Understanding tire traction involves many factors and aspects. I don't want to condense it too much as to loose valuable information, so I decided to create a multi part series. Lets start out by covering seemingly random basics.
Tire traction considerations:
Tires have two basic principals that create, what is loosely called traction: 'Adhesion' and 'Mechanical Keying'. Adhesion is just what you think of it, the rubber sticks to the surface. Obviously this is not a huge amount of traction added, but it still is enough to matter. The biggest contributor is mechanical keying, aided by the deformation characteristics of the rubber tire. See the pic below for a visualization.
The mechanical keying is in effect on the molecular level of the surface and on the larger scale of the tread pattern of the tire. You also see in this image, how increased pressure on the tire surface, forces more of the rubber into the driving surface, thus increasing the contact area and therefore increasing traction.
Driving Surface considerations:
'Loose surface' would generally be considered moderate snow, sand, gravel, dirt.
'Deep or soft surface' would be mud, deep snow (not compacted), loose sand, deep loose gravel. Basically everything that your tire sinks into by just standing, and would dig down into when spinning.
'Hard surface' would be asphalt, concrete, rocks, hard packed dirt (rock like).
General rule of thumb: you want to drive on hard surface to get the most traction. You want to avoid soft surface as it provides the least amount of traction.
Roll Resistance Considerations:
In addition to the incline of the terrain and the obvious obstacles of a trail, you also have to overcome the roll resistance of your tires in order to move. Once you find yourself in a low traction scenario, all the sudden this little bit of resistance plays a big role.
I would call all of the following the roll resistance factors:
- width of your tire
The wider your tire, the more inherent roll resistance it experiences.
- treat pattern
A street tire tread pattern (small gaps between blocks) has less roll resistance than a MT pattern (large gaps between blocks).
- deformation of your tire
Low tire pressure allows the tire to 'sag' and the resulting deformation draws energy from the motion, therefore increasing roll resistance.
- ramp angle in front of your tire
If the tire sinks into the ground, you basically have to drive up a ramp in font of your tire in order to move. This ramp is based on the tire shape (diameter) and how deep you sink into the surface.
Reality of mixed surfaces:
Typically you will encounter a mix of all of the above. In other words: you will have a hard packed trail (good traction) with a layer of mud on top (bad or no traction). Or a rough rock surface (good traction) covered with algae and water (bad traction).
In The Next Part:
The factors that create traction on the road, are very different from what creates traction off-road. You will see that in our second part when we evaluate what factors are in effect in specific scenarios. We will also draw conclusions on how to improve traction in the given scenario, and spoiler alert: 'airing down' is not the right answer.