Greg's post on variable linkages and castor etc made me think again about an article I re-read from a 1967 Motor , written by Charles Bulmer in the days when it did solid technical stuff.
Basically he was discussing power steering feel but he had a simple graph with lateral acceleration . (G) on horiziontal axis, steering arm torque in ft lbs on left vertical ,and slip angle on the right vertical .
The castor torque was linear with G force but the SAT rises to peak then falls before the maximum slip angle, making the steering feel or the feedback of steering arm torque up to the steering wheel critical.at the limit.
The total steering arm torque is obviously the sum of castor and SAT. What surprised me was the effect of the castor angle torque on the ratio between peak SAT and then drop-off over a 20% increase in lateral G.
These were 1967 tyres so we are talking a peak SAT of 36 ft/lb at 0.5 G falling to 28 ft/lb at 0.7 G - an 8ft lb or a 23% reduction.
In contrast the castor angle torques rose from 16 ft lb to 22 ft lb at 0.7 G. So a 6ft lb or 37% increase.. Thus the key drop in SAT of 8ftlb was 75% eliminated by the linear castor increase.
Sadly I don't know what eh castor angle was but it makes me ask why car makers use very high castor angles when they will reduce the key feedback of the SAT dropping approaching the slip angle breakaway?
Or maybe with modern power steering nobody can feel the SAT anyway?