HB280ZT Posted October 22, 2007 Share Posted October 22, 2007 Hi All, I have a geometry question concerning the LCA to body mounting point. You are looking at the adjustable LCA and T/C rod, complements of Mikelly. Well the question is what will happen to the caster if I change the bushings around so that the large one is in front and small in back on the heim joint? Thanks for your help. HB280ZT Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted October 22, 2007 Share Posted October 22, 2007 Not a damn thing. You're not moving the wheel at all, simply changing the way the lateral loading is transferred to the body. Doing what you propose will put more tensile force on the castor rod. Dave Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 22, 2007 Share Posted October 22, 2007 What is the amount of tensile force increase on the T/C rod (percentage wise)? Quote Link to comment Share on other sites More sharing options...
johnc Posted October 23, 2007 Share Posted October 23, 2007 You will reduce the amount of positive caster and change the SAI by moving the LCA pivot point back in relation to the upper strut mount. The LCA should move as perpendicular to the chassis centerline as possible or you'll get wheelbase changes during suspension travel. Because of this, you will also have to shorten the TC rod. Quote Link to comment Share on other sites More sharing options...
TimZ Posted October 23, 2007 Share Posted October 23, 2007 You will reduce the amount of positive caster and change the SAI by moving the LCA pivot point back in relation to the upper strut mount. The LCA should move as perpendicular to the chassis centerline as possible or you'll get wheelbase changes during suspension travel. Because of this, you will also have to shorten the TC rod. This will also have an effect on the bumpsteer curve, since you are changing the relationship between the LCA and the tie rod. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted October 23, 2007 Share Posted October 23, 2007 John, Sorry to be a pain, but could you please explain how changing the LCA pivot would affect the castor/KPI/SAI/etc. I must be missing something? The castor/radius/etc rod holds the bottom of the strut a fixed distance the bracket. The only way I can see moving the LCA pivot affecting the castor is by it pulling the LCA inwards a fraction more (assuming it was perpendicular to start with), which would cause the end of the castor rod to move inwards with it. As this end is an arc, it would change the overall castor. Or am I way off base? Dave Quote Link to comment Share on other sites More sharing options...
johnc Posted October 23, 2007 Share Posted October 23, 2007 Your assumption that the wheel (spindle) stays in the same place is mistaken, unless the LCA and/or the TC rod lengths are changed. Think of the LCA and the TC rod as two sides of a triangle. The third side is a line from the LCA pivot point on the chassis to the TC pivot point on the chassis. If one intersection of that triangle is changed (the LCA pivot point moved back and that side of the triangle shortened), the other two intersection points in the triangle have to change their position in space unless at least two sides of the triangle change their lengths. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted October 23, 2007 Share Posted October 23, 2007 Yep I get that. Thats a much better description of what I was trying to get at. I would raise two points though, 1. The amount of movement would be SFA. 2. I think that given the angle the castor rod is on, an inwards/forwards movement would increase positive castor ( by SFA ). Dave Quote Link to comment Share on other sites More sharing options...
HB280ZT Posted October 26, 2007 Author Share Posted October 26, 2007 Great information guys. Now the question is how do the race guys get as much as 7 deg of caster in there cars and keeping the LCA perpendicular to the chassis centerline? HB280ZT Quote Link to comment Share on other sites More sharing options...
wheelman Posted October 26, 2007 Share Posted October 26, 2007 You move the top mount point of the strut back relative to the LCA or lengthen the TC rod which pushes the outer end of the LCA forward relative to the top strut mount. Wheelman Quote Link to comment Share on other sites More sharing options...
HB280ZT Posted October 26, 2007 Author Share Posted October 26, 2007 I have an adjustable T/C rod and have pushed it forward so that the tire looks forward in the wheel well and is getting very close to rubbing on the fender. With it setup this way I am only at +3.8 deg of castor. So to get 6 to 7 I would have to push it into the fender? However I also have camber biscuits that I can change the camber and castor some but still not enough to get even close to what is recommended by the racing people. Thanks for the information hope this also helps someone else! HB280ZT Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 26, 2007 Share Posted October 26, 2007 You will have to cut or bend the fender out of the way or run a smaller diameter tire in order to run big caster numbers. Not really an easy way around that, other than adjusting it at the top with your biscuit camber/caster plates. Quote Link to comment Share on other sites More sharing options...
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