How does weight affect handling?
Which car handles/brakes better? A light car or a heavy car?
What does rim size affect? Should I change it?
Thanks! 
Better speed and acceleration, but can have less traction if you don’t tune the gear ratios correctly.
Wont a lighter car brake better?
Short answer: More weight means your car will have more inertia, and will take longer to change direction. This means that a heavier car generally takes longer to slow down when braking and for its direction to change once you start turning. A light car generally handles better than a heavy car.
Rim size is mostly aesthetic, but the game does model tire deformation during cornering, so a larger rim (and thus a smaller tire) won’t deform as much and improve handling.
Long answer: How a car handles is affected by a lot of different things. Weight is only one part of the equation, tires, aerodynamics, weight balance and suspension are more important to a good handling car than just making it light.
The force that changes your car’s direction is applied by the tires. If you have poor tires (or are driving on dirt), your car is more likely to drift/slide than follow where the front wheels are pointing. Poor grip also affect acceleration, since the tires convert the force from the engine into forward motion, and if you don’t have enough grip for the amount of power your car has the tires will spin out instead of accelerating your car. Same thing applies for braking, if your tires don’t have enough grip, the wheels will lock up and stop spinning (at least if you have ABS off), but due to inertia the car will keep sliding.
Aerodynamic downforce pushes the car down onto the road to improve the grip the tires can generate without adding extra weight to the car, however if you lose your speed then you also lose the downforce with it.
Weight balance indicates where the center of mass for the car is located. 50% means that that 50% of the weight is over the front tires, which is the ideal situation. When changing direction, the car generally wants to rotate around its center of mass. The farther this is from the center of the vehicle, the more difficult the car will be to control when cornering. The engine is generally the heaviest component in a vehicle, and since most cars have the engine in the front that means they usually have more weight over the front tires than the rear. This is why most supercars have the engine in the middle since it makes it easier to get a perfect 50-50 weight distribution.
Suspension controls how the tires make contact with the road and how the mass is carried around a corner. Softer suspension means better grip since the tires can move more freely to stay in contact with the road, but it also causes the center of mass to shift more around corners as the body rolls to the outside. This impairs handling in a similar way to poor weight distribution.
Take a look at some of the tuning guides in the forums to get a better idea of how all these things affect how a car performs.
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Thanks!
54% is the best balanced car all around imo. While in motion it will be 50 50 and when breaking it will be around 57% to aid in quicker breaking with a tuned break bias. Wieght affects the grip of a car. A heavier car is going to have more grip going straight but less grip through corners. A lighter car will have less grip from a dead stop than a heavier car but more grip through a turn due to inertia. A lighter car will have better acceleration due to power to wieght ratio to a point to where you have too much power for your light wieght car it’s tires can’t put enough force down to keep contact causing burn outs. So 1500 hp in a 1500 pound car might not have as much grip as a 1500 hp 2300 pound car. It all depends on tire compound, size, spring travel, wieght and wieght balance, areo dynamics, and alignment. Does your car come together to get the most out of your tires and hold a decent angle with speed with out having to sacrifice response and stability. There is no one best tuning set up for every situation, all though awd at 54 front wieght distribution around 2700 pounds will handle well in all classes with good speed if you can find a car capable of that set up.
All comments above make perfect sense in real life and some comments appear dashed on experience from in game. I generally try and keep weight down for all the reasons mentioned above. I have never tested the same car with heavier vs lighter build assuming you can keep the power to weight ratio the same. Would be interested if anyone has done this direct comparison.
If not I might do it myself when I get time and see if there is any noticeable difference in feel and lap times.
Generally the game favours higher power to weight ratios (assuming awd) and handling requirements are more track specific. The lateral ‘g’ stats in the build section also appear less accurate a measure of grip / handling than the handling bar. Stats regarding 0-60 / braking distance in tuning section can also be misleading. Best way to test acceleration change is against your own ghost in rivals assuming you have a track that the car in question is your fastest attempt.
The physics of the game are excellent so removing weight does decrease the energy required to change speed or direction. However, HP is required to overcome wind resistance. So, for a higher top speed in a situation that doesn’t require as much changing speed or direction, it’s better to keep the weight and add HP to the motor.
In general, I remove weight before adding to the motor, however, for a few special tunes, I’ve kept the weight and focused on adding HP if I need more top speed.
Cheers
Let’s take the Lancia 037 as an example:
442HP/319LB-FT 1995cc I4
2,386lb, 48% Front (Mid-Engine, RWD)
Without the weight reduction, with the stock block and everything else maxed out, you get insane swingaround on every corner. With the weight reduction, however, you start evening out the power-to-weight ratio. This causes excessive swingaround in RWD cars. (The reason why everyone says 700HP to 2400lb is a decent ratio for RWD drift tunes.)
If you drive cars from stock but slowly upgrade them, specifically weight reduction, you’ll see how different each car acts at different weights.