MadSci Network: Engineering

Re: Why are curves on roads banked?

Date: Fri May 21 22:17:21 1999
Posted By: Jim Stana, , Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 927123499.Eg

To understand why road curves are banked requires a little education on 

When you slide a box on the ground, the amount of force it takes to push 
the box is dependant on the weight of the box and how slippery the surface 
is.  This is because all surfaces exert what we call friction when they rub 
against each other.  Pushing a heavy box requires more effort than a 
lighter weight box.  If the surface is smooth, like a tile floor, it will 
be easier to push than on a rough floor, like a carpet.

The amount of force required is equal to the weight of the box times the 
coeficient of friction, which is an engineering way to describe how 
slippery a surface is.  The coeficient of friction of teflon, which is very 
slippery,  is .1 or lower.  The coeficient of rubber, which is what we make 
car tires from,  can be greater than .5  So pushing a 100 lb box accross 
teflon would take 100 lb * 0.1=10 lb.  Pushing that same 100 lb box across 
rubber would take 100 lb * 0.5 = 50 lb.

If you try to push the box up a slope, it gets harder to push.  That is 
because, in addition to pushing against the friction, you are also pushing 
against part of the weight of the box itself. If you imagine an arrow 
coming from the center of the box and pointing straight down towards the 
ground, that is the force of gravity pulling on the box.  When the box is 
on an incline, most of the weight pushes against the incline, but a small 
portion pushes down the incline, trying to make the box slide down the 

When something is made to travel in a circle, like a rock spun on the end 
of a string, it takes a force to keep it traveling in a circle.  IF you 
spin a rock at the end of a string, the string pulls the rock and keeps it 
from flying off into the distance.  The amount of force needed depends on 
how heavy the rock is and how fast you spin it.

When you drive a car around a curve, the same principle applies.  The car 
wants to go straight, so something must push on the car towards the center 
of the curve.  Otherwise the car will go off the outside of the curve.  If 
the roadway is flat, the force of the friction of the tires on the road 
push in the direction of the inside of the curve to keep it on the road.  
(The same force you feel when trying to push the box across the floor.)  If 
you drive too fast, or the road is slippery from rain (low friction), the 
car can lose traction and will slide to the outside of the curve.

If the roadway is banked, the car is like that box on the incline.  The 
weight of the car will try to slide it down the incline.  But that is in 
the same direction the force needed to keep it going in a curve must push. 
 So the banked curve allows the weight of the car help keep the car from 
sliding off the road.  The steeper the slope, the faster you can go without 
worrying about the tires losing traction and letting the car go off the 
road.  Many fast race tracks like Daytona Speedway are banked very steeply 
to allow cars to go over 200 mph and still go around a curve safely.

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