Bigger/smaller tires

Question

On a track, bigger tires have more contact therefore more traction. However, it comes at a cost of more unsprung weight. Therefore, the 0-60 time is slower as tested by Car And Driver. This was tested in a 2.5L engine with about 200 horsepower.

So my question is, if say, a 5.0L engine with 400 horsepower, would the 0-60 still be slower with bigger tires? I have this question because traction control limits power to prevent slipping. Bigger tires have more points of contact therefore more traction.

Answer 1

Wider tires are only a benefit if you have been running out of traction. If you have been running out of traction, there are tires that offer an increased contact patch with the same size, like slick tires, or you can move to a wider tire. I always recommend to maintain the same circumference using a tire sizing app so that the odometer and speedometer still read correctly. Having the wrong circumference tire could also cause issues with some traction control systems.

Answer 2

Contact patch is always the same size no matter what size the tire is and can be calculated by dividing vehicle weight by tire pressure. Going to a taller tire or a wider tire changes the shape of the contact patch, but not the area. If the tire has the same composition but a different physical size, it will still have the same coefficient of friction. External forces to the tire will change that.

Unsprung weight makes it more difficult for the suspension to keep the tire equally planted during motion because of inertia. The less inertia, the easier it is to keep that tire planted when the tire moves in relation to the road surface. Even tiny road imperfections will cause rebound and partially unload a tire. An unloaded tire (such as one moving up after a bump) will have a lower coefficient of friction, and thus be able to handle less power transmitted though it to the road before slippage.

The combination of unsprung weight, the direction of the forces acting on the tire, and the suspension will change the coefficient of friction between the tire and the road surface. Changing the tire compound (such as going to a slick) will also have a large effect on the coefficient of friction.

Basically, the suspension is tuned for a specific unsprung weight. Changing the weight outside of that range without a corresponding suspension change will generally have negative consequences. If too heavy, the shock will have trouble retarding the motion of the unsprung weight; in severe instances the tire will leave the ground on a rebound.

General rule of thumb is that 1 pound in unsprung weight equals 2 pounds in vehicle weight. So if you add 10 pounds of larger tire, it is effectively the same as adding 20 pounds of weight to the vehicle. The reason for that is because you are accelerating that weight twice. First, it is part of the car, so it must accelerate with the vehicle. Second, it has to be accelerated on its own rotation at a speed equal to the vehicle speed.

The final thing about 'larger' is that 'larger' can mean several things. Of note is that if the tire gets larger in diameter, it effectually changes the gear ratio to a higher numerical ratio. An example would be your 3.73 rear axle effectively being a 3.55. Generally, this will further slow the 0 to 60 time because you lose mechanical advantage except in rare cases, such as a shift occurring just before 60. In those cases, it may be better time-wise to delay the shift until just after 60 so that the shift time is not included within the 0-60 time. That's a pretty rare case, and I only include it for completeness sake.