DIY-racetuning.net The ultimate tuning resource for Forza Motorsport 3

TIRE TUNING FOR FORZA MOTORSPORT

TIRE SELECTION

Basically, when choosing a tire, wider means more grip, but as far as which ones to go with, you need to experiment with different set-ups. Considering the points limitations we're working with, regardless of the class we're running our car in, going as wide as possible is only part of the equation that needs to be considered. If you wonder why a C6 Corvette has 295mm tires on the back, while you'd never put that much rubber on the rear wheels of a Miata, it's because we need to match tires with available wheel torque.

Not enough tire relative to available power is going to result in wheelspin; it'll be most noticable with the inability to put power dwon on turn exit, but too much tire with a lack of available power is actually worse because it kills your car's acceleration since there's too much friction between the tire and road and not enough hp at the wheels to utilize all that grip.

The best news about our ability to mix and match front-rear tire sizes is we're not completely reliant upon our Anti-roll bars and suspension to dial out oversteer and understeer.

If we have a car that is understeering and pushing into the corners, and we want better turn in, we can go with a wider front tire. If we have a car that is breaking loose on turn exit, we can go with a wider rear tire. In the case of AWD, we pretty much want a squared tire set-up which is the same size tire on all four corners. AWD cars tend to push, so a wide front tire will hook up better and get the nose of the car pointing into the corner.

We should note the relationship between Anti-roll bars and tires:

Anti-roll bars regulate how much lateral load is placed on the outside tires when we corner. Tire width and tire compound determine how our lateral loads are utilized on the road surface.

Springs transfer loads laterally as well, but to a much lesser extent. ARBsare our primary source of regulating load transfer because they directly connect the lower control arm on the left side to the lower conrtol arm on the right side without any damping.

The stiffer the Anti-roll bar, the more load is transferred to the outside, laden wheel when we're cornering. Lateral load transfer is bad, but it's a necessary evil because our ARBs main function is to reduce body roll. The more load we transfer laterally, the more capable a tire we need to absorb that load and use it to generate cornering force, be it a wider tire or a more aggressive compound.

Let's say we have chosen a tire and as we're testing and tuning as we set the stiffness on our ARBs. We notice the car has corner entry understeer. This means:

1- We're overloading the outside front tire beyond its tractive capacity.
2- We don't have enough load on the inside front wheel, (body roll) so our wheel pair is not giving us enough overall grip.

Let's assume the problem is laod transfer and not body roll:

One fix would be to soften the front ARB to reduce the lateral load transfer, keeping the load more evenly distributed between the front wheels. But a vialble choice would be to go with a wider tire if I could work it into the build. A tire change would be a good choice because it'll increase our track width slightly, and give us more overall grip. Keep in mind however, that more tire means more cornering force, so we need to monitor our tire compliance on the inside, front wheel, which runs the risk of being lifted off the ground during hard cornering. We can fix this by adjusting our spring/shock combo, especially the shocks by reducing our rebound on that wheel pair.

If we have oversteer on corner exit, we're probably overloading the rear outside wheel. A wider rear tire will give us additional grip, again, if we can tweak our build to fit a better tire within the allotted P.I. Softening the rear ARB and/or rear suspension will help here, to try and even out the loading of the rear wheels. But the stagger that we choose with our front/rear tires is a great tool we can use to balance our car's handling.

As stated in the tuning Basics section. When we have a problem with one end of the car, we should work on that end without touching the opposite end of the car as a fix. If we need a wider tire in the rear, we shouldn't compensate (if the PI will allow for it) by increasing rear tire size and decreasing the front. If we do this, we're reducing front tractive capacity and we run the risk of the car getting pushy, which defeats the purpose.

RUBBER VERSUS WEIGHT REDUCTION - WHICH ONE?

Quite often in Forza Motorsport 4 you're faced with this dilemma; your two choices are:

1- Reduce vehicle weight and run less tire.

2- Keep the additional weight and mountstickier tires.

Weight reduction with less tire = better acceleration and improved handling. Less weight helps handling by having a lighter car, and a lighter car obvoiusly accelerates better. A heavier car doesn't accelerate as well, but a heavier car on race tires will handle much better than a lighter car on sport tires.

It's all track dependant, and class dependant. Lower class cars don't really need race tires because you'll have more grip than available whp to get them moving. Higher class cars, IMO need race tires. I'd say dealing with B-Class I'm on the fence sometimes, but A-Class and higher, race tires are the way to go.

As far as the situationality of it all, when dealing with high hp cars with big engines that make a lot of tq (like Corvettes) the amount of handling points you gain by keeping extra weight and big-grippy tires will outweigh the acceleration loss from not sheeding a few extra pounds. Powerful cars with high tq engines are better at overcoming carrying extra weight.

If you're dealing with a short track like Tsukuba and a light car like a MX-5 superlight, acceleration is the most important thing. If the situation allows for less tire to gain the advantage of being able to cut weight, you might be faster with less tire.

The only way to know for sure is to try different builds and test. In any scenario, your lap times will reveal the best course of action.