Why Vehicle Slip When It is Travelling on a Slope

7/30/2019 Why Vehicle Slip When It is Travelling on a Slope

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CONTENTSTopic page no.

WHY VEHICLE SLIP WHEN IT ISTRAVELLING ON A SLOPE 1BASIC TERM USED IN DETERMING

ROLLING RESISTANCE 2

AIR RESISTANCE 3

GRADE REISTANCE 4

TRACTION AND TRACTIVE EFFORT 5

BASIC EQUATION GOVERNING THESTABILITY OF THE VEHICLEON THE SLOPE 7

SOLVED EXAMPLE 9

REFRENCES 11


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WHY VEHICLE SLIP WHEN IT ISTRAVELLING ON A SLOPE

Actually, it's static friction. Even though the vehicle is moving, thepoint of contact the tire makes with the road is stationary. It isstatic friction which drives the vehicle.

If the wheels were spinning, then there would still be a frictional

force (kinetic in this case) driving the vehicle, but it will be lessbecause the coefficient of static friction is greater than thecoefficient of kinetic friction. This is why street racers and dragracers try not to spin their tires when they launch. This is whyautomakers have implemented anti-lock brakes.

If the car is keeping its speed, it has a constant acceleration

forwards.

If a car is keeping it speed, that is, maintaining a constantvelocity, then its acceleration is 0.

This acceleration causes that the tire is trying to friction with thesurface to maintain speed and this could cause that the tire slips.

And even more, if the driver decides to accelerate in a slope sothe car increases speed, the tire is increasing its work on thesurface making it more probably to slip. How can those things besimulated in a simplified formula? Simplified I mean considering acar is a weighted point and the tires are not four points of contact


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but one, and the tires have an infinite radius. Just some conditionthat can be easily programmed by a novice first course studentnot much experienced in programming, where the program can

decide that at this moment, the tires slip and traction control mustbe activated.Stability of a Vehicle on a Slope

BASIC TERM USED IN DETERMING

THE STABILITY OF VEHICLE ON ASLOPE

ROLLING RESISTANCE

Rolling resistance, sometimes called rolling friction or rolling drag,

is the resistance that occurs when a round object such asa ball, tire, or wheel rolls on a surface. It is caused mainly by thedeformation of the object, the deformation of the surface, andmovement below the surface. Additional contributing factorsinclude wheel diameter, forward speed load on wheel, surfaceadhesion, sliding, and relative micro-sliding between the surfacesof contact. It depends very much on the material of the wheel or

tire and the sort of ground. What might be termed "basic rollingresistance" is steady velocity and straight line motion on a levelsurface, but there also exists rolling resistance when accelerating,when on curves, and when on a grade.
http://en.wikipedia.org/wiki/Frictionhttp://en.wikipedia.org/wiki/Ballhttp://en.wikipedia.org/wiki/Tirehttp://en.wikipedia.org/wiki/Wheelhttp://en.wikipedia.org/wiki/Rolling_resistance#Depends_on_diameterhttp://en.wikipedia.org/wiki/Rolling_resistance#Depends_on_diameterhttp://en.wikipedia.org/wiki/Wheelhttp://en.wikipedia.org/wiki/Tirehttp://en.wikipedia.org/wiki/Ballhttp://en.wikipedia.org/wiki/Friction


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The "rolling resistance coefficient", is defined by the followingequation:

whereis the rolling resistance force (shown in figure 1),

is the dimensionless rolling resistance coefficient or coefficientof rolling friction (CRF), and

is the normal force, the weight of the vehicle on the wheel(including the weight of the wheel)

AIR RESISTANCE

This is the resistance offered by the air to the movement of thevehicle the air resistance has influence on the performance of thevehicle and on the the stability of the vehivle and depends on theshape and size

Air resistance Rr = Ka . A.V 2

A = are of the frontal prjoctionV= speed of the vehicleKa= coefficient of air resistance

GRADE REISTANCE

The component of the weight of the vehicle parallel to the gradienton the slope on which it moves is termed as grade resistance .Thus depending on the steepness of the grade of the grade is
http://en.wikipedia.org/wiki/Normal_forcehttp://en.wikipedia.org/wiki/Normal_force


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expressed as 1in 5 it means that for every 5 metres the vehiclemoves it is lifted up by 1 metres hence

Rg= W. sin

Theta is the inclination of the slope with respect to the horizontalW = the weight of the vehicle in kgf

TRACTION AND TRACTIVE EFFORT

The force available at the contact between the drive wheel tyresand road is known as tractive effort. The ability of the drivewheels to transmit this effort without slipping is known astraction. Hence usable tractive effort never exceeds traction. The

tractive effort relate to engine power as follows.


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When the tractive effort F>R, the total resistance on level road,

the surplus tractive effort is utilized for acceleration, hill climbingand draw-bar pull


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BASIC EQUATION GOVERNING THE STABILITYOF THE VEHICLE ON THE SLOPE

Let the vehicle rest on a slope of inclination 0 to the horizontal.This alters the distribution of the weight between the front andback axle and gives rise to reaction which can have com-ponents along the perpendicular to the inclined plane asshown in Fig. 31.8.Now, resolving forces parallel and perpendicular tothe slope respectively,

Fig. Force acting on a vehicle at rest on an inclined plane.
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It should be noted that when the vehicle is being driven up,the angle of overturning is, in general, smaller than in the presentcase and also the condition of instability becomes different from

those

ELLUSTRATIVE EXAMPLE

Example . A vehicle of total weight 49050 N is held at reston a slope of 10 . It has a wheel base of 2.25 m and itscentre of gravity is 1.0 m infront of the rear axle and 1.5 m abovethe ground level. Find(a) what are the normal reactions at the wheels ?(6) assuming that sliding does not occur first, what will bethe angle of slope so that the vehicle will overturn ?(c) assuming all the wheels are to be braked, what will bethe angle of the slope so that the vehicle will begin to slide if thecoefficient of adhesion between the tyre and the ground is 0.35 ?


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REFRENCES

P.K Mishra ,Automobile engineering

G.B.SNarang ,Automobile engineering

R.K Rajput Automobile engineering

william crouse, automobile engineering

Simon Iwnicki, ed. (2006). Handbook of railway

vehicle dynamics
http://books.google.com/books?id=Im0ZjhI3a-cC&printsec=frontcover#PPA256,M1http://books.google.com/books?id=Im0ZjhI3a-cC&printsec=frontcover#PPA256,M1http://books.google.com/books?id=Im0ZjhI3a-cC&printsec=frontcover#PPA256,M1http://books.google.com/books?id=Im0ZjhI3a-cC&printsec=frontcover#PPA256,M1


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Why Vehicle Slip When It is Travelling on a Slope

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