Tyre load index?

[dewarcraig]

Guys,

I have managed to get myself thoroughly confused with regards tyre load ratings etc...

So as far as I can tell the minimum load rating of the tyre is the maximum axle load divided by 2, then looked up on a load table to equate? Therefore the weights of the Z4M are -

Z4 M Roadster
Curb weight lbs/kg 3,203/1,453
Approved gross vehicle weight lbs/kg 3,821/1,733
Load lbs/kg 617/280
Approved front axle load lbs/kg 1,852/840
Approved rear axle load lbs/kg 2,006/910
Cargo bay capacity cu ft/liters 7.1/200
With convertible top compartment
panel folded up
cu ft/liters 7.8/220
Never exceed either the approved axle loads or the approved gross vehicle weight.

Z4 M Coupe
Curb weight lbs/kg 3,230/1,465
Approved gross vehicle weight lbs/kg 3,814/1,730
Load lbs/kg 584/265
Approved front axle load lbs/kg 1,885/855
Approved rear axle load lbs/kg 2,061/935
Cargo bay capacity cu ft/liters 8.7-10.6/245-300
Never exceed either the approved axle loads or the approved gross vehicle weight.

So taking the coupe as an example the max front axle load is - 855kg and the max rear axle load is 935kg. Therefore the minimum tyre load rating according the the table on Blackcircles is 79 front and 82 rear???

http://www.blackcircles.com/general/load-rating
dewarcraig
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[Half Colin]

Can't really comment other than why are you calculating the load index? Surely, you just need to ensure that your tyres are equal to or higher than the load index stated by the manufacturer.

 

[dewarcraig]

Half Colin - I was looking into it because the Z4M comes with Conti SportContact M3 tyres. These tyres are the same as those fitted to an E46 M3. Obviously the 2 cars are differnt weights and the load index on it is pretty high (95). When I was looking into how the load index is assessed it seems that as long as the tyre is over 79 front and 82 rear then this is ok. Basically gives a much wider choice of tyres as limiting the tyre choice to those over load index 95 is very slim! Especially when changing wheel size. For instance the Porsche spec N1 Pilot Sports are only load index 87...

 

[mmm-five]

I don't see where this is heading either - unless you're thinking of fitting some sticky tyres with a very low index - however I'm sure your insurance company would not look kindly on fitting anything BELOW the manufacturer's load index.

The load index on a tyre is simply the load it has been tested to carry when the tyre is BRAND NEW, and at the same tyre pressure (35psi), so there has to be some leeway for the tyre's age, inflation, history, hence the initial over-specification. The speed rating is also relevant to what speed that same tyre can carry that weight, so a 90W is different to a 90Y, as the 90W is tested to carry 600kg at 168mph where the 90Y is tested to carry 600kg at 186mph.

Cars are normally fitted with tyres that have a load rating higher than what the figures suggest as they need to take account of you hitting bumps, kerbs, potholes, etc. where the g-force can double the load on the tyre for an instant - would you want your tyre to be compromised & require replacement every time you hit some bit of bumpy road?

Please tell me that you don't want to fit a tyre that only meets the MINIMUM requirements for your vehicle?

 

[a11y]

Cars are normally fitted with tyres that have a load rating higher than what the figures suggest as they need to take account of you hitting bumps, kerbs, potholes, etc. where the g-force can double the load on the tyre for an instant - would you want your tyre to be compromised & require replacement every time you hit some bit of bumpy road?

As far as I understand it, the figure common used as "sensibly higher than axle ratings" is axle rating + 10%. That's certainly the figure applied when working out load rating for van tyres - as you say, to take account of dynamic load rather than catering just for static load. My winter van tyres are on that +10% limit but the summer ones I've just bought are a bit higher (around 13% I think) - and that's despite my "van" actually being a campervan and NEVER getting within 400kg of the max permitted weight of 3000kg.

I'd imagine that - on the same weight car - a tyre with a lower (but still safe/legal) load rating with provide better ride comfort and better performance via the softer sidewall being better able to handle bumps, etc. Just my thoughts though, not necessarily fact...

 

[dewarcraig]

No I most certainly don't want to fit a tyre with the MINIMUM requirements/specification. I am mearly confused as to what is acceptable, especially considering 911's are fitted with 87 load rating compared to a Z4M with a higher load rating but less performance...

The main point is that unless you are fitting like for like - OEM tyres then it is difficult to have a good range of others to choose from with an equal or higher load index. I also seriously doubt that an insurance company can not honour your policy because you fitted 93 load index tyres rather than 95's or whatever. Hence I was trying to see what the minimum spec is that would be required and know what I/we have to work with!

Therefore based on a11y's comments the Z4M should be fitted with a minimum of 82 front and 86 rear - making a porsche spec 87Y N1 tyre as an example perfectly adequate? Or a 91, 93 etc... Thats all I'm trying to establish.

Another difficulty, and probably the main reason I asked is because, when changing wheel size, if I wanted a 225/40/19 tyre then Camskill's whole range has a load rating of 93Y - are you saying I cant fit any of these including a Michelin PS2?

 

[mmm-five]

No I most certainly don't want to fit a tyre with the MINIMUM requirements/specification. I am mearly confused as to what is acceptable, especially considering 911's are fitted with 87 load rating compared to a Z4M with a higher load rating but less performance...

What is acceptable to a user is not the same is what is acceptable to the the law or an insurance company. You'll probably find 911s are generally LIGHTER than Z4s so will require a lower load index anyway.
You'd have to read through the 'Construction & Use' guidelines that MOT/Police/VOSA use for checking cars to see whether there's any rules on using tyres of lower load/speed rating than the car came with. I know brake pads have to be within +/-15% friction performance of standard pads (i.e. as fitted by the manufacturer) to be classed as road legal, so there could be something similar for tyres. Usually, something of higher spec than standard will not be looked on adversely by an insurance company, but lower spec items will.

I also seriously doubt that an insurance company can not honour your policy because you fitted 93 load index tyres rather than 95's or whatever. Hence I was trying to see what the minimum spec is that would be required and know what I/we have to work with!

If you have an accident, I bet the insurance company will look for anything that could have contributed to that - including tyre pressure, condition & age, speed & load rating. Would you expect an insurance company to pay out in an accident if your tyre blew because you were running a H-rated tyre as you never go over 130mph anyway, the same rules will be applied to load rating too.

Another difficulty, and probably the main reason I asked is because, when changing wheel size, if I wanted a 225/40/19 tyre then Camskill's whole range has a load rating of 93Y - are you saying I cant fit any of these including a Michelin PS2?

This confuses me even more than, as you claim to want to use a 93 tyre (as there are plenty available) but are worried because they're below the standard tyre rating, but a standard front tyre on a Z4M is a 91 rating, so I'd see no problem running a 93 rated tyre.

I'm note sure a court or insurance ombudsman would take a 'because there were more tyres available in the wrong rating' statement as a valid argument for fitting the wrong tyres.

 

[a11y]

What is acceptable to a user is not the same is what is acceptable to the the law or an insurance company.

What I'm interested to know is whether what is acceptable to the law is the SAME as what's accceptable to an insurance company.

There shouldn't be any difference as insurance companies operate using legal standpoints - I doubt a legal standpoint would be over-ruled by a manufacturer spec (but we are talking insurance companies here so it might be the case!). I'm just not sure where manufacturers fitments fit into that, i.e. BMW speccing a tyre that FAR exceeds the required load rating in the case of the Z4...

 

[mmm-five]

What is acceptable to a user is not the same is what is acceptable to the the law or an insurance company.

What I'm interested to know is whether what is acceptable to the law is the SAME as what's accceptable to an insurance company.

There shouldn't be any difference as insurance companies operate using legal standpoints - I doubt a legal standpoint would be over-ruled by a manufacturer spec (but we are talking insurance companies here so it might be the case!). I'm just not sure where manufacturers fitments fit into that, i.e. BMW speccing a tyre that FAR exceeds the required load rating in the case of the Z4...

That's why I worded it that way, as we know insurance companies aren't the most philanthropic of businesses

Without having reference to it, the law - i.e. the Road Vehicles (Construction & Use) Regulations 1986 which has been amended over the years (the original of which is not available online) - might say you can have any tyre load/speed rating as long as it is suitable for the car (but might leave you in a grey area by not defining what 'suitable' means), or might say that they need to be 'as prescribed by the manufacturer' in their type approval application (in much the same way as the manufacturer lays down the maximum loading for a vehicle which is used by the 'law' to see if you're overloaded. An in insurer on the other hand may simply refuse a payout if the load/speed rating is below those of the recommended/original tyre.

 

[bigdog]

I apologize, But I'm going to do a highjack of sorts. All this talk of load ratings, Being a psi or two off on your inflation causing the insurance companies to not pay a claim, Sounds strange to my American ears.
What I hear you guys implying, Is that if you dent one of your wings. A contingent of accident investigators and Scientists will descend upon your bent auto. Spend more on the investigation than what the claim would cost. Just to deny your claim.

Are the English insurers that bad?

In America. The cost of my insurance was half of what I pay in Germany. And They generally don't go over every claim with a microscope. Yet the insurance companies in America make huge profits.

Just wondering

Cheers
Jerry

 

[mmm-five]

I apologize, But I'm going to do a highjack of sorts. All this talk of load ratings, Being a psi or two off on your inflation causing the insurance companies to not pay a claim, Sounds strange to my American ears.
What I hear you guys implying, Is that if you dent one of your wings. A contingent of accident investigators and Scientists will descend upon your bent auto. Spend more on the investigation than what the claim would cost. Just to deny your claim.

Are the English insurers that bad?

It's not quite that bad...yet.

There'll always be some leeway for things like tyre pressures due to their nature to go up and down as they warm/cool, but in the event of a major claim (especially with added cost of the almost inevitable personal injury claims), I'm sure more insurers are looking for ways to 'lessen' their outlay by using the owner's/driver's 'negligence' to offset some of their liability.

If your tyre is a PSI or two down then there's no argument as it's likely to be down to conditions. But if you're tyre has no tread left, or is running flat, or you were driving over 50mph on a spacesaver, then the insurer is likely to come after you for contributory negligence. What isn't clear is how far down this line they would go if you're using consumable which their assessors consider below par - such as tyres with lower speed/load ratings despite you never having a fully-loaded car or going above 70mph anyway.

Look what a 'small' tyre issue did for Ford in the US and you'll see why small differences can make a lot of trouble.

 

[lacroupade]

I apologize, But I'm going to do a highjack of sorts. All this talk of load ratings, Being a psi or two off on your inflation causing the insurance companies to not pay a claim, Sounds strange to my American ears.
What I hear you guys implying, Is that if you dent one of your wings. A contingent of accident investigators and Scientists will descend upon your bent auto. Spend more on the investigation than what the claim would cost. Just to deny your claim.

Are the English insurers that bad?

In America. The cost of my insurance was half of what I pay in Germany. And They generally don't go over every claim with a microscope. Yet the insurance companies in America make huge profits.

Just wondering

Cheers
Jerry

Jerry I think its a mix of fact and urban myth to be honest. I mean ask us to point to people we know who've had claims refused because of any of the issues we frequently flag and there may well be a collective deathly hush. But the reality is that the risk isn't often worth taking in many peoples minds.

I think such things would come into play in the event of a fatal accident (in fact I know they do) in which case the police will strip the vehicle carefully and eventually issue an official opinion on the causes and contributory factors - that opinion may well then persuade the insurer that they have cause to decrease or even refuse the owners claim on them.

I mean heres a good example. Its not illegal to remove your rear anti-roll bar, and in the 4x4 community its common to do just that in order to get more articulation. But the insurer would frown on it, even if they were advised, because its a degradation of the original manufacturers spec - even if to many people its just a comfort thing (I say try removing your front ARBs and see how long you stay on the road around a bend!!!).

But if you then caused a fatal accident with a vehicle so modified, the police would flag it and the insurer could well take advantage.

So in summary I strongly suspect that its a bit of hysteria in the context of most run-of-the-mill insurance claims - but given the tiny chance of it being something much worse, its not worth taking the risk IMHO.

A few years ago my wife was involved in a fatal RTA caused by a drunkard crossing the motorway in the dark and rain...she was completely absolved of blame, but had she been speeding, or her car modified in anyway that could have influenced the outcome, no matter how small, the story could have been very different.

I just leave you with this as a reminder.....

 

[bigdog]

I thought as much. I.E. The urban myth and story enhancement. I suppose it's akin to talking about the "American crime wave"
I've lived in the USA for all but 5 of my 53 years. I've never witnessed a crime, Gang fight or shooting. But to hear others talk about it. One could justify the need to buy a bullet proof vest.

It's a terrible thing to even be near death. I prey that your wife was able to overcome the emotions involved with being in that terrible situation.
All the best to her.


Cheers
Jerry