Positive Caster
10-02-2002, 09:26 PM
#1
Three Wheelin'
Thread Starter
Positive Caster
There seems to be conflicting write-ups as to which direction the eccentric bolt creates positive caster. I'm hopping an alignment guru can verify this:
If positive caster is when the top strut bearing is toward the rear in relation to the lower ball joint as viewed from the side of the vehicle.
Then when adjusting the caster eccentric, positive caster is when the eccentric tab is turned left (looking from the back of the car) and right for negative caster.
Please correct me if I'm wrong.
If positive caster is when the top strut bearing is toward the rear in relation to the lower ball joint as viewed from the side of the vehicle.
Then when adjusting the caster eccentric, positive caster is when the eccentric tab is turned left (looking from the back of the car) and right for negative caster.
Please correct me if I'm wrong.
10-02-2002, 10:59 PM
#4
Okay - upper and lower pivot points (strut and control arms in our case) departure from vertical is caster. Top pivot to rear - negative - more self center. Top pivot to front - positive - less steering effort. You can actually see the effect that turning the adjusting tabs has on the control arm position. Its been awhile, but I believe mine face each other horizontally - thereby providing the easiest steering for a manual rack car.
10-03-2002, 02:17 AM
#5
Race Director
I don't think you can adjust the top pivot on a stock 951. So you're left with the bottom one.
Now imagine a flat plane (parallel to ground) that goes through your A-arms. This plane connects all three pivots on your A-arms (the balljoint, the front pivot, and the rear caster pivot). Now imagine yourself looking down at your A-arms from the top. Then imagine what would happen to the balljoint if you were to move the rear caster pivot outwards? The balljoint would move forwards, giving you more positive caster. So the answer is to pull the caster adjuster outwards for more caster. With the updated TSB on 17" wheels that increase caster by 1-degree, the little adjuster tab is almost all the way out (50-60 degrees outwards).
Now imagine a flat plane (parallel to ground) that goes through your A-arms. This plane connects all three pivots on your A-arms (the balljoint, the front pivot, and the rear caster pivot). Now imagine yourself looking down at your A-arms from the top. Then imagine what would happen to the balljoint if you were to move the rear caster pivot outwards? The balljoint would move forwards, giving you more positive caster. So the answer is to pull the caster adjuster outwards for more caster. With the updated TSB on 17" wheels that increase caster by 1-degree, the little adjuster tab is almost all the way out (50-60 degrees outwards).
10-03-2002, 02:29 AM
#6
Three Wheelin'
Join Date: Oct 2001
Location: Seattle, WA
Posts: 1,805
Likes: 0
Received 0 Likes
on
0 Posts
How can this caster adjustment work with solid metal bushings? It seems that the a-arm to crossmember bushing wouldn't accomodate this caster adjustment. What gives (literally)?
I suppose it is possible that this adjustment is lost with solid bushings and camber/caster plates are required to get adjustment.
I suppose it is possible that this adjustment is lost with solid bushings and camber/caster plates are required to get adjustment.
10-03-2002, 04:01 AM
#7
Addict
Rennlist Member
Rennlist
Site Sponsor
Rennlist Member
Rennlist
Site Sponsor
Join Date: May 2001
Location: Virtually Everywhere...
Posts: 4,820
Likes: 0
Received 5 Likes
on
5 Posts
Caster on early cars is only adjustable by moving the rear bushing clamp - on late cars, it is adjustable by both the clamp/bushing and caster eccentric. Solid metal bushings such as Racer's Edge use lateral bushing clamp movement only. Some brands of adjustable camber plates also allow for caster adjustment (flat-plate type, not raised-plate type) by moving the plate fore and aft.
Trending Topics
10-03-2002, 04:19 AM
#8
Three Wheelin'
Join Date: Oct 2001
Location: Seattle, WA
Posts: 1,805
Likes: 0
Received 0 Likes
on
0 Posts
[quote]Originally posted by Skip:
<strong>Caster on early cars is only adjustable by moving the rear bushing clamp - on late cars, it is adjustable by both the clamp/bushing and caster eccentric. Solid metal bushings such as Racer's Edge use lateral bushing clamp movement only. Some brands of adjustable camber plates also allow for caster adjustment (flat-plate type, not raised-plate type) by moving the plate fore and aft.</strong><hr></blockquote>
Skip, thanks for shedding more light on how the caster is adjusted on early/late, solid, oem caster mounts, etc.
But, as I was originally asking, what about the A-arm to crossmember bushing. This bushing is allows the a-arm to arc about its mounting point. If this solid bushing is really zero compliance, there would only be a single position that the rear caster mount/eccentric could be in for the front control arm mount to not be bound. Rubber can provide enough compliance for this adjustment, but providing for this adjustment on metal bushing would result in play rather than compliance.
<strong>Caster on early cars is only adjustable by moving the rear bushing clamp - on late cars, it is adjustable by both the clamp/bushing and caster eccentric. Solid metal bushings such as Racer's Edge use lateral bushing clamp movement only. Some brands of adjustable camber plates also allow for caster adjustment (flat-plate type, not raised-plate type) by moving the plate fore and aft.</strong><hr></blockquote>
Skip, thanks for shedding more light on how the caster is adjusted on early/late, solid, oem caster mounts, etc.
But, as I was originally asking, what about the A-arm to crossmember bushing. This bushing is allows the a-arm to arc about its mounting point. If this solid bushing is really zero compliance, there would only be a single position that the rear caster mount/eccentric could be in for the front control arm mount to not be bound. Rubber can provide enough compliance for this adjustment, but providing for this adjustment on metal bushing would result in play rather than compliance.
10-03-2002, 04:50 AM
#9
Addict
Rennlist Member
Rennlist
Site Sponsor
Rennlist Member
Rennlist
Site Sponsor
Join Date: May 2001
Location: Virtually Everywhere...
Posts: 4,820
Likes: 0
Received 5 Likes
on
5 Posts
Oops, it's late - I didn't RTFQ 
The front bushing is a multiple piece cassette type metal bushing. The outer is installed rigid to the arm, but the center pops in and allows ~3 degrees of movement. Also, the solid caster block for the late model cars retains the use of a caster eccentric.
Skip
The front bushing is a multiple piece cassette type metal bushing. The outer is installed rigid to the arm, but the center pops in and allows ~3 degrees of movement. Also, the solid caster block for the late model cars retains the use of a caster eccentric.
Skip
10-03-2002, 11:55 AM
#11
Three Wheelin'
Thread Starter
Ok guys. Thanks for the help.
To reiterate, for more positive caster, move the eccentric caster adjuster outwards, or point the tab to the left (looking from the back of the car).
Actually I just realized it depends on the wheel, right or left. For the driver side wheel the tab would be turned left, and the right side would be turned to the right. I currently set mine to the max, 210 degrees on the left side (driver) and 60 degrees on the right.
Now I wait for the smart camber gauge to arrive, and dial in more negative camber.
To reiterate, for more positive caster, move the eccentric caster adjuster outwards, or point the tab to the left (looking from the back of the car).
Actually I just realized it depends on the wheel, right or left. For the driver side wheel the tab would be turned left, and the right side would be turned to the right. I currently set mine to the max, 210 degrees on the left side (driver) and 60 degrees on the right.
Now I wait for the smart camber gauge to arrive, and dial in more negative camber.