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It has been suggested that this article or section be merged with Weight transfer . (Discuss) |
In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral).[1] This includes braking, and deceleration (which is an acceleration at a negative rate).[2] No motion of the centre of gravity (CoG) 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.[3]
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Weight transfer
Load transfer is often misguidedly referred to as weight transfer due to their close relationship.[4][5] Load transfer is change in load due to acceleration, in which mass inertia causes a torque to appear whose forces are the tyres' traction forces at road level, and the equal but opposed force of the mass inertia located at the CoG where the arm is the distance from the road surface to the CoG. The difference is that weight transfer involves the actual (small) movement of the vehicle CoG relative to the wheel axes due to displacement of liquids within the vehicle, whereas load transfer is conceptual. All result in a redistribution of the total vehicle load between the individual tires.
Cause
The major forces that accelerate a vehicle occur at the tires' contact patches. Since these forces are not directed through the vehicle's CoG, one or more moments are generated whose forces are the tyres traction forces at pavement level, the other one (equal but opposed)is the mass inertia located at (CoG) and the arm is the distance from pavement surface to CoG. 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. Although it is interesting to note that a perfectly rigid vehicle without suspension that would not exhibit pitching or rolling of the body would still undergo load transfer. However, the pitching and rolling of the body adds some (small) weight transfer due to the (small) CoG horizontal displacement with respect to the wheels axis suspension vertical travel and also due to deformation of the tyres i.e contact patch displacement relative to wheel.
Lowering the CoG 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 wheel base (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 wheel base and track.
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
References
- ^ Pacejka, Hans B. (2006). Tyre and vehicle dynamics (2nd ed.). SAE International. pp. 14–15. ISBN 978-0768017021. http://books.google.com/books?id=wHlkbBnu9FEC&pg=PP1&dq=Tyre+and+vehicle+dynamics. Retrieved 2009-03-31.
- ^ Jazar, Reza N. (2008). Vehicle Dynamics. Springer. pp. 72. ISBN 978-0-378-74243-4. http://books.google.com/books?id=Pvsv78xj7UIC&pg=PP1&dq=Vehicle+Dynamics+Reza#PPA73,M1. Retrieved 2009-03-31.
- ^ Cossalter, Vittore (2006). Motorcycle Dynamics (Second Edition ed.). Lulu.com. pp. 84–85. ISBN 978-1-4303-0861-4.
- ^ Foale, Tony (2006). Motorcycle Handling and Chassis Design (Second Edition ed.). Tony Foale Designs. pp. 9–1. ISBN 978-84-933286-3-4.
- ^ Cocco, Gaetano (2005). Motorcycle Design and Technology. Motorbooks. pp. 40–46. ISBN 978-0-7603-1990-1.
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