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GWhy so much castor angle?

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#1 mariner

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Posted 24 August 2020 - 15:25

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?




#2 gruntguru

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Posted 24 August 2020 - 21:07

Sounds interesting. It would be great if you could post an image.


Castor angle has a lot more effects than just SAT. Camber change is probably the most significant in performance terms. Next would be corner-jacking (when combined with scrub radius) - lifting the inside front, dropping the outside front - moving weight to the outside rear.


Both effects improve front grip and help get the car rotating in tight turns (high steering angle) without adding undesirable oversteer in higher speed turns.

#3 Greg Locock

Greg Locock
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Posted 25 August 2020 - 02:19

Corvette C7 and the Ranger both have castor of about 3 degrees. On Ranger we'd have reduced it further but didn't want it to go to zero under braking-  we're used to 7 or 8 degrees, but Ranger FGAWR /EPAS constraints drove us towards the smaller figure. So the interesting question is why some cars end(ed) up at 11 degrees. It's hard to talk about caster without thinking about KPI, scrub radius, and mechanical trail, all at the same time. Between them they define the relationship of the steering axis to the wheel. Many cars have zero longitudinal offset of the stg axis at the hub, this is handy for FWD but it is no particular advantage for RWD. i don't think any car has zero lateral offset (ie enormous negative scrub), so I have my doubts that it is important to keep it at zero.

#4 mariner

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Posted 25 August 2020 - 12:50

Sorry, I am to dumb to scan the graph  and then post it, but here is the data in ft lbs and remember 1967 tyres.



Lat accel G   Castor angle torque  self aligning torque  total steering arm torque   slip angle (degrees


 0.3                       10                                24                          34                                    3


 0.4                       12                                29                          41                                    5


 0.5                       15                                33                          48                                    7


0.55                      16                                38                          54                                    9


0.6                        18                                34                          52                                   12


0.7                        21                                28                           49                                   -



Hopefully that makes some sort of sense !!

#5 gruntguru

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Posted 26 August 2020 - 21:21

I assume steering angle was constant?


Re your comments about steering feel - I think the limit case of zero castor with a 26% reduction in both SAT and total steering arm torque would amount to more than just "feedback" or "feel". Perhaps this level of reduction in total steering arm torque would produce an undesirable increase in steering angle - the preferable response to imminent breakaway would be a reduction in steering angle.