup is within specification ?. Without knowing what "extra" I should add to the base 545Kgs spec how can I be sure I have a valid within spec tyre setup.
Btw I am ignoring the fact that some jurisdictions have a tyre placard in the car that purports to show the tyre spec,
Anyone got any ideas, or
disagree with the Hartge supplied base load calculation method ?
So first I got BMW's specs (from published Specs and brochures)
Front Axle Max Load = 1090Kgs
Rear Axle Max Load = 1270Kgs
I asked Hartge what is the relationship between these numbers and the tyre spec they chose and they explained it
as : BMW's Axle Max Load/2 (divide by 2 for 2 wheels per Axel)
so using that formula we get
Load Index /Tyre load Front = 1090 /2 (divide by number of Tyres) = 545Kgs per Tyre
Load Index/ Tyre load Rear = 1270 /2 (2 Tyres) = 635Kgs per Tyre
So bases specs are Front 545Kgs, Rears 635Kgs
Now I was told (local source) that some "extra" is factored in for increased load on front wheels when breaking hard and on Rear Wheels when accelerating hard. But I cannot find any source, info , test or published explanation of what that "extra" is and how you figure out what it is ?
That leads to a grey area - of well how do you know a tyre is within spec ? I'll give you an example based on figures we know
Base Load Front = 545Kgs
BMW 19" Original Equipment Front Tyres Load =710Kgs
Hartge supplied Conti in 21" Tyres Load is 650Kgs
So without knowing how the "extra" is arrived at, just looking at base specs, then both tyres look more then up to the job , as the both are capable of dealing with more then 545Kgs. The question is , whether there is a pertinent/important reason t for BMW's choice of a 710 Kgs capable tyre ?
So faced with a Tyre and Wheel choice how can you verify that your chosen setup is within specification ?. Without knowing what "extra" I should add to the base 545Kgs spec how can I be sure I have a valid within spec tyre setup.
Btw I am ignoring the fact that some jurisdictions have a tyre placard in the car that purports to show the tyre spec,
Anyone got any ideas, or
disagree with the Hartge supplied base load calculation method ?