Caster Reconsidered

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Doug states, at around :58sec into the video that caster has a lifting effect on the vehicle.

I disagree. Just by tilting a coin back and forth along an axis diagonal to the table, the center of the coin - the president's nose (!), seeemed to move closer to the table the more extremely I rotated the coin. Rotating the coin along an axis of 45deg actually cause it to "flop" down as I rotated it from the vertical. In effect, this leads me to believe that both positive(desired) and negative caster have the effect of LOWERING the nose of a vehicle when excessive amonts of caster are selected(10 degrees or more!).

This is offset by SAI, which tends to RAISE the nose of the car as the front wheels are steered away from straight-ahead.

Can anyone visualize what I'm talking about in regards to the former (caster)?

-CC

Reply to
ChrisCoaster
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I believe it is actually a combination of the king pin angle and also the scrub radius which causes the lifting. (if you had a non-zero SAI but a zero scrub radius, you still wouldn't have a lifting/lowering effect.) If you have a negative scrub radius you might actually have a lowering effect. Throw caster into the mix and whether the car actually lifts or lowers depends on the interaction of all three parameters.

(I hope I'm thinking this through right, this was all off the cuff.)

nate

Reply to
Nate Nagel

I think.....The reason caster "works" is because it raises the vehicle as you turn the wheels off center. The raising of the vehicle is what creates the self-centering force.

Reply to
Ashton Crusher

Only if the car has positive caster. Some older vehicles actually had negative caster so as to allow for light steering in cars with a heavy front end that didn't have power steering as standard. This obviously came at the cost of a little high speed stability. I know Studebaker for one spec'd negative caster up until 1961 or thereabouts.

nate

Reply to
N8N

_______________________ I just don't see how it would lift the front end of the car. Take a record - or a CD, and place it vertically on a flat surface. Next, locate 11 o'clock on that disc and put one finger there. Locate five o'clock on the disc and place another finger there. Then, rotate, or steer, the disc about this imaginary axis you thus created, and see where the spindle hole goes - up or down.

How caster creates self-centering is probably more related to the "trail" and how much the tire "flat-spots" behind the steering axis contact point with the road.

-CC

Reply to
ChrisCoaster

The strut or A frame that the spindle attaches to is not perpendicular to the ground. The top leans inward at maybe a 10 or 15 degree angle. To see what happens lean a dowel at an exaggerated 45 degree angle then attach the cd to it vertical to the ground and out from the dowel using putty or something. In other words, simulate a real mcphearson strut set up. Then rotate the dowel only on its long axis.

Reply to
Blank

What you are referring to is kingpin angle (or more lately, SAI since most cars don't actually have kingpins anymore)

nate

Reply to
Nate Nagel

Oh! You are right Nate. I never even thought of that.

Reply to
Blank

ttach the cd to it

 In other words,

its long axis.- Hide quoted text -

_________________________ Ahaaa, Blank, I think I know what happened! You are confusing caster with SAI - or Kingpin for the old-schoolers reading this.

The top leans inward with SAI, the top leans rearward(or rarely, forward) with CASTER. SAI definitely imparts a lifting force as it attempts to force the steering tires "below" the plane of the road.

-CC

Reply to
ChrisCoaster

__________________ Caster is engineered to aim the steering axis toward the leading edge of the contact patch of a specific sized tire at a particular inflation in PSI. That is why we are supposed to use the auto mfg recommended tire pressures, not the placard for max inflation on the tire itself.

By "dragging" that contact patch behind the steering axis, we are inducing straightline stability at speed. If anyone here believes, and can demonstrate, that caster imposes a lifting force on the front end of a car, Isaac Newton will come to haunt you when it's your time to go. :)

-CC

Reply to
ChrisCoaster

Ashton Crusher wrote in news: snipped-for-privacy@4ax.com:

That is not correct. the castor angle itself is what self centers the wheels. the extreamly small amount of lift does nothing at all. the amount of lift is so slight as to be almost unmeasureable. KB

Reply to
Kevin

I believe that when you look at the geometry of how things are moving you see that when there is a caster angle AND the suspension pivots (ball joints) and center of the tire are offset from each other that turning the wheel to the side creates an uplifting force. I tried to find a decent verification of that but the closest I came is in Fred Puhn's 1976 book "How to make your car handle". He discusses the effects of caster with the same comments you make but goes on to describe, on page 73, how to use a gauge to measure caster and states that "The caster gauge converts the rise and fall of the spindle into a caster angle measurement." I might be imagining it but I thought I had noticed in the past that when you are just sitting and not moving and you turn the wheels from side to side you can actually see the hood of the car go up and down with the turning movement. The more you turn, the more up you get. Easy enough to try sometime.

Reply to
Ashton Crusher

If you saw my other post to Mr. Coaster it refers to measuring the rise and fall as the way to indirectly measure caster. So it must be sufficient to measure caster to a fair degree of accuracy. I'm not sure how much you are thinking it would rise and fall, I doubt it would be more then an inch.

Reply to
Ashton Crusher

_________________________ Ashton: You need to get your head around the concept of SAI(steering- axis-inclination) AKA kingpin. If you look at SAI/KPI from ahead or aft, you will see that the steering axes are closer at their tops and further apart(toward the outside of the vehicle, where the intercept the road surface.

SAI is typically angled in a similar fashion to negative degrees of Camber(but don't confuse it with camber that is the angle of the wheel/ tire itself!).

On a rack, turning the wheels should cause them appear to drop lower as they angle away from straight-ahead. Once the car is on a surface, of course, this becomes impossible, and so as the wheels attempt to go below the plane of the road, they RAISE the nose of the car(how much varies with SAI angle - 10, 12, 14deg, etc).

You cannot get me to believe that Caster raises the nose of a car at GUNPOINT! If anything, the nose of the car is highest with the wheels straight ahead. Caster uses TRAIL(the area of the contact patch of the tire AFT of the intersection of the steering axis with the road surface) to provide straight-ahead stability.

It is the relatively high angle of SAI(typically 3 - 4x the amount of caster) that contributes to the lifting effect, NOT caster!

-CC

Reply to
ChrisCoaster

quoted text -

It ain't your imagination, I can clearly see it happen on my '55 Stude (I know this because I've distinctly noticed this effect while bleeding the air out of the P/S system after replacing/cleaning/ regasketing components) However, whether this is due to the (negative) caster or to the kingpin angle I am not sure. I suspect a little bit of both.

nate

Reply to
N8N

de quoted text -

____________________ Of course I've seen it happen, N8N(Nathan?). Both underway and standing still. But I'm telling you, caster cannot possibly contribute to nose lift. Perform the CD exercise I suggested, or better yet, pickup a toy "chopper" bike(with a lot of rake/caster) in a toystore and turn the handlebars. Betchya the whole front end of that bike goes down as you turn the front wheel away from center - left or right. No automobile commercially produced has anywhere near

30+ degrees of caster/rake(as in a chopper), but the bike example will clearly demonstrate this principle.

-CC

Reply to
ChrisCoaster

quoted text -

I think the reason you don't see it with the CD and do see it with a car is the offset between the ball joint CL and the Tire CL. It adds another set of geometric interactions. If you picture a shopping cart with those aptly named caster wheels you don't see the rise and fall but you don't have the offset.

Going back to Fred Puhn's book and his statement about measuring caster, ... if we accept that people can and do measure AND SET caster by measuring the rise and fall of the spindle then clearly caster affects the amount of rise and fall.

Reply to
Ashton Crusher

Going back to Puhn's book he discusses this and the bottom line is that "kingpin" IS Steering Axis if you have kingpins, if you don't it's an imaginary line between the upper and lower ball joints. So I think it's really all parts of what ultimately is the same end point. Caster is the angle between the SA and a vertical line. Whether you use kingpins or ball joints its still the same thing. It sounds like what is being called SAI is the same as Caster.

As to the inside outside issue, I think that's an entirely different aspect separate from caster/SAI just like the use of long/short A-arms which have the effect of changing the inside-outside relationship as the wheel goes up and down.

I'm going to keep the Guns in the safe for now......

I uploaded two pages from Puhn's book...

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Reply to
Ashton Crusher

_____________________ Fine. So SAI and Caster are essentially the same thing. It's official, it's gotta be! And Saddam destroyed the WTC, Joe DiMaggio poisoned Marilyn Monroe, and Babe Ruth is really buried under Fenway Pack.

Anything else we need to know?

-CC

Reply to
ChrisCoaster

If you are interested I can tell you who shot JFK.

Reply to
Ashton Crusher

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