Whoodoo
Well-known member
One thing I have been wrestling with is exactly how caster is defined and why it is defined that way. Of all my searching and reading, caster has been defined as the tilt of the axis created by the top and bottom steering pivots.
Caster primarily affects straight line stability because it moves the contact patch (and drag force) either in front of (like on an office chair) or behind (like on a car) the steering axis. Changing caster will also change dynamic camber. Dynamic camber is the amount of change in camber you get when you increase steering angle. Caster and camber are related because they are both measures of the angle of the steering axis, just looking at the car from a different perspective. This means that they are related and as you turn the wheels from center, the caster angle begins to affect the camber. Defining caster by the angle of the axis created by the top and bottom steering pivots seems like it doesn't quite encompass all of whats going on.
Let me explain. In the evo suspension chapter 2 thread I'm sure you all have read lately, the idea of adding caster by putting the strut ear spacers either in front of or behind the knuckle's strut mount has been discussed. Because the steering axis will remain unchanged, essentially what you are doing is rotating the knuckle about the lower ball joint. If you rotate the knuckle backwards by putting all your spacers in the front, then you are actually increasing caster because you are moving the tire's contact patch even farther back. However, you are not changing caster because the steering axis hasn't changed, and there will be no affect on dynamic camber.
Also, another point to make is that by rotating the knuckle backwards, you have moved the steering arm upward, which will affect bump steer.
So first, how exactly is caster defined? Is it merely the angle of the steering axis? If so, then what would you call moving the contact patch either forwards or backwards independently of the steering axis?
Second, how beneficial is an increase in caster for our cars? According to Dennis Grant at Far North Racing, caster isn't all too important. Is the point of an increase in caster to get the resulting increase in dynamic camber? Is the drawback of increased steering effort a reason to not increase caster? By rotating the knuckle in the fashion described above, you could either increase or decrease steering effort (and feedback) at the expense of only changing steering arm height (hence affecting bump steer).
Thirdly, is it even worth the effort of discussion? From my measurements, you only need to make up a few millimeters between the strut ears, so is it really going to make any difference at all? I kind of doubt it, but discussion is good and if someone really has a conviction to move the contact patch back, perhaps this discussion is worthwhile.
I tried to make this as concise as possible, but suspension is very complicated indeed /ubbthreads/images/graemlins/hsugh.gif
Caster primarily affects straight line stability because it moves the contact patch (and drag force) either in front of (like on an office chair) or behind (like on a car) the steering axis. Changing caster will also change dynamic camber. Dynamic camber is the amount of change in camber you get when you increase steering angle. Caster and camber are related because they are both measures of the angle of the steering axis, just looking at the car from a different perspective. This means that they are related and as you turn the wheels from center, the caster angle begins to affect the camber. Defining caster by the angle of the axis created by the top and bottom steering pivots seems like it doesn't quite encompass all of whats going on.
Let me explain. In the evo suspension chapter 2 thread I'm sure you all have read lately, the idea of adding caster by putting the strut ear spacers either in front of or behind the knuckle's strut mount has been discussed. Because the steering axis will remain unchanged, essentially what you are doing is rotating the knuckle about the lower ball joint. If you rotate the knuckle backwards by putting all your spacers in the front, then you are actually increasing caster because you are moving the tire's contact patch even farther back. However, you are not changing caster because the steering axis hasn't changed, and there will be no affect on dynamic camber.
Also, another point to make is that by rotating the knuckle backwards, you have moved the steering arm upward, which will affect bump steer.
So first, how exactly is caster defined? Is it merely the angle of the steering axis? If so, then what would you call moving the contact patch either forwards or backwards independently of the steering axis?
Second, how beneficial is an increase in caster for our cars? According to Dennis Grant at Far North Racing, caster isn't all too important. Is the point of an increase in caster to get the resulting increase in dynamic camber? Is the drawback of increased steering effort a reason to not increase caster? By rotating the knuckle in the fashion described above, you could either increase or decrease steering effort (and feedback) at the expense of only changing steering arm height (hence affecting bump steer).
Thirdly, is it even worth the effort of discussion? From my measurements, you only need to make up a few millimeters between the strut ears, so is it really going to make any difference at all? I kind of doubt it, but discussion is good and if someone really has a conviction to move the contact patch back, perhaps this discussion is worthwhile.
I tried to make this as concise as possible, but suspension is very complicated indeed /ubbthreads/images/graemlins/hsugh.gif