How to choose foam tires for 4WD Touring Sedans (Part 2)
by Eric Perez
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Foam Tires
Intro:
The other main type of tire is the foam tire. Foam tires
unlike rubber ones don't use an insert, instead the entire tire is
actually made of a homogeneous foam-rubber compound that is molded in the
shape of a doughnut and glued to a plastic rim. Good foam tires are more
expensive than rubber slicks, but they already come assembled with the rim
and trued. They are basically ready-to-run. With rubber tires you need to
buy rims, plus inserts and do a good job assembling the tires. When you
add up the dollars, rubber is more expensive but will tend to last longer.
More on the cost of ownership: The main disadvantages of foam tires are that they tend to give less run-time than a rubber tire, and the rims are not as robust as those used for rubber tires. If you tap a wall or get hacked, the soft foam tires could be damaged or chunked (torn). Sometimes the tire can be fixed with thin CA, but usually lightly chunked tires become "practice" tires.
On hot asphalt, foam tires will give you the highest
level of traction available. Foam tires can be run more aggressively
because of better traction recovery characteristics when you spin the
tires. Rubber tires have a harder time regaining their grip after they
break loose. Another benefit of quality foam tires is that you don't need
to worry about unglued edges. I still recommend checking the tires
rim edge between rounds; but it's not as critical as with glued rubber
tires.
Shore Rating:
Foam tires are rated by the hardness of the foam. Most of the
time you will be working in the 30-60 Shore range. The manufacturer
measures the shore rating by using a tire durometer. This tool has a small
steel pin that is spring loaded and indexed to a circular gauge, very
similar to a tire pressure gauge. To measure the tire hardness you roll
the durometer on top of the tire; the compression force that the foam has
exerted on the steel pin can be read on the dial. Repeat it over several
locations on the tire and the average is the tire's shore value. There are
other features that must be considered when describing a foam tire; like
tire construction, composition and rubber/silicone content of a foam tire.
These will typically be similar if you stick to a particular tire
manufacturer. Foam tire construction can be one single piece of foam or it
can be multiple rings glued together to cover the rim surface. Multi-ring
tires are more expensive to manufacture but are less prone to suffer
damage from the occasional rub or run-in with another car.
Types
of Foam: 
Another important
characteristic of the tire is the silicone/rubber content of the foam. Hi
silicone/rubber content tires are sometimes classified as exotic foam or
long-wear foam. These as the name implies have longer wear life. They also
provide better traction than pure foam tires because the tire doesn't dry
out as fast. Yes, foam tires do have a shelf life; this starts as soon as
you open the package and expose the foam to air. Keep your foam tires off
the car and inside a plastic zip-lock bag or other air-tight container and
keep them away from sunlight… and don't feed them past midnight :)
Typically, you can guess the rubber content of the foam by the weight of
the tire. Pure foam tires are very light and spongy, whereas
silicone/rubber tires are a bit heavier and more elastic.
Tire
Wear:
Another thing to consider when
running foams is tire wear. The number one enemy of foam tires is the
abrasiveness of the track surface. Smooth tracks make tire durability a
non-issue, rough asphalt parking lots might force you to run harder
compounds to let the tires last through the end of the day!
Another element of tire wear is related to the suspension geometry itself. If the suspension is not adjusted correctly or if the material used to make the rim is too flexible, the foam tire will not wear evenly. This means that the tire diameter won't be the same across the contact patch. This can induce handling problems in the car by creating an apparent camber angle or stagger (tires of different diameters on car). Typically, the softer the tire, the faster it will wear, and the easier it is for the tire to deform and for the contact patch to get coned out. All of these characteristics lower the useful life of a foam tire.
If you have access to a tire truer, it will greatly increase the useful life of the tires. It will also allow you to reduce the tires diameter to reduce side wall flex and make the contact patch more consistent. For most, a tire truer is a very big investment in the neighborhood of $200-350 depending on quality and features. If you are going to be running foam tires for most of the season, then a tire truer will almost pay for itself by allowing you to run the tires for a longer time. Not only this, but traction will feel more consistent from run to run. If you happen to have a lot of time on your hands, you can purchase foam doughnuts and mount/true your own tires at half the cost of buying the tire RTR.
Foam
Tire Selection: 
Most racers run a front tire with a shore rating in the 60-40
range. This is more commonly referred to as tire dot color codes. The dot
code for 60-40 is blue, red and orange. For the rear 30-40 shore or pink,
purple, orange is more common. Tire selection is going to depend heavily
on surface roughness, temperature, track preparation and
chassis setup. Keep this in mind when you are going into a late afternoon
main. The track is going to be hotter than in the morning. It's not
uncommon for the cars that were hooked-up in the morning to be traction
rolling in the afternoon. If you are on the verge of rolling over when
track temps are not up to par, then change to harder tires or make
suspension adjustments to anticipate higher traction levels later in the
day.
Chassis Setup for Foam Tires:
We already know that foam tires have the highest possible level of traction than any other type of tire -Under the right conditions. What will all this extra traction mean in the setup department? The extra traction will be translated into extra body roll. If the chassis dampening is set too soft, then the car will roll-over.
Roll Over problems seem to be one the primary problems of beginning racers when they switch to foam tires. They forget to stiffen the shocks and the car spends more time on it's lid than on the tires! What can you do to "stiffen" the car if you don't have a box full of shock springs and a gallon of shock-oil at hand? The first thing is to do is to lower the ride height of the car. This will lower the center of gravity of the chassis, and make it harder for it to roll-over. The next thing to do, would be to decrease the droop on the suspension arms. This will limit suspension arm movement and help keep the car from swaying too much on the corners.
When I'm setting my car up for the large asphalt tracks I usually take my parking lot setup and set the camber to -2 deg all around. This is done to prevent the tires from coning out. Another setup change is done in the suspension department. I increase the shock fluid viscosity and go to the next stiffer springs front and back. I do this to take some sensitivity out of the chassis. If I still need less traction, then it's either a tire change or I stiffen the shocks some more or I dial out some traction with anti-roll bars to get the car tuned for the corners.
I like to keep tabs on tire wear with a caliper. I measure the inside and outside diameter of the tire. If the outside is wearing more than the inside then you need to increase the negative camber. If the inside of the tire is wearing faster then I reduce negative caster. On tracks with a lot of high speed right hand turns I also like to rotate the tires left to right to promote more even tire wear and extend their life.
Racing with Foam Tires:
I tend to do less tire swapping when I run foams, than when running rubber. This is because there are only a few tire choices available when you are running foams -that is, after you find a brand that yyou are happy with. Once you find your brand you will probably only need 3 to 5 different compounds. There is going to be a Hot/Warm/Cold base tire selection, but after you make your in initial choices, the rest is done with chassis setup.
It's not uncommon for electric racers to use traction additives on their foam tires. Electric race times are relatively short when compared to a 10-15 minute nitro main, This is one of the reasons why I don't recommend their use for racing conditions. Results are just not repeatable enough for me. Why introduce an additional sometimes uncontrollable variable into your traction formula?
Where do you start? Whatever the guys wining the races are using -that's what I start with! After the initial "roll-over" phase use your judgment as to the tires traction/wear/cost and take it from there. I like Super G's and TRC tires, there are a couple of new brands out there. If the tires aren't lasting you at least 4 Sunday races then you might have a very rough track or you are using very soft compounds. If you are clue-less as to what the compounds that you need then run RED (50) fronts tires and PURPLE (40) rear tires -that should be a good starting point.
A special note: If the track is very cold with outside air temps in the 40-50F then you may be better of using low-temp belted rubber tires. I've done this in the past with great results. While the "foam" guys where complaining about no traction I was clocking the laps with my rubber tires.
Once again -When running foam tires you will tend to have more traction that what you need and taking traction away with suspension settings is a lot easier than "creating" traction with suspension settings. This allows the beginner more room for error when setting up a car that runs on foam tires. The only drawback is that you need to stay away from the walls!