Caster Question

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BOOT
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Caster Question

Post by BOOT »

I was reading a thread on another forum and it was suggested that caster was off because of an upper ball-joint replacement. On a double a-arm type suspension.

So I'm far from an alignment guru but If I wanted more pos caster on a double a-arm setup, is moving the ball-joint back some a feasible option to add a bit w/o going all $,$$$ setup? Like altering the a-arm some.

Doesn't seem like it would work to me But I'm not afraid to ask dumb questions if I learn stuff :D
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Re: Caster Question

Post by Tom68 »

More Positive.....Top joint back, bottom joint forward, any safe way you like.

P.S. changing caster changes bump steer.
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Re: Caster Question

Post by David Redszus »

Tom68 wrote: Sat Dec 31, 2022 11:29 pm More Positive.....Top joint back, bottom joint forward, any safe way you like.

P.S. changing caster changes bump steer.
Right.
In addition, the combination of SAI and caster will affect camber in a turn.
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Re: Caster Question

Post by Rick! »

...and jacking, and steering wheel return, and possibly roll center (if a large enough change to a hard point is made), and KPI. One of the few things that may not change is Side View Swing Arm (SVSA) angle so front end anti-dive should stay pretty much the same.
Changing one of the six main kinematic hard points usually will affect a significant amount of Kinematics and Compliance (K&C) curves.

In the big picture, changing to a more positive caster will help high speed steering feel and help keep the front end pointed in the intended direction. It also needs power steering to keep from needing a truck sized steering wheel at low speeds. On light, mid engine cars you can get away with lots of positive caster, like upwards of 5*, ala Ferrari. Most 'Merican muscle cars, +3* is typical with power steering. I stopped doing alignments in the early '80s so things may have changed since then.
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Re: Caster Question

Post by BLSTIC »

If camber gain on corners is your goal, it's worth looking into why you could be losing camber as well.

Braces, control arm flex, bushes (especially offset ones that adjust castor at the same time), even the wheel and tyre itself, are all places you can recover dynamic camber loss.

On a particular flimsy car at high g forces you might be losing (total guess here) 3-4 degrees on top of body roll
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Re: Caster Question

Post by PackardV8 »

On the Studebakers I work on, 1-degree negative caster is the most common, as this makes non-assisted steering and parking lighter. There is not enough adjustment to get most of them to zero, much less 1-degree positive. I've built tapered spacers to install between the A-arm cross pins and the frame to gain positive caster. But as previously mentioned, changing any one thing changes everything.
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Re: Caster Question

Post by agertz1 »

C5 Corvettes move lbj, forward in squash. upper reward = roll castor gain. By inverting a composite, quarter elliptic mono-leaf springs and using as
upper leading links on straight front axle, roll caster gain, is the plan.
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Re: Caster Question

Post by EXCELLENCE AUTO »

I highly suggest watching some of Kevin Wilsons videos on Youtube. Samson Sbc.
Heres one to get you started.
https://www.youtube.com/watch?v=ucpsXPWF4iw
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Re: Caster Question

Post by David Redszus »

10 Commandments of vehicle dynamics

#1 All vehicle forces involve either a change in speed or direction, or both.

#2 All forces that determine vehicle handling must act through the tires.

#3 The forces that determine vehicle handling arise from gravity, aerodynamics
or mechanical forces directly applied through the tires (acceleration and braking).

#4 Mechanically applied forces at the tire take preference over all other forces.

#5 Aerodynamic forces may be either positive (downforce) or negative (lift)
and will vary with the square of the air speed.

#6 Increases in vertical forces applied to a tire will increase lateral grip forces
but not proportionally nor linearly. All lateral weight transfer reduces grip.

#7 All static settings have value only as a beginning point; actual vehicle
handling depends entirely on dynamic forces - that is, forces arising due to
vehicle motion.

#8 All vehicle dynamic forces are transitory and fleeting in nature; they will
change rapidly and continuously.

#9 Vehicle dynamic behavior cannot exceed a certain limit determined by the
derivative of its combined acceleration forces.

#10 Vehicle dynamic forces operate at certain natural frequencies and when
combined with applied forcing frequencies can produce very substantial
amplification of forces which will severely affect handling.

The bottom line:
If we don't use suspension sensors and data logging, we know nothing about
actual vehicle dynmics.
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Re: Caster Question

Post by David Redszus »

Kingpin/Caster vs. Camber angle
When the front wheels are turned into a corner, the camber of the wheels will change.
The effect is called dynamic camber and it will modify the pre-set static camber of
both front wheels. It is important to know the actual tire camber in the middle of the turn
in order to understand tire performance.

Kingpin inclination angle will produce a positive camber change at the outside road
wheel when the steering wheel is turned. If the initial camber angle is negative, the outside
wheel will become more positive as the wheel is turned into a corner. A change in camber
will cause a change in load and ride height at the tire. Geometry induced load changes
can be ignored unless very wide wheels and tires are fitted.

Caster angle will produce a negative camber change at the outside road wheel when
the wheel is turned. Caster will also slightly lift the front wheels during a turn. The amount
of lift will be determined by the camber change due to Ackerman angle. Usually the lift
amount is small and can be ignored due to the large loading of the outside wheel in a turn.

It is apparent that a combination of caster and kingpin have opposite and canceling (but not
equally so) effects upon resultant camber. The actual road wheel camber is a result of static
camber setting, kingpin angle, caster angle and road wheel steering rotation.

Caster....Kingpin.....Camber @ 6deg
4............10..........+.40
6............10..........+.27
8............10..........+.13
10...........10...........0.0
10............8..........-.13
10............6..........-.27
10............4..........-.40

In order for the outside wheel to maintain a negative camber during a turn,
caster angle must always be greater than kingpin angle.
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Re: Caster Question

Post by rebelrouser »

More positive caster makes a vehicle handle better at speed, keeps it going straight. If you want to understand how caster works just watch the front fender sitting still as you spin the steering wheel. You will see the fender rise and fall. The more positive caster the more it rises and falls, so the weight of the vehicle helps keep the wheels going straight ahead. But as you increase the caster you can make the suspension prone to caster shimmy or what some call the death wobble. Where the tires start to shake. Push a grocery cart fast and watch the casters shimmy to get the idea. 4 to 6 degrees is what most a-arm suspension seem to like. A few vehicles were designed with more but no many. I know on Mopars they make an offset control arm bushing that adds a degree of caster pretty easy and cheap, especially if you are rebuilding the front end anyway. I use them on most of my project cars.
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Re: Caster Question

Post by Tom68 »

KPI makes the fender rise and fall evenly.

Caster skew's that and makes it rise one side and depending on the amount of KPI drop on the other side.

Very important when you're crossed up.
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Re: Caster Question

Post by David Redszus »

The rise effect is due to the camber change caused by a combination of KPI and caster.
We have many possible combinations: positive KPI, zero KPI, negative KPI, positive caster,
zero caster and negative caster.

The rise effect It is accentuated by tire width. It may be offset by lateral force induced roll
during a corner.
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Re: Caster Question

Post by David Redszus »

The rise effect is due to the camber change caused by a combination of KPI and caster.
We have many possible combinations: positive KPI, zero KPI, negative KPI, positive caster,
zero caster and negative caster.

The rise effect is accentuated by tire width. It may be offset by lateral force induced roll
during a corner.

Turn the wheel and check the camber of the outside wheel compared to the straight position.
If it goes positive, the suspension is designed to understeer in a corner like most street cars.

A race car should have negative camber in a turn, even after the effect of body roll.
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Re: Caster Question

Post by Krooser »

On the dirt late models I work on I have seen up to 8* of caster split on the front ends. Used to be 2-4*...
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