Tyre pressures when towing?

[MickG]

About to head to Fraser Island for the first time and also tow a camper trailer for the first time with my '99 D2 and the family. Driving from the Gold Coast to Fraser and wondering what pressures people run when on road towing a fully loaded trailer.
I have a manual '99 TD5 with 33" mud tyres on 16" steel rims, springs all round - currenty sitting at 38psi all round on the car and 32psi on the trailer - how does that sound for the road there at 110ks? Should be right on the sand pressures so really just after road pressures if anyone has any suggestions for my set up.

Aye, Mick

[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.

[MickG]

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

[noddy]

What is the total weight of the trailer?

[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.

[walker]

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. :D

[MarknDeb]

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.

[Dante]

Have you been watching the 4WD Monthly video too. :D

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.

[JDNSW]

..........

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

[Dante]

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


John,
I have tried the method to do it by feel, but had to stop it because my tongue always ended up being black and the taste wasn't too greate either.

As for the difference in load between front and rear ... not sure how that would make a difference, you can have different pressures front and rear.

As for feel and handling ... I (that is me personally, just for me) wouldn't be using the tyres to accomodate handling and steering. Unless where circumstances require it (the usual spots where you let your air down ... eg sand, off roading, etc).

The method above is obviously only any good if you travel speeds > 30km for longer than 30 mins.

In other circumstances of course you will modify your pressure different to above. Here again I think all depends on the circumstances (eg to make that last sanddune you might have to drop to 15psi, but for the other major part of the trip, because it is outgoing tide, you will use 25psi (faster speeds, safer, eg) and when you get to the exit you will see how cut up it is and change accordingly.
I believe that the goal is to maintain as high a tire pressure as possible where circumstances allow it(with the maximum being the pressure established as above). This can mean that at times you will only have 16psi in the tyres (or 16psi in the rear and 18psi in the front or whatever).

But coming back to the original question, method above is more or less useless for trailers as their axle is not driven.
I think the meassuring approach goes back to seeing the axle of the trailer as the 3rd axle of your truck and therefore you should try to match the characteristics of the trailer tyres to the charasteristics of the rear tyres. Pressure again will depend on the load, so that is how the length of pattern comes into play.
I believe that this is only valid if you are using same or similar tyres on your truck as on your trailer. We had on our trailer the bolt pattern, rims and tyres changed to be identical to the one on the car. More spare tyres ;-)

At least that is how I do it. Used to be easy ... you get the experience and you just know after 4 years of being permanent on/off the black stuff.

Now we got a new truck, the old one is gone, maybe a second coming, the whole experimentiering (I know .. that word doesn't exist) starts again.

I have to say that letting the tyres down and putting air back in is not such a drama for us, we use a big scuba tank to do this, takes just a few seconds.
I can imagine that if you (or we when the tank goes empty) have to use a compressor will eventually go to "bugger this, she'll be alright' mode.

[JDNSW]

John,
I have tried the method to do it by feel, but had to stop it because my tongue always ended up being black and the taste wasn't too greate either.

As you darn well know, I meant I use my hand, not my tongue!

As for the difference in load between front and rear ... not sure how that would make a difference, you can have different pressures front and rear.

Yes, the temperature/pressure method will automatically increase/decrease the pressure according to the load. But handling/ride may require a different pressure difference, for example to give less sway when heavily loaded than a proportional pressure increase in the back gives.

As for feel and handling ... I (that is me personally, just for me) wouldn't be using the tyres to accomodate handling and steering. Unless where circumstances require it (the usual spots where you let your air down ... eg sand, off roading, etc).

Most people would require handling to be a major consideration - and a lot would also put ride quality above tyre life!

The method above is obviously only any good if you travel speeds > 30km for longer than 30 mins.

.............
But coming back to the original question, method above is more or less useless for trailers as their axle is not driven.
I think the meassuring approach goes back to seeing the axle of the trailer as the 3rd axle of your truck and therefore you should try to match the characteristics of the trailer tyres to the charasteristics of the rear tyres. Pressure again will depend on the load, so that is how the length of pattern comes into play.
I believe that this is only valid if you are using same or similar tyres on your truck as on your trailer. We had on our trailer the bolt pattern, rims and tyres changed to be identical to the one on the car. More spare tyres ;-)

.........

Can't argue about the desirability of having the same tyres on the trailer, as you say, more spares


While there is good reason to have the same tyre track on a trailer as the towing vehicle, I am not sure that (apart from the spare question) there is any real reason to have the same or even similar tyres on the trailer, especially if it is unbraked. The four wheel drive towing vehicle is driving on all wheels, but the trailer is not, and furthermore, in most cases, the load on each tyre will be very different from either the front or rear of the towing vehicle. If the trailer is unbraked, the tyres have no need for any sort of mud grip, and would be better with a pure road tread. Even if braked, circumstances where better grip than given by a road tread is needed will be very rare - most heavy braking happens on hard surfaces, and the less aggressive tread will give better mileage and lower costs on hard surfaces. But there are real advantages in having the same tyres on both the towing vehicle and trailer from the point of view of spares, so this is usually the best way to go if you can, particularly if really offroad.

John

[MickG]

Wow....some very scientific methods there indeed. All makes perfect sense however, the footprint/drive around for an hour then check etc is not an option as I am packed ready to go but am also interstate and arrive back late the evening before we travel. However based on all the info above, we have a 30min highway drive to our convoy meet point, so, my plan is now to obviously check cold tyres pressures before setting off and then again when we reach our meet point. If there are any changes to pressures in this space of time I will adjust accordingly.

Thanks heaps guys for the input, very much appreciated and just to answer one question above, I would be way off if I guessed at how heavy the trailer is....never been good at guessing weights/distances etc:eek:

So to recap, currently running 33" car tyres at 38psi all round and trailer at 32psi, may keep these and check at meet point and adjust accordingly.

Thanks again and will report back after my trip.

Aye, Mick

[Jeff]

Having the trailer the same track as the vehicle may be desireable when on rutted road or sand but is not always possible, as you may need a wider trailer.
I do a lot of long trips with a large trailer, about 1100kg, and as I have a very accurate pressure gauge( don't believe servo ones) and a pyrometer to use on my race bike, I did some tests of car and trailer tyre temperatures and pressures. I found the trailer's left side got hotter, either due to my large tool box or road camber pushing more weight on that side, so I tried more pressure in the left. It seemed to work, but I don't know if everybody would approve of different pressures on the same axle.

I usually run 36psi front, 60 rear and 55 right and 60 left on the trailer.
My LR has Michelin X and the trailer Yokohama 14" light trucks.

Jeff