aaadave
Well-Known Member
in the alignment of a front end, i know what camber and toe in does, but could someone tell me what role caster plays in the alignment? thanks, dave.
caster
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mikeysmopars
Well-Known Member
The more caster you have the streighter and easyer it will steer, too much caster and you'll have issues like backing up turning
pishta
I know I'm right....
Shopping carts front wheels-- LOTS of caster! Sort of like the ability of the wheel to be dragged behind the pivot point to assume straight steering automatically. If your A arms put your upper ball joints more forward than stock , you would have increased your caster.
aaadave
Well-Known Member
so on the upper control arm, with the rear bushing all the way out, and the front bushing all the way in you would have maximum caster even though camber would probably be off?
Caster is the angle of the steering axis as viewed from the side of the car. If the top of the steering axis is behind the bottom the angle is said to be positive. If the top is in front of the bottom of the axis, the angle is negative. Imagine what the wheels would do with no steering linkage with positive caster. In other words, with the upper ball joint behind the lower one as viewed from the side. The wheels would turn in on each other. Since they cannot, because the steering linkage is there, this gives the "returnability" you feel when you come out of a turn and the steering wants to find center all by itself. This is good for a street car or one going in a straight line like a drag car. It gives great high speed stability. Positive caster is not always good though. It all depends on what you are doing with the car. Roundy round cars and road race cars benefit greatly from negative caster. Since they are set up to turn more, negative caster helps them to get into the turn more easily. Just as positive caster helps to keep the wheels straight, negative caster helps induce turns. Caster is not the only angle involved by a long shot, but it is one of the most important. SAI (steering axis inclination) is also of great importance. That is the angle of the spindle or steering knuckle when viewed from the front of the car. SAI can also have great influence on caster, but is usually a "built in" angle and is not adjustable.
Mopar to ya
Well-Known Member
Think of caster as the fork on a motorcycle. The more raked it is, the easier it is to drive straight and true. If it was straight up and down, the motorcycle would wander all over the road and need constant correction. Caster is not a tire wear angle, but it helps the vehicle track. And caster that is way out of whack is often indicative of bent front end parts.
aaadave
Well-Known Member
thanks fellas, i know now what to be aware of. i've always managed to get my toe and camber close so that it's easy on the tires, but never was sure about the caster. down where i'm at, if it does'nt have a mcpherson strut on it, these idiots dont have a clue what to do with it. you mention torsion bars and they give you that '' deer in the headlights'' look. lol. thanks again, dave.
Somewhere on this board, there's a couple of threads on front end alignment. The agreement is, with modern radial tires, you do NOT want to run the original alignment specs on these old cars. AllPar.com had a write up about that, as well.
Here's one I had saved:
http://www.forabodiesonly.com/mopar/showthread.php?t=60712
and a quote from somebody I copied and saved:
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"Have them start with the front cam all the way out and the rear cam all the way in. Move the rear cam out until camber is in spec (1/2 deg. neg is good). This is the most positive caster that is available without new parts (offset bushings, tubular upper control arms, etc.). All of this is AFTER setting ride height. I usually set ride height with about an inch between the LCA bumper and the frame, then even the sides up."
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and some more:
http://www.allpar.com/history/mopar/front-end-alignment.html
http://www.moparmusclemagazine.com/howto/mopp_0701_1968_plymouth_gtx_suspension/index.html
Here's one I had saved:
http://www.forabodiesonly.com/mopar/showthread.php?t=60712
and a quote from somebody I copied and saved:
======================================================================
"Have them start with the front cam all the way out and the rear cam all the way in. Move the rear cam out until camber is in spec (1/2 deg. neg is good). This is the most positive caster that is available without new parts (offset bushings, tubular upper control arms, etc.). All of this is AFTER setting ride height. I usually set ride height with about an inch between the LCA bumper and the frame, then even the sides up."
======================================================================
and some more:
http://www.allpar.com/history/mopar/front-end-alignment.html
http://www.moparmusclemagazine.com/howto/mopp_0701_1968_plymouth_gtx_suspension/index.html
BillGrissom
Well-Known Member
Good explanations. My terminology before in FABO was not quite right. The "shopping cart effect" is actually termed "trail", as the wikipedia article explains. Positive "trail" causes the front wheels to tend to follow the current direction of the car, as in pulling it with a tow-bar.
Caster is the vertical angle of the wheel pivot axis in a side view. I might be wrong (again), but it seems that both caster angle and trail are involved in the tendency for the wheels to self-center. Shopping cart wheels have large positive trail, but zero caster and don't self-center. If there was large positive caster but zero trail, it seems the wheels would also not self-center.
I know that in bicycles (and motorcycles), the steering axis must angle back (positive caster) and hit the road at a point behind the tire contact patch for the steering to self-center, i.e. you can ride with no hands. That is why beach bikes have forks that curve at the bottom and motorcycles have the pivot behind the forks. Many high-tech road bicycles have straight forks and are very unforgiving if you let go of the handle bars. Bikes are thus stable with "negative trail", which is the opposite of what wikipedia suggests is needed to self-center. I suspect the SAI angle also comes into play in cars.
Designing a front suspension is very involved. When the wheels turn, the inner and outer front wheels must angle differently. I get concerned when people on FABO suggest mod'ing a rack and pinon steering from a junkyard car to a Dart. You might get it to fit, but what about all those subtleties, some of which could put you in a ditch.
Caster is the vertical angle of the wheel pivot axis in a side view. I might be wrong (again), but it seems that both caster angle and trail are involved in the tendency for the wheels to self-center. Shopping cart wheels have large positive trail, but zero caster and don't self-center. If there was large positive caster but zero trail, it seems the wheels would also not self-center.
I know that in bicycles (and motorcycles), the steering axis must angle back (positive caster) and hit the road at a point behind the tire contact patch for the steering to self-center, i.e. you can ride with no hands. That is why beach bikes have forks that curve at the bottom and motorcycles have the pivot behind the forks. Many high-tech road bicycles have straight forks and are very unforgiving if you let go of the handle bars. Bikes are thus stable with "negative trail", which is the opposite of what wikipedia suggests is needed to self-center. I suspect the SAI angle also comes into play in cars.
Designing a front suspension is very involved. When the wheels turn, the inner and outer front wheels must angle differently. I get concerned when people on FABO suggest mod'ing a rack and pinon steering from a junkyard car to a Dart. You might get it to fit, but what about all those subtleties, some of which could put you in a ditch.
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