Tyre load rating versus max axle weights

[AnotherGareth]

I have always understood that tyre load ratings need to be at least capable of supporting the maximum axle weight of a car, as marked on the ministry plate, and that normally car manufacturers would specify minimum tyre load ratings to include a reasonable margin to allow for uneven loads across an axle.

 

Today I looked at a Mk1 Fabia vRS -- the plate in the engine bay says the maximum permitted weight across axle-1 (front) is 960 kg, and across axle-2 (rear) is 800 kg.

 

Skoda's official documentation says that minimum load index for each tyre is 83, which is equivalent of 487 kg. The maximum evenly-distributed load supported by the two front tyres, therefore, would be 874 kg, substantially less than the 960 kg maximum axle weight on the plate.

 

Can anyone explain this?

[Huskoda]

Yes, the 100kg you've lost in your calculations.

 

487x2 = 974.

[gadgetman]

So not that much margin

[AnotherGareth]

Yes, the 100kg you've lost in your calculations.

Oops, but still not much margin ... I'm thinking tyres with 86 or 87 load index would be a better choice, (being the standard XL tyres in the two sizes specified by Skoda).

 

BTW this compares with about 23% margin specified for tyres on our Octavia.

[Jeeves]

A slim margin nominally, but that's if the car is loaded to max capacity and even if slightly overloaded that mass would normally be biased towards the rear axle (where there is more "headroom" anyway).

I may be right in thinking that the maximum plated axle capacities total more than the maximum gross vehicle weight or whatever i.e. there's a further margin. Don't know the exact terms without getting out my V5C and Soda brochure but you get the idea.

And finally there must be some margin of safety in the tyres so they don't burst if overloaded by 1kg at their maximum rated speed.

That's why I'm happy to use tyres with a LI based on the larger of the two plated figures. From memory that meant that my FL Octavia PD Scout could have run on 91 LI tyres although most suitable sizes were well in excess of that. The OE Dunlops were 94 and I have used these briefly (and legally) on a much heavier 3.0 A6 avant automatic as they were just (15kg per Tyre) within spec.

[AnotherGareth]

Of the ones I've seen, the maximum axle weights add up to a bit more than than the maximum vehicle weight, (assuming my maths is up to scratch, which we've seen it might not be).

 

For example, the weights listed for our Octavia are 1900, (gross vehicle weight), 3200, (gross train weight), 1000 and 1000, (front and rear axle weights).

 

On the Fabia in question the numbers are 1720, 1720, 960 and 800. The second figure seemed odd; if the car isn't permitted to tow I'd have thought it should be zero, but having the gross train weight the same as the gross vehicle weight suggests that if the car isn't loaded then, theoretically, it could be used to tow a very light load.

 

So far as margin of tyres is concerned, if the load across an axle isn't evenly distributed, you'd need tyres rated for a higher load to handle the imbalance. Cornering at speed is one instance where the weight might not be spread equally!

 

Against that, though, I think the Load Index for a tyre applies at the maximum speed for that tyre, so for H-rated tyres if you don't go at 130 mph there's some extra margin, although I don't know what the margin varies with actual speed.

[Jeeves]

Yes, you're right about the load dynamics. With sedate driving this shouldn't be a problem unless you suddenly do an unexpected "Dukes of Hazzard" manoeuvre.

For track day use I wouldn't sail as close to the limit. But then I wouldn't take a Scout when I have something better suited potentially with a specific set of wheels/tyres.

Its a bit like weather conditions in that you drive appropriately and logic prevails, knowing the sensible limits for your car. None of us like driving up kerbs or through pot holes, especially at speed.

Above 130mph the LI has a factor relating to speed. But as it is only small, that suggests to me that either the additional dynamic loads don't push the margins massively or that the calculations assume straight line speeds.

If that's how I drove all the time, I'd be wanting the best tyres for that job rather than a compromise that covers the bulk of my normal/sensible driving.

[Gaz]

Load rating has me perplexed.

 

Without looking at the car's plate, my car seems to be listed as having an axle weight of 1095kg.

 

Load rating on tyre websites range from 91 to 94.  91 is 1230kg load rating per axle.

 

So is a 94 load rating more resilient to pot holes and speed bumps, or is 91 more able to deform and absorb without sustaining damage?

 

It's exhausting looking at the range of tyre names out there, so I'll probably stick with the Pilot Sport 3's I'm used to (225/45 17).

 

Gaz