m5-black

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 ?

Wolverine

Well, here's my thinking:

BMW developed these tires with Conti specifically for the M5. Their load ratings are 710kg front, 775kg rear. They designed the tires with those load ratings for a reason - I imagine that was the spec that BMW gave them. Or possibly BMW gave Conti a spec, and they exceeded it. I don't know if you can ever figure that out though.

When substituting tires, I would not go with a lower load rating than the tires that were specifically designed for the car. I'm sure there is a safety factor built in, but I'd rather have that factor than not. When I talked to Tirerack, they said the same thing - they would not recommend lower load ratings than the OEM tire specs.

m5-black

Funny thing is that if you look on Tirerack you'll see that the non OE version of the tyre has an even higher load rating - higher by approx 199Lbs - which I find confusing.

I wish this tyre stuff was more transparent and straightforward.

Amjad Ali

Hi Guys

Let me try and help here. I used to be in the alloy wheel business and was responsible for researching exactly this kind of thing when we started to look at new applications for our wheels.

The axle load rating that manufacturers give is the maximum including the extra amount that m5-black is querying.
The way manufacturers calculate this is by testing for maximum acceleration and braking, as well as lateral acceleration. You will also note that the rear axle load rating is higher than the front, when in reality the car is actually heavier at the front than the rear. The reason for this is to allow for all possibilities including towing something. So if the car was ever fitted with a tow bar then the additional weight has also been factored in.

If you add the load ratings that bmw give this gives a total weight of 2360kg's. Now we all know that an M5 does not weigh that much, the true weight I believe is sub 1800kg's.

M5-black you don’t have to worry about the extra weight for braking or accelerating as this has already been factored in.

As to the difference in load ratings of the standard 19" tyres and the 21" Conti's the explanation is most probably as follows, and this is based on my experience with dealing with conti for exactly this type of exercise:

The standard 19" tyre when built by the manufacturer already has an inbuilt strength and when conti then test to give a load rating these tests then conclude that the tyre in that particular size has a certain load rating. Most of this is based on carcass strength and construction. The OE tyres that have been developed for the M5 use this basic caucus and then the tyre is fine tuned by altering the type of compound and making alterations to sidewalls to match the dynamic requirements of the BMW engineers. Once this has been finalised the tyre is then retested and the final load rating is given. But it is fairly normal for tyres of this size and width to have this kind of load rating.

There is one factor that is also crucial to the load rating of the tyre that no one as yet has mentioned and that is the speed rating of the tyre.

For instance these tyres are Z rated, which means over 149mph, but as cars have got faster there has been three new additions that of W up to 168mph and Y which is up to 186mph. The other is (Y) in brackets which means that this tyre has normally been tested at over 186mph for its particular application.

Now it could be that the load rating of the OE tyres is 710kgs and it is Y rated, that means it will have that load rating up to 186mph. As the car exceeds this then the load rating also comes down, so it could be that by 205mph the tyre mat still have a load rating that is greater than BMW's load rating. I don’t recall this formula they used as speed increased, but I had these problems when I was developing a 20" wheel and Tyre for the Bentley GT Coupe.

I hope I have been of help.

One thing I could never understand, and this is aimed at the USA members, why is it in your country where people will sue you for something really trivial, do most of the industry have a blatant disregard for this type of safety on wheels and tyres. An example of this was when I was trying to develop the 20" wheel and tyre package for the Bentley and also the Aston Martin Vanquish I could not find the correct load rated and speed rated tyre. I could find the correct sizes but not the ratings that were required. I visit SEMA in LAsVegas that year and to my horror there were Bentley's running 22" wheels and tyres, where I know for a fact that the tyre did not have a load rating or speed rating anywhere close to what was required for that car to be safe!!!!! Absolutely mad.



So in short m5-blackk the Hartge wheel and tyre combo will be correct for the car. Hartge are a quality tuner and will have tested this.


Anyway I hope I have helped.

Thank you all for reading this far down.

frylord

Thank you for the explanation, one thing on my mind however.

How can the tyres under the M5 be Z rated when the limiter kicks in at 250km/h (155mph)? And BMW inidividual planning on offering a speed limiter removal so it can run upto 305km/h (189mph). I thought the conti's were rated to go upto 305km/h wouldn't that give it a Y rating and not a Z rating?

Since you seem to be fimiliar with these tyres and also fimiliar with the industry could you give us your opinion on the conti's? Things like comfort, noise level, grip, durability and such.

sydl

I think this is a very dangerous and reckless recommendation. The only thing that is improtant IMO is whether his insurance company will compensate him for damage that occurs when he is on a tyre that does not conform to the manufacturers' specifications.

The tyre load rating spec for the M5 is clearly stated as 710 and 775kg - so if you are on a tyre that is load rated at less than that in some countries the car is deemed to be unroadworthy. The consequence of that is that insurance policies are void. Obviously it's still safe to drive but that is not the point from the point of who will be responsible for damage that occurs to the car or by the car.

If you are OK with driving your M5 without insurance, then you can put whatever rubber you want - but if you want to know that you're insured - just make one call and ask your insurer "if I have an accident in this car with these +2 wheels and tyres is my policy valid and hey say yes and give it to you in writing" then go for it.

Frits

The Y is an addition, so the tyre will probably have a ZY rating...

Amjad Ali

HI Fry as explained above the tyre is usually described as follows:

285 35 ZR19 98Y

This is the tyre width, aspect ratio and then the speed rating before the rim size. The 98Y is the load rating and additional speed rating.
As to the new limiter you have indeed made an interesting point. Can any members please post the exact tyre rating for me. If the car is going to run at 189mph then the tyre will have to be tested on the car for this. In this case it should end up with a (Y) rating. In brackets. This is exactly what Yokohama did with the Bentley tyre at 20". It has a (Y) rating meaing it has been tested on the car.

Many Thanks

Amjad Ali

Sydl you make a very valid point. Yes you should only use rubber that is recommended by the insurance company. But you should also notify your insurance company that you will be using aftermarket wheels.
The reason I have recommended the Hartge package to m5-black is because it is by Hartge.
In Germany if you are going to fit any wheel or tyre on your car the package has to be TUV approved. Germany has the strictest rules and regulations anywhere in the world for wheels and tyres. If it is not TUV approved then not only is your car insurance invalid the car is not allowed on the road. TUV are the equivalent of the UK's Ministry of Transport. TUV testing is expensive and also very tough.
Hartge's package has TUV approval. The process for TUV approval is as follows:

From Memory
you have to supply approx 12 wheels. All bar 4 are destroyed in testing.
They then test for load rating, and they test until they physically break the wheel.
It is tested for corrosion and is subjected to corrosion tests.
Impact test where a predetermined weight is slammed into the wheel in a manner that represents the wheel hitting the kerb.
There are other tests but it’s too early and I have had no coffee yet.
The main part of the test however is making the tyre choice and fitting the complete package to the car in particular and road tested.

When I mean particular car I mean that car, I used to make the same wheel for Mercedes and BMW but because one had a PCD of 112 and the other 120, it required separate tests.
The tyre load rating that you are mentioning as 710 and 775kg's would mean that the M5 would weigh 2970kg's, I don’t think that is correct.
Hartge's 21" package will have the TUV certificate not only for the wheels but also for the Car, because this is stated on the certificate.

TUV charge you for tyres, petrol used in the test, the hire of the car if need be. Everything has a charge. To give you an idea of this world, when you next complain about the price of your wheels consider this:

Design and CAD testing approx €100,000
Mould manufacture €60-75,000
TUV Testing €35,000

that’s before they even get to a point where they can sell a single wheel.

Now Sydl like I said I agree with you 100% and you are correct, but my recommendation is based on my knowledge of Hartge's TUV criteria. And it is on that basis that I have recommended this package.

Ashok Arora

A very interesting & educational thread - thanks guys

__________________
Ashok
2002 E39 Carbonblack/Caramel M5 (best car I ever owned)
2005 E60 Blue Onyx/Caramel/Light Figured Ash M5 (worst car I ever owned)
2007 E90 Montego Blue/Black Alpina D3 (sublime)
Let me tell you what SMG stands for...