Weight transfer
From Wikipedia, the free encyclopedia
- For other uses, see Weight transfer (disambiguation).
In automobiles, weight transfer (often confused with load transfer) refers to the redistribution of weight supported by each tire during acceleration (both longitudinal and lateral). This includes braking, or deceleration (which can be viewed as acceleration at a negative rate). Weight transfer is a crucial concept in understanding vehicle dynamics.
Weight transfer occurs as the vehicle's center of gravity (CoG) shifts during automotive maneuvers. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoG. Front-back weight transfer is proportional to the ratio of the center of gravity height to the vehicle's wheelbase, and side to side weight transfer (summed over front and rear) is proportional to the ratio of the center of gravity height to the vehicle's track. Liquids such as fuel readily flow within their containers also causing changes in the vehicle's CoG. Furthermore, as fuel is consumed not only does the position of the CoG change, but the total weight of the vehicle is also reduced.
By way of example, when a car accelerates, a weight transfer toward the rear wheels is said to occur. An outside observer can witness this as the car visibly leans to the back, or "squats". Conversely, under braking, weight transfer toward the front of the car will occur. Under hard braking it is clearly visible even from inside the car as the nose "dives" toward the ground. Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn occurs.
Weight transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. For example, because of the forward weight transfer under braking, the front wheels do most of the braking. This bias to one pair of tires doing more `work' than the other pair results in a net loss of total available traction. The net loss can be attributed to the phenomenon known as tire load sensitivity.
An exception is during positive acceleration when the engine power is driving two or fewer wheels. In this situation where all the tires are not being utilized weight transfer can be advantageous. As such, the most powerful cars are almost never front wheel drive, as the acceleration itself causes the front wheels' traction to decrease. This is why sports cars always have either rear wheel drive or all wheel drive (and in the all wheel drive case, the power tends to be biased toward the rear wheels under normal conditions).
If (lateral) weight transfer reaches the tire loading on one end of a vehicle, the inside wheel on that end will lift, causing a change in handling characteristic. If it reaches half the weight of the vehicle it will start to roll over. Some large trucks will roll over before skidding, while passenger vehicles and small trucks usually roll over only when they leave the road. Fitting racing tires to a tall or narrow vehicle and then driving it hard may lead to rollover.
Weight transfer is generally of far less practical importance than load transfer, for cars and SUVs at least. For instance in a 0.9g turn, a car with a track of 1650 mm and a CG height of 550 mm will see a load transfer of 30% of the vehicle weight, that is the outer wheels will see 60% more load than before, and the inners 60% less. At the same time, the CG of the vehicle will typically move laterally and vertically, relative to the contact patch by no more than 30 mm, leading to a weight transfer of less than 2%.
[edit] See also
- Car handling
- Center of mass
- Drifting
- Electronic Stability Control
- Fishtailing
- Inboard brake
- Oversteer
- Steering
- Suspension (vehicle)
- Tire
- Understeer
- Unsprung weight
- Vehicle dynamics
- Weight transfer
