8th Scale Buggy Tires

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.2 Off-road tires
Off-road tires operate quite differently from on-road tires. They usually have some sort of tread pattern with pins that dig into the soil, or a series of small pins that scrape the top surface. This is entirely different from the friction model described above, where you had a smooth, hard surface and a uniform rubber contact patch.
Off-road traction is generally more complicated, the curves have more complex shapes, there are more types of soil, more transients, and there are more variables.
1.2.1 Tread Pattern
Size of the pins/blocks
There's a basic rule of thumb that says:"The softer the dirt, the bigger the pins need to be.". Long pins work by penetrating the (soft) soil, and short pins usually work by scraping off the upper layer of the soil.
Bear in mind that long pins and very soft compounds don't mix very well, then the spikes just bend over instead of penetrating the dirt.

Density of the pins/blocks
For a given tire width, the density of the pins is inversely proportional to the weight supported by each pin. There's usually an optimum, where the tire works best. For example: if a certain tire works very well when it's heavily loaded, but doesn't feel right when it isn't loaded, the tread pattern is probably too dense. This can happen in very dusty or soft conditions.
Tires for sticky mud usually have a very low spike density, because too much spikes cause too much stiction for the amount of traction they create, slowing the car down.
Pins or blocks
Round pins provide grip that feels the same in all directions, it feels consistent and it's somewhat easier to slide. Very useful on difficult, low-traction, bumpy tracks.
Square blocks feel more 'edgy', they can give the impression of generating slightly more grip, especially on smooth, hard surfaces. The forward traction they generate also feels nicer.

Center tread
Some tires have a larger tread pattern in the center, often shaped like an X, an I, or a T. In all cases, it increases forward traction dramatically.
Note that on 4WD cars, more forward traction from the front tires can also mean more steering.
1.2.2 Rubber compound
"The softer the surface, the harder the compound, and vice versa."
Some synthetic surfaces, like carpet or polished floors require specially formulated compounds, such as Schumacher's Yellow compound, and Medial Pro's Indoor compound.
Hard compounds
E.g. Losi Gold, ProLine XTR, Schumacher Blue & Green.
These work well on very soft surfaces, such as mud, damp, loose dirt and fresh grass. The idea is that the tire doesn't move, but the soil does.
Hard compound tires aren't sensitive to changes in foam insert.
Medium compounds
E.g. Losi Silver, ProLine M2, Schumacher Silver.
These compounds work well on most dirt surfaces. They're the best choice if the track is very dusty, or is starting to break up.
Soft compounds
E.g. Losi Red, ProLine M3, Schumacher Pink.
Soft compound tires work very well in blue-groove conditions, when there's enough rubber deposit on the track to make it darkish gray or black. They also work well on very hard surfaces, where the rubber compound is more important than the tread design.
1.2.3 Carcass shape
Round carcass
These tires have a rounded contact area, so they're not at all sensitive to camber changes. They're excellent for bumpy, rutted conditions; they'll provide consistent traction and won't hook into the ruts and make the car flip over. The downside is that they don't generate as much forward traction.
Flat carcass
These tires have a flat, or almost flat contact area. They provide excellent forward traction, and if camber is correct, also excellent cornering traction, but only on smooth surfaces. In bumpy sections, they feel inconsistent and can make the car flip over easily.
1.2.4 Rim size
Taller rims
If you use a slightly taller rim, for example if you use an old 2.0 inch tire on 2.2 inch rims, you'll stretch the tire's sidewall a little, making it stiffer and flatter. If you overdo this, the tire's carcass is bent out of shape, and traction is very poor. But done correctly, it can make the tire feel just a little more responsive and sure-footed, but maybe not as good in bumps.
Wider rims
Using slightly wider rims seems to be in fashion now, probably because they combine very well with very soft compound tires. Using a slightly wider rim stretches the carcass, making it lower, wider and more firm. This makes the tire feel more direct, and a little better for smooth tracks.
1.2.5 Foam inserts
All tires, except maybe hard coumpound tires, need foam inserts in order for the carcass to keep its shape. The softer the carcass, the denser the foam needs to be. It's best that the shape of the foam is matched to the shape of the carcass, so often the foam will need some trimming around the edges. Also, when you're using wide rims, you might need a wider foam.
The foam that comes with the tires is usually the best choice, you could use a slightly softer one for bumpy tracks and a slightly harder one for smooth tracks, but you'll get a very similar result by just trimming the stock foam differently. If the variations are relatively small, using a bigger or a more dense foam has the same effect.