Tony Posted January 14, 2007 Report Share Posted January 14, 2007 Often people replace wheels for cosmetic reasons maybe going from 15" to the bigger 18" and it looks wonderful.... But! To retain the correct rolling radius the tyres aspect ratio would need to be lowered, historically this would also mean a wider tyre and rim and the desired diameter.... "So what most would say". Designed into the Geometry is the "scrub radius" this is the destination for the Geometry at the tyres footprint... Key: White:- Tyre Red :- Actual camber Blue :- Kingpin inclination or (SJI SAI) depending on your language. So by design there is a destination for the "Scrub radius" if an incorrect off-set wheel is fitted then the position of the SR is also incorrect resulting in this...... This particular car is not Geometrically adjustable so there is nothing we can do to re-direct the SR... Be wise off-sets will lead to upsets if not correctly calculated. Quote Link to comment Share on other sites More sharing options...
Mark H. Posted January 14, 2007 Report Share Posted January 14, 2007 So Tony, take a supra(/soarer) ET50 on 16"s, and say you want a set of 18"s you make the tyres rolling radius the same by lowering the profile, however the tyres are say 50mm wider on the rear and 20mm wider on the front, they were 225 all round, now 245/275. Would you keep the ET at 50 or would you change it either way and why? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 14, 2007 Author Report Share Posted January 14, 2007 So Tony, take a supra(/soarer) ET50 on 16"s, and say you want a set of 18"s you make the tyres rolling radius the same by lowering the profile, however the tyres are say 50mm wider on the rear and 20mm wider on the front, they were 225 all round, now 245/275. Would you keep the ET at 50 or would you change it either way and why? Good question. If the front and rear wheels are not receptively equal by design then this is of no concern.. The manipulation of the off-set applies to a non-adjustable chassis where the scrub radius is fixed... On cars that have adjustable chassis then although the front kingpin destination is pre-determined the counter position via the camber can be manipulated narrowing the width of the radius and limiting the damage. Quote Link to comment Share on other sites More sharing options...
Sam@TDi Posted January 15, 2007 Report Share Posted January 15, 2007 Just to add, good quality coilover kits (example, Cusco) MacPherson strut systems are designed with adjustable top mounts and camber snails at the base. This rather neatly allows you to achieve your desired camber setting whilst keeping the king pin inclination as correct as you like. The process of setting these 2 parameters simultaneously though is easier said than done Quote Link to comment Share on other sites More sharing options...
jon Posted January 15, 2007 Report Share Posted January 15, 2007 I'm a little confused why wheel offsets were mentioned twice but never actually explained... (ie the distance of the center line of the wheel to the distance of the mounting surface typically something like 35, 22, 15) or are we talking about another offset? How is offset by definition ever geometry adjustable? Quote Link to comment Share on other sites More sharing options...
Tony Posted January 15, 2007 Author Report Share Posted January 15, 2007 I'm a little confused why wheel offsets were mentioned twice but never actually explained... (ie the distance of the center line of the wheel to the distance of the mounting surface typically something like 35, 22, 15) or are we talking about another offset? How is offset by definition ever geometry adjustable? Jon i am sure you are well versed in all things relating to off-sets.. My point is that most times the wheel fitment is calculated not to be in conflict in areas like strut platforms or calipers, rarely is there consideration to the "scrub radius". Manipulating the KPI at the lower wishbone will affect the inclination of the KPI and Camber, small deviations will correct the S/R but the corrections do not measure receptively to the off-set? meaning a 20' inclination at the KPI and 5' at the camber could equal -15mm at the off-set regaining the equilibrium, brilliant assuming the car is Geometrically adjustable. Quote Link to comment Share on other sites More sharing options...
jon Posted January 15, 2007 Report Share Posted January 15, 2007 Quote Link to comment Share on other sites More sharing options...
Tony Posted January 15, 2007 Author Report Share Posted January 15, 2007 "Phew" Quote Link to comment Share on other sites More sharing options...
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