Tyres

4X4 · 30-07-2008, 06:03 PM

For all intents and purposes, tyres are a compromise; there are no tyres that will handle every situation. Having said that, tyre pressures are critical in the performance of any tyre. Over inflation reduces the ability of the tyre to absorb road shocks, resulting in a much harsher ride. In fact excessive over inflation will cause excessive wear of the centre of the tyre and may lead to impact fracture or other casing failures. Under inflation results in excessive wear on the shoulder of the tyre and heavy steering allowing excessive internal heat to build up, eventually weakening the casing.

Here is a simple method of determining the correct pressure for your vehicle, tyre, and axle load and speed combination. Always check tyre pressures when the tyres are cold, preferably first thing in the morning, before they have been driven on. Drive for half an hour at highway speed and check the tyre pressures. You require a 4psi increase. If less than a 4psi increase, the tyres are over inflated; you need to decrease the cold pressure. If more than 4psi increase, the tyres are under inflated; you need to increase the cold pressure. Allow the tyres to cool down, overnight preferably, and check pressures again. Once you have the correct pressure it is simply a matter of maintaining that pressure. If you load up for a camping trip or tow a trailer, any changes in the axle/speed combination will necessitate a recalculation of your operating pressure.

A lot of people have two sets of tyres, one set for the road and one set for offroad use. Whilst this may seem extravagant to some, after the initial cost of rims and tyres, it is no more expensive to operate than having a single set of tyres. You can only drive on one set of tyres at a time, so you actually double your tyre mileage. In fact, it may save you money; as you can operate a tyre that is designed for a particular environment and not be as susceptible to damage as a road tyre may be.

There are three distinct types of 4WD tyres, Road tyres, All terrain tyres and Mud terrain tyres. Road tyres that are fitted to new 4WD’s are designed to spend 80% of the time on the bitumen. The pattern is similar to passenger car tyres, and don’t have any lugs on the side wall. They have good characteristics on the road and don’t generate much road noise. Offroad they don’t perform very well and are susceptible to cuts in the tread pattern, side wall damage and punctures.

All terrain tyres have a more open tread pattern and small lugs on the side wall. They are designed for 50% on road use and 50% offroad use. In theory, they should be the perfect tyre, and for some people, they are. It all depends on what type of 4WDriving you do. They have slightly more road noise than road tyres and have predictable handling on road. Offroad they are better suited to medium 4WDriving on dry tracks. In muddy situations the tread clogs with mud and offer little, if any traction. If operated at road pressures on gravel tracks, the tyre can develop cuts across tread pattern.

Mud terrain tyres have a very open pattern and aggressive lugs on the side walls. On the road they have bad manners; they are very noisy and have a tendency to aquaplane on wet roads. Offroad they afford maximum traction, especially when deflated. Mud terrain tyres use the lugs on the side wall to bite into the mud and the tread pattern is designed to be self cleaning.

The numbers and codes which appear on tyre walls, known as “markings”, are the international identification system for tyres. They allow accurate identification of the tyre by describing its dimensions, structure and main applications. Size format may be imperial or metric.
Imperial Markings: Eg:10R15LT/109N 10 Tyre width in inches. R Radial Ply Construction 15 Rim dia. In inches. LT Light Truck 109 Load Index N Speed Rating (N=140km/h)
Metric Markings: Eg: LT 235/85R 115/113N denotes Lt Light Truck 235 Section width in mm
85 Aspect ratio (the ratio of the section height over the section width)
R Radial construction 115/113 Load Index N Speed Rating (N=140km/h).

frosty · 30-07-2008, 09:47 PM

4BY! You are just so worthy!!!!!!!!!!! There was stuff there that I think I did'nt know! BUT!! What about manufacture and use by dates? C'mon, don't hold back! anyone, anyone!

Are you a school teacher by any chance?

centaur · 31-07-2008, 07:37 AM

I know what Im doing this weekend......

Pure Yobbo · 31-07-2008, 05:12 PM

wow war & peace on tyres.

Actually their is some really useful information in there - thanks for that Rossco.

Cheers

Ven · 31-07-2008, 05:53 PM

good post, very usefull info

remus · 31-07-2008, 08:37 PM

great info and some more useful stuff. Well done

grit · 11-08-2008, 04:02 AM

Converting Metric to Imperial can be tricky for some as Imperial measurements state the overall diameter of the tyre as in 32 x 10.5 x = 32" Diameter and 10.5" wide. One does not need to know the rim size to determine overall diameter (approx.).

With metric its another story....

Example: 265 x 75 x 15 =

265mm wide
75 Ratio = height of rubber from edge of rim = 265 x 75% = 198.75 (x2 for either side of rim)
15 (rim size is always stated in Imperial - just to confuse the sums)

As comparison will be to an Imperial size... & as the rim is stated in Imperial..... we need to divide twice the aspect ratio by 25.4 then add the rim diameter...

198.75 x 2 =397.50mm

which is 397.50 / 25.4 =15.65" (close enough)
add 15" for rim = 30.65" overall diameter....

This would be close to the diameter of a 31" tyre - but don't mix & match tyres!

Pure Yobbo · 08-10-2008, 12:44 PM

I just used the above formula to work out the size of the tyres & Rims I just purchased from Quozie - yep they are 33".

The formula works a treat and it very easy to use - Thanks Grit great info.

Cheers

jack48 · 24-03-2010, 11:52 PM

Who's got an answer? Tyre pressures.
The car Bundera, tyres 285/75r16 Silverstone MT 117 Sport.
What pressure for highway & what for Off Road?
Let's open the discussion.
Running 35psi currently, and the back keeps trying to pass me !!!!!!!

Disco3SE · 15-08-2010, 05:10 PM

Quote:

Originally Posted by jack1948

Who's got an answer? Tyre pressures.
The car Bundera, tyres 285/75r16 Silverstone MT 117 Sport.
What pressure for highway & what for Off Road?
Let's open the discussion.
Running 35psi currently, and the back keeps trying to pass me !!!!!!!

On Land Rover Discovery 3 4.0 SE
3 tonne on the road.
265/60R18 Yokohama Geolander GO12 AT-S.
Sand = 12-15PSI
Dirt/Rock = 15-18PSI
Mud = 15PSI
Highway = 36PSI Rear & 34PSI Front

Hope this helps!
CHEERS