Why Low Tyre Pressures? « 4xforum.com, the 4×4 Info-Tainment Channel

Why Low Tyre Pressures?

Does the size of a car tyre print vary significantly depending on the pressure of the tyre?

Research, text and pictures by Duncan Walker,

International School of Hout Bay, April 2010

Reducing tyre pressures helps improve traction, smooths the ride and reduces penetration when conditions get soft. Why? This article will help explain this

Aim+ Hypothesis

My question is: ‘Does the size of a car tyre-print vary depending on how many bars of pressure are in the car’s tyre?’ I am investigating this because if we are out in the middle of nowhere, in the Kalahari desert for example, and we get stuck in thick sand, we have a problem. My dad then will let down the pressure in all the tyres of our Landcruiser and usually we will be able to drive away after doing so. I have always assumed that this works because the weight of the vehicle is spread over a larger area, decreasing the point loading. I wanted to find out how large the difference of the cars tyre print area could possibly be, depending on how much pressure is in it.

Method

To collect my data, I decided to use my family’s off-road car, a Toyota Landcruiser, so that I would be using the same type of tyres as those that would be used in the real situation of getting stuck in sand (as explained in the introductory paragraph). I followed this procedure:

First I pumped the tyres to 3.5 bars and jacked up one of the wheels of the car. Once that was done, I took shoe polish and painted the polish onto a section of the jacked-up tyre. Next I made sure the polished section of the tyre was facing the ground and I placed a piece of A3 paper under the tyre. After that I lowered the jack so that the tyre was on the ground, putting the car’s full weight onto the piece of paper and creating a print. I did this twice with the pressure at 3.5 bars. Doing it twice helped me to make the area data more accurate.

Next, working with the same tyre, I let the pressure down by half a bar and repeated the process. I kept repeating this until the pressure was down to 0.5 bar.

Results

To create written results and graphs with my data, I took each tyre print (A and B for each half bar) and found the area in cm². I did this by multiplying the width by the length for each print. Next I found the average for each half bar by adding the A and B area together and then dividing by 2. After doing this, I put the results into a table and a graph.

I then calculated the percentage increase in the area as the tyre pressure decreased and plotted this on Graph 2.

Pressure (bar)	Area#₁ cm²	Area#₂cm²	Area-_Average	Percent increase in area
3.5	320.0	292.0	306.0	0%
3.0	320.0	310.0	315.0	3%
2.5	367.5	374.0	370.8	21%
2.0	397.8	396.0	396.9	30%
1.5	451.0	472.5	461.8	51%
1.0	517.0	528.0	522.5	71%
0.5	682.0	686.3	684.1	124%

After I collected my data and put it into a table, I found two main facts.

The area in graph 1 rises steadily as I expected. When I drew a best-fit line onto the points, it showed a non-linear relationship. Once I had reached 0.5 bars, the tyre print was almost twice the size of the first 3.5 bar print.

In graph 2, comparing the width and length of the tyre prints, something unexpected happened. The rate of change in the length is much greater than the rate of change in the width. From 3.5 bars to 2.0 bars, the measurement of the footprint along the tyre was always a bit smaller than the measurement across the width of the tyre. At pressures lower than 2 bars, the length overtook the width by a long way.

I wasn’t expecting the difference to be so dramatic and I was surprised by how much bigger the size of the tyre footprint was when the pressure dropped from 3.5 bars down to 0.5 bars.

Conclusion

Now that I have finished all my data collecting and summarizing, I am satisfied that I have successfully answered my question of ‘Does the size of a car tyre-print vary depending on how many bars of pressure are in the tyre?’

In the end, my data showed me that the reason for being able to drive out of thick sand after you’ve let the tyre pressure down, is that the area of the tyre on the sand surface has greatly increased. This spreads out the car’s weight over a larger point of load so the pressure on the ground is reduced and the vehicle will thus not sink into the sand so deeply. The larger surface area also means that there is more contact with the ground and thus greater traction. These two factors combine to allow the vehicle to drive out of the thick sand.