Okay a lot are probably saying give it a rest, tough, I thought I would provide some proof/calculations to support the wide tyre case for consideration.
Following the debate and the enormous number of follow ups and responses to post number 30344 from Troll81 on Thursday, I had a very lazy day on Friday (at work) and thought I would do some calculations on this issue.
Some basic parameters were used
A single wheel with 700mm diameter (16” rim with tyre) equates to circumference of 2200mm.
Same diameter in both cases.
Load on single tyre (vehicle divided by 4) - 500kg.
Assume sand can support load of 0.01kg/square mm.
Skinny tyre – 175mm across width of tyre
Wide Tyre – 235mm across width of tyre
Assume no bulging of side wall for both tyres (stiff side walls)
Both tyres have same pressure of 25psi (172kPa).
As a starting point lets have 1/16 of the circumference as the contact area with the sand (will probably not happen in real life) of 2200/16 = 138mm (rounded off).
Wide 235mm tyre has contact area of 235 x 138 = 32,430 square mm
Skinny 175mm tyre has 175 x 138 = 24,150square mm contact area.
Load per square mm
Wide at 500/32,430 = 0.015kg/sqmm
Skinny at 500/21,150 = 0.021kg/sqmm.
Sand can only support 0.01kg/sqmm so both will tend to sink into the sand.
How much each will sink into the sand?
For the load of 500kg and the sand able to support 0.01kg/sqmm the area at which the sand theoretically will support the load with no sinking would be 500/0.01 = 50,000sqmm.
Wide tyre at 235mm wide to have a contact area of 50,000 sqmm it would need to have (235 x 2200/50000)th of its circumference in contact with the sand, ie one 10.34th of the circumference which equates to 2200/10.34 = 213mm.
Skinny tyre the same 50,000 sqmm contact area needs to have (175 x 2200/50000)th of its circumference in contact with the sand ie one 7.7th of its circumference which equates to 2200/7.7 = 286mm.
Using some fancy trigonometry I calculate
Wide tyre would sink some 32mm into the sand
Skinny tyre will sink some 57mm into the sand.
Again allowing for the unrealistic sand that stays level around this sinking tyre the angle from the bottom of this perfectly round tyre to the edge of the sand at the tyre
Wide tyre has an angle of some 17 degrees
Skinny tyre has an angle of approx 23 degrees.
So for the wide 235mm tyre the bank of sand in front of the tyre that has sunk some 32mm into the sand would be 235 x 32 = 7,520 square mm whilst the skinny tyre at 175 mm and in at 57mm in the sand would have 175 x 57 = 9,975 square mm of sand in front of the tyre.
For the supporters of skinny over wide how can a tyre of 175mm width create less of a wave of sand in front of the tyre at the same pressure as a 235mm wide tyre on the same sand and same load when it sinks further into the sand? Which slope is easier it drive up from a standing start, 17 degrees or 23 degrees?
Okay we let down the tyre to increase the area of the tyre in contact with the sand. But in order to increase the area of the skinny tyre to match that of the wide tyre we would need to let the pressure down more to achieve a similar contact area, but at what point does the contact area not increase, buggered if I know. I think I would rather drive on sand with a wide tyre at 16-18psi than a skinny tyre at 10-12psi and have the tyre still on the rims.
In this example for the skinny tyre to match the required support area it needs to increase its contact area by over 200% whilst the wide tyre only needs to increase its contact area by some 154%, and the volume of air contained in a skinny tyre is a lot leas than for a wide tyre so where does the extra volume come from to deflate the skinny tyre to a greater amount than a wide tyre, maybe the spare tyre under the car?
So to tim_s, Shawn, Bonz, the explorer, and robak think about that whilst you try to drive those skinny tyres (less than 200mm wide tread) across the soft sand).
By the way I spent the weekend driving around the sand dunes at
Beachport and Canunda with my wider tyres fitted to a Courier dual cab turbo deisel and had no issues of bogging in soft sand.
Cheers and happry driving