### 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
Message:
```
To understand why road curves are banked requires a little education on
friction.

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
slope.

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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