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JasonL220

Lateral Weight transfer

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Could someone help me understand lateral weight tranfer please? The two sides of the car have a different raduis cirle to follow which would result in a smaller force on the outside tires but how does this help with weight transfer? so the two forces are: left: F = mv^2 / r+x right: F = mv^2 / r [Edited by - JasonL220 on December 16, 2006 3:13:31 AM]

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Your pic is broken, so it's hard to see what you're asking. However, if by 'weight transfer' you mean the feeling of being pushed to the outside of the corner, it is to do with the centripetal acceleration a=v²/r – that force needs to be applied to the car somehow, and it tends to be through the outside tyres. (See next para for why!)

There is also a torque effect; because the centripetal force is applied to the car at road level, below its centre of mass, a torque is applied which results in more of the upward force (or downward force on the road) being applied to the tyres on the outer side of the car than the inner.

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i've drawn a diagram of what i think is going on but i'm confused as to how the centripedal acceleration is affecting the wieght on each tire as it is completely perpendicular to the forces on the tires.



thanks

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Think about it like this (force graph):

Front Wheels <-> Road, sideways force due to friction + inertia, slightly more than the back since they are spinning non-parallel to direction of inertia.

Rear Wheels <-> Road, sideways force due to friction + inertia.

All wheels <-> Chassis, force equal magnitude but opposite direction of the Wheel <-> Road force.

Since the application point and force isn't pointing towards or away from the Chassis' center of mass, the chassis tries to rotate (as if you skewered the center of the car with an axle and then pulled the bottom of the car toward the inside of the turn).

The resulting rotation causes everything attached to the chassis to move accordingly:

Chassis <-> Upward force on inner tires, added to the usual gravity pulling the tire down. (The shock absorbers are what directly transfer this back to the wheels)

Chassis <-> Downward force on outer tires (in addition to gravity, also via shock absorbers).


When the rotation overpowers gravity...

DO A BARREL ROLL!!

If you had a car where the Chassis' center of mass was exactly on the wheel axle, the tires wouldn't try to rotate the chassis at all. Your car would never roll (it would skid sideways first as the forces would overcome the tires' static friction).

Also, since there is a bit more force on the front wheels, the chassis does not rotate evenly from front to back. The front outer tire will get more downward force than any of the other tires if the car is not also accelerating.

[Edited by - Nypyren on December 16, 2006 4:39:30 AM]

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Draw a free body diagram, a big block to represent your cars front/rear profile.

Label your CG.
Apply upward forces RL, RR at the tires to counter the downward force from the CG.

Apply sideways forces at the point where the tires contact the ground. This is the force that would make you turn.

Notice the sideways forces do not act colinear with the CG (if you drew something realistic!). This means there is a moment around the CG of the cars body.

Now solve for RL, RR, so that the body remains static. This will tell you the weight on the tires (and thus the weight transfer).

Having done this excercise you could work out a formula using the acceleration of the CG of the car (and other geometric things, like track width) to find your weight transfer. It also appears in most vehicle dynamics books.

Also try to ignore the effects of BODY ROLL at this point, as this is a counter-in tuitive bit that many people misunderstand. There is a fictitious point called a roll center which is based on particular suspension geometry bits. The roll center height relative to CG, spring stiffness in roll, ARB's, etc will determine the body roll.
Note that weight transfer is independent of body roll! (unless it is so severe that it causes the CG to move alot). You CAN (and WILL) have weight transfer even if there is 0 body roll. (Go-Kart!). You CAN (and WILL) have weight transfer if you have ALOT of body roll. (offroad truck). Please do not get confused into thinking body roll and lateral weight transfer are directly related.

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