Weight transfer

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Chevrolet Camaro performing a wheelie during drag racing.
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A motorcyclist performing a stoppie.
A Toyota MR2 leaning to the outside of a turn. TargaTasmaniaRacer1.jpg
A Toyota MR2 leaning to the outside of a turn.

Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: [1]

Contents

In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration. [2] This would be more properly referred to as load transfer, [1] [3] and that is the expression used in the motorcycle industry, [4] [5] while weight transfer on motorcycles, to a lesser extent on automobiles, and cargo movement on either is due to a change in the CoM location relative to the wheels. This article uses this latter pair of definitions.

Load transfer

In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral). [3] This includes braking, and deceleration (which is an acceleration at a negative rate). [6] No motion of the center of mass relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. Load transfer is a crucial concept in understanding vehicle dynamics. The same is true in bikes, though only longitudinally. [4]

Cause

The major forces that accelerate a vehicle occur at the tires' contact patches. Since these forces are not directed through the vehicle's CoM, one or more moments are generated whose forces are the tires' traction forces at pavement level, the other one (equal but opposed) is the mass inertia located at the CoM and the moment arm is the distance from pavement surface to CoM. It is these moments that cause variation in the load distributed between the tires. Often this is interpreted by the casual observer as a pitching or rolling motion of the vehicles body. A perfectly rigid vehicle, without suspension that would not exhibit pitching or rolling of the body, still undergoes load transfer. However, the pitching and rolling of the body of a non-rigid vehicle adds some (small) weight transfer due to the (small) CoM horizontal displacement with respect to the wheel's axis suspension vertical travel and also due to deformation of the tires i.e. contact patch displacement relative to wheel.

Lowering the CoM towards the ground is one method of reducing load transfer. As a result load transfer is reduced in both the longitudinal and lateral directions. Another method of reducing load transfer is by increasing the wheel spacings. Increasing the vehicle's wheelbase (length) reduces longitudinal load transfer while increasing the vehicle's track (width) reduces lateral load transfer. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track.

One way to calculate the effect of load transfer, keeping in mind that this article uses "load transfer" to mean the phenomenon commonly referred to as "weight transfer" in the automotive world, is with the so-called "weight transfer equation":

or

where is the change in load borne by the front wheels, is the longitudinal acceleration, is the acceleration of gravity, is the center of mass height, is the wheelbase, is the total vehicle mass, and is the total vehicle weight. [7] [8]

Weight transfer involves the actual (relatively small) movement of the vehicle CoM relative to the wheel axes due to displacement of the chassis as the suspension complies, or of cargo or liquids within the vehicle, which results in a redistribution of the total vehicle load between the individual tires.

Center of mass

Weight transfer occurs as the vehicle's CoM shifts during automotive maneuvers. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. Front-back weight transfer is proportional to the change in the longitudinal location of the CoM to the vehicle's wheelbase, and side-to-side weight transfer (summed over front and rear) is proportional to the ratio of the change in the CoM's lateral location to the vehicle's track.

Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced.

By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. An outside observer might witness this as the vehicle visibly leans to the back, or squats. Conversely, under braking, weight transfer toward the front of the car can occur. Under hard braking it might be clearly visible even from inside the vehicle as the nose dives toward the ground (most of this will be due to load transfer). Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur.

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 CoM 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. Total available grip will drop by around 6% as a result of this load transfer. At the same time, the CoM 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%.

Traction

Load transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. 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 load 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 usually 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).

Rollover

If (lateral) load 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.

See also

Related Research Articles

Vehicle dynamics is the study of vehicle motion, e.g., how a vehicle's forward movement changes in response to driver inputs, propulsion system outputs, ambient conditions, air/surface/water conditions, etc. Vehicle dynamics is a part of engineering primarily based on classical mechanics. It may be applied for motorized vehicles, bicycles and motorcycles, aircraft, and watercraft.

<span class="mw-page-title-main">Unsprung mass</span> Portion of a vehicle not supported by its suspension system

The unsprung mass of a vehicle is the mass of the suspension, wheels or tracks, and other components directly connected to them. This contrasts with the sprung mass supported by the suspension, which includes the body and other components within or attached to it. Components of the unsprung mass include the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. Brakes that are mounted inboard are part of a vehicle's sprung mass.

<span class="mw-page-title-main">Car suspension</span> Suspension system for a vehicle

Suspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. Suspension systems must support both road holding/handling and ride quality, which are at odds with each other. The tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the road or ground forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.

<span class="mw-page-title-main">Slip angle</span> Term or maneuver in vehicle dynamics

In vehicle dynamics, slip angle or sideslip angle is the angle between the direction in which a wheel is pointing and the direction in which it is actually traveling. This slip angle results in a force, the cornering force, which is in the plane of the contact patch and perpendicular to the intersection of the contact patch and the midplane of the wheel. This cornering force increases approximately linearly for the first few degrees of slip angle, then increases non-linearly to a maximum before beginning to decrease.

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of the vehicle to changes in steering angle associated with changes in lateral acceleration. This sensitivity is defined for a level road for a given steady state operating condition by the Society of Automotive Engineers (SAE) in document J670 and by the International Organization for Standardization (ISO) in document 8855. Whether the vehicle is understeer or oversteer depends on the rate of change of the understeer angle. The Understeer Angle is the amount of additional steering that must be added in any given steady-state maneuver beyond the Ackermann steer angle. The Ackermann Steer Angle is the steer angle at which the vehicle would travel about a curve when there is no lateral acceleration required.

Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly during cornering, acceleration, and braking as well as on the vehicle's directional stability when moving in steady state condition.

<span class="mw-page-title-main">Highsider</span> Motorcycle calamity

A highsider or high-side is a type of motorcycle accident characterized by sudden and violent rotation of the bike around its longitudinal axis. This generally happens when the rear wheel loses traction, skids, and then suddenly regains traction, causing the rider to be thrown head-first from the side of the motorcycle or over the handlebars.

<span class="mw-page-title-main">Wheelbase</span> Distance between the centers of the front and rear wheels

In both road and rail vehicles, the wheelbase is the horizontal distance between the centers of the front and rear wheels. For road vehicles with more than two axles, the wheelbase is the distance between the steering (front) axle and the centerpoint of the driving axle group. In the case of a tri-axle truck, the wheelbase would be the distance between the steering axle and a point midway between the two rear axles.

<span class="mw-page-title-main">Aquaplaning</span> Loss of traction due to water buildup under tires

Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from when water on the surface of the roadway merely acts as a lubricant. Traction is diminished on wet pavement even when aquaplaning is not occurring.

<span class="mw-page-title-main">Anti-roll bar</span> Device that reduces the body roll of a vehicle

An anti-roll bar is an automobile suspension part that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It links opposite front or rear wheels to a torsion spring using short lever arms for anchors. This increases the suspension's roll stiffness—its resistance to roll in turns.

<span class="mw-page-title-main">Tilting three-wheeler</span> Tilting three-wheeled vehicle

A tilting three-wheeler, tilting trike, leaning trike, or even just tilter, is a three-wheeled vehicle and usually a narrow-track vehicle whose body and or wheels tilt in the direction of a turn. Such vehicles can corner without rolling over despite having a narrow axle track because they can balance some or all of the roll moment caused by centripetal acceleration with an opposite roll moment caused by gravity, as bicycles and motorcycles do. This also reduces the lateral acceleration experienced by the rider, which some find more comfortable than the alternative. The narrow profile can result in reduced aerodynamic drag and increased fuel efficiency. These types of vehicles have also been described as "man-wide vehicles" (MWV).

<span class="mw-page-title-main">Adhesion railway</span> Railway relying on adhesion to move trains

An adhesion railway relies on adhesion traction to move the train, and is the most widespread and common type of railway in the world. Adhesion traction is the friction between the drive wheels and the steel rail. Since the vast majority of railways are adhesion railways, the term adhesion railway is used only when it is necessary to distinguish adhesion railways from railways moved by other means, such as by a stationary engine pulling on a cable attached to the cars or by a pinion meshing with a rack.

Traction, traction force or tractive force is a force used to generate motion between a body and a tangential surface, through the use of either dry friction or shear force. It has important applications in vehicles, as in tractive effort.

<span class="mw-page-title-main">Beam axle</span> Automobile mechanism

A beam axle, rigid axle, or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically, they have also been used as front axles in four-wheel-drive vehicles. In most automobiles, beam axles have been replaced with front (IFS) and rear independent suspensions (IRS).

<span class="mw-page-title-main">Trail braking</span> Driving and motorcycle riding technique where the brakes are used

Trail braking is a driving and motorcycle riding technique where the brakes are used beyond the entrance to a turn (turn-in), and then gradually released. Depending on a number of factors, the driver fully releases brake pressure at any point between turn-in and the apex of the turn.

<span class="mw-page-title-main">Fishtailing</span> Vehicle handling problem

Fishtailing is a vehicle handling problem which occurs when the rear wheels lose traction, resulting in oversteer. This can be caused by low-friction surfaces. Rear-drive vehicles with sufficient power can induce this loss of traction on any surface, which is called power-oversteer.

<span class="mw-page-title-main">Bicycle and motorcycle dynamics</span> Science behind the motion of bicycles and motorcycles

Bicycle and motorcycle dynamics is the science of the motion of bicycles and motorcycles and their components, due to the forces acting on them. Dynamics falls under a branch of physics known as classical mechanics. Bike motions of interest include balancing, steering, braking, accelerating, suspension activation, and vibration. The study of these motions began in the late 19th century and continues today.

All Wheel Control (AWC) is the brand name of a four-wheel drive (4WD) system developed by Mitsubishi Motors. The system was first incorporated in the 2001 Lancer Evolution VII. Subsequent developments have led to S-AWC (Super All Wheel Control), developed specifically for the new 2007 Lancer Evolution. The system is referred by the company as its unique 4-wheel drive technology umbrella, cultivated through its motor sports activities and long history in rallying spanning almost half a century.

<span class="mw-page-title-main">Wheelie</span> Vehicle maneuver

A wheelie, or wheelstand, is a vehicle maneuver in vehicle acrobatics in which the front wheel or wheels come off the ground due to sufficient torque being applied to the rear wheel or wheels, or rider motion relative to the vehicle. Wheelies are usually associated with bicycles and motorcycles, but can be done with other vehicles such as cars, especially in drag racing and tractor pulling.

<span class="mw-page-title-main">DW-link</span>

The DW-link is a subset of the common four-bar system used widely in bicycle suspension. The four-bar system has been used on mountain bikes since the early days of suspension. Similar suspension systems to the DW-link have been used by Schwinn, Fisher and Karpiel. Currently a similar system is used by Giant and named "Maestro". DW-link gets its name from the designer and patent holder, mechanical engineer Dave Weagle. Currently the DW-link has been licensed to the following bicycle companies: PIVOT Cycles, Ibis, Independent Fabrication, Turner Suspension Bicycles, and Iron Horse Bicycles. The DW-link suspension design was used to win six Elite level UCI downhill World Championships from 2005 to 2007, the highest contested level of the sport. This winning streak made the dw-link the most successful linkage suspension platform in the history of the sport of downhill. Dave Weagle also developed the Split Pivot suspension and Delta System which are both used in cycling.

References

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