Drivetrain

Snow and ice driving, as you know, requires that your wheels turn without slipping. The group of components that deliver power to the wheels is called the. drivetrain. This is how the wheels of your vehicle get power. 

Drivetrain components vary by manufacturer design and type of vehicle. Four-wheel drive vehicle components often include a clutch, gearbox, transfer case, transmission brake, propeller shafts, differentials, driveshaft, and axles.

Understanding the drivetrain and why power to your wheels is so important requires a quick physics refresher

Torque, Traction an Wheel Slip

Torque, traction and wheel slip work together, and their joint effect impacts what your wheels do on snow and ice. Torque is the twisting force, produced by your engine, transferred and multiplied through gears in a car’s transmission and differential. Traction is the amount of force the wheels can apply against the ground. Traction is in turn impacted by additional factors like weight, coefficient of friction and wheel slip.

Most race clinics and professional driving classes involve an hour or more on a skid pad. A skid pad is a large round or squarish area. Surfaces are smooth. Occasionally, the surface may be gravel (depending on the focus of the training). The course instructor will have the students make aggressive moves in their vehicle. They generally have the student turn the wheel ninety degrees and gun the engine. This turning motion applies lateral force to the tires beyond the tires available tractions and the wheels slide causing the vehicle to slide or spin out. They’ll also have the student practice fast stops starts. The student is essentially peeling out and sliding wheels before stopping. From a physics standpoint, longitudinal force beyond the available traction to the tire is causing the car to slip and slide. The instructor is giving the student a feel for the how aggressive moves impact vehicle response.

Front wheel drive cars, all wheel drive cars, and four-wheel drive cars respond differently under aggressive conditions.

Weight

Weight helps traction. Heavier vehicles apply more weight to wheels and create more traction. However, weight shifts to different area of the wheel as the vehicle moves. In a turn, weight shifts to the outside of a car. When a car stops, weight shift to the front of the wheel.

Coefficient of Friction

This is the friction between two forces. For our purposes, the tires and the road. The weight from the vehicle presses on the tires, pressing the tires against the road. Flat tires on dry, flat, concrete roads have very high coefficient of friction. This allows race cars to move at very high speeds. Put those same tires on ice and things change very quickly. Put studs into tires and the coefficient of friction on ice increases.

Wheel Slip

This refers to the contact a wheel makes with the road. If the wheel and road are not slipping, the contact is called static. When the wheel is slipping, relative to the road, the contact is called dynamic. Slipping occurs when force applied to a tire exceeds the available traction. The skid pad example above talked about the two forces: longitudinal applied in starting and stopping; and laterally, applied when a vehicle turns. Drivetrain (components responsible for delivering power to wheels) design determines how and why wheels slip and usually offers countermeasures to minimize negative effects of slip, or enhance the positive effects.