Thread: Cooper AT3 now in 265/60/18

Originally Posted by TerryO

One workshop strongly recommend sometime ago 40 psi for them, they kept saying that was the right pressure for the AT3's

Comments like that just **** me off. The person making it either doesn't know what they are talking about or are too lazy to explain things properly. I know this isn't the case with you Terry, but people who don't know better may then take these statements as gospel because they are made by someone who fits tyres.

Two things to consider:
There is no "right" pressure for a tyre. This will always vary with the load and the conditions.

Discos always benefit from running a higher pressure in the rear than the front. More so if heavily loaded - the placard is a good starting point. Terry's pressures are a good example.

Cheers,
Jon

Hi Jon,

I guess I have always run the same pressure front and rear for all previous vehicles (cars and 4x4s). I haven't got my head around the Disco pressures yet so just went for what we used to run in the old 100 series for bitumen: 40 psi (and 30psi for dirt). When I bought my car it had 36 in the front and 42 in the rears which seemed a bit too much of a variance for me. After all, doesn't that mean the axles are turning at different speeds?

I left it that way until the tyre guys told me to change it to 35 all round, which I think was wrong but went by their advice and now I have a permanent spare because it has tiny cracks all over it (it was my passenger rear).

So I am back to 40 out of habit, but I am the first to admit that I have no clue with tyre pressures for this car! I have to head up to Marree in SA in a few weeks, what pressure would you recommend for dirt? The car will not be loaded up.

Thanks

James

Originally Posted by morpheus

When I bought my car it had 36 in the front and 42 in the rears which seemed a bit too much of a variance for me. After all, doesn't that mean the axles are turning at different speeds?

That sounds like its probably about right, if the back is loaded up. Remember the load on the front tyres is different to the rears, so running the same pressure front to rear would only give you identical rolling diameters if the load is identical. Either way the differences are going to be mostly negligible.

Originally Posted by morpheus

So I am back to 40 out of habit, but I am the first to admit that I have no clue with tyre pressures for this car! I have to head up to Marree in SA in a few weeks, what pressure would you recommend for dirt? The car will not be loaded up.

When you say not loaded up, what does that mean? Two up and empty? Have you got any heavy accessories fitted, like bull-bar, rear bar, LR tank?

I don't know your vehicle, nor have I run this tyre so I can't recommend a specific pressure. I have run 18"s on a D3 so I can give you some guidance to setting them yourself.

The best starting point is the placard pressures. I can't remember what they are on a D3, but the D4 is 33 front and 36-40 rear so I'll assume they are the same.

I have found I prefer a little more pressure in the front. Without bar work 34 seems to be about right. Add a bar and winch and you probably want to go up another 1-2 psi. Note Terry is running 36 front with bar and winch - sounds about right.

On the rear, 36 is the starting point if the vehicle is unladen. You may find it better with another 1-2 psi in it. Put anything in the back and it starts going up from there. What you ultimately run will depend very much on the load. Again, Terry is running 38 which sounds about right - for his vehicle. I wouldn't go beyond about 42, especially in the heat.

You should be able to set your pressures by feel. If they are too high the ride will be harsh and the front particularly will feel very skatey. Too low and it will wallow. Experiment by trying a few different pressures and see how the vehicle feels. Ultimately though, if you are more than a few psi away from the placard numbers, something is not right.

As for dirt, I like to lower my pressures relative to the highway pressures I have been running. Again, you will run higher pressures in the rear depending on load. You are trying to find the right compromise between tyre flex and excessive bulge. Lower profile tyres like to run at higher pressures than more conventional 4wd tyre sizes. For the good dirt roads heading up to Marree I would probably start by dropping around 2-4psi and see how it feels. for the rougher stuff, maybe 6-8 but you need to see how the tyre looks and feels. Again, I haven't run this tyre so I may be out by a couple of psi.

Hope this helps, just don't take any of theses numbers as gospel. They should be close, but you should get out and try it for yourself.

Cheers,
Jon

Thanks, Jon. Useful information for me. The Centurion does have a long range tank and rear wheel carrier. It also has a plastic LR nudge bar and safari snorkel. By not loaded up I meant it would only be carrying me, not a whole family and gear but I take your point re all the accessories and 100 litres of extra fuel.

With a long range tank and rear wheel carrier, you are already heavily loaded. The rear wheel carrier puts a fair bit of weight a long way out from the rear axle too so you are already probably more heavily loaded than me with just a full load of camping gear. Hence my point about there being no one right answer.

Good luck with it all. I think its very important for anyone doing any remote touring to understand about tyre pressures what influences them.

Hope you enjoy Marree. If you have time, spend a night at the Prairie Hotel in Parachilna on the way. The food is excellent.

Cheers,
Jon

Thanks Jon,

I grew up out of Marree and know the Prarie Hotel, and its owners, well. It is a cracker of a place - more people should get out and do the Flinders.

Sadly it will probably only be a quick trip up to see family, have a potter around the property , and then back South. Now that I am back in SA I plan to get up to the Far North of SA much more though, with a vehicle that can take all of us comfortably.

Thanks for the tips re pressures. I will see how I go.

James