SaabCentral Forums banner

Status
Not open for further replies.
1 - 14 of 14 Posts

·
Registered
Joined
·
431 Posts
Discussion Starter #1
Last week, i picked up some alloys with tyres at a toogoodtoresistprice:)

I was aware that the tyres were nankang ns-2 ultrasports(pleasedontlaugh) and i am seriously going to run with these as my previous michelin pilots (i wont mention formula 1) put me in some bad positions on wet roundabouts without full throttle. Nankang have been producing tyres since the forties so they have some pedigree and every new thing deserves a chance.

When i received my wheels last week, i gave them a thorough check out (only 3 months old) and i noticed that 2 of the tyres were 87 loaded and the other 2 were 91s like my michelins.
I was aware that the loading was related to loading on the tyres so i thought that i would stick my 87s on the rear which should be less loading than the front of car. But off course things are never so logical, as the tyres are rotational, and were obviously fitted by a ----head, they fited the tyres so that i can only put the 2 87s on the right side, and 2 91s on the left side.

This then required more investigation on the loading situation which i started on another forum.
I had checked the loading situation on the tyreindex, and
87 = 545kg
91 = 615kg
94 = 670kg

I know that my cars kerbweight is around 1600kg but i did not know the proportion of weight distribution from front to rear.
Luckily the good sir called SJ on another forum had these figures available and they are unladen
each Front wheel = 494kg
each rear wheel = 315kg

Now the most interesting information will follow in that whilst removing my 3 spoke saab alloys last week, i noticed that there was a maximum static weight stamped on the wheels and this was 575kg

Now if i put 4 very heavy people in my car, then my front wheel weight must surely approach 575kg :eek:

I think i would also want my tyre wall to collapse before the wheel collapsed.

As i drive alone most of the time, ive decided to run with my 1 87 load on the front of car which will be running close on the stated limits.

But i am a daredevil at heart so i feel fine on this as manufacturers usually build in good margins.

There was also talk that some viggen wheels were collapsing, and i wonder if this information is relevant

Any thoughts or more indepth knowledge on these issues are welcome
 

·
Registered
Joined
·
22,211 Posts
Fascinating stuff ;)

I wonder if an insurance assesor would have a problem with the wrong tyres? :(

I agree and would hope a manufacturer builds in safety margins, I also thought, maybe incorrectly, that the cars declared weight was including occupants :confused:
 

·
Registered
Joined
·
431 Posts
Discussion Starter #3 (Edited)
Car weight seems to be a bit of a grey area.

Ive checked my handbook , and it seems to define kerbweight as the
"weight ready for driving with a 70kg driver onboard" otherwise car with all fluids (fueltank) full and 1 driver at 70kg onboard (im 76kg:) )

It does recommend the sizes of tyres,but
It has no recommendation for the loadrating of tyres ??????????

For the 9-5 saloons, it gives 1520kg to 1665kg dependent on spec, the estates are heavier
It also gives the front to rear weight proportion as 60 / 40


So im taking the heaviest 1665kg and 60 / 40 that gives me 999kg on the front wheels and 666kg on the rear wheels.
This gives 499.5kg on each front wheel, and 333kg on each rear wheel.

The 87s are rated at 545 kg static load so i would be running well within the static loadlimit.

If the insurance assessor enquires about my tyres, i would put forward this calculation and state that my handbook does not recommend any specific loadrated tyre.
I would then put to him that my fitted tyre has a maximum static load rating of 545kg and i also know for sure that the 17inch saab wheel has a maximum static load rating of 575kg. I would then say to him that given the 2 choices of 87 rating @ 545kg and 91 rating @ 615kg that i chose 87 rating.
My reason for this choice was that i beleive the wheel max static load rating should always be higher than the tyre max static load rating.
The reason for this Mr insurance assessor is that in the very unlikely circumstance that some person should somehow load the car up such that it approached these maximum static loads (which to be fair in the wheels case, is not unreachable) then i would rather that my tyrewall collapsed before my wheel collapsed which is what would happen if a 91 tyre was fitted .

I should have been a lawyer in a soft easy chair ;)



Danger Alert :nono; Aero estate owners with 3 spoke 17 alloys and maybe even all saab alloys - you guys should even be more careful with a front wheel kerb weight of 522kg, then you are running much closer to that 575kg alloy wheel max static weight :eek: especially if you load these wagons up like a lot of you do
 

·
Registered
Joined
·
22,211 Posts
Thedriver said:
Danger Alert :nono; Aero estate owners with 3 spoke 17 alloys and maybe even all saab alloys - you guys should even be more careful with a front wheel kerb weight of 522kg, then you are running much closer to that 575kg alloy wheel max static weight :eek: especially if you load these wagons up like a lot of you do
Worse than that!

What about a Stage 4 with shed loads of stainless steel bolted to it and a very overweight driver.............:eek: :eek: :eek:

I better prise that M3 out of the front grille too :cheesy:
 

·
Registered
Joined
·
431 Posts
Discussion Starter #5 (Edited)
(o: (o: (o: (o: (o: (o: (o::cheesy:

Whilst Laughing, another thought (a serious one) appeared in my mind

Does the alloy wheel support the weight of the vehicle or is it the air pressure in the tyre ? and if so, then the alloy wheel would only support the vehicle in the event of a puncture

This would move the goalposts on this subject of tyre and wheel static max loads :suprised;



Chris 9-5 said:
Worse than that!

What about a Stage 4 with shed loads of stainless steel bolted to it and a very overweight driver.............:eek: :eek: :eek:

I better prise that M3 out of the front grille too :cheesy:
 

·
Registered
Joined
·
431 Posts
Discussion Starter #6 (Edited)
Well, as nobody apart from chris came up with any info, i decided to do some research (should have done this earlier but im a little lazy sometimes)and can now conclude the following.

The load index rating for tyres is for "static and dynamic (moving)loads" up to a speed of 135mph, apparently after around 135mph, the maths calculations get more complex.

The "max static load" stated on my saab 17 alloy of 575kg is probably only relevant when you have a puncture as in normal use, the air pressure takes the weight of car and therefore this figure is not really relevant to normal road usage with fully or unladen car.
So apologies to any wagon owners if i spooked you;oops:

But out of curiosity, if a 500kg weight is on a tyre at say 35psi, then i assume that the alloy wheel is under the force of 35 pounds per square inch of wheel ? which could easily be coonverted into metric weight if i could be bothered.

But im sure somebody somewhere will learn something from this post;)




Thought i should answer my own question
Tyre pressure of 35psi - what force or weight is on the alloy ?
Perimeter of alloy (or tyre contact area ) = 17 x pi = 53.4 inches
Area of wheel(in contact with tyre) = 53.4 x 7 inches = 373.8 square inches
Total force or weight on alloy wheel at 35psi = 373.8 x 35 = 13083 pounds
This equates to 5946.81 kg:eek:

But max static weight of alloy wheel = 575kg is only applicable to the ground contact area of wheel which im guessing is about 28 square inches.
That contact area is about 1/13th of total area of wheel.
Therefore 13 x 575kg gives maximum force or total max force(applied weight) on wheel = 7475kg

So 5946.81kg is easily within the limit

If i raised the tyre pressure to 40 psi then this totalforce or applied weight goes up to 6796.36kg which is still well within the limit :) enjoy

Final bit - Although the air pressure in effect takes the weight of the car. the alloy wheel is off course under pressure from the air at all times, and this equates to the sums as shown for 35psi and 40 psi. So the alloy wheel is always taking weight, under the guise of pressure ;)

Off course when you drive your car fast on a hot day then the tyre pressures go up - No i dont want to know that tonight.

Sweet Dreams
 

·
Registered
Joined
·
11,030 Posts
Bottom line is Saab recommended a load index of 91 on earlier 9-5's, they increased this to 94xl (extra load) for later models.
 

·
Registered
Joined
·
431 Posts
Discussion Starter #8
Kermit said:
Bottom line is Saab recommended a load index of 91 on earlier 9-5's, they increased this to 94xl (extra load) for later models.
This is true kermit and i had heard that myself, but it was interesting finding out the science behind it and making some ----ups on the way to this understanding.

Is it just a verbal recommendation from the dealers as its not in my handbook or do they put it in the handbooks now
 

·
Registered
Joined
·
11,030 Posts
I believe its one of those things they (Saab) don't broadcast too often. I know of one person who bought an MY03 model that came with 91 rated tyres, he took it back to the dealer and they were obliged to change them for the Saab recommended load ratings. (A point they should have checked in the certified 2nd hand Saab contract) :roll:
 

·
Registered
Joined
·
119 Posts
Hello Thedriver,

You're confusing yourself about the wheel not carrying the load. The tyre sidewall in combination with the air pressure do form part of the load path from car to ground, but this sidewall/air combination transmits the load from ground to wheel rim. The wheel itself then carries the load from the rim to the hub, the hub carries the load to the bearings, the bearings transmit the load to the hub upright.............

Regarding the wheel rating, I was surprised when I found this, but if you put 5 x 100kg persons in the car, which is about as much as practical, this will only add about 170kg to the front axle load. This would take my measured wheel load of 494kg up to about 579kg - not far away from the 575max. Checking the saab manual may reveal a maximum passenger load. Any boot loading (up to 200kg permitted I think) will be taking weight OFF the front axle.
 

·
Registered
Joined
·
2,534 Posts
Also remember those 87 rated tyres will wear at an higher rate compared to the 91's, and will generate more heat because they will be 'working harder', so much more chance of a blowout.

I wouldn't take the risk of putting less than 91's on a 9-5, and certainly not put a mix of ratings on a car.:eek:

And all those calculations of yours wouldn't cut any ice with an insurance man, or the Police for that matter.
 

·
Registered
Joined
·
431 Posts
Discussion Starter #12
HILDEBIRD - I do not believe that I am confusing myself, as you can see from the post, I started off without much knowledge of tyre loads etc and by thinking on my feet, I came to an evolved understanding and maybe it is you who are confused by my post, so I shall try and make it more clear.

Firstly the max static load of 575kg that is stamped on the saab wheel , is only applicable if you had a puncture, and the weight of the car at rest was then sitting on the one contact area of the wheel, so if you do put 5 adults in your car, and load the boot, there is no danger to the wheel.

Secondly, the wheel does not carry the loadweight of the car , as shown from my maths calculations, the wheel is always under a near constant loading which is created by the air pressure in the tyre.

It is purely the tyre pressure that carries the load of the car and obviously the tyre is what seals this air pressure allowing the car to move on the air so to speak and therefore the tyre is subject to friction wear. The wheel itself is hermetically sealed by the tyre in pressure from the air. This can be proven by the fact that if the car is standing still or moving, the wheels are not making contact with the ground due to the air pressure being greater than the load of the vehicle that is pushing on this air pressure, and so the wheel itself is always under a 360 degree pressure from the air and not from the direct loadweight of the car .

Aeropilot - The 87s are rated for 545kg at a constant 130mph or more, and I know my front wheel weight is under 500kg as I always drive alone or 2 max and never load my car. With regard to heat in the tyre, there are so many factors that cause this , air pressure, hard cornering , ambient temps, speed etc etc and I wont be racing round any race tracks. I certainly would not have chosen these 87s myself, I only have 2 of them due to the series of events based on ignorance. The guy I got the wheels from got them from a genuine garage 3 months ago and I have seen the receipt, but unfirtunately the receipt does not specify the load ratings as im sure many tyre receipts do not.

This guy had no idea about load ratings and just trusted the garage. The garage either tried a fast one on him, or just had a fitter who was also ignorant. But by these events, this post has come about and I certainly have load ratings welded in to my mind now. So every good lesson learnt has a price.

But I feel happy with my maths, and will run the 87s then upgrade to 91s on next buy or sooner depending on the wear state which I will monitor very closely.

In a court of law, you may be surprised to hear that maths calculations are taken as very serious as they are a more evolved explanation than a simple recommendation which in itself is meaningless in court.

To be fair, I really think that these load ratings should be stipulated in the handbook as they are quite an important area.The fact that they are not is either down to gross ignorance on manufacturers side, or absolute confidence in the tyre specs being very conservative. I wonder if they do a 17 inch tyre lower than 87 rating , probably not.
 

·
Registered
Joined
·
2,534 Posts
Thedriver said:
In a court of law, you may be surprised to hear that maths calculations are taken as very serious as they are a more evolved explanation than a simple recommendation which in itself is meaningless in court.
True, if said calculations are signed off by a professional, so I take it you are then.........so, AMIMechE, MIMechE or maybe even FIMechE?
 

·
Registered
Joined
·
11,030 Posts
I'm locking this thread... the original subject matter has run its course, we've no need to open a can of worms about our mathematical prowess. :(
 
1 - 14 of 14 Posts
Status
Not open for further replies.
Top