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Off-Road Tire Tuning

 

 

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.

1.1. Tread Patterns

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. 

'Stubbiness' is also a factor. A thick, short, 'stubby' pin will have a hard time penetrating the soil. But, on the plus side, it will hold its shape very well. So there are basically two conditions in which 'stubbies' work well. Condition 1: for some reason, it's important to have a lot of rubber in contact with the surface, to really lay the rubber down. Some surfaces are like that, some artificial surfaces, like gym floor, or some types of clay that have either very very small grain, or rounded grain. 
Condition 2: broken-up surfaces, with 'rubble'. When clay dries out, it becomes like cement. Sometimes it stays together nicely, sometimes it breaks up. Most of the time, it breaks up first in landing areas after jumps, and in turns. But anyway, when a chunk of clay is ripped out, what's left is some dust, a hole, and a very sharp edge. A stubby type of pin holds up best of all over those edges. The pin isn't ripped off easily, and it won't 'catch' the edge either. 

There is another basic rule. The higher the level of grip, the shorter the pins will have to be. Or: the more power that's available, the shorter the pins have to be. Pins shouldn't bend over too much, that's all there is to it. 

Shape of the pins/blocks

Most pins are tapered. That's very good: it makes them a little more rigid, and gives them a nice, wide base on the tire carcass. Cylindrical pins would bend over easier. 
There is one special tread pattern, the Step Pin. 

Like the name implies, and the pictures show, the pin is stepped. It has a wide, stubby base, and another fine pin on top. Step pins are great tires. For very loose, heavy, or damp soil, they're the best. The narrow tip of the pin digs in very easily, and quite deep, and the thick, solid base gives good support, so the pin doesn't bend over too much. But there's something else as well: because the thick base pin takes up quite some space, the pin density is quite low. So you've got a fine pin, that's well supported, with plenty of pressure on it. That's what's needed for heavy, but soft soil. 
There are other, more 'funky' pin shapes. But it seems they can only be used in the center section. For the sides, you need something non-directional. Like a circle. Or a square if it needs be. 

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. Which is probably because each block has a wider 'edge', which scrapes off more material. (Of all 2D figures, circles have the smallest circumference for a given surface area.) (That's also valid in 3D: spheres have the smallest surface area for a given contained volume.) (I promise not to bore you any further with geometry.) 

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 with lots of pins don't like dusty conditions: there's not enough space between the pins for the dust to get out. 
Tires for sticky mud usually have a very low spike density, because you need plenty of pressure on a spike to push it into the mud. Too many spikes would mean not enough pressure on each one of them. That, and also because too many spikes cause too much stiction for the amount of traction they create, slowing the car down. 

There's also something to the alignment of the tread pattern. You can have a pattern of evenly spaced pins, but there's a certain directionality to it. Just look at the picture, you'll see what I mean. Look at the left side (inside) of the tire. The pins are densely packed when viewed from the front or rear of the tire. This 'direction' of tread pattern gives a lot of forward bite. Now look at the right side of the tire. (outside) When viewed from the front or back, there's a lot of space in between the rows of pins. When viewed from the side, the pins are densely packed. This arrangement gives a lot of sidebite; it's easy to control a slide, and it inspires more confidence in the turns. 
Now, you'd think it was a stroke of genius combining the two arrangements in one tire. It's not. The tire pictured is OK, but not great. The grip it provides is too dependent on camber angle, and the distortion of the tread is not evenly distributed over the tire carcass. 
What does this teach us? Basically that there are 2 kinds of tires, when we classify them by pin arrangement.

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. When the front tires are pulling the front end of the car through the corner. 
Why does a big center tread pattern increase forward traction? When you hit the throttle, it's the center of the tire that's doing most of the work. At speed, the center of the tire is the only part touching the ground now and then. It doesn't help if the rubber compound is soft, and you get the 'pizza cutter effect'. That's when tire balloons because of the centrifugal force. 

Anyway, for a center tread you want something like horizontal bars, or a wide 'X'. It can be wide, since all it's good for is forward traction, and it can be pretty big as well, since not much of the other pins are touching the ground, so there's plenty of pressure. That, and at speed there's usually some downforce. I like an 'X' better than a horizontal bar; (An 'I') it has a sort of self-aligning thing going on and a bar hasn't. 

1.2. Rubber compound

"The softer the surface, the harder the compound, and vice versa." That's the basic rule. There is one exception. 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. Wheher you use a firm one, a soft one, or none at all, it doesn't really matter.

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 soft, or is starting to break up.

Soft compounds

E.g. Losi Red, ProLine M3, Schumacher Pink. Soft compound tires work well on very hard, slick surfaces, where the rubber compound is more important than the tread design. 
Choosing the right compound for the track conditions is not easy. It takes a lot of experience to 'read' the track. The best way to start is to kneel down and feel the dirt with your fingers. If it's hard as a rock, with very small grains, it's almost certain you'll need a soft compound. Even if there is some fine, loose dust. 
However, if there is a serious 'blue groove' (rubber deposit), a medium compound is probably better. If there's plenty of traction, there's no point in using a soft, floppy tire. It would just flop around and bend over too much, giving a very imprecise feel. 
If you're racing on dirt that's damp, rubber compound is not very important. The only difference is that a soft tire will feel stickier, and a medium compound tire will feel more direct because the carcass holds its shape better. 
It gets difficult when a track is drying up, or when it has hard and soft parts. In that case, it's probably best to do some back-to-back testing with different tires.

 

1.3. 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. 

 

1.4. 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. 

1.5. Foam inserts

Full size cars use rigid carcass materials and air pressure, R/C cars use foam inserts. IMHO, the latter is more interesting. I'm just saying that because air pressure works the same in all directions; either it's high or it's low. A shaped foam insert can put high pressure on some part of the tire and little or no pressure on another. Opinions aside, foam inserts are very important. 
So... soft, hard, wide, narrow, .... where to begin? 
Wide versus narrow is probably the easiest matter. Wide is good. The foam has to be at least as wide as the tire. 10 to 20% wider seems to work well. 
Hardness: this is always a compromise. Firm is good because it supports the tire carcass better. (The tire doesn't fold all over itself in corners) Firm is bad because it takes away some 'bump absorbency'. (I don't think that's a word) 
So, for smooth tracks, it's easy: go very firm. For bumpy tracks you'll have to compromise. Too firm and the car will bounce all over the place, too soft and it will slide in the corners. 
I have found there's another very important factor: inner diameter of the foam. The foam has to fit very tightly over the rim, so it doesn't move around when driving. 
The most mysterious factor of all is height. Or in other words, the 'air gap' between the insert and the tire. There are basically 2 systems that work. System 1: firm, wide, but low insert. This way, the tire carcass is still well supported, but at the top (contact patch), there is an air gap. So the part of the tire where the contact patch is, can move around. This way, you make the carcass work, like an on-road tire. You get 'internal slip' in the rubber. It shouldn't surprise you that this works very well on ultra-hard, no-grip surfaces. The surface is practically asphalt. 
This system is a very good way to generate grip on slick, no-traction surfaces. But, on the downside, it can make the tire feel a little floppy, and inconsistent in throttle transitions. 
System 2: really big, firm insert. This is the exact opposite of system 1: you lock the carcass, and let the pins do all the work. The oversize insert stretches the carcass like an elastic band. As a result, the pins are really standing tall, and resist bending very well. Which is nice. Downside: doesn't absorb bumps very well. But overall, the traction generated with this system is very high and very, very consistent. 

If you read this, that means you made it through the first, most important, longest, and most gruesome chapter.
Congratulations!