Thread: Tyre pressures when towing?

I have a 2004 Disco running MTR's.

When towing the trailer on the road I run the front at 38psi and the rear at 44psi and the trailer (which has muddies as well) at 44psi.

I have just installed a tyre pressure monitor and after using it on the last trip I think I fond out I needed to increase the pressure in the rear wheels.

I started with the front at 38 and rear at 40 (which is what I had always run before) but after half an hour on the open road the front was at 40psi and the rear had gone to 47psi. This means the rear tyres had increased by 7psi. On the way home I increased the rear tyres to 44psi cold and they only went up to 46psi when warm.

Originally Posted by walker

I have a 2004 Disco running MTR's.

When towing the trailer on the road I run the front at 38psi and the rear at 44psi and the trailer (which has muddies as well) at 44psi.

I have just installed a tyre pressure monitor and after using it on the last trip I think I fond out I needed to increase the pressure in the rear wheels.

I started with the front at 38 and rear at 40 (which is what I had always run before) but after half an hour on the open road the front was at 40psi and the rear had gone to 47psi. This means the rear tyres had increased by 7psi. On the way home I increased the rear tyres to 44psi cold and they only went up to 46psi when warm.

Thanks Walker, sounds like a handy bit of kit the tyre pressure monitor!! Might take the rears up slightly and see how I go.

Cheers for the input, Mick

What is the total weight of the trailer?

My thought:
you should run the trailer with the same footprint as the rear of your truck.

The easiest way to measure your footprint is to put something straight (eg piece of wood) on the ground infront one of you rear tyres and one behind, where the tyre touches the ground.

Measure that distance.

Deflate the tyres on your trailer to the same footprint.

Just a psi number is not usefull. The trailer will displace more/less at a different weight.

Originally Posted by Dante

My thought:
you should run the trailer with the same footprint as the rear of your truck.

The easiest way to measure your footprint is to put something straight (eg piece of wood) on the ground infront one of you rear tyres and one behind, where the tyre touches the ground.

Measure that distance.

Deflate the tyres on your trailer to the same footprint.

Just a psi number is not usefull. The trailer will displace more/less at a different weight.

Have you been watching the 4WD Monthly video too.

We tow a 2t caravan and on the car we still have the michies it came with, front are 35psi & rear 45psi, the van runs 50psi on nankang 4x4/8plys.

Originally Posted by walker

Have you been watching the 4WD Monthly video too.

I did, but actually I learned about this a while ago.
I had this phase of wanting to know everything exactly to the point, so I wanted to know what is the deal about right tyre pressure.

You know ... you ask and they go ... "20psi, 18psi ... nah .. I run mine with 17psi, no worries".

Then you have the ones which go ... "what is the weight" and then they use some formula/experience to give you a psi value.

My conclusion out of my research (and please note that I am not saying it is the correct way, just the correct way for me):

1) Establish a baseline for the car (eg tyres, approx load, surface you will be driving on)
2) Start with cold tyres
3) establish the current psi
4) drive for an hour
5) check the new pressure

After driving for an hour or so, they tyre (and the air in the tyre) would have heated up. The air would have expanded increasing the pressure (Charles's law). For a correct starting pressure the average truck construnction tyre 31" ish should (!) increase by at least 3 psi, but not more than 4 psi.

If it increases less, your starting psi is too high, if it increases by more than 4 psi your starting psi is too low. If your pressure has not increased at all or is less than your starting pressure you are losing air.

What the actual margin (3-4 psi) is, depends actually on your tyre (eg passenger tyres would be less (2-3 psi), a 38" tyre would be 4-5psi.

It depends on the overall volume of the tyre, the construction and material, and how many plys. I also only applicable if you want to travel distances (eg > 30 mins) or speed changes (> 30 kph).

That is how I do it, usually too much hassle for others.

Originally Posted by Dante

..........

After driving for an hour or so, they tyre (and the air in the tyre) would have heated up. The air would have expanded increasing the pressure (Charles's law). For a correct starting pressure the average truck construnction tyre 31" ish should (!) increase by at least 3 psi, but not more than 4 psi.

........

More scientific than my method, which is to simply check the temperature by feel! (both methods are unreliable if there is a big air temperature difference between when you do the first check and the second - e.g. your dawn start in the desert at 0C and 25-30C an hour later!)

But the problem with both these methods (and also applies to recommendations by tyre manufacturers) is that it really only looks at the effect of pressure on tyre life, whereas handling or ride may dictate a pressure different to the one you get from this, particularly where there is a big difference in load between front and rear.

John