Article on The Internet Mini Encyclopædia
If the lower swivel is further forward than the upper you
have positive castor (aka trail). In this situation the centre of the tyre's
contact patch trails behind the point where the king pin axis intercepts the
the steering self centres, the drag on the tyre will pull it back behind the king pin axis. Just like a bike (push or motor).
With things the other way round the steering would tend to un-centre itself (just like reversing).
The castor has no effect on the roll. Body roll is caused by the contact with the road being lower than the centre of mass of the car so when cornering the momentum of the vehicle trying to carry on in a straight line pulls the vehicle and tends to tilt it over. What we know as centrifugal force, but which doesn't really exist.
"The factory race team drivers almost always use 25 or 30 degree castor blocks.
30 degree will add some turn-in steering, but it will cause understeering when exiting corners at speed or on high-speed sweepers. The 30 degree blocks will also stabilise the car on fast, bumpy conditions.
Using 20 or 25 degree castor will give the car more low speed steering and more corner exit steering. When using higher amounts of castor, raise the swivel hub by placing 2 'xyz' spacers under the outer ball joint to keep bump steer to a minimum"
I dunno if this helps at all? IIRC, castor also affects
how much the wheel on the inside of a corner 'flicks out'. i.e. If you turn
right to full lock, the right wheel will be tilted out a the top a little and
it will be turned slightly more than the left."
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