Tyres

The surface aria that is covered by a wide and tall set of tyres when compeered to a set of skinnies is far greater. Therefore the weight of the vehicle is distributed over a larger surface aria and so the tyres float over the sand and don’t sink in as much. But tyre pressure plays a big role in this as well, and I think it has got more to do with the rolling diameter rather than the width of the tyre.

I don’t totally agree with the second statement, but I suppose it could be because the skinny tyres will find the harder base (if there is one) a lot faster than fat ones and then they don’t need to move as mush slush to power throe it.

However I feel a fat and tall tyre will prefer better in both situations

Luggy,

When a tyre is at road pressure it may have a footprint of say 100 square centimetres The weight of the vehicle is spread over this area. For this, lets say that the vehicle weighs 2000kg, so each tyre has 500 kg of weight, and has 5kg / square centimetre.
If the tyre is deflated the foot print is now larger, say 200square centimetres. The vehicle weight is still the same but now spread over a larger area.

Tyre weight= 500kg foot print= 100 square centimetres 5kg/ square centimetre
Tyre weight= 500kg foot print= 200 square centimetres 2.5kg/ square centimetre

The wide tyre on sand is not as good as a narrow tyre. A wide tyre even though it has a large footprint, also has a wide area across the tyre. This causes a wedge of sand that the tyre is trying to go over all the time. A narrow tyre has a smaller wedge to try and roll over so there is less effort to drive on the sand.
The footprint of the tyre should be long and narrow.
Mud tyre or tyres that have an open block are not as good as road tyres, and tyres that don't have much tread are even better.

As for skinny tyre in mud, it is the same as spreading the weight over a small area, more weight / square centimetre.

Wayne

1. Floatation. Wide tyres with low pressures in them exert less force on the sand, therby creating less of an indentation in it. You therefore don't have as far to drive "uphill' to get moving, and have generally less chance of digging youself in.

2. Penetration. Skinny "pizza cutter" wheels sink further into the mud for the exact opposite reason of 1. above. They are also able to exert more force onto the hard surfaceat the bottom of the mudpit, giving overall better traction....sometimes.

If the mud is deep, there is some justification for the "float and wellie" approach of lowering the tyre pressures and giving it a bit of right foot to skim over the surface.

Wayne and GaryinOz,

I think I am totally confused with your answers re wide tyres vs skinny tyres in sand. I am not that hard to confuse though :)))) Your answers seem to be diametrically opposed.

A LONG footprint is what counts on sand. Thats achieved either by letting air out, or fitting taller tyres. Width doesn't matter.

And as an aside, ever wondered why the old VWs and Kombis were pretty good on sand???

Mud: it all depends on where you are - if you want the tyre to dig in (often the case in the high country), then skinny will do that better. If you want the tyre to sit on top, (say in the desert) then lower pressures help that.

Cheers
Phil

LOL .. I did warn you that I was easily confused :)

1. Luggy, my apologies for taking over your thread.

2. Wayne and Phil G, thanks for explaining it to me so that I finally understand :)

3. GaryinOz, sorry I thought you inferred that wide tyres were definitely better in sand.

Geez I hate being the designated driver when all around me are enjoying themselves. Must have been all those lemon squashes I drank today. Bad for the grey cells :)

Phil's on the money here.

When you let air out of a tyre you significantly increase the length of the footprint and only marginally increase the width.

Hence a tall skinny tyre at low pressure is better than a short wide tyre at similar pressure.

The stiffness of the sidewalls prevents sideways bulge.

Similarly skinny tyres in mud will cut through the slush to the firm base ... if there is one.

Unfortunatley bit fats are usually accompanied by something physically similar ... both not necessarily to any real benefit, other than the pose perception.

Cheers

BEEEEEP
Too many factors to symplify this topic. ( You got to be patient with me here for a bit.)

Grip and surface area are what works. Weight to surface area is the formula that will determine optimum traction. Even on bitumen, too wide a tyre can have less traction for a light vehicle than a thinner tyre - causing spin outs and accidents in the wet. Well documented but not the question asked by Luggy.

But lets sink the tyre in a few inches of mud and sand and see what changes in the equation.

A thin tyre in Mud works well when there is a solid base and you have momentum. and need to keep direction.

Otherwise why do these companies like BFG Mickey Thompson etc sell big fat aggressive Mud tyres and not tall thin ones?? (Im sure it can't all be compensation!!)

So yes I can see why You dont see how thin tyres in mud work. generally they dont.

BUT............Gotta consider. (This will bore some of you so skip over this)

1 The Driver - do you know what your tyres are actually doing - are they gripping or spinning? are they digging??

2 Suspension/Articulation Rigid vehicles will get stuck much sooner than a well articulated vehicle.

3 Drive train diffs LSD lockers etc (Open diffs equate to 2 driving wheels in sand and mud. A Good Rear LSD makes that 3 wheels, a Locker in the front finally makes true 4 wheel drive. So this comes into play before they tyres even get a look in.

4 Tyres - Height , width Aggressivness its not what you've got but how you use it.(any girl will tell you that)

5 Terrain (Sand Mud Soft /Hard Base - No base Dry/Wet Up hill down hill Slippery, better watch myself here - but lubrication makes a lot of difference too)

6 Intention, (Where is this going --Yikes) What you intend to do and expect the best result from your choice of .............tyre.(Long drives on soft sand, (Moonlight roses champaign - oops back to the topic - dont want to be modded) stops starts and turning, recovery, using heaps of power in short distances etc etc (driving off the barge onto soft rising sand on Fraser is another challenge that stumps many high pressured tyres fat and thin on great vehicles)) Intended speed and direction will determine what works best, for some intentions tall and skinny is great, for others tall and fat will work better.

A rigid vehicle may very well spin and dig itself in, a well articulated vehicle will put a wheel down a hollow and keep traction.

In a stock standard 4WD the final word on this will be had by the best driver in the vehicle with the best rear LSD (or even any LSD) with the best surface to weight ratio running the lowest air pressures and having the longest legs (best articulation/suspension.). Elimimnate the best driver and it would then be a toss up between the least aggressive tread and the most aggressive tread. Take out the good LSD The most aggressive tread will win only when excessive wheel spin is eliminated. Otherwise use a less aggressive tread and then rev the brains out of the vehicle - However - same result -- bogged.... but will take 20 seconds longer maybe enough time to think.-

Still comes back to the driver.

Add a diff lock - all the other factors change - so lets just look at the bog ('scuse the pun) standard 4bys on the trail.
So if you have a 4by without a LSD or front lockers go for a non aggressive tall tyre, wide is good, skinny ok too - just dont dig them in. Tall tyres give you that extra clearance till the diff drags. You also get that long foot print - a bit like 4 mini caterpillar tracks. As for the wedge in front of the tyre - great theory but I personally dont find it to be the real determining factor. It may come into play more at low speeds and in determining fuel consumption over long distances.
Too many other things to consider. (The driver with a sense of what the wheel's are doing is going to win here.)
Sorry guys but I have been driving in sand and mud all my life and only on some occasions is a skinny tyre an advantage in really SOFT sand and thats when its low in pressure, the sands not too soft and loose and when you are already moving and want to keep momentum in a fairly straight path. Then you go for the skinny tyre (That is a big advantage of thin tyres when doing long drives in the soft stuff keeping straight - as you loose a lot of momentum when swanning back and forth in somebody else'es meanderings - really best to pick your own path - and if picking your own path - a wide tyre will keep you on top of the sand if its low in pressure - so back to wide tyres in your own track -well you did ask!).

For most stop and start beach work fats will be easier.(as long as you dont dig them in and pressure is low.)

For starting off manouvering and recovery and really low speed stuff - that old surface area will win out every time.

In mud a skinny aggressive tyre finds the depth to a point - but once you are on your diffs - thats it. Break out the winch. Or Get some wide muddies. (10s of thousands of competition junkies cant be wrong) And dont dig them in (too much). On clays and firmer mud, manouvering and stop and go driving aggressive tread and even snow chains will win the day.

The best tip I can give while negotiating that steep muddy hill or uphill sand track - LOW TYRE PRESSURE on any tyre (but preferably a muddie if all the tracks Ive seen recently around the hills after the rain are an indication - lots of road tyres sliding back down the hills and into the gullies)) and know which gear will get you all the way up (even if it seems tooo low), gain momentum and start hard but lift your foot off as you progress so as to maintain maximum traction, (this is where real drivers are defined), dont damage the track, but dont be afraid to give it a nudge if you are slowing. Most importantly - if you stop - dont bother revving the engine - if possible just go back and try again -with more speed - lower pressures and more knowledge, -- or else - break out the winch or snatch strap.

hope this helps

Always driving the last vehicle moving, (Unless we're parked on a headland in the romantic moonlight)

Nick

Another perspective on the above , Basically I think everyone is pretty well spot on
but I will add another dimension to the discussion re sand driving.

better weight distribution (low pressure)
low ramp angle (big diameter tyres)
are the fundamentals on sand IMO.
but add this to the discussion , when you lower the pressure ,the centre of the tread area on the tyre compresses more that the outer tread area , this actually forces the sand to the centre of the tread area from both sides ,this compressed sand creates a "rail" of hard sand directly under the tyre area.
The reverse happens with over inflated tyres with the tread pushing the sand out from the centre (and in front hence the ramp) and therefore digging in instead of riding on the compressed sand
the best I have seen on sand is an old Hilux with 750X16 (razor blades ) let down to 10 PSI , The tyres would fold in on themselves and ride high . Don,t go round corners though.

Dave L