Tyre pressures for wet mud

Nick Williams uttered summat worrerz funny about:

It would give you a wider footprint which may or may not add to your problems dependant upon the type of surface.

On some ground you ideally want to cut through the top slop to reach the harder surface below for which narrow tyres will be better and also offer less resistance to forwards motion. However it you have no firm lower surface as it sounds here then you may gain the edge having lower pressures to give a wider footprint to prevent cutting into the surface and bogging down to the axles.

I'd think if Farmer giles tractor with no doubt heavy aggricultural tyres on was having problems then in your cirs it may have made little difference.

This is one of those questions that you could give to 20 different people in a sound proof booth and they all come out with differing answers none of which will be wrong but all of which will be opinion.

Having bottled air is fine but have they the correct valves? Worth checking before deflating.

Depending on the distance from the road / firm ground then consider multiple long tow ropes, shackles and leaving one of the vehicles closer to the dry area as a contingency. Especially if it starts to pee down. Who ever is in command of the RVP needs to build this in to the plan I'd say. Being on a big flat surface which is slightly undulating is often worse than good old hills which despite offering different problems tend to drain well to somewhere or other... though if your in the hole ;-)

Also consider having enough waffle boards to build a temporary road for the largest vehicles, drive forward to end... pick up ones at back place in front... drive forwards , pick up ones at back etc etc. May look like a circus act / reinactment of stone henge in the making but if it save someone putting there back out it's worth it.

Lee

I've seen the technique mentioned in a few off-roading books, also used on modded trucks with fat tyres for off-roading nuts in snowy countries. Also some trucks, the Humvee (proper one) have tyre inflation and deflation devices built in so the tyres can be deflated and re-inflated to gain traction. Never needed to try it myself yet but I reckon it's worth a crack. On the fat-tyred snow trucks the tyres were tubeless but apparently you can go to lower pressures with tubes as the bead can slip on the lower pressures which would break the seal with tubeless. Bear in mind that letting your tyres down reduces ground clearance.

One technique I have tried, picked up from this very boutique, is driving with the brakes on. The idea is to even up the resistance across the axles to stop one wheel soaking up all the power by spinning, I've used it to reverse out of a stuck situation where I couldn't move at all, and to get out of a cross-axle situation. Keep the revs up with one foot and with the other, either bosh up and down on the brake quickly as if you were cadence braking, or try pushing down on the brake slowly, keeping the revs up. I found the former easier than the latter.

It usually helps. FYI the minimum pressure for ARC CCV trials is 12psi, from memory. Anything much lower and you run the risk of the tyre and tube slipping on the rim, which neatly cuts the valve off which gives you 0 psi!

Gordon

Always an embarassing one that! Flat field, water and no go.....

Lowering the tyre pressure would help if the ground was all soft, but knowing the area you were in I'd venture that it was clay-ish soil on rock with a nice coating of Peak Park Special greasy grass, in which case lowering tyre pressures won't make a lot of odds.

This is one case where tyre choice can make a significant difference, something with a failry open block tread like Pirelli's are very effective, or if damaging the surface is not an issue then Michellin OR's are good as they are very good at removing the surface (without truning into slicks) and gripping on the sub-surface.

Alxe diff locks are the best solution in these circumstances, but for a once-in-not-very-often senario thats' probably over-kill. You can use a little pressure on the brake pedal to make a "temporary" axle diff lock, but it doesn't always work on that sort of surface.

If you have sufficient people on hand to push then thats the easiest (if embarrassing) solution - winches are all well and good but, in these circumstances, a lot of faffing about compared to a couple of minutes cringing.

Just my 2p

Richard

On Mon, 27 Mar 2006 09:54:44 +0100, beamendsltd wrote (in article ):

Amen to that - I've long been a proponent of the theory that these things are often better managed with lots of manpower and some common sense rather than a more technological solution!

Thanks Richard, useful comments (and the rest of you, as well!)

Someone from Australia (thanks Ron) has sent me an e-mail response which refers to 'Stuan deflators' which I presume are a handy widget which lets the air out to a pre-determined pressure. Are these available in the UK, and if so how much and where?

Nick.

In my experience of being bogged far too often for my own good, dropping tyre pressures dont do squiddly dit. In mud the tyre tread pattern is the major factor, does the mud drop out of the tread as you move forward? Road tyres dont do that, they just fill up with mud and slide all over the place. Dropping tyre pressure only increases the likelyhood of the tread retaining the mud, it also reduces the approach angle of the tyre over the surface which makes it harder for the tyre to keep on top and rolling forward.

In fact I'd say that in most situations dropping tyre pressures is a waste of time and effort, even on sand where the larger the diameter (to reduce approach angle) of the tyre is far more important than the spread of the tyre by a couple of mm.

Quick google search for "Staun Deflator UK" will give you links. Their main website is at

Alternatively, it makes the tyre flex at more acute angles at the point at which the tyre leaves contact with the ground, so helping it get rid of mud.

Alternatively, it allows the tyre to "step" over the edge of an obstacle, particularly useful when rock crawling which is why rock crawlers use low pressures, as well as those who drive in the desert and on beaches, snow, mud etc.

Alternatively, the tyre becomes concave not convex so doesn't push the sand away but compacts it underneath, and the reduction in pressure increases the footprint of the tyre by a very significant amount as it's both wider by a few centimetres when flat, and the contact point is longer by a few centimetres too. A larger diameter tyre has a larger contact footprint than a smaller diameter tyre, unless you let the smaller tyre down.

So either one of us is talking out of our hats, or you've not been letting your tyres down enough ;-)

I have relatively little experience of mud, but I would be inclined to agree with you, although where dropping the pressure makes the difference between going over the top and digging in it will work, and I have experienced this.

But I do have about two years of full time desert work, and I can assure you that in sand tyre pressure is by far the most important factor, more important than tyre size or type or tread type or even 2wd or 4wd. And pressures have to be well down - 12-15psi tops. JD

Or both of you.

Sand, rocks, snow irrelevant in this discussion as the OP was concerned solely with trying to get of a fairly flat very wet grassy, quickly becoming muddy field.

The big problem as has been identified is that road tyre treads fill up with mud and become like slicks. In fields usually the further down you go the harder it gets, unless you have failed to look ahead and driven into a bog, so narrower tyres will probably do better than wider ones.

Top gear did a test awhile back with various 4x4s as supplied from the showroom. Of course the shiny ones came along with road tyres and were useless, whereas a Series LR with nice knobbly 750x16s won hands down.

The lesson for the OP's team is simple - if your work requires you to go off the tarmac then make sure you are properly equipped.

On or around 27 Mar 2006 00:06:12 -0800, "gordon" enlightened us thusly:

bearing in mind that on newer vehicles the tyres are just as likely to be tubeless, which can get unseated with the same effect.

On or around Sun, 26 Mar 2006 21:30:23 +0100, Nick Williams enlightened us thusly:

Had this once at Margam park watching the rally. many cars all parked in field, during the stage, mucho pissingdownrain, later all cars stuck in field. Amused by one bloke in a ford ranger pickup trying to tow something like a fiesta and getting stuck himself.

After a bit of amusement and a bit of pushing of other things and so forth, I got into the 4x4 sierra with fast road-pattern tyres, and drove straight across and out the gate, almost took out a bystander when the front wheels suddenly gripped and proved to be at an angle to the direction of travel mind.

How? viscous couplings in the rear and centre diffs. It's unbelievably good on slippery stuff. Obviously, as an off-roader it'd be hopeless, with ground clearance of about 4" and not much suspension travel at all. But on flat slippery (or even steep slippery, such as icy roads) I've not found much to beat it.

And the good news is that the bloke in the garage has almost finished the work on it for the MOT, so it'll be on the road again soon. Bugger, forgot to phone the insurance idiots. Bloody Firebond. They've been told, at least 4 times, that the motor has been off the road, not in use, and as such has no MOT, that it's being repaired and MOTed and when that's done I shall tell them the MOT number and the MOT mileage figure as requested, and take the 6 photos they want to prove the condition for the agreed value bit. I can't take the bloody pictures when it's not here, FFS!

But that doesn't stop them sending letters about every 2 days wanting to know where the rest of the info and pictures are.

On or around Mon, 27 Mar 2006 22:17:25 +1100, JD enlightened us thusly:

The trials boys wouldn't agree - they typically go as low as about 8psi and get a lot more grip on some surfaces. Mind, they also have bolts or screws in the rim to secure the tyre on the bead, quite often.

Lowering pressure, in the OP's circumstance, would almost certainly have made sod-all difference unless the mud was both deep and soft. On hard stony slippery terrain it make some difference and on very soft going it gives you a bit more flotation effect.

But you do have to let a lot of air out.

Yeah I know, don't worry, I've done enough off-roading to know that there's so many variables that hard-and-fast rules don't tend to exist, pretty much the same as the rest of life: it's all shades of gray, not black and white.

And there was I thinking it was only Adrian Fux who got their knickers all twisted and did that sort of thing!

On or around Mon, 27 Mar 2006 19:15:43 GMT, "GbH" enlightened us thusly:

I expect they're all reselling the same policy :-)

Yep, Sureterm cancelled the policy on my S2, despite me telling them that I'd need a feckin long lens and x-ray vision to photograph the damned thing. Got cross and went to firebond.

They did exactly the same!