Ye Olde Rangie wrote:yeah i understand that castor is measured in degrees. Im a suspension tech by trade and im just trying to come to terms with how the castor changes without lengthening the arms, as castor is measured from the top to bottom of the pivot points ie top of spring to bottom
If you are a suspension tech, I will not try and explain what castor is, but you were incorrect in your earlier post where you stated "
castor is technically wheelbase so without lengthening the castor wont change" and also incorrect in the context of Land Rovers with solid axles where you stated "
top of spring to bottom". Perhaps your wording has not explained what you mean very well.
A Land Rover with radius arms measures roughly 840 mm from the centre of the axle to the point where the radius arm pivots on the chassis (some, such as disco II have longer radius arms than this). The radius arm is attached to the axle housing with a pair of bushes and bolt - one in front and one behind the housing. A stock Land Rover with radius arms has approximately 3 degrees castor.
Now if the suspension is raised approximately 50 mm, simple trigonometry will show you that in this situation the angle that the radius arm makes to the horizontal plane through the pivot point is then a little over 3 degrees lower than it was at the stock ride height position (arctan 50/840 = 3.4 degrees). The attachment of the radius arm to the axle housing forces the axle housing to rotate through the same angle change as the radius arm - thus the castor changes.
Radius arms for castor correction resemble/duplicate a stock radius arm that is bent/cranked upward where the radius arm attaches to the axle housing. This rotates the axle housing back to be the same as before the suspension lift. Normally they will also be bent in the area where the chassis bush is, to remove the deflection in those bushes, not for castor correction.
The problem with this is that it rotates the diff pinion down and increases the angle of the drive shaft, leading to vibrations. This is why (at least with Land Rovers) it is better rotate the swivel balls to correct castor and not do it with corrected radius arms or eccentric bushes in the radius arms.