Caster: Mo's Last Imponderable

Discussion in 'General Automotive Questions' started by mo.herbfarm, Aug 19, 2015.

  1. TestDummy

    TestDummy In Maximum Overdrive SILVER MEMBER

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    I'll wait for the series to come out on DVD.
     
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  2. ribald1

    ribald1 Banned PLATINUM MEMBER

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    If the vehicle is moving in a straight line on a flat road the vehicle will not be affected by a difference in camber between the wheels. What it will do is tend to follow with or against the camber of the road. As the camber of the road is generally down to the right, your proposed vehicle should want to tend leftward until it reaches a zero or leftward camber in the road.
     
  3. azcolin

    azcolin In Maximum Overdrive

    Are you still taking your medication Mo???? seems to me you might need to double the dosage ???
     
  4. ribald1

    ribald1 Banned PLATINUM MEMBER

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    Cut him some slack.
    He got suckered by a guy that goes after new members offering to 'show them the ropes'.
    Most run away after figuring out what was done to them, he has the courage to hang in.
    He may never present himself as he really is, but after enduring that trial by fire he deserves at least some grudging respect.
     
  5. mo.herbfarm

    mo.herbfarm In Maximum Overdrive

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    After that, how can I realistically cut you down about the double-talked "straight ahead on a flat road" which suddenly becomes cambered downwards off to the right? mo.
     
  6. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    Yes, i think i had gotten it confused Mo, they are all attached i guess, but it was the 'way it is facing' bit, i think i misunderstood.
     
  7. ribald1

    ribald1 Banned PLATINUM MEMBER

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    My bad, I guess not everyone understands how roads are designed.
    Other than gravel roads or certain race tracks, no road is flat. Most roads are made with a double camber so that water flows away from the center.
    Keep in mind that this situation relates to motor vehicles only due to the Ackermann steering geometry. A cart or other similar system does not interact with road camber regardless of differences in caster between the front wheels.
     
  8. tony o

    tony o In Third Gear

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    In my first response I was trying to show that thru the concept of trail that the stability dynamics in effect are the same between an automotive steering with positive castor and a front shopping cart castor. In the case of the shopping cart castor it appears backwards. Because a shopping cart castor has a zero degree castor or steering head angle, trail is created by offsetting the wheel axel from the centerline of the steering angle. If a person was to add enough positive castor angle to a shopping cart castor to generate a trail dimension large enough to overcome the axel offset, it would rotate to the front and exibit the same stability dynamics as a car or motorcycle. In the case of a shopping cart, it is much easier to manufacture by using a zero degree steering angle and generating the trail in the wheel assembly.
    Herb. Please keep the imponderables coming. I've enjoyed them. Tony
     
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  9. mo.herbfarm

    mo.herbfarm In Maximum Overdrive

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    Thank you, Tony! Well-said. Caster is important for stability in a suspension/steering system. Hardly of any value on a shopping cart, though, as the damn thing would want to always go in a straight line! One item crossed my mind during this melee: Caster angle actually changes, as the wheels are steered from side to side. This is due to what we used to term "King Pin Inclination". That came from the old solid axles. Used to also be a more common problem with steering, as the kingpins and bushings got badly worn, the "Ackerman" principle no longer was held close; that made for poorer steering and more tire wear. The amount of "inclination", that is, tilt from true vertical of the steering axis of the knuckle, viewed from either front or back, is dependent on wheelbase, and is built-in on todays steering spindle/knuckles. IOW, the top ball joint is tilted inwards toward the other side of the vehicle when compared to the lower ball joint (if it has 2).

    Lots of hot rods are advertised as having different than stock front suspensions, Mustang II comes to mind as often used. Often wondered if builders attempted to main true Ackerman steering in principle, didn't have a clue, or didn't give a damn! mo.
     
  10. burninbush

    burninbush In Maximum Overdrive

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    Uhmmm ... isn't king-pin inclination identical to caster in a solid front axle?
     
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  11. jwarden

    jwarden In Third Gear

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    The question I have is why is it that I always get the shopping cart with the bad caster, "seems" like ya always get the one wheel that never tracks right or sticks. Of course shopping carts are highly engineered, well tooled and in fine working condition, NOT. Sorry, didn't have anything intelligent to add, to the discussion.
     
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  12. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Yeah, pretty much...um, more or less. I won't go into it as it gives me a headache just thinking about it (steering geometry was the one principle I had so much trouble wrapping my head around, even though everything else was easy to understand, including electronics), but the upshot is that in steering alignment, you want the best SAI to give the largest 'scrub' area (basically the tire's contact patch) in order to give the tire the longest amount of tread life.
     
  13. mo.herbfarm

    mo.herbfarm In Maximum Overdrive

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    This says much more concisely what it took all my gobbledy-gook to not be able to! Of course, the solid axle is "tipped" slightly backwards as well, to achieve caster angle on both ends. So, in a way of speaking, the king pins are tipped inwards as well as backwards! One could argue a bit with the latter part of the statement below, in that the steering wheel must always return to straight-ahead coming out of a turn, or the damned vehicle would continue turning, no?

    From: http://www.engineering-dictionary.org/Kingpin_inclination
    Kingpin inclination
    [​IMG]
    Dictionary of Automotive Terms
    An alignment adjustment where the tops of the kingpins are tipped inward toward each other. This places the center line of the steering axis nearer the center line of the tire-road contact area. Thus when the vehicle comes out of a turn, the steering wheel returns to the straight-ahead position. Also called "steering axis inclination."

    Below is a diagram illustrating the wheel location necessary for true Ackerman geometry: the principle states that in order for the 4 wheels to roll smoothly during turning, perpendicular lines through the center of each wheel must meet at a common point. Clearly, in order for that to happen, the inside front wheel must rotate farther inward than the outside wheel, because they do not travel around identical circle diameters. The two front wheels are "tied" together (tie-rod?). If their steering axes were parallel, they would both turn the same amount, not conforming with Mr. Ackerman's idea. King pin inclination makes the inner wheel rotate farther inward than the outer wheel. Note that the longer the wheelbase (distance between front and rear wheels), the farther away from them will the common intersection point lie. IOW, the difference in the angles turned by the front wheels becomes less the longer the wheelbase is; that requires less king pin inclination. (For infinitely long wheelbase, k.p.i. becomes zero, let's not even go there!). mo.

    [​IMG]

    EDIT: I AM WRONG. King Pin Inclination is NOT solely responsible for achievement of the Ackermann Principle. mo.
     
    Last edited: Aug 22, 2015
  14. tony o

    tony o In Third Gear

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    I think something we need to add another element to the discussion. In an automotive application the wheel is offset from the steering or castor axis. The distance from the center line of the contact patch of the tire to the center line of the steering axis forms moment arm or lever. This moment arm in conjunction with the caster and camber cause the end of the spindle, when viewed from the side, to scribe an arc like an upside down happy face with the high point when the steering is on center. This creates the natural return to straight when you let go of the wheel.
    Ackerman angles are almost entirely generated by steering linkage geometry and not castor of camber. It is the result of the angle between the spindle and the steering arm and the length of the steering arm. If the angle is 90 degrees, Ackermann is non existent. In most cars he steering arm angle is offset about 10 degrees or the spindle to steering arm angle is around 100 degrees. To answer your question Mo about street rod suspensions. Most spindles have the steering arm cast integral to the spindle so there is no way to adjust the Ackermann angles. The wheelbase and wheel track determine the needed Ackermann geametry. So I believe the after market sees close is good enough as far as Ackermann goes. Tony
     
    Last edited: Aug 22, 2015
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  15. ribald1

    ribald1 Banned PLATINUM MEMBER

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    The wheel is not always offset from the steering axis. Many straight axle and Ford dual I beam axle set ups, as well as some sports cars, came from the factory with the wheel centered over the steering pivot. My 58 Triumph TR3 (sadly lost in 1972) came with just under 3/16" of offset, which I removed. This theoretically allows for zero toe in. As bearings and front end parts are not perfect, a slight amount of toe in is used in practice.
    I suspect that the only reason wheels are offset outward is that people like the look of a recessed wheel interior.
     
  16. tony o

    tony o In Third Gear

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    I wasn't talking about wheel center offset in and of itself. I was referring to the moment arm created from the steering axis centerline to the wheel mounting flange on the hub and +- the wheel offset. A distance of 3.00-4.00 inches for the moment arm, steering axis to hub flange, is common on most cars, this includes straight axel, ford twin I- beam ,Triumph TR-3 and Ford Ranchero's. Using a positive offset wheel with an offset equal to the steering axis to hub moment arm would almost completely remove any return to center effect at the factory castor setting and completely remove any feel of feed back from the steering. With a wide enough tire it could create the situation once you started to turn it would be harder to turn back than turn in unless you were to compensate with a very high positive castor. Say 10+ degrees. At that high of a castor setting I would expect to generate excessive shoulder wear on the front tires if you did much turning. Sorry for my lack of clarity. I hope this clears things up. Tony
     
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  17. plumcolr

    plumcolr In Maximum Overdrive SILVER MEMBER

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    Please keep it up, Mo. All this learnin' makes my brain hurt, and since masochistic sex is no longer possible - - -

    In other words, any day I learn sumpin is a good day, and your imponderables tend to promote that.
     
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  18. ribald1

    ribald1 Banned PLATINUM MEMBER

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    Just like the fork balancing bit, the moment arm is the relationship of the center of the tread to the center of the steering axle. Were it otherwise a big rig (often with more than 1 foot from the lug nuts to the steering axle) would handle like a ricer that has the wheels sticking out 6" from his fenders.
    In fact, there are two moment arms. The distance between the steering axle and the lug nuts, and the distance between the lug nuts and the center of the wheel. Otherwise the goofy ricer would handle just as well with the wheels sticking out. These moment arms can add, as in the case of the ricer, or deduct, as in the case of the big rig.
    Imagine a unicycle with a V shaped bar between the seat and the wheel support. Other than the weight of the bar, would the operation of the unicycle change?
     
  19. mo.herbfarm

    mo.herbfarm In Maximum Overdrive

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    I am working on one that will not only be imponderable, but also intangible, to boot! Thanks for the support! mo.
     
  20. TestDummy

    TestDummy In Maximum Overdrive SILVER MEMBER

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    We tolerate your imponderables, but may not accept your intangibles. It's a slippery slope you navigate, Mo.
     
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