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View Full Version : what is caster when aligning a car?


Magna
12-27-2002, 09:55 PM
ok, i think i know what camber and toe is, but i don't know what caster means? negative camber is when the top of the tire lean toward each other. toe-in is when front of the tires are pointing in. i search around the internet but it's not clear to me. one site said caster is similar to the orientation of bicycle wheels turning.

Magna
12-27-2002, 09:58 PM
forget to ask how does the dealer align the car. i went to the hunter website but it didn't make much sense to me. according to the site a camera linked up to a plastic dish sensor that's attached to all four wheels somehow aligns the car.

doeboy
12-27-2002, 10:47 PM
Originally posted by Magna
ok, i think i know what camber and toe is, but i don't know what caster means? negative camber is when the top of the tire lean toward each other. toe-in is when front of the tires are pointing in. i search around the internet but it's not clear to me. one site said caster is similar to the orientation of bicycle wheels turning.

I believe caster is how much the wheel position is forward or back in the wheel well relative to where the suspension mounts to the rest of the body....

For example.... / | or \ looking from the side.... a lot of cars will have the caster set a little towards the front.... so it does sorta resemble the orientation of the wheel in relation to the suspension's load point on the body....
\
(o) --> (front of the car)
wheel

ok... I know... crappy attempt at an illustration but I hope this sorta explains it... :D

operknockity
12-27-2002, 11:37 PM
Found this with a simple Google search... Camber, Caster and Toe: What Do They Mean? (http://www.ozebiz.com.au/racetech/theory/align.html)

Haven't read it through yet, but enough to find out what castor is... "Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative."

doeboy
12-27-2002, 11:44 PM
Originally posted by operknockity
Found this with a simple Google search... Camber, Caster and Toe: What Do They Mean? (http://www.ozebiz.com.au/racetech/theory/align.html)

Haven't read it through yet, but enough to find out what castor is... "Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative."

Yeah.... that's what I meant... :D hehe....

very well put.... :thumbup:

kster
12-28-2002, 12:44 AM
I prefer as much positive caster as possible on my cars because:

1. Makes the steering wheel stiffer
2. Static caster becomes dynamic negative camber under conering.
3. Makes the steering wheel more self centering

Pinecone
12-28-2002, 04:44 AM
To understand caster, go to the supermarket.

Get a cart. Push the cart watching the front wheels. Caster angle sets the force that tries to make the front wheels point in the direction the car is moving.

In the case of a shopping cart you can see how the pivot point (steering axis) is in front of the wheels contact point. That means that the drag of the wheel pulls the wheels to in line with the force.

In a car the steering axis is tilted so that the point where a line through the steering axis hits the ground is in front of the tire contact patch.

On BMW strut type front suspensions the steering axis is a lien front the top of the strut at the shock nut center to the lower ball joint where the strut assembly attaches to the control arm.

Caster also makes camber change as you turn the wheels. Since the steering axis is angled, the wheel turns about this angle so with positive caster, you gain camber as teh wheel is turned towards the center of the car, you lose camber in turning it away. So in a right turn the left wheel gains camber the right wheel loses camber just due to the steering angle.

To measure caster you use a camber guage and move the wheels from about 20 degreees either side of straight ahead. You then use the difference in the camber between the two to calcuate caster.

As for how an alignment machine works, it is just a facny laser system to do what you can do with a level and few pieces of string. To measure camber you measure the difference between the distance to a level held vertically from the top of the rim and the bottom of the rim. Toe is measured front to back ro a line parallel to the middle of the car. Caster I mentioned already. The fancy machines just hook a laser gadget to each wheel to measure these things faster and more accurately.

Magna
12-28-2002, 11:56 AM
ok thanks!! got some reading and experimenting to do. going to the supermarket is going to be more interesting than usual. :-)

e28Will
12-30-2002, 07:17 PM
Here's a good write-up about caster by people who deal with the intricacies of suspension setups all day long.

Caster (http://www.ground-control.com/gctalk5.htm)

:thumbup: