Can any one tell me why riders when racing move their bums right over to the left or right on cornering?Is it to keep the bike more upright so as they have more tread on the surface of the road.

It lets the bike lean less for a given speed. By sliding off the inside of the seat, the rider's body weight is moved towards the inside of the corner. This means the bike needs less lean for a given speed and turn radius. As ground clearance is often the limiting factor in cornering (particularly at higher speed) this allows the rider to corner at higher speeds.

yes

More from the interweb:

Here's some nerd info that describes turning geometry:

t= arctan [ v^2/(g*R) ]

v is your velocity, R is the radius of your turn, g is the

gravitational constant. t is the "lean angle." It's the angle between

1. the horizontal, and

2. a line from the contact patch of your tires through the center of

gravity of the bike-rider system.

Hanging off and sticking out your knee moves the CG of the system to

the inside, while leaving the bike more upright, so you don't run out

of ground clearance.

Aside from running out of ground clearance, a second limiting effect

is caused by excessive lean. At lean angles below 45 degrees from the

horizontal, tires no longer purely roll and are rotating more about a

vertical axis rather than a horizontal one. This makes them act more

like a rotary brush of a street sweeper than a rolling wheel. With

steep lean, the contact patch twists in place scrubbing away traction

and power so that leaning farther reduces cornering ability and

ultimately causes a washout in the turn. Incidentally, since cars do

not lean, they do not exhibit this behaviour, and can corner with

greater G-forces than motos.

2. Some riders like to 'feel' the road so they know their lean angle.

(You can't afford to look down). It doesn't hurt - they wear pucks on

their knees to avoid damage. Only problem sometimes is the rumble

strips on car circuits.

3. Top riders can use their knee to change the balance of the bike, for

example taking some of the weight off the rear wheel when they want to

induce a bit more slip.

4. Some will claim it also aids braking (because of air drag) going into

a corner. Every little bit helps....

More from the interweb:

Here's some nerd info that describes turning geometry:

t= arctan [ v^2/(g*R) ]

v is your velocity, R is the radius of your turn, g is the

gravitational constant. t is the "lean angle." It's the angle between

1. the horizontal, and

2. a line from the contact patch of your tires through the center of

gravity of the bike-rider system.

Hanging off and sticking out your knee moves the CG of the system to

the inside, while leaving the bike more upright, so you don't run out

of ground clearance.

Aside from running out of ground clearance, a second limiting effect

is caused by excessive lean. At lean angles below 45 degrees from the

horizontal, tires no longer purely roll and are rotating more about a

vertical axis rather than a horizontal one. This makes them act more

like a rotary brush of a street sweeper than a rolling wheel. With

steep lean, the contact patch twists in place scrubbing away traction

and power so that leaning farther reduces cornering ability and

ultimately causes a washout in the turn. Incidentally, since cars do

not lean, they do not exhibit this behaviour, and can corner with

greater G-forces than motos.

2. Some riders like to 'feel' the road so they know their lean angle.

(You can't afford to look down). It doesn't hurt - they wear pucks on

their knees to avoid damage. Only problem sometimes is the rumble

strips on car circuits.

3. Top riders can use their knee to change the balance of the bike, for

example taking some of the weight off the rear wheel when they want to

induce a bit more slip.

4. Some will claim it also aids braking (because of air drag) going into

a corner. Every little bit helps....

Thanks for the info guys, I got a bit more from GT Rider as well. Very informative.