Mitsubishi 4WD Club Trev's Tip 23. 

Trev’s Tips

Feeling the pressure

Tyre pressures are something that all competent 4wders spend a lot of time talking about and some time testing but for most of us our knowledge of tyre pressures is limited to if you get stuck in soft sand let your pressures down to around 18psi and hopefully that will be enough to get you going again.  If you were to be asked the question “how much difference would it make if the tyre pressure was lowered from 18psi to 14 psi”? many could only give a vague answer.

The Christmas holiday trip of 2006 to Denmark saw a couple of our club members get to test this question in real life when they attempted to tackle the famous Bornholm sand hill.  I remember talking to Terry about the challenge shortly after this event which saw him express surprise that just lowering  the pressure from 12psi to 8 psi made a huge difference to the ease with which the cars had to work when going up the hill.  You would think that a drop of 4psi would make minimal difference but the reality is that this actually represented a 33.3% drop in pressure which is quite significant.  Does this mean that a 4psi drop does not always mean what it seems?  Absolutely.  If a tyre pressure was dropped from 40psi to 36psi (a drop of 4psi) you would hardly notice any difference in the cars handling or off road ability at all because of the

small percentage decrease in pressure where as the drop from 12psi to 8psi represented a large percentage decrease in pressure and consequently a big change in the shape of the tyre.

My physics has never been fantastic but I do remember a formula that was taught to me in school that suggested a link between pressure, area and force.  From memory I think it was

P = F/A

(Pressure = Force divided by Area)

or for what I’m about to discuss it would be better if we had

A = F/P

My application of this might be a bit wayward here but from this it would suggest that there is a  relationship between these variables.  Stay with me while I try to explain some technical “stuff”.  To get some meaningful units we really want the area to

be measured in m2. To have this happen means that pressure should be measured in Pascal (1 Pascal = 1 Newton meter2 (nm2 )),

and force should be measured in Newton (1000 Newton = 1 ton).  I always refer to pressure (inside your tyres) in pounds per square inch (psi) but this is easily converted to kPa or for the purposes we need for this application Pascal using the following conversion.

Multiply your psi measurement by 6894.7

Anyway, imagine if the force applied to a tyre (by the weight of the car was 500N (0.5 tones) and the tyre pressure was

206841Pa (30psi).  This would mean (according to the formula) that area (tyre in contact with the road) should be 0.002m2.  I

know there are many other factors involved here so this simplistic idea is not all that accurate but here comes my point.  If the force was left the same but the pressure was halved to 103420.5 (15psi) , according to the formula the area would increase to

0.004m2 .  This is double the original area indicating that for each time you half the pressure in the tyre you increase the area

of the tyre by 2.

Sounds simple enough but does it really work like that.  I remember having a conversation with Chris S at Step Point last year where he had actually tried to investigate how the footprint of his tyre changed as he decreased the pressure.  Stealing this idea from him I to decided to do a bit of playing with pressures and looking at the footprint of the tyre for myself.

To do this I jacked the rear tyre of my car up, spread a thin layer of sand under it then lowered the tyre onto this.  I then jacked the car back up and measured the length and width of the imprinted rectangular shape the tyre had left in the sand so the contact area of the tyre could be calculated.  I then let some of the air out of the tyre and repeated the whole process.  The results of this are shown below. 

By the way my tyre is a 31x10.5r15

Some interesting results especially the huge increase in area when the pressure was dropped from 15 to 10 psi.  Didn’t quite match up with my theoretical calculations that if the pressure halved then the area would double so the only theory I managed to prove here was that my physics is still not as good as it should be.  The technical “stuff” sounded good for a while.

Keith P also gave me a table similar to the one above that had been produced for a different sized tyre 235r16.  Here it is.

So there you have it.  Proof that a small decrease in tyre pressure can make a big difference to your tyres footprint and your cars overall ability to get out of a sticky situation.  As always if you are planning to drop pressures to below 10psi be very careful not too drive too hard and cause the tyre to come off the rim.  Only do it just to get through a bad spot then put them back up again.

Till next time

Happy 4wding

Trevor

My search engine ran out of gas.