Why does reducing weight improve handling and braking in Forza?

The laws of physics say that handling and braking should be independent of weight.

Frictional force = coefficient of friction * ( mass * g + aero downforce )

Centripetal force = mass * v^2 / r

When cornering, these must be equal, so the mass terms cancel out in the absence of aero downforce and we have:

g * coefficient of friction = v^2 / r

i.e. the speed at which you can turn around a given radius depends solely on the coefficient of friction (and the gravitational constant, but that varies only a relatively small amount over the planet, and isn’t something that can be altered by tuning the car).

A similar argument applies to braking.

If you have aero downforce, then that increases the frictional force without increasing the centripetal force, and hence allows faster cornering and reduced stopping distance.

A related issue is that wider tyres shouldn’t increase grip, as the frictional force equation doesn’t depend on contact area. As wider tyres don’t change any of the terms in the equation, frictional force should remain constant. I’ve actually rationalised that one by pretending that when you make the tyres wider, the game is silently using a softer compound, i.e. the compound for “race tyres” in size 245 is a softer compound than the same tyre in 225, for example. As far as I’m aware, this has some basis in reality as explained here:

“Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. Softer tires have a larger coefficient of friction, therefore better traction.”

So I’ll leave the tyres, but I’m struggling to rationalise the effect of weight in the game.

Maybe we could see it as accounting for tyre wear even though the game doesn’t explicitly model tyre wear? A heavier vehicle will wear out the tyres faster due to the increased force at the tyre-road interface, so if you have a finite tyre life for the race, you’ll have to corner more slowly to make them last the race if the vehicle is heavier. So we could perhaps look at it that way, that reducing weight giving better handling and braking isn’t a direct effect, but rather the game enforces equal tyre wear over the race duration and therefore the heavier vehicle must be given inferior braking and cornering ability to achieve it?

You assume the only factor at play is friction between tyre and surface.

It’s exactly the same in real life, lightweight cars turn and stop quicker.

I suggest looking up weight transfer. The force felt by any tyre is not static, and cars tend to roll when turning.

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Inertia.

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I would say it’s just a simple matter of mass?

It takes far less force to stop a moving object with a mass of 1000Kg than it does to stop a moving object with a mass of 2000Kg travelling at the same speed.

So if you apply an equal amount of braking force to both objects, the object with the lesser amount of mass will stop first. That’s how I see it at least.

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It takes twice the force to decelerate 2000kg at the same rate as 1000kg, but you have twice the frictional force available from the tyres.

However, and this may be what you had in mind, I’d overlooked the force between pads and discs, I was only thinking of the force between tyres and the road. You would need double the force between pads and discs, and the vehicle being twice the mass would provide no additional force. It could of course have a braking system that provides that extra force, but there’s nothing in the game to suggest that it provides any corresponding brake upgrade to go with any weight variation, so that could be seen as the limiting factor and an explanation for that.

GreatFlea: yes, I have wondered if the frictional force equation is actually a simplification and not totally valid in all situations. One issue with really narrow bicycle tyres, for example, is you have to run them at a high pressure, and they become really skittish on a rough surface. If the whole tyre bounces into the air, obviously there’s no friction.

Tires grip better with more weight, but there are diminishing returns. If a car is changed from 0% lift to 100% of car weight in downforce (double the load on the tires at a given speed), you won’t see +100% roadholding. In the same, halving the weight will result in more than half the tire friction, with only the square of the previous centrifugal force… big difference.

Ti Hsien is right too. Yaw inertia in low-speed corners can be a significant factor. If you drive a long-bed pickup truck on ice, you’ll notice in 2WD it can struggle to make right turns from a stop, yet in 4WD the front tires can generate more yaw of the vehicle by driving the front end in the steered direction.

I read of a study done with formula spec cars, I think it was. They found that wider tires do in fact marginally increase contact patch area with identical pressures, and it’s well-known that wider tires are better suited to racing (to a point). Exactly why, is a tougher question.

Edit: wider tires=better because wider and shorter contact patch and cornering grip wins more races than braking and accelerating grip. Also contact patch stability, and wear area. I’ll stop barging into threads with unrelated information one day. :rolleyes:

The force/friction relationship of tires is not linear. Doubled load doesn’t double friction. Some of it is contact-patch deflection. The rest could be molecular shear; absolute limits of rubber to hold itself together.

If tires weren’t mysterious, sims would have perfect tire physics. Even the best are based on sampled data, further tuned in testing to approximation of reality, or taste.

I just did some more searching, and found this thread:

Multiple people there say that tyres don’t obey the normal friction force equation. This

describes it as an approximate model, so I think it must just be substantially invalid for tyres.

Agree, a tyre has a peak amount of grip at a specific load depending on temps, pressures etc.

The load is applied via weight transfer, a lighter car can transfer a greater percentage load and so can turn or brake harder at the peak tyre load.

Preface: if this post hurts to read, it hurt to re-read too, sorry. Running low on sleep right now.

Normal friction is a surface-area thing. When a tire is on pavement, it molds like a sponge to the microscopic contours of the surface… the rougher the pavement, to a point, the more surface area there is, and softer tires can better mold themselves on a molecular level to cover more of it. A tire on metal or glass has almost no grip, because the metal has a fraction of the effective surface area to the tire.

Anyway, weight effects how well the rubber can molecularly conform to the texture of the surface, except, rubber is full of spring and damper forces (molecurlarly) as well as the air pressure as a spring, and sidewall construction… the variables are so far beyond what we can measure (if anyone has, it’s more guarded intellectual property than the basic samped data sets sims like Forza use), simulating it accurately or even comparing tires and surfaces is impossible beyond objective lap-times and driver feedback. Some sims may opt to prioritize driver feedback and real-world comparisons, behind the wheel of the sim, to intuitively set the basic friction/shift/aligning moment curves.

Many sims in the past have used sampled data from the tire itself, independent of the reactions with the vehicle in totality. This method causes a number of unresolvable conflicts between the forces the simulated tires generate and the nature of the car’s inertia/moments/suspension. I found this video interesting. this link will start at the explanation of a hub-centric model, dubbed “The VMS system”, that I believe it altogether more effective than models like Pacejka. I don’t know what Forza uses or how it works, only that it’s tuned for fake difficulty this gen more so than realism.

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There are so many factors that contribute to a cars handling with friction being just one of them. Even with that you have to factor in temperature, tire tread, tire compound, psi, tyre wear etc. Weight is one of those factors due to centre of mass and centre of gravity.

That’s all very well for the subjective feedback that a driver experiences, but for what the game terms “handling” it’s just talking about the g force at a given speed. That g force is the centripetal force, and that force can only be provided by the frictional force between the tyres and the road.

IF the formula
frictional force = normal force * coefficient of friction
were correct, then that g force would be independent of weight. The issue, it seems, is that the formula is only an approximation, and is substantially incorrect for the case of tyres against a road surface.

With all due respect, but it’s a high school formula (at least I’ve learned it in high school) that provides maximum theoretical cornering speed under perfect conditions. It also does not seem to take an elastic object into account. Road driving isn’t exactly perfect, as demonstrated by fellow posters, though I believe the roads in Forza Horizon to be a gross simplification of actual roads.

Light cars in this game do have trouble putting power to the ground, but softening the suspension can usually mitigate this. Engine acceleration is a longitudinal load, though. In a braking situation I imagine it’s easier to lock up the tires in a lighter car but this doesn’t equate to being more difficult to stop. If there’s grip, a lighter car will stop quicker with the same force, no way around it.

The game however does simulate results in accordance with what this formula dictates. You can have very different behavior with the same handling stat. Let’s say you run a Honda S2000 with the widebody kit on Street tires and adjustable aero, with a handling rating of 7.5 or so. This car, despite its high score in handling, will have much trouble navigating slow corners, where the downforce is limited, because its mechanical grip is bad next to its aero grip.

Meanwhile, if you achieve the same 7.5 rating with Race tires and no aero, you’ll have a car that handles exceptionally well on tight corners but might not be very stable at high speeds due to the balance now favoring mechanical grip. Race tires provide a higher coefficient of friction, which lets cornering speeds have a higher limit, when speaking strictly of mechanical grip. At high speeds, however, lift might decrease the load on the surface, decreasing mechanical grip.

Simplifying the argument, take any car, push it to a value in handling with weight reduction, then do the same with better compound. It won’t handle the same in both situations and the lighter car will have more difficulty taking 90 degree corners at speed. However, even though the formula isn’t dependant on mass, the lowered inertia provided by weight reduction will impact other areas. It makes the car react quicker, for one, which helps boost handling since you have more control over the car. The heavy car might also not be physically able to reach the maximum speed determined by the formula due to other factors.

The handling stat is in no way a true reflection of car behavior, it’s just a number and it’s best treated as such. Even the theoretical results in the tuning menu should be taken with a grain of salt. Anyway, the biggest gains in the handling stat are had with better compound, followed by width with a far smaller increase, and then weight and chassis stiffening, the latter of which helps modulate load in a more even way, which helps making the car fulfill its potential.

Aero (full areo) seems OP in FH4. I use it on practically everything. The only builds I don’t, I expect to let go of the idea of taking any place higher than 3rd on average in PVP, and another place for every couple of Shelby Daytonas.

I use the handling stat and roadholding expectations to build cars. Once they’re built, their competitiveness is determined by many more factors like CG height, track width, suspension travel…

This, and, tires do abide by normal friction… on the molecular level, as the rubber conforms to every microscopic texture of the road surface it can. I think the shear force you’re talking about is also normal friction, within the rubber itself? Could we ever measure that?.. Maybe.

This is one of the reasons it’s obvious that FH4 and FM7 have unreasonably amplified load-grip curves. Heavy trail-braking without locking the inside front tire is almost impossible in FH4. Heavy trail-braking in FM7 is almost impossible period. Derp.

Aero is OP in Forza, period. It’s like that in all Forzas. In fact, FH4 is the first Forza I’ve played where aero has a bigger impact on PI, which has led to cars needing aero just to reach the top of their starting class. Aero gives a huge boost to braking and is the only thing able to keep cars on the road at high speeds. The Renault F1 car in FM7 is the greatest example of how wrongly simulated aero is in Forza.

If I had some time with Turn 10’s team, the first thing I’d ask would be how they plan to solve these inconsistencies that hurt the physics as well as the PI system. Forza is similar to Pokémon in this regard, but I’ll gladly sacrifice backwards compatibility for improved physics and rationalized upgrade system, especially when the tunes seldom carry over to the newer games without requiring adjustments.

You cite some laws of friction but incorrectly assume that those laws are the only laws of physics that affect handling and braking. More mass means more momentum when cornering and braking, this applies to brakes, suspension, roll, yaw center of gravity etc. Friction coefficiant of the tires are important but more mass significantly hurts handling and braking. Even without citing laws of physics this is pretty much common sense in everyday life, lighter is quicker.

No they don’t !
You’re just wrong. VERY wrong.

Have you ever driven a car in real life ?

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Okie! Physics!

The hypothesis of frictional force is correct. The frictional force is the downward force (mass, area and force of gravity), so the heavier vehicle on same compound and tire should have same traction for acceleration and decelleration.

Accel is obvious, more weight is more power needed. Problem answered plus the stuff talked in a bit here.

Braking. Okie. Like with acceleration, there is more energy the heavier you are. So we need to dissapte that energy. This is via heat through friction. There is a finite level of energy a rotor is capable of dispersing. Heavier is more heat and poorer performance there. But this is about tires and assuming ceramic brakes which can lock up racing slicks on the dry.

There are two types of friction forces. You have the micro that is your coefficient which is your formula, then you have your shear force.

The shear force is the catch. This is when your tires lock and are now leaving black streaks into that corner wall. Heavier car means more frictional force than one with less weight and same contact patch so will shear sooner.

This doesnt even get into additional science of contact patch, inertia and deformation.

Seriously?

Go outside and put 800lbs worth of people/stuff in your car and see how fast you stop from 60-0. Now take all that out and try again.

See how it stopped faster?

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Actually, that formula is spot on. But it just isnt the only variable. It is true up to a point. For example, my car cannot accelerate like my motorcycle due to engine power and weight shift on a fwd. But!!! My car will stop way faster than my motorcycles and way better tight and at speed maneuvering.

Contact area, tire pressure, tread design, etc…