Axle load/tyre pressures

Submitted: Monday, Jul 15, 2013 at 10:44
ThreadID: 103249 Views:2284 Replies:8 FollowUps:11
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Would anyone out there have a formula for calculating tyre pressures based on axle loads??
Our vehicle has a total weight (loaded) of 3680kg- Front axle load is 1590kg and rear axle load is 2090kg.
Tyres are 235/65R16 (singles all round)
Thanks
Wombat
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Reply By: Lyn W3 - Monday, Jul 15, 2013 at 11:08

Monday, Jul 15, 2013 at 11:08
Would be having a look at the load ratings and pressure stamped on the tyre. Most 235/65R16 have a load rating of around 900-1000kg at around 44 psi. Based on that you may be a bit overloaded.
AnswerID: 514822

Follow Up By: wombat100 - Monday, Jul 15, 2013 at 11:31

Monday, Jul 15, 2013 at 11:31
Thanks Lyn
The load rating is 121/119 which I read as about 1450kg per tyre so safely an axle load of 2900kg.
The speed rating is R (170kph)
Thre max pressure 80psi.
So what are your recommendations ??
Thanks
Wombat
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FollowupID: 793962

Reply By: Ross M - Monday, Jul 15, 2013 at 11:17

Monday, Jul 15, 2013 at 11:17
wombat100

When I read the loading and thn the tyre size my immediate thought was, tyres far too small.
I had a 60 series LC and when was loaded it never got anywhere near that weight on 265/75/16.

What vehicle are you referring to? I can't think of a vehicle with that size tyre would operate at that weight. I don't think it a matter of tyre pressure selection at all.

Ross M
AnswerID: 514823

Follow Up By: wombat100 - Monday, Jul 15, 2013 at 11:33

Monday, Jul 15, 2013 at 11:33
Hi Ross
See my reply to lyn
The vehicle is a Mercedes Benz Sprinter.
The weights are true weighbridge read outs (not assumed).
Thanks
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FollowupID: 793963

Follow Up By: Lyn W3 - Monday, Jul 15, 2013 at 15:04

Monday, Jul 15, 2013 at 15:04
Ross M,
Now you know why when you're following a Sprinter down the road on those skinny little tyres they look so unstable.
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FollowupID: 793976

Follow Up By: wombat100 - Monday, Jul 15, 2013 at 16:35

Monday, Jul 15, 2013 at 16:35
So what you're saying Ross & Lyn-
you have no real constructive input into my question !!



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FollowupID: 793982

Follow Up By: Lyn W3 - Monday, Jul 15, 2013 at 17:34

Monday, Jul 15, 2013 at 17:34
Wombat100

What does the tyre recommendations say on the Sprinter?, As you are travelling pretty close to max weight this would be a good starting point. Other people have posted information which should help you.

There are many variables, are you using it as a delivery/service van in the city? Do you have it set up as a motorhome? Where are you driving it, straight up the highway or on twisty mountainous roads?

All there factors will determine what pressure you run the tyres at,

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FollowupID: 793986

Reply By: Member - Andrew - Monday, Jul 15, 2013 at 11:37

Monday, Jul 15, 2013 at 11:37
Hi Wombat

Tyre load limits and the pressure that it is set at can be found in the Tyre & Rim Manual put out by Standards Australia.
Your local tyre service should (but probably hasn't) have a copy.

The manual will tell you what the load rating is from the code on the tyre and also what the minimum pressure is to carry that load.
It also has tables that let you work out how much you can lower the pressures for low speed continuous operation.

The load rating takes into account ply rating and tyre construction. One size tyre might come in many different load ratings depending on its intended use, so you need both to do your calculations.
Regards
A
AnswerID: 514827

Reply By: Rockape - Monday, Jul 15, 2013 at 12:09

Monday, Jul 15, 2013 at 12:09
Wombat,
here is a link to a tyre pressure/axle load calculator.

TYRE LINK

AnswerID: 514830

Follow Up By: DiscoTourer - Monday, Jul 15, 2013 at 12:42

Monday, Jul 15, 2013 at 12:42
Rockape, a lot of work has gone into this report.
Thanks for sharing.
Brett...
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FollowupID: 793966

Follow Up By: Jeremy W - Tuesday, Jul 16, 2013 at 10:49

Tuesday, Jul 16, 2013 at 10:49
Rob Dobson' s reports are superb in their detail and descriptions.

Many thanks for providing the link to this excellent work.

Jerry.
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Follow Up By: Echucan Bob - Wednesday, Jul 17, 2013 at 09:36

Wednesday, Jul 17, 2013 at 09:36
RA, thanks for the link. I run 245/75 R16s. The report mentions 225 and 265 in the 75 profile and with 1000kg load both need 60 psi for 104 kph running. That fits with my real world experience. When the car is loaded up for a trip (who knows the weight on each tyre?) I inflate the rears to over 50 psi to prevent excessive sidewall bagging and overheating when cruising on bitumen. Many's the time where I've been criticised for running the tyres at too high a pressure.

Bob
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FollowupID: 794072

Reply By: The Bantam - Monday, Jul 15, 2013 at 15:21

Monday, Jul 15, 2013 at 15:21
What you are looking for is a load V pressure table for the tyre in question.

Now the tyres are 235/65R16.....just checking are they a passenger or light truck rated tyre...it makes a difference.

Unfortunately my copy of the wheel & tyre manual is a couple of years old & I cant find that particular tyre in it

The load v pressure characteristic should be pretty much the same from brand to brand......but note the maximum load capacity of the particular tyre

A toyo document written for the american market lists the following

235/65R16 pasenger radial
PSI...load in pounds
26....1598
29.....1675
32.....1764
35.....1819
remember to half loads for KG

Some one with a current coppy of the manual may be more help...every tyre shop should have a current coppy.

or check the manufacturer.

cheers
AnswerID: 514835

Reply By: BFreer - Monday, Jul 15, 2013 at 15:54

Monday, Jul 15, 2013 at 15:54
I was given this formulae some years ago by a tyre techno, works well for me.

Actual tyre load (kg) divided by the tyre load rating on side wall (kg), multiplied by the max psi shown on the side wall. Less 20% on rough roads @ 80 km/h.
AnswerID: 514837

Follow Up By: Jeremy W - Tuesday, Jul 16, 2013 at 10:37

Tuesday, Jul 16, 2013 at 10:37
BFreer , Hello.

1. I Like the formula in its simplicity and seems logical for what it says, in that the tyre pressure should be changed as the axel load is changed and by how much. ----- Formula remembered.

2. The 20% reduction in tyre pressure for rough roads ? I will also note this recommendation. But, for my information, did the techno mention how this number was arrived at?

3. Is there a similar derating number for sand ?

Jerry.



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FollowupID: 794015

Follow Up By: wombat100 - Tuesday, Jul 16, 2013 at 13:02

Tuesday, Jul 16, 2013 at 13:02
Thanks heapd mate- exactly what I was after..
Had seen it somewhere before !!
Cheers
Wombat
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FollowupID: 794024

Reply By: BFreer - Tuesday, Jul 16, 2013 at 11:46

Tuesday, Jul 16, 2013 at 11:46
Hi Jerry,

1. the formula uses tyre load, not axle load, so axle load/2. Using that formula I find that tyre psi increases about 6-8 psi which is OK for LT tyres going from cold to midday ambient temp. at highway running. The so-called 4psi rule is OK for Highway tyres not carrying much load but I find too low for my rig.

2. not sure where the 20% originated from but seems to be the number used by off-road drivers (20-30%). The late Adam Platt also suggested that figure, so good enough for me.

3. for soft sand driving, most say another 20% reduction is OK. I have done the Simpson a few times and found that 20psi worked for me, with 16-18 psi on a few tough dunes.
AnswerID: 514866

Follow Up By: Jeremy W - Tuesday, Jul 16, 2013 at 13:25

Tuesday, Jul 16, 2013 at 13:25
Yes ofcourse (axel load)/2. ..... No worries.

OK on the 20 to 30 % derating for rough roads ... I'll split the difference and call it 25% !!

And

A further 20% reduction for sand driving .... Got it !

OK on your experience and am glad to benefit from it. I understand the use of formulae very well and it is very easy for me to remember and apply them.

Every thing is so obvious AFTER someone gives one the answer.

Jerry.

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FollowupID: 794026

Reply By: Keith H7 - Tuesday, Jul 16, 2013 at 17:02

Tuesday, Jul 16, 2013 at 17:02
See if this helps
http://www.peakhillcaravanpark.com.au/Tyre%20Pressuresa.htm
AnswerID: 514888

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