Steering Wheel Return

Ever push a grocery cart? Did you notice that no matter which way you turn the cart, the front wheels line up with the direction in which you are headed? Same principal. The wheels horizontal axle is aligned to pass behind its vertical axis of directional rotation. Moving forward, the friction of the road surface is applied laterally to the vector of travel, thereby rotating the wheel to align directly behind the vertical axis.

It is also true for the oposite direction. The example would be a bycicle, where the front axis is well in front of the steering axis. It is self centering too. If you pay attention (in a bycicle it is easier to note), the steering action of the wheel tends to lift the weight of the vehicle, that forces the wheel back to aligned position.

Some suspensions have positive, some negative caster. Check out http://www.carbibles.com/tyre_bible.html

My first thoughts were "caster", "camber', & "toe-in",.... toe-in being very important once you link the wheels as one does in a wagon, or a motor vehicle.

Then, before speaking up, I checked out the link you provided and I'm very impressed. I have bookmarked it, and must peruse it when time permits.

I might stress that dkamesh11 be sure to take the time to check that link out and be sure to look at all three of these factors to help in understanding the reasons for steering and control of vehicles being so effortless and reliable.

Not meaning to imply that dkamesh11 may not study the link with out my input, but these factors are detailed near the end, following bucket-loads of interesting information on tires, so just a helping hand (I hope).

Interesting point you made about the caster being functional either rearward, or forward of the swivel centerline. I had a Farmall Super C trike with a belly mount mower on which I reversed the caster direction to ease the steering effort. I was afraid it would be a hazzard to steer on the road, but, it proved to still have excellent steering control.

Interesting point you made about the caster being functional either rearward, or forward of the swivel centerline.

That's not really precisely true. The problem being that posters here have left out some key aspects of steering geometry. (The Car Bible article does, too, because it is concerned with alignment, rather than design.) The best illustration of caster, taken alone is the grocery cart: there is a strong tendency for the contact point to follow the pivot point, when the cart is going forward. If cars had only caster, and you reversed it, then the steering would tend to turn away from center at the slightest provocation - you'd have negative stability. But cars have both caster and king pin inclination. This is most easily seen on classic hot rods, with exposed front axles. Viewed from the front, the steering pivot axis is angled in at the top. (This angle is called king pin inclination.) Therefore, when you turn the steering wheel either way from center, you are lifting the vehicle, providing a strong centering effect. To appreciate the effect, imagine the pivot axis pulled in extremely far at the top: then a great deal of your steering effort would go to lifting the car. You can see that if this angle were too great, you could not make the car do anything but go straight.

Toe-in or toe out has no appreciable effect on steering feel. Typically it is there (at rest) to enable the car to travel with the wheels perfectly straight when underway. In the few cars with very rigid components (and little or no elasticity in the pivots -- e.g. some Citroens, in which suspension pivots were actually roller bearings -- then toe-in can be set to precisely 0.)

If dkamesh doesn't have access to a classic hot rod, he can open up the hood of any car with McPherson strut front suspension. The upper pivot can be seen easily, and is behind and inward from the lower pivot, which is inside the road wheel, very close to the wheel rim and roughly centered above the center of tread contact. Typically, king pin inclination in McPherson strut cars can be quite high: 19 degrees in a Mustang (used as an example file in a suspension simulator I have on this machine). Caster angle (which can be visualized from the side of the car) will be less: 5 degrees or so, typically.

The front end of a car has caster built into it's geometry,that is what tries to center the wheels (straight). The front fork on a moter cycle has a caster rake of 10 to 15 degrees, a car only has 3 to 5 degrees.

We run Ford Capri's on the track, and to improve turn in and stability we increase the caster angle by moving the anti roll bar forward a few mm. Adjusting toe in and toe out has a smaller effect, at least on this vehicle, so we run with the wheels parallel. Camber can be a nightmare, just remember that to much camber means that the tyre contact patch is smaller.

Actually, the situation with a motorcycle is a little different. Designers talk mainly about rake and trail, and rarely mention "caster" (although you are correct that the effect is there). "Rake" is the angle that the steering head makes with the vertical (viewed from the side). This angle is typically 28 degrees +/- 2 degrees in stock road-going motorcycles. (In choppers, this angle can be much larger, mainly in the interests of appearance, but some like the very slow steering response provided, too -- it may depend on how much time you spend riding drunk.) Typically, racy bikes have steeper steering head angles, which promotes quicker steering. Cruisers have just the opposite.

Trail is related, in that it will increase as rake is increased, if all else is held constant -- but in the design process, all else is not held constant, and trail can be set independently from rake, at the design (or modification) stage. Trail is the distance from the front tire contact patch (i.e., directly down from the axle) forward (usually)to the point on the road surface through which a line through the center of the steering head will pass. To reduce trail, one would move the axle forward relative to the steering head, by mounting the axle forward of the fork tubes, and/or by moving the fork tubes forward with offset triple clamps (the brackets at the top of the fork tubes).

Designers and racers obsess about the effects of rake and trail (and, in general, most will say that increasing rake slows steering response, and increasing trail increases steering stability and effort) but Tony Foale has done a lot of interesting testing, finding that the values can be varied dramatically from the standard values while still providing good handling.

Hi Ken, I had a dysfunctional cerebellum, there! I meant of course rake, not caster.

Uh Oh! Your use of "caster" (re Capris) seemed fine to me. Here, we separate "rake" and "trail" only in the motorcycle world, where both are designed into a bike for slightly different but related reasons. Together, they give what is essentially a caster effect. Of course, when you consider that to bank right on a motorcycle you must turn the handlebars to the left, the dynamics are clearly a bit different than they are in the four wheel world.

Every time I see your location, I think of the Ford Anglia (which was sold here in the 60's and maybe into the early seventies). I assume it was named for the town... which got me thinking about other town-named cars. We have a whole brand, Pontiac, named for Pontiac, Michigan. The Ford Fairlane was named for a place -- Ford's Fair Lane estate. Then there is the Chrysler New Yorker, named after a town which is named after another town...

Hi Ken (again), East Anglia is a region, not a town, and I'm not quite sure of the derivation of the name, but I can tell you that A very good friend of mine is restoring one as we speak. (See Picture) As a motorcyclist, it;s nice to see the effect of counter steering is understood, though I suppose this site is the place for that.

Counter steering is becoming increasingly understood, but it is still surprising how many riders do not understand it. The Motorcycle Safety Foundation here puts out books on learning to ride that many states use. In it they say "Push right, Go right." and explain a little bit about how that works -- but still, there are people convinced that you turn the bars to the right (push left) to go right. EVEN some racing schools still debate using body english alone vs counter-steering. One school has built a couple specialized motorcycles to prove that you can't make it around a race course without counter steering: one simply has an extra set of bars mounted to the fairing instead of the steering stem, and using weight shift alone, the bike is essentially uncontrollable (unless your reference is the turn rate of a tanker coming into port).

We still have plenty of accidents here where people bank directly into oncoming traffic while thinking that they are steering away. The problem is subtlety, I suppose. When you are not panicked, the control motion is so subtle as to be almost undetectable: it feels like you just do the natural thing, and lean to to right. But when you are panicked, your brain is shouting "turn away!" and you crank the bars that way, only to find that the bike won't lay down as expected.

Interestingly enough, the Wright brothers knew that one counter-steered to turn a fast moving bicycle.

Does that make you an "Angle"?

i'll probably never b able to truly afford Norton's "Manx" (but i can dream!)

i seem to encounter you often; bravissimo on your good taste.

Cyclists use the term 'rake'; 4-wheel (&Morgan) types use 'caster' for the same geometry. Serve exactly the same function: self centering of the front wheel.

Where cyclists complicate matters is the addition of 2 more variables;

1) an Offset of the wheel center from the "kingpin" centre.

2) The ability to change tire diameter at a whim; this alters both the 'projected intersection of axes, & the rake angle (relatively large to 4wheel, shorter wheelbase).

Hence the term "trail"; easily physically measured w/ straightedge + scale. Is the∫ of all variables.

In case of M'Cycle, the total contact patch lag value is reduced by axle offset

The easiest way to understand the principal of how caster works is to see that the points where the spindle or axle of the wheel are mounted back to the frame are tipped either forward or backward from being directly in line above or below each other. When you turn the wheel in either direction one wheel is being moved farther away from the bottom of the frame as it rotates about that axis of its pivot points. The other wheel typically is moving closer to the frame at the same time. This transfers weight to the wheel moving away and takes weight off the tire that is moving toward the frame. When you let go of the wheel the weight imbalance corrects itself and since the balance point occurs when both wheels are the same distance from the frame the car is steering straight again at that point. Hold your hand out in front of you with the fingers up and together and both palms facing each other. Tilt both hands away from you at 10 or 15 degrees. Rotate your forearms from the elbow to turn your hands and what how your hands move in relation to each other. This is basically what is happening with the front wheels on your car.

Simply put, the turned wheels on the pavement, which are actually casters, force the steering gear, hence the driver's wheel, to return to more or less center. Casters follow the rear wheels--the direction of momentum of the steered vehicle (or, in the case of crowned/banked pavements, the path of least resistance, even off the road)--because a caster wheel trails behind the caster pivot point on the vehicle. You can readily observe the dynamic by observing that, when the car is driven in reverse (where the wheel does not trail the pivot point), the steering wheel does not center on its own when let go. Accomplished drivers exploit this for making quick u-turns, by driving in reverse.

Please indulge me in a Query:

I (reasonably well) understand how these components of 4our wheel geometry effect handling on a RWD unit (has served me well, until now.

Am most interested in the DRIVEN front axle; how does power/trail effect the dynamics of a FWD/AWD arrangement?

(specifically: i'm thinking RWD to ~250KPH, then AWD thereafter; it's @ 230 kph that i'm interested in)