Saturday, Dec 20, 2014 at 12:10
I only speak from personal experience on this
forum, and I have to comment further on this "misconception" which seems to relate more to older tyre choices and individuals own opinions or limited experiences in correctly optimising the use and choice of tyres in sand.
I should mention I was always an adventurous soul and learned to drive on sand and mud in 2WD vehicles as a kid so I have a bit of experience behind me.
There is no question that the length of the tyre alone - adds substantially to the tracton available - this is why tracks are so effective on earthmoving equipment - but a dozer on narrow tracks will still sink
well before the same dozer on wide tracks.
The way I commonly use to explain, to novices, the effect of deflated tyres when looking for extra traction is to view them as being 4 mini Caterpillar tracks on your vehicle. This makes sense to them when visualising how the tyre should look for soft sand.
The "bag out" I refer to actually includes tyre length and width ...and yes, with the extra surface area, "width" does assist greatly in sand despite opinions to the contrary.
The "bag" depends on a lot of things but I was referring to a 33' BFG MT which as I stated has soft walls and "bags"
well when needed. The Photos above are quite correct but are not the type of tyre we are discussing on this post.
The aired down length available is directly proportional to the available height or diameter of the tyre and the overall volume of the tyre determines how low a pressure the tyre can be reduced to before the weight of the vehicle is no longer effectivey supported by the tyre. IE A low profile small diameter tyre needs to maintain a higher pressure to keep its shape for a given weight than a high profile larger diameter tyre. simple physics as 10 PSI over 10 sq in will carry more weight than 10 psi over 5 sq In.
In essence a 33" Muddie has 1/ height - for a longer deeper footprint, 2/ Width - extra volume (allowing even lower "let down" pressures) and of course extra cm2 for extra traction. 3/ Pliability to allow the side wall to hang out a bit wider when the pressure is low and take pressure of specific points on the bead so the tyre is less prone to popping off the rim.
When driving on soft dry sand (or any surface for that matter), until the tyre sinks into the sand, this bulge is irrelevant. However if climbing a sandy
hill or towing another vehicle in soft sand and the vehicle begins to sink into the sand and this bulge sinks below the surface it becomes something like a paddle and provides extra surface area and obviously extra traction. Albeit only marginally extra traction but as the vehicle is still moving this marginal amount of traction AND surface area is often enough to keep the vehicle from sinking further AND in motion. Many aggressive tyres have protective lugs in this bulge area which also help.
From my experience - a similar diameter thinner tyre has little or nothing left - the vehicle stops.
The other issue relates to the driver who controls the throttle. It is also said MT tyres are useless in the sand - this is untrue. They are very effective if they are not forced to dig themselves onto a deep hole by an unskilled driver.
EG
A few years ago at the first cutting on Noosa North shore, I came across a 4X4 Wagon towing a small caravan bogged 1/2 way up the cutting in soft dry sand, below it were two other 4x4s bogged - facing down the cutting - they were both hooked onto the 4X4wagon and caravan and all three 4X4s had become hopelessly bogged. As I had a horse trailer on and was escorting my daughter and friend on a beach ride I had no interest or time to intervene. However, eventually money was offered and the kids encouraged me to assist. So I unhitched the horse float and dropped my tyres till they looked right for the job - so somewhere around 10 PSI. Much to the disbelief of the gathered crowd I pulled in front of the Prado and said "we do this once - we go up
the beach not down". (I had better things to do - honestly) I gave them a shovel and tyre guage and insisted on 15 PSI in all tyres and they needed to get the sand away from the front of all their tyres. I had 2 Maxtrax and a front Difflock in my vehicle with 33" BFG MTs. I gave him a handheld 2 way and talked him through the revs we needed to maintain - I.E. when to ease off and when to apply throttle.
Needless to say - It Worked - Once we got moving I actually pulled them all the way to the top without stopping once. At times barely moving but still making progress.
I did know from experience how resilient the MT tyres on my vehicle were to bogging is sand because of all the above factors. Unfortunately we didn't record the recovery on video.
I would like to see how (to quote the Explorer) "A large diameter, bald, skinny tyre would perform as good on sand as a wide, brand new, deflated "muddy""
Consequentially you may forgive me as to why I dispute the "misconception" regarding wide (aggressive) tyres being for "looks" alone.
Maybe I am wrong and any vehicle with 10m psi in their tyres will do this, (if they can go that low and still support the vehicle and stay on the rim) I have recovered quite a few vehicles over the years and personally I have not seen anything else do it yet.
Why do desert racers use wide tyres and not bald skinny ones?
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