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Discussion Starter · #1 ·
Okay, first off, credit goes to Huckleberry5 because I chopped his picture of his UCA for my diagram.

So, I'm unsure how the caster should be setup on my Roadster. Donor LCAs, Fox Spindles, and the "Gold" colored UCAs from Factory Five. If the Factory5-suggested caster setting is used (and I don't remember what it is), should the UCA's and geometry resemble Scenario "A" or Scenario "B"?

 

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you want it to lean back (right pic) to allow the steering to self center. 2-3 with no power steering and as much as you can get with PS, 5+degrees
 

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I think it's Scenario B too.
 

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Yep,

Having positive Caster basically means the spindle leans back when viewed from the side. This makes the steering self center.

To get more, you increase the length of the front arm and / or decrease the length of the back one. (trying to get your Camber right also)

Welcome to the goose chase! :D

R :D

[ August 08, 2007, 09:21 AM: Message edited by: ChevyCobra - Randy J. ]
 

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Discussion Starter · #5 ·
Interesting. I started getting interested in this, because mine's set up like Scenario A, and when Huckleberry5 posted his pic (Scenario A, as well), I had just finished a conversation with Richard Oben about caster, and we looked at his Coupe, and it's set up like Scenario B, so I was all confused.

I have noticed on my car, when I hit the brakes while turning a corner, I can feel the steering wheel twitch, and my car is all over the road when I hit a crown in the road surface. I know a lot of that has to do with the 255's on the front, and maybe some bumpsteer (I'm using the kit-supplied outer tie rods with a Flaming River rack), but it seems pretty twitchy when I drive. It sounds like my alignment guy went the wrong direction when setting the caster, no?

-Greg
 

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WOW!! This might explain why a few guys find that their cars are so twitchy! "B" is definitely correct.
 

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Randy, I think you have the negative VS positive reversed. We have always run -1 camber and +3 or 4 caster. From my understanding + caster is what we see in B. It gives a positive feel to wheel. We run 3 to 4 degrees on street cars as the more caster you have the less the bump steer is prevelent. HTH, Cheers Richard.
 

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Somebody said once.. look at a bicycle... if the front forks were straight down.. it would be a beotch to ride.. hence the curve and rake to the front forks.. kinda made sence to me.
Whats a MK1 owner to do
 

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Richard, thanks! When I was doing the alignments, I got it right. Just had the term wrong!

I fixed it in the above post.

Also, here's a picture that shows how it is supposed to be:



R :D
 

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If it was setup like A, I think you would crash. The wheel would turn with uneven forces (i.e. brake disk clamping forces, irregularities in the road etc). I lawn mower doesnt have any caster, if you let go of the wheel it stays there. This makes the machine easy to steer.
 

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Originally posted by DMW:
How much caster oil should we use?
I believe that depends on your diet.
 

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Two questions -

1) For those of us doing home alignments with string and a tape measure, what is the best way to actually physically measure the caster value?

2) Am I correct in thinking that the further back the the spindle leans (IE, the more you exaggerate scenario B) the car is more stable and self centers easier, and the trade off is increased steering effort?

Good info here!
 

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It should also be mentioned that tire diameter will affect how much caster you should use. The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.



The steering axis (the dotted line in the diagram above) does not have to pass through the center of the wheel, so the caster can be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping trolley (shopping cart in the US) wheel, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.
 

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Hi Greg, I got your note. Good catch. I noticed this about a week ago and since I'm not driving the car yet I put it on the back burner. I did pull out the alignment report and pulled the tire off to check some measurments for myself this morning.

The alignment was done the first week of May and I havn't driven the car since. Can't comment how it handles on the road. The specs show the front caster as +3.9 on the left and +3.6 on the right. Yes they are supposed to be different, can't remember the reason the tech gave but driver weight vs doing alignment on empty car or something like that.

So any way the alignment report says that it is correct. However looking at the position of the top control arm I am also wondering. On the right side not shown in the picture above the measurments between the front and rear arms have the rear arm about 1/4" longer. On the left shown above in both pics the rear is about 1" longer. This really doesn't seem right.

The pics above don't show the ball joint on the lower control arm so it is difficult to get the right perspective between caster angle and pivot line. Believe it or not as close as I can eye ball it with a couple straight edges and a string line it looks like the caster is pretty close. I'm going to do some more checking. Maybe I have a lower control arm on upside down if that is possible. (Forte's LCAs) I'll do a lot more checking until I'm completly satisfied it is correct.

Again, Good catch!

Mike M.
 

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Mike with these cars that difference in length you see when checking side to side. One side being 1/4" and other being 1" long. Not unusual.
The placement of the control arms and where they mount is not exsactly the same side to side or car to car. As long as the final numbers/settings can be reached. Reason why when trying to get more normal 3* caster for power steering cars. We only end up with 5 or 6* because one side runs out of threads before other side does. All's good.
The reason your alignment guy dialed in 3/10th a degree on the left is to compensate for the "crowning" of most roads. Roads are not level. They have a crown in the center so water drains. Many factory specs will list a different setting for left and right side wheel. Helps keep the car from wandering to the left if you were to release the steering wheel. This difference side to side takes that pull to the right out.
Personally found these cars like equal settings. So little suspension travel plus it's "firmness" really don't need the offset,but it not going to hurt either. Auto crossing or road raceing you would want eual side to side settings. Up to 1/2* difference unless measureing would probally never notice driveing.
Greg get a realignment. Sounds like yours may be off. A quick check to see if you have negative or posative caster now. Sit in the car of level,flat ground. Without moveing forward turn the steering wheel to the left. The left side of the car should raise up further as it's turned. This is the jacking effect of caster at the wheels. Caster wants to force the inside wheel down,outside wheel up. Also how weight is initially transfered to the outside tires for turning traction. Reason it gets played with on track cars from recommended settings.
 

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"The reason your alignment guy dialed in 3/10th a degree on the left is to compensate for the "crowning" of most roads."

I think that was the reason he gave me now that I'm thinking about it. Thanks for clarifying!

Mike M.
 

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You can't measure caster directly. I tried and it won't work.I also tried to find my instructions on how to do w/o success.basically, from memory, you turn the road wheel(not steering wheel) left 20 degrees from straight ahead and measure the camber, then 20 deg. right and measure camber.Then do a subtraction or addition and end up w/ a number that is the caster.hopefully someone has the detailed instructions and can post here. I had a piece of plywood about 18x18 and cut one side off at 20 degrees.Then figured a way to set this next to the front wheel to eyeball straight down from above til the wheel was parallel to the 20 degree side.Flipped it over to measure the other direction. After you do this 3-4 times on the first wheel you will find you are comfortable w/ the results cause you can repeat them.
Also w/ power steering you can't get too much caster. The front link in the uca will get too close to the coilover spring. So then you start moving the coilover cause what ever you got made the car so much better that you want more.Good luck
 
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