Towing tyre pressures

Submitted: Tuesday, Aug 02, 2011 at 15:32
ThreadID: 87941 Views:12559 Replies:8 FollowUps:14
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I know this issue has been done to death before, but my topic search seems to throw up 305 threads, all but 5 being archived - and I can't find the active ones (I must be doing something wrong)! Specifically, I am seeking feedback on why my rear car tyres seem to increase in pressure by at least twice the amount that the front tyres do, and the tyres on the caravan do. The pertinent facts areas follows:
Car: 200 Series Landcruiser turbo diesel weighing approx 3460 loaded (without caravan attached).
Suspension: has been upgraded by ARB with 200kg system (heavier coils front and rear and OME Sports shockies at each corner).
Rear coils have Coilrite, inflated from 12-16psi (still experimenting).
Tyres: Bridgestone D694LT 285/65R17 (on both car and caravan - approx 9,000km since new); load rating 1400kg each.
Caravan: Free Spirit off road; weight approx 3100kg loaded; Simplicity 4500kg load sharing suspension.
Hayman Reese WDH 750lb bars, with 100mm drop hitch.
Ball weight of caravan on car = 280kg.
Tyre pressures: Car - 40psi front; 45psi rear. Caravan: 40psi all around
(Note: this is for bitumen highway driving. Bridgestone fitters recommended 40psi all around).
All wheels and tyres have behaved as expected (approx 4psi increase when hot). Rear car tyres increase by 7-8psi from cold when towing - this is the problem.
This happens despite the WDH transferring some of the caravan load (ball weight) to the front wheels. It also happens on gravel/dirt roads when I remove the WDH bars for slow going over very uneven ground and creek crossings (e.g. on the Gibb River Road and Savannah Way from Roper Bar to Borroloola.
I am unsure of the recommended maximum inflation pressure for these tyres and am a bit wary of e.g. increasing cold pressures at rear to 50psi to see if this makes a difference. Dropping them to 40psi still results in them increasing by 6-8psi when hot. Any simple suggestions to get a more acceptable rear tyre pressure increase when hot? Mike

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Reply By: snoopyone - Tuesday, Aug 02, 2011 at 15:41

Tuesday, Aug 02, 2011 at 15:41
Sounds about right actually

I have tyre pressure monitors when we did our trip and was as follows.

Fronts 36 cold
Rears 40 cold

Van 45 cold

Weights 100ser 3260 loaded inc ball weight Van 2850 ATM

Fronts hot 45
Rears hot 45-48 depending on ambient.
Van 50 hot.

One thing that would make your rears get hot is a full width mudflap

I started off with one but after the day when the rears got up to over 65 deg I chucked it.

They went straight back to about 7-8deg over ambient and stayed that way for the rest of the 50,000k trip

6 lb increase seems normal for 4wd tyres .

I wouldnt worry if they are wearing OK
AnswerID: 461607

Reply By: Rockape - Tuesday, Aug 02, 2011 at 15:44

Tuesday, Aug 02, 2011 at 15:44
I think 45psi maybe a tad high, another reason for the higher pressure is the tyres are not getting the same cooling and airflow as the fronts.

I am not saying you have low profile tyres but I have seen trucks that have had low profile drive tyres fitted and these have increased the temps in the diffs markedly because of air flow.

Just a thought,

AnswerID: 461609

Follow Up By: Rockape - Tuesday, Aug 02, 2011 at 15:49

Tuesday, Aug 02, 2011 at 15:49
This may assist you. have a look

FollowupID: 735305

Reply By: CSeaJay - Tuesday, Aug 02, 2011 at 15:50

Tuesday, Aug 02, 2011 at 15:50

On the one hand you talk about and ask about tyre pressure on highway, yet you also state the Gibb River Road and the like as examples.

Your pressure should be significantly less tan your 40psi for these roads! Even at your stated weights, I would drop the pressure by 8psi all round for these conditions from what you stated

Personally I do not believe 6-8 psi increase when hot is a problem, and if it is on these stated roads then perhaps slow down a tad


AnswerID: 461610

Follow Up By: newbies - Tuesday, Aug 02, 2011 at 16:01

Tuesday, Aug 02, 2011 at 16:01
Thanks for the feedback to date. In case there is any confusion, when on the rough roads (e.g. Gibb River Road) I was running cold tyre pressures around 30psi on all car and caravan tyres, both for comfort and safety. My point was that there still seemed to be twice the 'normal' (if there is such a thing) increase in pressure on my rear car tyres on the rough tracks, as well as on the highways. Maybe the 6-8psi increase at the rear of the car in these circumstances is 'normal', as some have suggested.

FollowupID: 735309

Follow Up By: CSeaJay - Tuesday, Aug 02, 2011 at 16:03

Tuesday, Aug 02, 2011 at 16:03

Yes this makes sense
FollowupID: 735310

Follow Up By: Off-track - Tuesday, Aug 02, 2011 at 19:41

Tuesday, Aug 02, 2011 at 19:41
That is one opinion. Another is that 40 for the GRR is quite ok. Shakes the car a bit more but saves the tyres.
FollowupID: 735338

Follow Up By: CSeaJay - Wednesday, Aug 03, 2011 at 08:46

Wednesday, Aug 03, 2011 at 08:46
Each to their own - I would never have 40 psi in those conditions. 35 max in the rear, less in front and the van.
FollowupID: 735376

Reply By: ob - Tuesday, Aug 02, 2011 at 16:01

Tuesday, Aug 02, 2011 at 16:01
Not sure about the Bridgestones you have but most tyres have the max load per tyre and inflation pressure moulded into the sidewalls somewhere along with a bunch of other info. It would appear if the rears are heating up twice as much as the fronts that as long as you don't exceed the makers recommendations for max pressure I would pump another 5 psi into the rear and try again to see if the hot pressure is any lower. Most sources seem to agree that 4 psi increase from cold to hot is a pretty good indication that your pressures are about right.

AnswerID: 461613

Reply By: CSeaJay - Tuesday, Aug 02, 2011 at 16:13

Tuesday, Aug 02, 2011 at 16:13

Many people do not prescribe to the 4psi rule anymore with more modern tyres.
Specially not on dirt.
No use increasing pressures to suit that rule but then ending up shaking to bits and blowing shocks on gorrugations and getting punctures from rocks.

AnswerID: 461614

Follow Up By: ob - Tuesday, Aug 02, 2011 at 16:21

Tuesday, Aug 02, 2011 at 16:21
Fair enough what is the rule for modern tyres?

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Follow Up By: Member -Dodger - Tuesday, Aug 02, 2011 at 17:38

Tuesday, Aug 02, 2011 at 17:38
The 4psi rule is for car tyres but for LT truck tyres or 4wd tyres it is 6psi so you are about right.
These psi readings can also increase if the road is very hot.
EG. ambient temp 38% C road temp 60% C. so you can imagine that the tyres will raise in pressure due to the extra heat in the road.

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FollowupID: 735322

Reply By: CSeaJay - Tuesday, Aug 02, 2011 at 16:39

Tuesday, Aug 02, 2011 at 16:39
A Cooper Rep told me that the 4psi rule was before more modern tyre technologies/compounds etc were introduced.
He and others also confirmed the 4psi rule does definately not apply to dirt roads fro the reasons stated.

He said the best method for determining the correct pressure in a loaded situation on the blacktop is to use the loaded radius method. That means measuring the distance from the centre of the hub to the ground at cold temp using the specified unloaded pressure on the placard. When the load is applied then inflate the rear tyres until the distance or loaded radius is the same as when unladen. You can do the same for the front tyres also.

This makes sense to me as the FOOTPRINT is then the same as per spec. It also conforms to this advice Jol Flemming outback tyre pressure advice

AnswerID: 461619

Follow Up By: Sand Man (SA) - Tuesday, Aug 02, 2011 at 18:16

Tuesday, Aug 02, 2011 at 18:16
One problem with the loaded radius method is that the unloaded pressures on a tyre placard are usually too low. The manufacturers recommended tyre pressures is designed to suit "a comfortable ride" rather than correct pressure for longer wear, safer handling and better braking.

Therefore I apply the 4 psi rule for unladen bitumen use and it works for me.
The loaded radius method is then a good idea once the correct unladen weight has been determined.


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Reply By: youngharry52 - Wednesday, Aug 03, 2011 at 10:44

Wednesday, Aug 03, 2011 at 10:44
Hi Mike,

As a matter of interest, if your have the ARB GVM upgrade and your towball weight exceeds 180kg, I suspect your vehicle will be overloaded which could negate your insurance in case of an accident. Not good news, I know, but I am having the same issues myself.


AnswerID: 461686

Follow Up By: youngharry52 - Wednesday, Aug 03, 2011 at 10:46

Wednesday, Aug 03, 2011 at 10:46
Sorry, I meant 120kg. Maths is a bit out today.


FollowupID: 735387

Reply By: carts - Wednesday, Aug 03, 2011 at 13:19

Wednesday, Aug 03, 2011 at 13:19
Gooday newbies, In my opinion you may have overdone the suspension upgrade a little. I think that suspension coupled with a WDH you are not transferring enough of the van weight to the front wheels of the tow vehicle.
I suggest hooking up the van and putting it all on a weigh bridge.
You need to make sure the tow ball weight of the van is distributed fairly evenly over both front and back wheels. You may need to reconfigure the set up of the WDH.
I think I would be running the rear tyre pressure at 40 psi also.

There could be 100"s of options and combinations to look at, This would be one of my first options to look at from the info provided.

Keep us all posted on the outcome.
AnswerID: 461701

Follow Up By: newbies - Friday, Aug 05, 2011 at 20:30

Friday, Aug 05, 2011 at 20:30
Thanks to all who took the trouble to respond. I like the idea that I need to reduce the (cold) rear tyre pressures, and also adjust the WDH bars up another link to see what, if any, difference this makes. I'm on the road again tomorrow but may be out of internet range for a week or more, but will advise the outcome when possible.


FollowupID: 735653

Follow Up By: snoopyone - Saturday, Aug 06, 2011 at 20:15

Saturday, Aug 06, 2011 at 20:15
If you reduce the pressures you have a good chance of them rising up even further than before as soft tyres get hotter than hard ones.

As I have said 6lb is nothing and out BFG's werent worn out at 60,000k of running as I described
FollowupID: 735721

Follow Up By: carts - Saturday, Aug 06, 2011 at 23:23

Saturday, Aug 06, 2011 at 23:23
snoppyone, I am suggesting options including transferring more ball weight to the front wheels, then a further option of reducing tyre pressure as the extra 5 psi may not be required in the rear tyres. carts
FollowupID: 735735

Follow Up By: snoopyone - Sunday, Aug 07, 2011 at 08:40

Sunday, Aug 07, 2011 at 08:40
First pull the WDH up an extra link and see what happens.

Even at that you still have an extra 250kg hanging off the rear and so will load up the tyres creating more heat.

What you dont want to do is lower them too much and find out the temp goes up and you have a blowout.

Mine at about 46lb were perfect for the whole trip and I would not have considered lowering them.

Even on a cold morning when they had dropped a bit more than usual they came up to the 46 lb and stayed there all day every day.

The amount of weight transferred forward wont make all that much difference anyway.
FollowupID: 735738

Follow Up By: newbies - Tuesday, Aug 30, 2011 at 19:13

Tuesday, Aug 30, 2011 at 19:13
Since my last response, I have tried adjusting the WDH bars up another link, and reducing the car's back tyre pressures to 43psi. This seems to have had the desired effect, with a smaller increase in pressure in the back wheels of the car when hot than I previously encountered. It would seem this is a result of transferring a little more weight of the caravan to the car's front wheels. Thanks again to all contributors.

FollowupID: 737773

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