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Understanding caster in relation to camber

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  • Understanding caster in relation to camber

    caster is simple to understand but often hard to get right....

    consider the standard mac strut + tension rod.... they both have a swing, when they are level they are the longest, anything above or below level the shortest....

    so assuming you want negative camber in a turn at all times no matter the dive, the tension rod (assuming it's in front and not behind the control arm) would have to start level at full droop and swing up and forward pulling the control arm (hence the need for bushings)

    if you corner hard and the nose doesn't dive because you have big swaybars and stiff springs, then your caster won't affect you much, and you'll have to get stickier tires and load up the front with a weight transition when in the turn to use the caster (sometimes this means tapping or lightly riding the breaks mid-turn, or tapping the gas in a fwd car)

    the whole point is that because the normal "soft" car tilts so much in a turn that you need the extra caster angle to maintain negative camber, without it you'll be rolling over your sidewall and chewing the outer edge of the outer tire (problem on my k-car boohoo)

    how this affects the inside tire can vary, again it depends on the compression of the spring and where the tension rod is level.... if the tension rod is below level at full droop (and level when the car is sitting) you could start to go positive a touch on the inside wheel and drag it in the corner, which slows you down and again chews the tire.... but the dragging can help stability in that it acts much like driving into a puddle of water slowing your inside a touch

    having a car tilt/roll in a corner is not necessarily a bad thing, the loading/twisting actually helps you grip by moving some of the lateral force into the chassis and away from the tire, and the opposite is tru for race cars that don't roll, you transfer more force into the tire, so you need better tires and there's a very small "warning" threshold before you lose grip altogether...... and a side effect is that with softer springs/swaybars with the same tire you're less likely to break loose than with a stiffer suspension for the same loading of the tire's contact patch

    swaybars interact with the loading of the spring and in some cases where the swaybar can actually overpower your springs, you can for instance gain significant camber effect on the inside tire (this isn't the panacea it sounds, you also lose grip from less weight on that tire).... in the opposite scenario where the springs are very stiff but the swaybar is weak, you can actually gain some camber effect at the highest point of weight transition that will reverse and go more positive as the tension rod regains levelness before returning to the upper arc giving you more negative camber

    I won't get into the awful design tradeoffs domestics and other softly suspended vehicles have aside from pointing out that they will often have tension rods below level and inadequate swaybars for the springs, which are also usually way too soft, and thereby they can get away with using a cheaper variant of a retail tire which is the same in look and name only

    this is all usually justified in the name of safety, so the car will not exceed the performance level of the driver, and therefore even drivers with inadequate skill can feel confident but won't push the car beyond the very progressive limits

    - I wrote most of this on another forum, figured it topically suits this one even more so...

  • #2
    as an addendum: the camber you get with the wheel angled can also be adjusted on a mac-strut by moving the lower balljoint fixture point on the knuckle.... moving it more forward can give you more negative camber when turned outward, and less when turned inward, and vice versa by moving the fixture point rearward