I disagree with SLEENAD...
For a rubber tyre to work it requires an insert. To get it to work with an insert requires knowledge of the insert type, the air gap required and the proper method of mounting.
For a rubber tyre to control a car with its single homogenous contact patch requires a suspension system that places it in steering, acceleration, braking and cornering. A GT12 has arguably the least adequate suspension to achieve that, resulting in a car that would grip, grip, grip and then "bang" would let go at one end of the other without warning. That situation is made worse by its narrow track, short wheelbase and low weight.
A foam tyre has a discontinuous contact patch so it lets go progressively and is easy to control with rudimentary suspension. The transition between grip and slip is easy for a driver to detect and control, so making the car easy to drive and forgiving for the average driver. That's just what you need with a small, light car.
The grip you can get from a foam tyre is much more than from a rubber tyre. That allows the cars to be driven with a much wider margin of error and still stay on the track. Rubber tyres do not offer that level of grip and so are harder to drive.
Rubber tyre performance gets worse as the tyre wears. IME, new ones are not always the best, scrubbed ones work well for a few runs and then performance goes off. If new tyres did work best, then drivers would need new rubber tyres more often. Foam tyres in GT12 wear like iron and never drop in grip from the packet to the bin. They get changed because there is an accident and they get chunked beyond repair, or they simply get so small you can't keep the chassis off the deck any more! Foam tyres last longer than rubber with no performance drop-off.
A rubber tyre and its insert has at least twice the rotating weight of a foam tyre. On 1S, that would make them slow to accelerate and hard to slow down. On small indoor tracks that's not a good thing! It makes the car a bit driver-unfriendly to say the least. Combine a lethargic car with one that lets go and needs a lot of slowing up so it can regain grip and it's a bit of a handful!
If a rubber tyre worked that well with simple, cheap cars then 12th and the Oval classes would have had it decades ago. That fact that it hasn't is because it has been tried innumerable times and every time it is abandoned because the cars are so hard to drive. With a TC that is twice the weight, has full independent suspension and shed loads of power, or an Off-Road car needing a tread to get through the dirt to a solid surface for grip, rubber is the best solution. Both classes have raced foams in the past with marginal success.
Rotating weight, grip, very progressive transition from slip to grip and no deterioration in performance as the tyre wears are just four good reasons why foams are the low-cost choice for GT12, and rubber is a distant second best.
The tyre truer? Red herring. Every club and meeting I have ever been to in 35 years of 12th and latterly GT12 racing has had someone with a truer very willing to true anyone's tyres. And these days, the Contact tyres come in sizes that can be used straight out of the packet. And they cost £6 a pair mounted, trued and glued - a rubber tyre couldn't come close to that.
Foam tyres for GT12 - yeah!
HTH
