Weight transfer

Weight transfer

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, causing changes in the vehicle's CoG. 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 30% more load than before, and the inners 30% less. Total available grip will drop by around 3% as a result of this load transfer. 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%, and a corresponding reduction in grip of 0.01%.

Other Valid Interpretations

The incorrect use of the term "weight" is often misinterpreted. It can be argued that the term weight above is not really accurate. But this is a matter of semantics and using the terms for whatever force "load" is being referred to.

Weight is equal to Mass times Acceleration. If the Mass is at rest on the Earth's surface it is subject to 1G of acceleration. Subject that mass to a cornering force and you add additional force vectors and thus change the total acceleration vector. It is true the CG or Center of Mass moves slightly due to shifting liquids and chassis roll. This effect is usually very small. But the "weight transfer" can be equated to the "load transfer" due to the acceleration forces. The individual wheel weight transfers from one wheel to another during cornering are not simply a sum of the vector forces on the CM of the vehicle.

There are three major means of transferring weight "load" in a conventional vehicle. Unsprung weight transfer, Sprung weight transfer and Jacking forces due to suspension members.

The roll axis is the imaginary line through the vehicle from front to rear that the vehicle rotates about during cornering. The tire contact patches apply force vectors through this axis.

One force vector at each wheel is due to the unsprung mass being subjected to the total acceleration vector. It should be noted that unsprung weight still transfers from one tire contact patch to another in vehicles with independent suspension. The most foolproof method of visualizing this is to realize if the tire contact patch on one side of the vehicle is preventing the tire on the other side from falling over, then the weight does "transfer" to the other contact patch.

The second force vector is due to the "vertical component" of the sprung mass force vector pushing on the roll axis. The tire contact patch experiences a change in load due to this vertical force vector or Jacking force.

Lastly, is the sprung weight transfer. This transfer is a result of the sprung mass rolling about the roll axis. The CG is usually above the roll axis and is coupled or resisted by the springs or "wheel rates" as seen by each tire contact patch. That is to say as the car is twisted around the roll axis by the lever arm from the roll axis to the CG and the springs resist this roll. Due to the wheel rates being higher in the front in most cases, this roll resistance is coupled more at one end of the vehicle than the other. Thus the sprung weight transfers higher loads to the tire contact patch at the end of the vehicle with the stiffer wheel rates. Front roll resistance often exceeds 75% of the total roll resistance. This is called the roll couple percentage.

There are also similar sets of forces applied to the pitch axis during acceleration or braking resulting in front to rear weight "load" transfers at each tire contact patch.

One other means of applying force to the tire contact patches is a result of the moments of inertia. Measuring the moments of inertia in a vehicle is quite difficult, but is routinely done by many organisations. A lower moment of inertia means the vehicle's mass is concentrated more closely around the center of mass. A higher moment is the opposite. Imagine a car sitting on a rotating platform positioned so its CG is directly above the rotational axis. Now push sideways on the front tires and the car begins to rotate on the platform. The forces are only applied at the end of the vehicle that is doing the steering.

See also

* Anti-roll bar
* Braking

External links

* [http://www.safercar.gov/Rollover/pages/RatSysVCompare.htm DOT rollover ratings by vehicle type] *

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