Tyre pressures when towing

Submitted: Monday, Apr 21, 2008 at 22:32

ThreadID: 56872 Views:6128 Replies:3 FollowUps:13

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Scubaroo

Towing a Kimberley Kamper at the moment (in Port Pirie on the way to Central Australia and beyond) - NM Pajero with a camping load in the back. Running rear tyres at 44psi cold - they hit 54psi hot today (and have dropped back to 44psi since - no punctures). Seems a big jump in pressure, which would imply the cold pressure was too low? Or is that acceptable for LT construction? They are rated to 65psi cold.

What do other travellers inflate the rear tyres to when towing? Tyres are 265/70R16 BFG AT, LT construction.

thanks

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Reply By: Member - Uncle (NSW) - Monday, Apr 21, 2008 at 22:42

Monday, Apr 21, 2008 at 22:42

G'day Scubaroo, at present we've been using 28 on the camper, and 32 all round on the Patrol, quite loaded. these are hot running pressures, so far no dramas . Over the last few days on the dirt these have been fine. cheers Unc

AnswerID: 299739

Follow Up By: Scubaroo - Monday, Apr 21, 2008 at 23:30

Monday, Apr 21, 2008 at 23:30

Should have mentioned that was 44psi on bitumen - haven't hit the gravel yet. Was expecting to drop them to between 28 to 32 cold depending on conditions.

FollowupID: 565892

Follow Up By: Gronk - Tuesday, Apr 22, 2008 at 01:04

Tuesday, Apr 22, 2008 at 01:04

When towing my KK along Cordillo Downs track last year I made the mistake of letting my tyres down to 28psi...

Would have been OK if I had reduced my speed, but we were regularly sitting on 90K's on the good sections and I think it contributed to a blowout ( plus they were Coopers )

If you are going fast then they will heat up the same as if you were on tar ( maybe more with corrugations etc )

FollowupID: 565899

Follow Up By: Member - Bucky (VIC) - Tuesday, Apr 22, 2008 at 05:41

Tuesday, Apr 22, 2008 at 05:41

A bit of a curly question to answer.
But this is what works for me.
We own a Navara Twin cab Diesel, and weigh just under 4 tonne fully loaded, with our Trak Shak.

34 psi all round on the bitumen ( cold ),, 25-28 all round on Gravel (warm ),,,in sand down as far as 15 psi ( warm )

As far as the camper goes, well I work on the vehicle and camper's foot-print. Generally our camper runs about 2 psi lower than the vehicle, for the same foot-print.

ie, at home I worked it out, before we left, and the rule is to make the camper ( or what ever you tow ) the same foot-print.
Have everything on level ground, and use 2 rods ( sticks will do )
Place one, touching the front of the tyre tred, one touching the rear tred , now measure the distance between the 2 rods/sticks

Repeat this to what you are towing, and keep dropping the pressure, to give you the same distance.

All done before we leave.....Mind you, things may change, as you travel, ie. weights, road conditions, ect. and you may need to be slightly more flexible, with tyre pressures , but you have the basics covered.

Hope I have not confused you too much, ( or myself, come to that ! )

Cheers
Bucky

FollowupID: 565905

Reply By: Thermoguard Instruments - Tuesday, Apr 22, 2008 at 08:40

Tuesday, Apr 22, 2008 at 08:40

Hi Scruba,

Common wisdom is that tyres should increase pressure by about 4-5 psi from cold to hot, so I'd say yours were under inflated.

Despite the best intentions of the other replies, anecdotal advice based on various different rigs is not very useful. But there is an easy way to get it right.

Get an accurate measurement of the weight on the trailer wheels at a weighbridge. In larger centres, the council tip/transfer station will often let you drive over their weighbridge and stop with just the trailer wheels on it to get a figure.

Next get the exact tyre data off your tyres. For example, 185R14LT LI 102 or 10.5R15 LT LI 110, etc.

Then ring the manufacturers info line or go to your friendly tyre dealer and get them to look up the Tyre & Rim Handbook and tell you:- For that particular size and construction of tyre, and for your particular load per tyre (i.e. half the weighbridge figure for a single axle, a quarter for a tandem) - what is the recommended cold inflation pressure. I then add 2 psi to this "for luck".

Every morning , about 20-odd km after we set off, we pull off the road and I walk around our rig putting my hand on each tyre. You soon get to know what's a 'normal' amount of heat gain for your rig. Also do the same every lunch or "pit stop".

Despite running the recommended (or slightly higher) pressures, our vans 185R14 LI 102 tyres always run considerable hotter then the Landy's 245R16LT LI113s. But they've lasted very well in some very tough conditions.

Hope this helps.
Ian

AnswerID: 299784

Follow Up By: Thermoguard Instruments - Tuesday, Apr 22, 2008 at 08:49

Tuesday, Apr 22, 2008 at 08:49

Hi again Scruba,

Sorry, I've just re-read your post and realise you were talking about your vehicle tyres, not the trailer. Anyway, the same exercise could be done if you have an accurate weight of the load on the rear axle of the vehicle (with the trailer attached, of course) and talk to a manufacturer or dealer who knows how to read the T & R Handbook.

Must say I'm surprised that they heat so much from a 44psi starting point. Do you have an accurate ball weight for the KK? Some camper trailers seem to carry an extraordinary amount of weight on the drawbar and can have ball weighs higher than some much larger caravans. That, plus a reluctance to use proper weight distribution hitches on camper trailers due to the perception of decreased off-road ability (maybe 1% of the use of most of them?) can lead to excessive loads on the tow vehicles rear axle, sometimes beyond the manufacturers ratings.

Let us know what you find.
Ian

FollowupID: 565929

Follow Up By: Member - Andrew (QLD) - Tuesday, Apr 22, 2008 at 10:08

Tuesday, Apr 22, 2008 at 10:08

The KK trailers weigh around 1500kg (+-) with a fairly large ball weight (150kg IIRC) dependant on loading characteristics.

I would think that having a WD hitch on a Treg hitch setup would be counterproductive when needed. With the IRS setup of the Paj, and towing a KK, some form of suspension upgrade is necessary....how this will affect tyre pressures, i am unsure.

Watch the companion shafts on the Pajero, with a KK, as they are known to pop out due to the weights invoved.

Andrew

FollowupID: 565940

Follow Up By: ddr - Tuesday, Apr 22, 2008 at 11:14

Tuesday, Apr 22, 2008 at 11:14

"I would think that having a WD hitch on a Treg hitch setup would be counterproductive when needed."

Simple Solution, Take the WDH OFF when the articulation requires the Treg Hitch abilities.

its a 30second job that can be done just as you leave the black top.

FollowupID: 565958

Follow Up By: DesF - Tuesday, Apr 22, 2008 at 16:12

Tuesday, Apr 22, 2008 at 16:12

hi, Put some PolyAirs in the rear coils ( I run18lb fully loaded & 5lb empty) and then check the running temps , usually they say about 4lb increase after a 100ks, thats what my Pajero usually shows .
with a full load of camping gear etc I run 30lb in the front and 36 lb rear ( cold) both on the dirt and bitumen, same when the van is on, it has 140lb on the ball, I usually find the rears seem to travel at 40lb when tested.
Running Desert Deulers 9R's , about 5 years from first set with no problems.
Cheers Des.

FollowupID: 566016

Follow Up By: Gronk - Tuesday, Apr 22, 2008 at 18:48

Tuesday, Apr 22, 2008 at 18:48

A KK at 1500kg's would be carrying some serious weight, but suppose some people will load them up that much ?
And with over 150kg's towball weight, maybe a bit too much "crap" that needs to be left at home..

FollowupID: 566045

Follow Up By: Member - Andrew (QLD) - Tuesday, Apr 22, 2008 at 19:36

Tuesday, Apr 22, 2008 at 19:36

Actually the wet weight for mine is listed at 1250Kgs...add some other essentials and 1500Kg might be possible for some people (mine is probably 1350Kg then). They are constructed to handle this if you look at them closely, hence some of the price.

Remember there is 130L of water, 100Kg or so for batteries, 150mm x 50mm hot dip galved drawbar etc which all adds up.

The ball weight is listed at 125Kg properly loaded, again heavy, yet workable.....at the limit of a stock Pajero IMO.

As for the "crap", i suggest that it a personal choice for everyone....i'm sure you carry too much "crap" in your own setup. ;-)

Andrew

FollowupID: 566057

Follow Up By: Gronk - Tuesday, Apr 22, 2008 at 23:01

Tuesday, Apr 22, 2008 at 23:01

Yep, I probably carry too much "crap" also, but I try to keep it to a minimum..
I have found the listed weights are not nesessarily correct......my Limited KK has a listed ( with water tank full ) of 125kg's but its more like 140kg's..

I think you could get to 2 tons if you wanted to throw some accessories at it ......rear swingaway = 3 jerrycans..
3 jerries in the front gullwing
optional 3 jerry can holder for the front of the gullwing
mariner pack with boatloader and boat, plus out board motor in the front gullwing
a generator plus a few other options and phew ! 2 tons easy !! And haven't even started on the weight of the full canvas annexe and other accessories like gas hot water and gas heater....or extra 70 litre water tank !!!

FollowupID: 566132

Follow Up By: Thermoguard Instruments - Wednesday, Apr 23, 2008 at 05:01

Wednesday, Apr 23, 2008 at 05:01

Hi DesF

While PolyAirs & equivalants are good products and definitely help carry extra weight on the rear of the vehicle, I can't see how they can have any effect on the temperature the tyres run at for any given load.

For example, say the vehicle has 1500kg weight on the rear wheels, made up of the vehcile itself plus passenegrs and/or gear in the back and/or ball weight of a trailer. What diference can it make to the tyre running temp at a particular pressure if the thing is almost sitting on the bump stops or sitting up at near full extension with 35psi in the polys?

OK, if the vehicle was continually bottoming out, I guess it would be stressing and heating he tyres more but I doubt that's the case with scruberoo's Pajero.

Ian

FollowupID: 566150

Follow Up By: DesF - Wednesday, Apr 23, 2008 at 17:14

Wednesday, Apr 23, 2008 at 17:14

hi, Ian .I didn't really mean it as you took it, what I meant was the PolyAirs would fix his rear sag and make it much better to drive and the tyre pressures are what we found to work well for us,
"But" , having all the sag and extra weight on the rears "could"
cause them to heat more than normal.
Cheers Des.

FollowupID: 566260

Follow Up By: Thermoguard Instruments - Wednesday, Apr 23, 2008 at 19:00

Wednesday, Apr 23, 2008 at 19:00

Fair enough Des. Sorry, but it seemed to me that you were saying polys would somehow reduce the load on the rear tyres...

I still find it amazing that, from a starting pressure of 44psi they could gain so much heat & pressure, no matter what it was towing.

So Scruba, me ol' china, what have you found out?

FollowupID: 566281