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Tyres - Not Again

Date:
By Mr Bill Collyer

Tyres – Not again!

 

I have just returned from a cross country jaunt (?) in the Simpson Desert, no people, no tracks and very very cold. But that is not what I want to write about, I want to return to the subject of tyres in the sandhills. In particular what has raised my interest yet again is the rise of another approach to tyre pressures, one which it would seem is at least in part based on the improved puncture resistance of high quality tyres, plus the fact that todays vehicles generally have much more power than needed.

 

First we need to have a brief look at “tyre theory”. For this I am going to stick to imperial measures, mainly because most of us still measure tyre pressure in pounds per square inch, lbs/sq in or psi.  Ok let us set our vehicle weight at 6500 lbs (just under 3000 kgms), this means each tyre must carry 1625 lbs. This means that if the tyres are at 30 psi then each tyre must have around 54 square inches in contact with the ground (54 multiplied by 30 psi gives 1620 lbs). If the tyres were at 15 psi (half) then the tyre will have 108 square inches in contact (double). This increase in contact area will be along the tread (lengthways) and to a lesser degree across the tread (sideways).

 

It is this lower ground contact pressure and the greater contact area that gives us the increased traction we need.

 

To increase contact area we can increase lengthways or we can increase sideways. Which is better? The answer is pretty obvious when you consider how well tracked vehicle perform in soft terrain – it will always be lengthways. Back in the very early days of my cross country driving, easily the best tyre was Michelin, standard highway Michelins. When we carried out “Contact Tests” by checking the footprint on a sheet of carbon paper, it was clear that Michelin tyres got their contact area largely by increasing lengthways as they were deflated. And this of course gives us a clue as to why they were so good. The rolling friction of tyres depends on the direction of travel profile, the smaller the frontal area the lower the rolling friction (check out the width and pressure of racing bike tyres when you are at the velodrome!). Thus the Michelin with their long ‘skinny’ contact profile provided the low rolling resistance plus the contact area needed for good sand performance.

 

If you think rolling resistance is not important then just try and pull by hand an empty trailer on the sand! Almost impossible, yet on paved surfaces easy Now the 4WD’s of today do have very high power to weight ratios so the rolling resistance of your rig is not as important as it used to be. However, when you are working at the margins in terms of vehicle capability then it does become important. PLUS the rolling resistance is what determines your fuel usage.

 

The problem with Michelin was unprotected sidewalls; when run at low pressure this made them prone to side wall staking (and they were damned expensive to replace!). The “bagging” of the Michelin was what gave them their long footprint AND exposed the sidewalls. To keep the sidewalls vertical and thus out of harm’s way, you need in theory to have a rim that has the same width as the tyre at the pressure you wish to run. Not possible of course since the width will differ for every pressure, but you can go for wide rims and narrow tyres. This is a good compromise. Thus for 285 tyres ie 11 inches you would need rims at least 11 inches wide – not possible - and you will need beads that clamp as well! So the best we can manage is to choose the widest rims that will fit our tyres, and for 285R18 this will be 9 inch (note that the recommended rim width for 285’s is 10’ anyway). You do have a wider choice with 16” tyres but the more modern 4WDs will have 17” or larger 18,19 or even 20”. (The larger rim diameter is needed to fit the bigger brake calipers required on modern high performance 4WDs.)

 

This is all a bit depressing, our modern 4WD will do 150 km/hr safely on the highway but has tyres that will give us grief in the desert! There is some good news, todays tyres are much more stake resistant than the old Michelin so even at low pressures they are much less prone to staking.

 

So what will I put on my Discovery3/4? Well I cannot go below 18 inch rims so am stuck with those. I can go out to 9 or 10 inch aftermarket wheels. So if I stick with the 9” then I should fit 235’s or at most 245’s – this is a lot narrower than the 255 I am currently running – and I should go for as high a ratio as possible, say a 70, for extra ‘bagging’.

 

So my fitment would be 235/70R18 in a high quality brand, Kumo, BFG, Cooper. A final comment – despite what you may hear or read the lower the tyre pressure the higher the chance of side wall puncture, and the lower the pressure the higher the fuel consumption. The “best” pressure is the lowest pressure that will get just you over the dunes. Any lower pressure just increases fuel consumption, increases the chances of rolling the tyre off the rim and increases the chances of punctures!

 

There you have it, bye for now,

Gurrumul.

 

PS You can view my last Simpson trip (a somewhat biased account!) on Youtube – Alloffroad - “Hay River and Simpson Desert”

 

Recommendations for Tyre width / required rim width (min.)

  • 195 – 235 / 7,5″
  • 215 – 245 / 8″
  • 225 – 255 / 8,5″
  • 225 – 255 / 9″
  • 245 – 275 / 9,5″
  • 255 – 285 / 10″

(Source:Expedition Equipment, Germany)