Operating Tyres Beyond Design Limits
Submitted: Wednesday, Mar 16, 2011 at 14:22
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Member - Rob D (NSW)
For those who read the Blog "Are You Operating Your Tyres Beyond Their Design Limits".
I have just added the graphical charts for a number of different common 4WD tyre sizes on the same website, under "4WD Light Truck Tyre Load-Speed-Pressure Charts"
http://www.netspeed.com.au/rob.dobson/
If you can not find your tyre size then you may find something close or else you can produce your own charts from the load vs inflation tables included.
I recommend that you print out the Load-Speed-Pressure graphs for your vehicle and carry them in the vehicle. I find it most useful when changing terrain, loads or speeds.
Reply By: Andrew & Jen - Wednesday, Mar 16, 2011 at 15:32
Wednesday, Mar 16, 2011 at 15:32
Hullo Rob
This is a very useful and timely article.
I only recently became aware of this issue when looking at getting some MTs and reading the Cooper's 4WD driver's guide. I guess the bells started to ring when I realised that the quoted max load was only applicable for a fully inflated tyre - something I have never done and am never likely to do (not wanting my vehicle to self destruct!)
I guess the only niggling thought I had with your chart was the assumption of a linear extrapolation to low pressures (20psi). My concern? A gut feeling that the rate of flexing and hence heating of the side wall may in fact be greater than that indicated by a linear functional model and hence a greater than indicated loss of load carrying capacity or need for a reduction in speed.
Thanks for doing this work and sharing it.
Cheers
Andrew
AnswerID:
448424
Follow Up By: Member - Rob D (NSW) - Wednesday, Mar 16, 2011 at 16:02
Wednesday, Mar 16, 2011 at 16:02
Hi Andrew,
Thank you for the
feedback it is appreciated. There is nothing worse than putting in many hours or even days on something you think worthy of sharing and either getting no
feedback or comments that show people have not read what was there.
On the issue of lower pressures. I have not found much data on pressures below 35psi and one of the reasons for this is that as the pressure gets way down, the sidewalls take a larger proportion of the load. The sidewall stiffness is affected by proprietary manufacturing techniques which means that it is different from manufacturer to manufacturer.
At very low pressures the stiffness of the sidewall has a significant effect on the tyre shape. There is an interesting article on
airing down at
http://www.4x4tirereview.com/airdown.html which reached the following conclusion:
“All of our
test tires were just over 6.5 inches tall at the sidewall at 50 PSI. At the 75% height, just under a 5 inch height, each tire had a different pressure, which indicates different sidewall stiffness. The pressures to reach the 75% height were:
• BFG – 6 PSI
• Hankook – 9 PSI
• Toyo – 8 PSI
FollowupID:
720725
Follow Up By: Andrew & Jen - Wednesday, Mar 16, 2011 at 17:10
Wednesday, Mar 16, 2011 at 17:10
Hullo Rob
Yes, I read that in one of your attachments. I still have a concern re the linear assumption (which is the best you could do given the absence of data to the contrary) and for the reason you have identified, namely the sidewalls playing an increasing role as pressures reduce.
BTW, you may like to copy in Rob Ackland who has done a lot of field work on this issue in conjunction with Bridgestone - see www.nuccaleenacottage.com.au/blog/
Cheers
Andrew
FollowupID:
720730
Reply By: Member - Captain (WA) - Wednesday, Mar 16, 2011 at 17:05
Wednesday, Mar 16, 2011 at 17:05
Hi Andrew,
I too appreciate this. I have been thinking at what tyres I should run my camper at, they are the same tyres all round but the cruiser has ~750kg per tyre while the camper has ~1100kgs per tyre. Most people recommend the camper pressure to be the same pressure as the vehicle, but as you rightly know, load rating is related to pressure. Will use this to work out how much higher my camper tyre presures should be.
Cheers
Captain
AnswerID:
448431
Reply By: Tonyfish#58 - Wednesday, Mar 16, 2011 at 17:15
Wednesday, Mar 16, 2011 at 17:15
That's great work :-)
Must say though that a will most likely still load my truck up to the max, drive on 100k on 36psi front and 45 rear until I hit the dirt, then I will put them to 36psi all round until I hit the sand and then I will run them at 18psi all round for the duration.
So far thew Mazxxi Bravo 753s 245/76/R16S have been up to the impossible task we give them
Cheers Tony
AnswerID:
448432
Reply By: Crackles - Wednesday, Mar 16, 2011 at 21:24
Wednesday, Mar 16, 2011 at 21:24
Quote: "I find it most useful when changing terrain"
Rob where does your chart show pressure for different terrain?
Cheers Craig................
AnswerID:
448438
Follow Up By: Member - Rob D (NSW) - Wednesday, Mar 16, 2011 at 21:40
Wednesday, Mar 16, 2011 at 21:40
I would expect that most people would have learnt this in a 4WD training course or a sand driving course etc.
Some of the tyre manufacturers publish information on recommended tyre pressures for different types of terrain. Cooper Tyres, for example, publish such a document.
Typical figures are:
Bitumen: 35 to 55psi depending on speed and load.
Gravel:25 to 35psi
Sand: 15 to 25psi depending on slope, softness of the sand etc.
FollowupID:
720740
Follow Up By: Crackles - Wednesday, Mar 16, 2011 at 23:07
Wednesday, Mar 16, 2011 at 23:07
Sorry Rob it's just you said you found the chart handy when changing terrain & I suppose that's just one of the problems I have with it that there is no reference to terrain, in fact by your 285 chart, 20 kph is as fast as you can drive safely at 20 psi & yet many travel the desert at 50 KPH running just 15 psi. Likewise your loaded example has a minimum pressure of 35 PSI in the rear to travel at 100 kph. This apparently is for any surface. Of cousre recommended pressure to avoid damage for a fully loaded wagon on outback dirt roads with that tyre is 28 psi.
This chart asumes far too much & does not line up with real life tried & accepted pressure/speed use nor does it take into account differences like brands, ambiant temperature or road surfaces to name but a few.
Cheers Craig....................
FollowupID:
720744
Reply By: Member - John Baas (WA) - Wednesday, Mar 16, 2011 at 23:45
Wednesday, Mar 16, 2011 at 23:45
Great effort Rob.
Your attachments answer a question I had on ThreadID: 79764 which wasn't really answered satisfactorily.The correct answer from your page is:
:
http://www.bridgestonetrucktires.com/us_eng/answers/doctor_facts.asp
In general, operating temperatures of radial tires can be anywhere between 65C-82ºC Under severe conditions the operating temperatures will range in the 93ºC+ area.
We know that when heat is applied gradually to a tire, the tyre will go through several stages. In general, as a compound is heated, molecular activity within the compound increases. At temperatures in the 120ºC range the tyre starts to lose strength and the influence of other stresses becomes greater.
Sometimes, with time as a factor, these other stresses will cause a tyre to be damaged.
During the manufacturing process, tyre vulcanization occurs at 120 - 160ºC. Depending on the factors mentioned above, tyre reversion also begins around 120 ºC. Of course, during the manufacturing process, these factors are highly controlled so we can produce a high quality product. As heat is, again with time as a factor, the molecular bonds disintegrate and the tyre starts to physically come apart (reversion). It is this physical destruction that leads to the "two halves" of the tyre that we sometimes see on vehicles.
Many thanks.
Cheers.
AnswerID:
448446
Follow Up By: Member - Rob D (NSW) - Thursday, Mar 17, 2011 at 14:33
Thursday, Mar 17, 2011 at 14:33
Hi John,
Thanks for the
feedback. It was the extremely high operating temperature of my 265 tyres on the
Canning Stock Route that prompted me to do the research to start with. I spent about 12 full days trying to find the information I needed.
FollowupID:
720760
Reply By: Member - Wim (Qld) - Thursday, Mar 17, 2011 at 09:44
Thursday, Mar 17, 2011 at 09:44
Hi Rob.
Nice read.
I have the same vehicle set-up as you.
I have been using "smart tyre" monitors for some time now. This allows me to see the relationship between Load, Speed, pressure and temp.
I can confirm your findings in relation to 285/75R16 LT's.
regards
AnswerID:
448465
Follow Up By: Member - Rob D (NSW) - Thursday, Mar 17, 2011 at 11:55
Thursday, Mar 17, 2011 at 11:55
Hi Wim,
Thank you for the
feedback. I noticed a big improvement of the 285s over the 265s as far as operating temperature at low pressures. I have bought a set of TPMS but have not fitted them yet.
FollowupID:
720755