Choosing Sedan Tires 101 Part 2
by Eric Perez
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!
