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Re:Tyre Pressure on Gen4 w BFG K02 - still a bit confused

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24 Nov 2020 12:47 #230351 by Groenewald
Glad I am not the only one....

My next big upgrade in the next year will be the Des Sol 50mm kit.

Glad I did the tyres first - 195's on a lift kit will look a bit weird on the road.

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24 Nov 2020 12:56 - 24 Nov 2020 12:57 #230352 by lookonimages
1.6 to 1.7 bar (24-26 PSI) perfect.

Where did you say you are in SA.?
Last edit: 24 Nov 2020 12:57 by lookonimages.

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24 Nov 2020 17:42 #230355 by Groenewald

1.6 to 1.7 bar (24-26 PSI) perfect.

Where did you say you are in SA.?


We are in White River, Mpumalanga

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25 Nov 2020 00:00 #230368 by Busta
There is no precise answer but the fundamental principles you describe are correct. Boyles law tells us the relationship between pressure and volume is inverse. The larger the volume, the lower the pressure required to carry the same load. If you want to be academic about it you could calculate the change in volume of the tyres and use that to adjust the pressure. But why not try different pressures and find out what works for you?
I now have the same tyres on my gen 3, and have had KM2s in the same size for the past 7 years. I find 21psi all around works for me, a good balance between grip and confort. If economy is your thing you might want to use higher pressures. If you want good traction then try lower pressures. There is no one-size-fits-all.
The following user(s) said Thank You: Groenewald

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25 Nov 2020 07:00 #230378 by Groenewald
The roof rack and bigger tyres already buggered up my fuel economy :laugh: :laugh:

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25 Nov 2020 10:34 #230383 by jadatis
This pigheaded Dutch selfdeclared tyrepressure-specialist" registered to answer here.

Lambert gave in his first reaction directly the right answer, but there are some bears along the road( if I use the right expression) .

The maximum pressure given on tyre is not the pressure for wich the maximum load is calculated.
For standard load passenger cartyre that so called reference-pressure is 250kPa/36psi with afew exeptions to lower. For XL/reinforced/ extraload referencepressure ( further Pr) is 290kPa/42 psi with lesser exeptions.

Then it is best to first add 10% to the determined load, for reserve.

The calculation of pressure for lower then maxload , I got hold of the official calc from ETRTO end 2007 , and went running with it.

Now I use my own calculation in spreadsheets I made , for travel-trailers and the one for motorhomes can be used for every vehicle with some adaption for speed .
The maxload is also calculated for a reference-speed of 160kmph/99mph, for tyres with speedcode Q and up.

It all has to do with the goal of not overheating any part of rubber when driving the speed constantly, for wich the pressure is calculated, and for that a certain deflection of tyre counts.

If you give all the needed info, I will calculate ut for you.
But here a simple do it yourself calculation that comes close .

A .determine axleload and devide by 2 to get weight on 1 tyre.
B. Add 10% to that for reserve.
C. Substract 45kg/100 lbs from outcome B , this is determine by me be carried at zero pressure.
D. Also substract that 45kg/100lbs from maximum load of tyre.
E. Do the division C/D
F. Multilpy E x Pr and this is your advice pressure.
Is highest pressure for the load , at wich comfort and gripp is still acceptable.

Mind that you calculate for highest reserve, but by all the inacuracies , you can end up with only yust enaugh to not overheat the tyre.

Especially determining the loads in your use, is the most tricky part in it all.

Then AT tyres have large profile blocks, that cover part of sidewall , wich gives more heatproduction a cycle as a onroad-tyre, but mostly has the same loadindex/maximum load, as that onroad tyre.
To ve safe mayby even 10% has to be substracted first from maxload of such a tyre is 4 loadindex steps, before you put it in the calculation.

This is also my introduction on this forum.
Greatings from the Netherlands.
( ja dat is ) Peter ( nickname from an old Dutch namesong, translates as Yeah that is Peter)

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20 Dec 2020 08:53 #231047 by Chalky the Jimny
I emailed michellen tyre company directly, they requested the weight of each axle and the exact tyre size of my BFG's, and they gave me the exact pressure per tyre (I did this on my previous cruiser, I will do the same when I get BFG's for my Jimny)

This email address is being protected from spambots. You need JavaScript enabled to view it.

Their engineering dept responded once I got through the normal bot replies.

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20 Dec 2020 09:49 #231050 by Soeley
Out of interest;
What was the original tyre size and pressure on your Cruiser?
What size tyres did you fit and what pressure did they recommend?

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20 Dec 2020 10:00 #231051 by Busta

This pigheaded Dutch selfdeclared tyrepressure-specialist" registered to answer here.

Lambert gave in his first reaction directly the right answer, but there are some bears along the road( if I use the right expression) .

The maximum pressure given on tyre is not the pressure for wich the maximum load is calculated.
For standard load passenger cartyre that so called reference-pressure is 250kPa/36psi with afew exeptions to lower. For XL/reinforced/ extraload referencepressure ( further Pr) is 290kPa/42 psi with lesser exeptions.

Then it is best to first add 10% to the determined load, for reserve.

The calculation of pressure for lower then maxload , I got hold of the official calc from ETRTO end 2007 , and went running with it.

Now I use my own calculation in spreadsheets I made , for travel-trailers and the one for motorhomes can be used for every vehicle with some adaption for speed .
The maxload is also calculated for a reference-speed of 160kmph/99mph, for tyres with speedcode Q and up.

It all has to do with the goal of not overheating any part of rubber when driving the speed constantly, for wich the pressure is calculated, and for that a certain deflection of tyre counts.

If you give all the needed info, I will calculate ut for you.
But here a simple do it yourself calculation that comes close .

A .determine axleload and devide by 2 to get weight on 1 tyre.
B. Add 10% to that for reserve.
C. Substract 45kg/100 lbs from outcome B , this is determine by me be carried at zero pressure.
D. Also substract that 45kg/100lbs from maximum load of tyre.
E. Do the division C/D
F. Multilpy E x Pr and this is your advice pressure.
Is highest pressure for the load , at wich comfort and gripp is still acceptable.

Mind that you calculate for highest reserve, but by all the inacuracies , you can end up with only yust enaugh to not overheat the tyre.

Especially determining the loads in your use, is the most tricky part in it all.

Then AT tyres have large profile blocks, that cover part of sidewall , wich gives more heatproduction a cycle as a onroad-tyre, but mostly has the same loadindex/maximum load, as that onroad tyre.
To ve safe mayby even 10% has to be substracted first from maxload of such a tyre is 4 loadindex steps, before you put it in the calculation.

This is also my introduction on this forum.
Greatings from the Netherlands.
( ja dat is ) Peter ( nickname from an old Dutch namesong, translates as Yeah that is Peter)


For the tyres in question (max load=730kg, weight on one tyre= 350kg, Pr=42psi) your calculation gives a pressure of 20.8psi, so the same as the 21psi I have found to be ideal from trial and error.

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20 Dec 2020 12:09 - 20 Dec 2020 12:23 #231058 by jadatis
That 350 kg a tyre, is your responcibility, to be the real weight, hope you did not just devided total weight by 4.
Mostly normal use front more weight then rear, but fully loaded rear more weight then front.

But OK , assuming the right weights detetmined, I checked that you used my on the road calculation, and added first the 10%
The official european calc gives 18.88 psi, and my motorhome spreadsheet gives 20.54 psi.

In theory, you then can drive 160 kmph constant with even 46/54 weightdivision an axle, without overheating any tyre, and comfort and gripp stil acceptable.

The pressure is for 18 degrC/ 65 degrF cold filled.
Some write upto 21 degr/ 70 degrF but difference in pressure it gives is that small ,that its not worth the discussion.

This means that you have to calculate the pressure back to that 18 degrC., but within a certain range of ambiënt temp, you can fill the calculated advice-pressure.

I think we are talking about South Africa here, but correct me if wrong.
So ambiënt temps of 35 to 40 degr C. can happen.
Then you have to fill cold ( yes still called cold pressure) a higher pressure to give it the reserves needed .

Also made spreadsheet for that, and filled in 1.6 bar/21 psi, see if I copied it right here. Edit: no , hard to read, but cant make it better, so attached a pdf.

This browser does not support PDFs. Please download the PDF to view it: Download PDF



BAR degr C. degr F. PSI
1,10 -38 -9 16
1,20 -27 6 17
1,30 -15 21 18
1,40 -4 36 19
1,50 7 50 20
advice-pressure 1,60 18 65 21 advice-pressure
1,70 29 80 22
1,80 40 94 23
1,90 51 109 24
2,00 63 124 25
2,10 74 139 26
2,20 85 153 27
2,30 96 168 28
2,40 107 183 29
2,50 118 197 30
2,60 129 212 31
2,70 141 227 32
2,80 152 241 33
2,90 163 256 34
3,00 174 271 35
BAR degr C. degr F. PSI

Then you see that at 94 degr F it needs 2 psi more, so never blead down to 21 psi then , the tyre needs the higher pressure to give lesser heatproduction, because cooling down is also lesser, because of the lesser temperature-differences between critical temp of rubber, and in and outside-tyre air.

When cold , say freesing, you dont need to highen up to advice for safety, but may do so for road-handling.
Attachments:
Last edit: 20 Dec 2020 12:23 by jadatis. Reason: Some writing errors

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20 Dec 2020 20:11 #231067 by lightning
These "fast fit" speed humps have appeared all over my area.
They are horrible things, but not as bad as the speed humps that have been put in around my FIL's house in Prestatyn
These are truly vicious and in anything other than a 4x4 you have to drive over them at walking speed.
Worse than that there's dozens of them, and travel on those roads is now dominated by the white "not my van" sat a metre from your rear bumper.

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21 Dec 2020 10:00 #231079 by Chalky the Jimny
Soeley. I had BFG KO2 265/70/17 - estimated weight of my cruiser was 3 tons. I got the below from their engineering dept:


"Thank you for the information. Considering your GVM of 3000kg on your vehicle with accessories we worked at a 40% / 60% load distribution between front and rear.



Front: 1200kg

Rear: 1800kg



Below would be the recommended pressure:



Font: 2.5 Bar

Rear: 3 Bar



The increase in pressure might seem high, however this is due to the change from P-Metric tyres fitted new once purchasing the vehicle and going to LT-Metric (light truck construction). The LT-metric has a higher carrying capacity than normal tyres, but require an increase in pressure to fulfil the load placed upon it.



Best regards, Bien cordialement,"

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