James, a softer foam tyre is different to a softer rubber tyre.
Usually, rubber tyres have the same sidewall flex irrespective of compound. Generally, whether TC or Off-Road, the insert remains the same even if the rubber changes. On a foam tyre, when you change the compound you change the sidewall flex.
When grip comes up you want the car to be soft enough to roll into the corner. That way, the CofG is lowered as you roll and the car will not try to roll over. A softer foam tyre has a more flexible sidewall so it has the same effect as fitting a softer spring allowing the car to roll more, allowing the CofG to be lower and resisting rollover.
A softer sidewall will also allow the tread to flex more. That allows the tyre to 'squirm' a bit on the track which makes it easier to drive - it is more forgiving between grip and slip. Going to a hard tyre will reverse all of this, making the car more inclined to roll and leaving a fine line between grip and slip that makes the car dart and harder to control.
The foam insert on a rubber tyre controls the flex and the sidewall 'give' so changing the compound and not the insert gives you a direct comparison between grip and tyre grade. More heat and more grip means a harder tyre works, but not when using foam tyres!
Foam tyres will give you grip all the way across the tread and down the sidewall! Superglue on the sidewalls up to the point where it meets the tread means that the car won't grip and try to roll over. I use it all the time and it works fine - the car is predictable and won't surprise you.
Additive would take a novel to explain. Here's the one-minute version...
On low-grip carpets, use SXT or Spider Blue. Always coat the rears full width, and start with half width (on the inside) for the fronts. Up that width if you need more steering.
On high-grip carpets Spider Green will work better. For a large event, start on SXT/Blue and when the car starts to feel a bit edgy, go to Green. Avoid Green on low-grip tracks as it will 'burn off' and leave you with not grip at the end of the race.
Start at 30 minutes soak time on the rears, and 15 minutes on the fronts. As the grip comes up you can back that off to 20 minutes rear and 10 minutes fronts. Dry the tyres off with a towel so that it is standing for one heat before you race. Never go on the track with wet tyres. The car should not slip and slide around at the beginning of a race, more drying time if it does! During the race a car will change its grip slightly as it licks up grip from the track. If it changes a lot then your additive regime is at fault - change the soak time and increase the standing time before you race, and think about changing the additive.
You have to experiment as the application of additive is a key part of the car's grip and handling. Find a regime that suits you and your car. There are no hard and fast rules for finding that last few tenths, so start here and see what works for you.
I am sure others can add to this with their experiences. Those might be different to this but where additive is concerned they are usually valid and worth trying. Note what works on what track with what tyres and you'll get the hang of it soon enough. HTH
