RaisedByWolves
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Matco
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That's a non-swiveling tailwheel.
It looks cool but IMHO it'd be less than satisfactory for actual use on an airplane.
Think how often you swivel your t/w around when ground handling or when maneuvering around in tight quarters.
The most common reason tail wheels shimmy is because of a negative caster angle. It may be positive when empty, but egative when the airplane is loaded.I have one of those, and used it for a while. Not worth the effort. Here is one of my lathes - a work of art!
The most common reason tail wheels shimmy is because of a negative caster angle. It may be positive when empty, but egative when the airplane is loaded.
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Does negative mean the bottom of the pivot pin is further back than the top of the pin? I can see this angle increasing as the weight at the tail increases and the spring angle to the ground decreases from its (45 degree) angle when load weight is added
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Yes you are correct. Bottom of pin is further back then the top. Re-arch the spring so that it is at a positive angle when loaded. Or you may be able to shim the tail wheel with a tapered shim.Does negative mean the bottom of the pivot pin is further back than the top of the pin? I can see this angle increasing as the weight at the tail increases and the spring angle to the ground decreases from its (45 degree) angle when load weight is added
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You beat me to it Raised by Wolves
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The geometry of the matco might be different than the API. Thickness of tires has nothing to do with the pivot point of the wheel.That's all great, but I think to the wide tire throws another dynamic into it. The API tailwheel was rock solid, but the fat Matco shimmied like a stripper, on the same spring.
I think you'll find those drawings are mis-labeled. Positive caster exists when the contact patch of the tire is behind the steering axis. All tailwheels have positive caster, the issue is how much. For tracking stability you want as much as possible without inducing shimmy. The acceptable amount varies with the mass of the tire/wheel assy. A heavier assy will tolerate less caster before occilations set in. I've been through this a million times with Ford trucks and their infamous death wobble. Excess positive caster causes wobble, excess negative caster causes wander.
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Take another look and youll see that the tire contact patch is behind the steering axis. Excesive positive caster does cause wobble on a vehicle where that wheel is leading. Excessive negitive castor on a leading wheel does cause wander, but on a wheel thats trailing it does cause a shimmy.I think you'll find those drawings are mis-labeled. Positive caster exists when the contact patch of the tire is behind the steering axis. All tailwheels have positive caster, the issue is how much. For tracking stability you want as much as possible without inducing shimmy. The acceptable amount varies with the mass of the tire/wheel assy. A heavier assy will tolerate less caster before occilations set in. I've been through this a million times with Ford trucks and their infamous death wobble. Excess positive caster causes wobble, excess negative caster causes wander.
Does negative mean the bottom of the pivot pin is further back than the top of the pin? I can see this angle increasing as the weight at the tail increases and the spring angle to the ground decreases from its (45 degree) angle when load weight is added
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I have studied automobile stearing and suspension geometry. As a matter of fact I design suspensions for street rods etc. What you call wobble is a shimmy. And I beg to differ with you on thst grocery cart. When the wheel gets bent backwards, it is a negative caster. As far as termanology is concerned, I dont see how its backwards. The drawing is exactly right.Certainly the contact patch is behind, obviously so. My point is, you guys got your terminology backwards. It doesn't matter which end of the vehicle it's on, the wheel doesn't know the difference. A tailwheel wobbles for the same reason a shopping cart (front) wheel wobbles. Too much positive caster. In the case of a shopping cart, it's usually had a curb strike and has been bent just a bit more positive than it was designed to be. Just like with a tailwheel, the mfgr. was going for as much stability as possible without wobble, and it got pushed over the edge. Same thing with a saggy tailspring, the effective caster angle has increased.
If you really want see something confusing, study automotive steering geometry and you'll be COMPLETELY convinced I'm wrong. With a typical auto steering system, leaning the top of the steering axis BACK causes positive caster. Looks exactly opposite of our shopping cart wheel, but it's not. In both cases the wheel meets the surface behing the axis, and the result is the same. Too much and it wobbles. It gets hard to equate because of the two vastly different means of achieving the same condition.
I have studied automobile stearing and suspension geometry. As a matter of fact I design suspensions for street rods etc. What you call wobble is a shimmy. And I beg to differ with you on thst grocery cart. When the wheel gets bent backwards, it is a negative caster. As far as termanology is concerned, I dont see how its backwards. The drawing is exactly right.
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Well wiki is wrong. If you study suspension geometry, if the steering axis is straight up and down ,it is at zero caster. If the top of the steering pin is back, it is positive castor. If the top of pin is forward it is negative castor. Look at any auto manual under wheel alignment and that is what you will see. It is true that the tire contact patch in both cases is behind the steering pin, however it is the distance between the contact patch and the axis intesection line that makes the difference. The closer that distance the less stable the wheel. The greater that distance(to a point) the more stable and self centering the wheel becomes. When you go around a turn in your car and you let go of the wheel it wants to straighten itself out and go straight. This is self centering caused by positive castor. If you have negative castor in your car, it will not self center. It will have a tendency to wonder.According to Wiki, both the tailwheel pics are positive caster angles https://en.wikipedia.org/wiki/Caster_angle and this looks to be a debate about how much positive is enough Gut feel says the pivot angle should have some bearing (excuse the pun!!) on stability and sensitivity etc, but I'm no expert although having said that, I've just moved from a touring type bicycle to a sportive one and the steering is WAY quicker/sensitive!! I suspect the head angle is the reason...........
Wobble vs. shimmy is tomato/tomato. It's also called caster flutter in some circles.I have studied automobile stearing and suspension geometry. As a matter of fact I design suspensions for street rods etc. What you call wobble is a shimmy. And I beg to differ with you on thst grocery cart. When the wheel gets bent backwards, it is a negative caster. As far as termanology is concerned, I dont see how its backwards. The drawing is exactly right.
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The reason your sport bike is quicker steering is because the head angle is less positive than on the touring bike, making that distance from the tire contact point to steering axis line shorter,ie quicker steering.Well wiki is wrong. If you study suspension geometry, if the steering axis is straight up and down ,it is at zero caster. If the top of the steering pin is back, it is positive castor. If the top of pin is forward it is negative castor. Look at any auto manual under wheel alignment and that is what you will see. It is true that the tire contact patch in both cases is behind the steering pin, however it is the distance between the contact patch and the axis intesection line that makes the difference. The closer that distance the less stable the wheel. The greater that distance(to a point) the more stable and self centering the wheel becomes. When you go around a turn in your car and you let go of the wheel it wants to straighten itself out and go straight. This is self centering caused by positive castor. If you have negative castor in your car, it will not self center. It will have a tendency to wonder.
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I think I see the problem with our discussion. If you look at an automobile diagram you will see that the steering axis passes through the center of the wheel. On a tail wheel or cart, the steering axis is way ahead of the wheel and will never pass through the center of the wheel. That is what I meant by a trailing wheel. Actually a brand new cart has zero castor. Castor is the inclination of the steering pin or king pin. The distance from the tire contact patch and the steering axis intersection is called trail.Wobble vs. shimmy is tomato/tomato. It's also called caster flutter in some circles.
As far as positive vs negative with regard to the shopping cart, I'm happy to differ. Study it some more and re-read the last couple sentenses of my prior post. Compared to a drawing of a typical automotive system a shopping cart LOOKS as negative as you can get, until you understand what actually defines caster.
I think I see the problem with our discussion. If you look at an automobile diagram you will see that the steering axis passes through the center of the wheel. On a tail wheel or cart, the steering axis is way ahead of the wheel and will never pass through the center of the wheel. That is what I meant by a trailing wheel. Actually a brand new cart has zero castor. Castor is the inclination of the steering pin or king pin. The distance from the tire contact patch and the steering axis intersection is called trail.
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According to Wiki, both the tailwheel pics are positive caster angles https://en.wikipedia.org/wiki/Caster_angle and this looks to be a debate about how much positive is enough Gut feel says the pivot angle should have some bearing (excuse the pun!!) on stability and sensitivity etc, but I'm no expert although having said that, I've just moved from a touring type bicycle to a sportive one and the steering is WAY quicker/sensitive!! I suspect the head angle is the reason...........
You got it Philly.Yes, that's really helped me, thank you. I went back to the Wiki post and see that they're defining Castor angle as the difference between the steering tube pin angle and the vertical and with that definition, the Positive/Negative labelling of the tailwheel pics is correct to me (I sincerely hope!!!!)
I'm guessing and then standing well back that the combination of Castor Angle and trail both have an influence on performance, length of levers effect etc?
Agreed! If you look at my edit time of post #50 you'll see we were typing basically the same thing at the same time. Oh well, I figure that horse is pretty dead. We'll have to find a new one!I think I see the problem with our discussion. If you look at an automobile diagram you will see that the steering axis passes through the center of the wheel. On a tail wheel or cart, the steering axis is way ahead of the wheel and will never pass through the center of the wheel. That is what I meant by a trailing wheel. Actually a brand new cart has zero castor. Castor is the inclination of the steering pin or king pin. The distance from the tire contact patch and the steering axis intersection is called trail.
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Agreed! If you look at my edit time of post #50 you'll see we were typing basically the same thing at the same time. Oh well, I figure that horse is pretty dead. We'll have to find a new one!
Cool great discussion.Agreed! If you look at my edit time of post #50 you'll see we were typing basically the same thing at the same time. Oh well, I figure that horse is pretty dead. We'll have to find a new one!