What Pressure 255 KM2’s ?

[James]

I have a nice new set of 255/85/R16 BFG KM2’s on Defender 130 rims (6.5” wide)
As a taller, narrower tyre do they require a higher pressure?

I would be interested in tyre pressures for (say) four types of terrain
Bitumen
Secondary roads – gravel & dirt
Sand
Mud

It’s a Defender 110 wagon, all up weighs 2,700 Kg

I have searched, please point me in the right direction if there is an existing thread somewhere.

James

[110 Pete]

Hey James, nice set of tires you've got :)

I would be interested in tyre pressures for (say) four types of terrain
Bitumen - I woulkd run 35-40 psi
Secondary roads – gravel & dirt - Same as Bitumen unless you are doing JUST dirt and gravel al day then probably drop them down a bit.
Sand - Around 10 psi
Mud - Depends on the mud but clay based mud (not slop) 20psi

Hope this answers your question, some of it does come down to trial and error too.

Cheers

Pete

[newhue]

I use the 4psi rule for bitumen and high speed dirt (80 to 90km/h) generally.

Bitumen 36 and 40 up to 600kg, 45 over 600kg
Secondary roads – gravel & dirt 26 and 30
Sand 15 and 18
Mud ?

Defender 130, somewhere between 2500 and 3500kg most days.

[Witchdoctor]

Bitumen 32-40 depending on load
Gravel 25-30
Sand 15

Cheers
David

[James]

Thanks Pete, Jason and David.

These figures provide some good guidance so I can look after the new tyres.

Jason,
Your figures are front and rear?

Are you referring to this 4 psi rule?
Using the 4psi Tyre Inflation Rule @ ExplorOz Blogs (http://www.exploroz.com/Members/216531.875/2/2012/Using_the_4psi_Tyre_Inflation_Rule.aspx)

Cheers
James

[steveG]

Thanks for that link James. I knew about the 4psi rule, but hadn't considered the effects of increasing ambient temperature during the day.

For those who don't want to plough through the maths in that article, I'll paraphrase:
The rule of thumb is 4psi difference between cold tyre pressure and hot(after travel), with an additional 1psi for every 6degC rise in ambient temperature.

Example:
40psi cold pressure at 20degC ambient should be roughly 46psi after travel at 32degC ambient (increase of 4psi + 2psi due to ambient temp increase of 12deg).

Hope I've got that right :D

Steve

[newhue]

Thanks Pete, Jason and David.

These figures provide some good guidance so I can look after the new tyres.

Jason,
Your figures are front and rear?

Are you referring to this 4 psi rule?
Using the 4psi Tyre Inflation Rule @ ExplorOz Blogs (http://www.exploroz.com/Members/216531.875/2/2012/Using_the_4psi_Tyre_Inflation_Rule.aspx)

Cheers
James

yep front and rear. The 4psi rule changes with load, speed, and surfaces a bit. It takes a bit of fiddling around initially but once you've run a few surfaces and weights you get a general feel for it. Each car will also vary due to people builds and tyre size.

I find you get what back what you put in. On a recent central Aus trip I got lazy on the last dirt road and didn't lower them, as a result I cut up the rears pretty bad. Quite annoying because for the whole trip I virtually had no cuts in all fours due to diligence, and that over many many cricket ball size rocks.

this guy also does a reasonable description of the 4psi rule.
http://www.aawen4x4.com.au/index.php?option=com_content&view=article&id=1:4psi-rule&catid=2:current&Itemid=7

[goingbush]

Your 130 rims will be tubed type, right ?, as opposed to the tubeless wolf rims,
Are you running them with tubes or tubeless ?

if tubeless i would be careful going to 10psi for sand, probably stay above 15 or you might pop a bead

(but easy enough to reseat with a can of WD40 and a lighter.)

[James]

Jason,
Thanks for the link and your help.
I will monitor carefully to learn the pressures required for my loaded vehicle over different surfaces.
Great to have a formula and some realistic psi figures as a starting point.

Goingbush,
I have just swapped to tubeless 130 wheels (previously using tubed).
Good point about popping beads at less than 15psi.
Have never tried the WD40 & lighter trick . . . so far I have always managed to re-seat beads with less exciting conventional techniques.

Cheers
James

[steveG]

FYI - WD40 sucks for reseating beads as it doesn't seem to be volatile enough. Aerosol deodorant worked well when we had a play around a few years back (whereas WD40 failed to ignite violently enough), and I'd expect that insect repellent or fly spray would also do the job.

Steve

[MLD]

110 on 7" disco 1 alloy rims (255/85 KM2's) quoted in psi

bituman F36-38 R38-40
high speed dirt/gravel F26-28 R30-32
low speed dirt/gravel (high range) F20-22 R22-24
low range off road 16-18 F&R depending on terrain but have comfortably run 14 on tracks like Mt Airley and Mt Walker.
sand i've been down to 12 without rolling a bead but was very weary.

i'm yet to find a happy medium on bitumen. At lower psi (low to mid 30's) the tyres tend to roll on the rim making corners interesting. At 40 psi it corners predictable enough but is a harsh ride and the rear chatters if there is no weight.

Not sure if others experience the same but my KM2's seem to keep form except when there is considerable pressure on a point on the tyre (ie driving over a rock) even at 15psi. Stated another way they don't seem to bag or to have a long footprint when driving at those low pressures. When you look at my photos when off-road you could swear i'm running bitumen pressure by the look of the tyre wall. I've seen a set of 285/75 KM2's at higher pressure bag more than mine.

just curious if I'm an odd one out or it's a KM2 characteristic.

cheers MLD

[newhue]

I run mine on 6.5" rims. For me they seem to bag an awefull lot. I often have rub well past the side brightest and up the side wall. You could alway measure foot pad length with two sticks at road pressure, then dirt pressures and so on. But yes some tyres don't seem to bag much compared others. Personally I'd prefere more side wall stiffness with less side wall bag. All the grip is in tread length but one contradicts the other.

[Yorkshire_Jon]

I tent to run about the same as David, a couple of psi less in the front.

Also, I'm seeing a lot of people running this size & model on 6.5" rims. I'm nearly certain the BFGoodrich site details that a 7" rim is the min width for these and a max rim of 8.5", its a year or two since I researched into rim sizes for mine, but I seem to recall I got 7" cos that was the smallest and I could still run 235x85 R16s on a 7" if I wanted also, which I couldn't on an 8".

HTH
J

Sent using Forum Runner

[rick130]

I tent to run about the same as David, a couple of psi less in the front.

Also, I'm seeing a lot of people running this size & model on 6.5" rims. I'm nearly certain the BFGoodrich site details that a 7" rim is the min width for these and a max rim of 8.5", its a year or two since I researched into rim sizes for mine, but I seem to recall I got 7" cos that was the smallest and I could still run 235x85 R16s on a 7" if I wanted also, which I couldn't on an 8".

HTH
J

Sent using Forum Runner

Nope, BFG have always listed 6.5"-8.0" for the 255/85 with the measuring rim width being 7" ;)

Maxxis list 7.5"-8.5" or something silly like that and it's a typo that keeps being repeated in each subsequent catalogue.

[edit] Here's an '03 and I think current (actually from '09) tyre size chart for BFG MT's below

[rick130]

Hmm, it doesn't look like the forum will let me post two pdf's in the one post...

[newhue]

I tent to run about the same as David, a couple of psi less in the front.

Also, I'm seeing a lot of people running this size & model on 6.5" rims. I'm nearly certain the BFGoodrich site details that a 7" rim is the min width for these and a max rim of 8.5", its a year or two since I researched into rim sizes for mine, but I seem to recall I got 7" cos that was the smallest and I could still run 235x85 R16s on a 7" if I wanted also, which I couldn't on an 8".

HTH
J

Sent using Forum Runner

tire rack US quote 6.5", I havent checked again and could be wrong but I think BFG Aus say 7". I can certainly vouch for the suckers stay on a 6.5" rims in all conditions and psi variants. I have not been below 15psi however.