Oodnadatta to Marla Road

Is the 250mm footprint generally applicable to most tracks in the area, eg Birdsville
or just specific to the subject vehicle?
T.R.

Generally applicable to those stoney roads, but it is imperative you slow down, as the vehicle will not handle as well wiht partially deflated tyres. So slow and steady wins in the outback.

Give it a test in the driveway if you like. Drop one front tyre to give 250mm footprint and see what pressure it needs. Message me back if you like with starting pressure and pressure at 250mm. Oh be sure to pump it up again :-)

Cheers
David

I've just done the exercise out in the garage on a level floor, front tyre, size
235/70R16

Starting pressure = 36psi
Footprint = 115mm

(loud hissing noise follows)

Finish pressure = 10psi
Footprint = 250mm

(and remembered to pump it back up again)

T.R.

OK thats kinda what I expected. After posting I suspected they must be very solid tyres and not much weight up front, thats why I asked you to do the excercise. 10psi is too low for that sort of work. Go for 28 psi max and about 26 psi minimum. Reduce your speed 20%.

Cheers

David

I concur with that David, I have 28psi offroad marked in my notebook of base settings for my tyres which are: 36psi tarseal, 28psi dirt, 20psi sand.
The tyres on the front are Simex 300i A/T Centipedes, hard rubber, and the whole vehicle unloaded only weighs around the 1600kg mark.
Thanks for the advice.
T.R.

Hi David (4WD Tag Along), Thanks for the flow of information. I am running Bridgestone R623 225/70R15 (as original). Running 40 psi front & 55 psi back as recommended by Bridgestone. The tyres are 5 ply with 2 ply on side walls.
My concern is the vulnerability of the side walls. Appreciate your comment on this. Thanks, Keith

Solly

Sidewall punctures are a red herring and as rare as hen's teeth in this sort of terrain. Almost 99% of punctures in this terrain are due to tread punctures ie stones penetrating the tread ara of the tyre due to tyre inflexibility and high speed.

If you lower the pressure, the tyre will flex around the sharp rocks. Lowering the speed by 20% decreases the risk of puncture by at least 40%. Sidewalls flex just like the tread only better, so lower pressure offers sidewall protection as well unless you maintain high speed in which cae you will run out of sidewall flex and puncture the tyre.

So lower pressure and lower speed, and keep weight down...it really is that simple.

Thanks David, I will have a play with pressures here and give it a go. Keith

Solly it may be worth measuring the footprint length at 26 psi on the frot and matching that length on the rear and seeing what pressure it gives you. My bet is it will be less than 30psi but more than 26 psi. If so then run with that pressure on the rear. This matching of footprint length is a good way to compensate for different loads front and rear. Feel free to message me if the pressures are outside my guestimate.

Just out of curiosity I went out and compared the front and back prints.
Both tyres have 36psi. The rear print is less than the front ie 115 and 111.
So unloaded, there is a bit more weight over the front axle.
I won't change the pressures just yet, as it will be interesting to remeasure the print when there's a load in over the rear axle. I think it's a great idea to adjust the pressures to suit the load and must help even up the wear on the tread.
cheers
T.R.

Hi David (4WD Tag Along), I did a measure and test of the footprint this morning, and it was interesting. I know my highway pressures are good as the Sprinter van handles very well on the highway and minor roads. I have also had each axle over a weigh bridge at my mates place, and then got pressure recommendations from Bridgestone. These giber stone roads are a new venture for me.
------Anyway current normal conditions are as follows:
Front: Pressure 45psi, Footprint 190mm, Side wall bulge 25 mm.
Back: Pressure 55psi, Footprint 190mm, Side wall bulge 25 mm.
-----Tyre Pressure then dropped to get a 250mm footprint
Front: Pressure 26psi, Footprint 250mm, Side wall bulge 30 mm.
Front: Pressure 35psi, Footprint 250mm, Side wall bulge 30 mm.
Both front and back had a drop of 20 psi to achieve the 250 mm. This would give me a 31% increase in tyre bearing area, which must help. My only remaining concern is the bulging sidewalls as these are road tyres (2 ply walls) as compared to some off-road where the walls are virtually straight. But despite that, what you are saying makes sense, so happy to run with it. By the way do you take conventual (other than 4WD) vehicles on Tag Alongs??????
Many thanks for your help. Keith

Solly

Ahhh that sounds a lot better. Run with those pressures. Sidewalls will be fine if you keep the speed down. All 4wd passenger tyres are 2 ply sidewall. Only the LT versions have more plys. It would be a good idea to fit Light Truck (LT) tyres if you plan to do a lot more of this kind of travel.

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David

Thanks David. That was most helpful. Keith