Tony Posted December 13, 2009 Report Share Posted December 13, 2009 I've always thought deeper static castor meant deeper camber gains on turn ? Well done, you are paying attention. This is where things get complicated, on the IS the Ackemann is aft of the TOOT plus the IS has a very low SAI (around 9 degrees) this means the SR migration is aggressive compared to most cars. The migration on the IS is almost instant lock-on-lock, whereas on most cars the migration is a shared transition. Put it another way... Why would a car with a OEM front camber setting of -30 duff the tyres in 5k? I think we would agree straight line camber position is fine so why the wear? Quote Link to comment Share on other sites More sharing options...
CIH Posted December 14, 2009 Author Report Share Posted December 14, 2009 Oh I agree. I'd have expected the extra castor to make the SR migration issues more pronounced, but clearly that's not the case ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 14, 2009 Report Share Posted December 14, 2009 It depends on the car and chassis design, you wouldn't want to math this on every car but a car like the Lexus and it's obvious problem invites investigation. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 14, 2009 Author Report Share Posted December 14, 2009 could you be more specific about what the SR is doing on turn ? I preume it would be the outer wheel (of a given turn) as that would move toward neg. camber ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 14, 2009 Report Share Posted December 14, 2009 The SR migrates to the outside of the tyre...... The SAI is declining as is the camber in addition the castor is reducing. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 14, 2009 Author Report Share Posted December 14, 2009 oh yeah sorry that's right. So it's the inner wheel SR that's incorrectly positioned ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 14, 2009 Report Share Posted December 14, 2009 oh yeah sorry that's right. So it's the inner wheel SR that's incorrectly positioned ? Sorry yes it is..... The problem we have is no one has the data for how everything should correctly migrate, if there is a visible tyre wear issue and the readings are near to stock then it's time to look outside of the box. In the case of the Lex i had the wheels turned 20 degrees and the inner wheels camber had dropped to around -1 degree 45' from the ahead position of -30', this as you are well aware is a massive transition for a 20 degree lock. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 15, 2009 Author Report Share Posted December 15, 2009 I was thinking about this today. We've already established static camber can cause incorrect placement of SR on turn, so is the extra castor you dial-in related to SR migration, or is it simply to compensate for the lower camber ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 15, 2009 Report Share Posted December 15, 2009 I was thinking about this today. We've already established static camber can cause incorrect placement of SR on turn, so is the extra castor you dial-in related to SR migration, or is it simply to compensate for the lower camber ? No it's to activate/ belay TOOT in order to control the migration. In the case of the Lex an increase of castor slowed the TOOT (steering arm behind pivotal point) which in turn reduced the migration. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 15, 2009 Author Report Share Posted December 15, 2009 That was my other thought but I had always thought of deeper castor = more migration which seemed a contradiction. "In the case of the Lex an increase of castor slowed the TOOT (steering arm behind pivotal point) which in turn reduced the migration." Is that consistant across all makes/models, and obviously the opposite for models with steering arm ahead of pivot ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 16, 2009 Report Share Posted December 16, 2009 Differences in camber position, toot design and arm position opens many different scenario's. Quote Link to comment Share on other sites More sharing options...
CIH Posted December 27, 2009 Author Report Share Posted December 27, 2009 Between KPI and castor which would have the greater jacking effect on turn (for any given vehicle) ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 27, 2009 Report Share Posted December 27, 2009 I would say KPI/SAI since this holds vehicular weight and mechanical energy, castor is a consequence not a real mechanical feature (argue that one later) The longitudinal function of castor overwhealms the real pivotal value of the SAI (perpendicular in action) (the mythical castor) meaning the longitudinal effect of the castor is much less than the vehicular weight pressing down on ther king pins. By convention it's globally recognized this transition holds real transitional free energy obliging a steering migration and ultimately a free return. Power steering...... Pah Quote Link to comment Share on other sites More sharing options...
CIH Posted December 27, 2009 Author Report Share Posted December 27, 2009 castor belays some of the on-turn SAI effects doesn't it ? Quote Link to comment Share on other sites More sharing options...
Tony Posted December 27, 2009 Report Share Posted December 27, 2009 Well not really...... The marriage between the castor and SAI is divorced once the "dead a-head" relaxed state is violated "yaw", here on you have an orbital suspension/ chassis configuration in action. To say the castor activates TOOT, yes it does..... Does it help steering return, maybe, is it more valuable than SAI? you tell me? Quote Link to comment Share on other sites More sharing options...
CIH Posted December 29, 2009 Author Report Share Posted December 29, 2009 my ikkle book says castor infulences the jacking effect created by the SAI rotating on it's axis ? Anyway, to answer you're question, I'd say SAI. For the same reasons you stated earlier. Quote Link to comment Share on other sites More sharing options...
Tony Posted December 29, 2009 Report Share Posted December 29, 2009 my ikkle book says castor infulences the jacking effect created by the SAI rotating on it's axis ? Anyway, to answer you're question, I'd say SAI. For the same reasons you stated earlier. Indeed, there is a complicated, dynamic going on during the yaw.... Remember also the SAI is longitudinally perpendicular during the turn, so it's almost orbiting around the rotational axis. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 4, 2010 Author Report Share Posted January 4, 2010 Is it fair to say it's the absence of castor (ie; too shallow a castor angle) that causes problems ? I can't ever recall, or imagine, a domestic car having too much castor, steering feel/weight aside. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 5, 2010 Report Share Posted January 5, 2010 The actual castor position is dependant on a whole host of factors, your thoughts on castor should concentrate on "weight, return, migration" and power!.... Why does a car like a 1.4l Fiesta need power steering? Quote Link to comment Share on other sites More sharing options...
CIH Posted January 8, 2010 Author Report Share Posted January 8, 2010 "power" ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 8, 2010 Report Share Posted January 8, 2010 "power" ? Yes power...... Why does this type of car need power steering? Quote Link to comment Share on other sites More sharing options...
CIH Posted January 8, 2010 Author Report Share Posted January 8, 2010 We've already established PAS allows for more castor....but is the PAS there because of the castor or vice versa ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 9, 2010 Report Share Posted January 9, 2010 With advances in suspension design it was possible to reduce the kingpin off-set to almost zero but although this made construction more economical a problem with road shocks evolved, the logical solution was to tilt the SAI to a perpendicular position (remember the kingpin was vertical) thus increasing the castor position, inevitably this made the steering weight heavy since the weight is no longer over a vertical pivotal point. The castor off-set although more productive needed assistance to reduce driver fatigue, hence the need for power assisted steering. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 9, 2010 Author Report Share Posted January 9, 2010 do you see much of centre point steering nowadays ? I'd guess not ? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 9, 2010 Report Share Posted January 9, 2010 do you see much of centre point steering nowadays ? I'd guess not ? LCV's and commercials still use CPS with tragic tyre wear issues, mainly with NSF outer tyre wear problems. Quote Link to comment Share on other sites More sharing options...
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