Question: Tyre ratings and letter codings

Mike Harding · Guru

Question: Tyre ratings and letter codings

Are We Lost · Guru

The dual load rating figures are for single axle and dual axle respectively. I don't think there is any distinction for suspensions with/without load sharing.

Tyre speed and load ratings

It's surprising how many help pages ignore this important piece of information.

I think the 10PR is ply rating, also covered in that page.

KJB · Guru

Are We Lost wrote:
The dual load rating figures are for single axle and dual axle respectively. I don't think there is any distinction for suspensions with/without load sharing.

Tyre speed and load ratings

It's surprising how many help pages ignore this important piece of information.

I think the 10PR is ply rating, also covered in that page.

You must be clear about the difference between - a Dual Wheel Axle vehicle (1 axle with 2 wheels on each end) and a Tandem Axle vehicle (2 axles with 1 or 2 wheels on each end ) - the different Ratings are for a Dual Wheel Axle (as on the rear of most trucks..) A lot of caravan owners do not appear to be aware of the difference and call a Tandem Axle caravan a dual wheel caravan........ KB

Mike Harding · Guru

Thanks guys! Great information and clearly presented.

So, I have a tandem axel vehicle with two wheels per axel therefore I assume the lower load rating of 113 applies to my tyres?

Just as a matter of interest; the quoted load figure applies at 180kph, does that mean at, say, 90kph the load capability of the tyre is higher? I don't fancy towing the van at 180kph to do my own test :)

Also why is the number of ply quoted, apart from the overall strength of the tyre what does the ply count mean?

KJB · Guru

Mike Harding wrote:
Thanks guys! Great information and clearly presented.
So, I have a tandem axel vehicle with two wheels per axel therefore I assume the lower load rating of 113 applies to my tyres?
Just as a matter of interest; the quoted load figure applies at 180kph, does that mean at, say, 90kph the load capability of the tyre is higher? I don't fancy towing the van at 180kph to do my own test :)
Also why is the number of ply quoted, apart from the overall strength of the tyre what does the ply count mean?

I think you will find that the higher Load Rating is the one that you can use as your wheels/tyres are not running as Duals.......

TheHeaths · Guru

My understanding Mike is that the speed and weight ratings are mutually exclusive.

That is the tyre is rated to a maximum load whatever the speed (below the quoted maximum), and the speed is the maximum allowable, whatever the weight, (at or below the quoted maximum weight).

I agree with KJB that the weight rating applicable to your unit is the higher one, being the first rating quoted.

The ply rating is not actual manufactured ply numbers, but the rated strength in ply numbers compared to older type tyres when individual plies were used. It is applicable as far as I know to the side wall strength. 10PR means it is rated the same as the old designated Light Truck, with passenger rated cars I believe are 6PR.

yobarr · Guru

KJB wrote:
Mike Harding wrote:
Thanks guys! Great information and clearly presented.
So, I have a tandem axel vehicle with two wheels per axel therefore I assume the lower load rating of 113 applies to my tyres?
Just as a matter of interest; the quoted load figure applies at 180kph, does that mean at, say, 90kph the load capability of the tyre is higher? I don't fancy towing the van at 180kph to do my own test :)
Also why is the number of ply quoted, apart from the overall strength of the tyre what does the ply count mean?
I think you will find that the higher Load Rating is the one that you can use as your wheels/tyres are not running as Duals.......

Kerry is right.The higher figure is for a single wheel setup,while the lower figure applies to a dual-wheel setup.Tandems still count as single wheels if there is only one wheel on each end of each axle,as opposed to dual wheels at each end of the axles. Cheers.

Radar · Guru

Mike Harding wrote:
I'm trying to establish optimum tyre pressures for my caravan so the first thing I did was to identify its tyres:
Maxtrek, Sierra S6, LT235/75 R15 10PR 116/113S
This seems to decode as:
Light Truck - 235 width - 75% height - Radial - 15" rim - ?? - ???
I have no idea what the 10PR means and although I believe the 116/113S is a load rating at 180kph I don't understand why it has two load rating at 1150kg and 1250kg?
The manufacturer's website is no help.
Any ideas?

Mike hi,

10 PR equal to the old 10 ply rating on conventional tyres.

Two load ratings, the highest 1250 kgs is for a single tyre and the low one is when you run dual wheels together.

Jaahn · Guru

Radar wrote:
Mike Harding wrote:
I'm trying to establish optimum tyre pressures for my caravan so the first thing I did was to identify its tyres:
Maxtrek, Sierra S6, LT235/75 R15 10PR 116/113S
This seems to decode as:
Light Truck - 235 width - 75% height - Radial - 15" rim - ?? - ???

Mike hi,
10 PR equal to the old 10 ply rating on conventional tyres.
Two load ratings, the highest 1250 kgs is for a single tyre and the low one is when you run dual wheels together.

Hi, I think the question has now been answered OK. biggrin

I just want to say that the air pressure is as important as all the other markings and should be on the sidewall also. This will be the max pressure at the max load. You can then proportion it to your actual load.

Actually a tire has NO functional load rating with no air pressure as the air actually carries the load. The "ply rating" just tells you how strong the carcass is and thus how much working pressure it can hold. So higher ply rating then a higher load rating for truck tires. It is an old rating system from the good'ol days but does not mean it has that many plies these days, but still used by lots of people. But the official rating is the 116/113s which actual real load can be found on any site as above.

Jaahn

Peter_n_Margaret · Guru

The load rating is the important bit. The "ply rating" is useless historical information. It tells you nothing that the load rating does not tell you.
I run Michelin tyres which will carry 3,075kg each @ 115psi. Their load rating is 147/145M. They have a single steel sidewall ply.
Cheers,
Peter

Are We Lost · Guru

Thanks KJB and others for pointing out the correct meaning for the dual load rating. As you say, the higher rating is for use with one wheel on each end of the axle. So the the lower rating would have no relevance for caravans. I should have read the link I posted.

Hylife · Guru

Forget about all that other stuff if you are after pressures.
Also ignore all the really bad advice from anyone who tells you what pressure to inflate to, unless they have the exact same van with the exact same weight with the exact same brand and model tyres as you.

What you need to look for has been embedded in every tyre manufactured for the last 40years.
The pressure and weight data on the tyre sidewall for your particular tyres should say something like "xxPSI at a max xxxxkg".
Every tyre is different.

Now you simply do the simple math for your vans weight. More weight = more pressure. Less weight = less pressure.

eg:

lets say your van was a single axel and weighed 2800kg loaded with 250kg of that on the towball and 2550kg on the wheels.
Just divide the weight on the wheels by how many wheels you have, in this case 2550 divided by 2 = 1275kg per wheel.
Lets also assume your tyres say 80PSI at a max of 1550kg
Now just divide the actual weight on each tyre by the tyre max weight to get the percentage of the specified pressure.
1275 divide by 1550 = 0.82
Now just multiply the tyres specified pressure by that percentage. 80 times 0.82 = 65PSI

That is how it is meant to be done for the last 40 years.

The tyre placard on any vehicle ONLY relates to the original OEM fitted tyres when the vehicle rolled off the production line new and empty.
The compliance plate contains pretty much worthless tyre data, but is required by law to have a data field for tyres, so most manufacturers fill the field with factory data.

KevinJ · Senior Member

I use this spreadsheet to calculate tyre pressures. Just enter your values in the yellow cells. The answer is in the blue cells.

The green and grey cells in the Conversion block are just to check the values in the yellow cells look ok.

The second TAB holds the table of Load Ratings if you need them to convert.

Mike Harding · Guru

Thanks again all.

Hylife's system sounds perfectly logical and for my van is:

Tyres rated at 1250kg at 80psi cold

Therefore each 100kg = 6.4psi

My van weighs 2800kg and has two axels = 4 tyres

Therefore 2800/4 = 700

Therefore 7 x 6.4 = 44.8psi

So each of my van tyres should be pressured to 45psi cold

Does that sound reasonable to all?

(PS. KevinJ: Thanks for the spreadsheet, I'll run it when I'm back on my PC).

yobarr · Guru

Mike Harding wrote:
Thanks again all.
Hylife's system sounds perfectly logical and for my van is:
Tyres rated at 1250kg at 80psi cold
Therefore each 100kg = 6.4psi
My van weighs 2800kg and has two axels = 4 tyres
Therefore 2800/4 = 700
Therefore 7 x 6.4 = 44.8psi
So each of my van tyres should be pressured to 45psi cold
Does that sound reasonable to all?
(PS. KevinJ: Thanks for the spreadsheet, I'll run it when I'm back on my PC).

You're right on the money there,Mike.Cheers

Mike Harding · Guru

Thanks Yobarr.

Interesting:

Today I remembered about the compliance plate fitted in the front boot of my caravan, it states 50psi when the van is at minimum load and 80psi when it's at maximum load. This conflicts with Hylife's quite plausible explanation of the markings on my tyres.

As 90% of my travel is on Bitumen and I never exceed 90kph when towing I've settled on 50psi... at least for the moment.

NB. It is *some time* since I have checked the van tyre pressures and I am too embarrassed to tell you what they were but I do suggest you check your van ASAP people.

Are We Lost · Guru

Mike Harding wrote:
NB. It is *some time* since I have checked the van tyre pressures and I am too embarrassed to tell you what they were but I do suggest you check your van ASAP people.

Tyre temperature is a quick and easy check. When travelling, and we stop somewhere, I usually do a quick walk around, feeling the tyres (and hubs) while my partner is gathering her things and getting out of the car.

It's no replacement for proper pressure testing but that quick walkaround may highlight you need to take action.

Hylife · Guru

If doing the math is too much effort, the same formula I mentioned, as a calculator, can be found on Australia's largest chassis manufacturers web site.

http://www.gnschassis.com.au/tyre-pressure-calculator/

Mike Harding · Guru

Hylife wrote:
If doing the math is too much effort

Nah, we're all sufficiently old such that we went to school before "progressive education" was introduced and as a consequence we can all read, write and do arithmetic.

Buzz Lightbulb · Guru

Hylife wrote:
If doing the math is too much effort, the same formula I mentioned, as a calculator, can be found on Australia's largest chassis manufacturers web site.

http://www.gnschassis.com.au/tyre-pressure-calculator/

"Nice one Centurion". I've bookmarked that website.