Suspension SAI (kingpin axis) and Caster (trail) relationship cleared up
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6th Gear
Joined: Jan 2003
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SAI (kingpin axis) and Caster (trail) relationship cleared up
SAI - Steering Axis of Inclination or Kingpin axis (an international term) will cause the sprung mass of a car to rise symetrically, that is, the left and right sides of the car will be raised by an equal amount...this also means that a tire turned left or right will cause an equal condition. Gravity, as we already know, and a little forward motion, causes the car to settle, creating the self center effect.
Caster, or trail as measure on the ground plane, does not raise the car symetrically. For example, a car with the wheels turned left; in this example, caster causes the car to roll to the right because the left wheel rises and the right does not. This also causes a weight shift to the right rear wheel - an old argument for caster; the outside rear wheel gains a little grip in a turn. However, this added weight can actually add to oversteer, especially in conjunction with heavier rear springs.
Curiously, positive caster will also cause the car to roll to the wrong side in an opposite lock, out of control, condition.
Just thought I would clear this up. I was a tad confused a while back, but I'm old...am I forgiven?
Caster, or trail as measure on the ground plane, does not raise the car symetrically. For example, a car with the wheels turned left; in this example, caster causes the car to roll to the right because the left wheel rises and the right does not. This also causes a weight shift to the right rear wheel - an old argument for caster; the outside rear wheel gains a little grip in a turn. However, this added weight can actually add to oversteer, especially in conjunction with heavier rear springs.
Curiously, positive caster will also cause the car to roll to the wrong side in an opposite lock, out of control, condition.
Just thought I would clear this up. I was a tad confused a while back, but I'm old...am I forgiven?
4th Gear
Joined: Nov 2005
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From: Springfield, Missouri
Originally Posted by meb
SAI - Steering Axis of Inclination or Kingpin axis (an international term) will cause the sprung mass of a car to rise symetrically, that is, the left and right sides of the car will be raised by an equal amount...this also means that a tire turned left or right will cause an equal condition. Gravity, as we already know, and a little forward motion, causes the car to settle, creating the self center effect.
Caster, or trail as measure on the ground plane, does not raise the car symetrically. For example, a car with the wheels turned left; in this example, caster causes the car to roll to the right because the left wheel rises and the right does not. This also causes a weight shift to the right rear wheel - an old argument for caster; the outside rear wheel gains a little grip in a turn. However, this added weight can actually add to oversteer, especially in conjunction with heavier rear springs.
Curiously, positive caster will also cause the car to roll to the wrong side in an opposite lock, out of control, condition.
Just thought I would clear this up. I was a tad confused a while back, but I'm old...am I forgiven?
Caster, or trail as measure on the ground plane, does not raise the car symetrically. For example, a car with the wheels turned left; in this example, caster causes the car to roll to the right because the left wheel rises and the right does not. This also causes a weight shift to the right rear wheel - an old argument for caster; the outside rear wheel gains a little grip in a turn. However, this added weight can actually add to oversteer, especially in conjunction with heavier rear springs.
Curiously, positive caster will also cause the car to roll to the wrong side in an opposite lock, out of control, condition.
Just thought I would clear this up. I was a tad confused a while back, but I'm old...am I forgiven?
Just as a note for people not familiar with the terminology.
SAI.. is the angle between true vertical and the steering axis. {on the Mini the steering axis is the center point at which the top of the strut attaches to the body carried down through the center of the lower ball joint}
SAI..uses the same points as those used to measure caster. However, SAI is viewed from the front of the vehicle while caster is viewed from the side.
SAI..causes the spindle to arc downward as the wheel is turned in or out and force the vehicle weight upward.--Assists in returnability of the steering wheel after a turn.--SAI also projects the point of load near the inner wheel bearing.
SAI..reduces road shock and provides directional stability.
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6th Gear
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...now, if I could figure out how to get some of my chicken scratch drawings into this box - my term for a computer - I would attempt to illustrate how many other suspension geometrys are tethered to SAI (for a Mac strut)...and then play a what if game like the big boys do. Camber is a very interesting one...Don't give it away BLIZZ
I'm gonna learn how to get drawings in here. It's magic
???
...we should actually have a dictionary of terms here with drawings. These should be easy to read and understand...
SAI..causes the spindle to arc downward as the wheel is turned in or out and force the vehicle weight upward.--Assists in returnability of the steering wheel after a turn.--SAI also projects the point of load near the inner wheel bearing.
...in fact, it projects along an axis which will almost always be on the inside of the bearing, otherwise known as positive spindle. If for some reason, beyond my pityful brain, this projection falls outside the bearing, the body would actually lower...I think I got that right. I gotta draw another picture...too much positive spindle, however, and kickback becomes a severe problem on bumpy roads; for example, if the right front tire is traveling on a glass smooth road and left tire over a very bumpy road, the left side tire must travel farther - over the arc of the bump - than the right side. This arc becomes more exsagerated with a longer spindle ( and smaller numeric wheel offset) This will cause tugs and kickback at the steering wheel. Adding negative camber increases positive spindle and this effect while at the same time adding to more negative scrub radius. But then, adding more negative camber can actually lower roll centers, fixing roll couple problems associated with lowering the car...but do the outer tie rods continue to align with the roll centers, a must for zero ride roll - no toe changes as the wheel moves up and down. The list goes on...
Spindle; in the front view the distance between the tire's centerline and the intersection of the SAI along the axle centerline. If you can draw this you will see why there is potential for kickback with more spindle, and, why we get more spindle with more negative camber. How do we offset this, no pun, by changing offset - bring the wheel and tire closer to the body.
And, too much caster will eventually cancel out the effect os SAI. FYI
I'm gonna learn how to get drawings in here. It's magic ???...we should actually have a dictionary of terms here with drawings. These should be easy to read and understand...
SAI..causes the spindle to arc downward as the wheel is turned in or out and force the vehicle weight upward.--Assists in returnability of the steering wheel after a turn.--SAI also projects the point of load near the inner wheel bearing.
...in fact, it projects along an axis which will almost always be on the inside of the bearing, otherwise known as positive spindle. If for some reason, beyond my pityful brain, this projection falls outside the bearing, the body would actually lower...I think I got that right. I gotta draw another picture...too much positive spindle, however, and kickback becomes a severe problem on bumpy roads; for example, if the right front tire is traveling on a glass smooth road and left tire over a very bumpy road, the left side tire must travel farther - over the arc of the bump - than the right side. This arc becomes more exsagerated with a longer spindle ( and smaller numeric wheel offset) This will cause tugs and kickback at the steering wheel. Adding negative camber increases positive spindle and this effect while at the same time adding to more negative scrub radius. But then, adding more negative camber can actually lower roll centers, fixing roll couple problems associated with lowering the car...but do the outer tie rods continue to align with the roll centers, a must for zero ride roll - no toe changes as the wheel moves up and down. The list goes on...
Spindle; in the front view the distance between the tire's centerline and the intersection of the SAI along the axle centerline. If you can draw this you will see why there is potential for kickback with more spindle, and, why we get more spindle with more negative camber. How do we offset this, no pun, by changing offset - bring the wheel and tire closer to the body.
And, too much caster will eventually cancel out the effect os SAI. FYI
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