Rear Caster?!? - Subaru Enthusiasts Car Club of the Sierras

sperry · 2006-01-10, 01:42 PM

So I got my new Ground Control rear top mounts in the mail yesterday. Get this, they're listed as camber/caster adjustable!

The tops are slotted, which might just be for fitment purposes, but I'll certainly be able to finagle some fore/aft adjustment out of them if I so desire.

What I can't figure out is how changing the caster in the rear does anything... since you don't steer with those wheels, positive caster isn't going to add (well, subtract) dynamic camber. Does rear caster have some sort of effect on the camber of the tire as the strut compresses?

Now I'm guessing that the invoice is just a generic printing, and the slots are actually just to handle tollerance issues in the car. But as a thought exercise, what would rear caster adjustment gain you?

MattR · 2006-01-10, 01:50 PM

weird. There are sizing application challenges that would need a huge change in shock angle, but rear castor ( on our cars at least) seems useless.

AtomicLabMonkey · 2006-01-10, 01:59 PM

It has a small effect on dynamic camber, since there is some designed-in compliance and roll steer. It is not a large effect like on the front wheels though. As I recall the rear response will be in the neighborhood of like <0.1 degree camber change over 2-3" of bump travel, whereas the front response can be 1+ degrees at a typical steering angle during steady-state cornering.

MPREZIV · 2006-01-10, 02:01 PM

Most cars that are 4 wheel align-able, only have toe and camber adjustments. As these are tire wearing and handling angles, and caster is a steering and handling angle, I can't see any caster adjustment in the rear making any actual driving difference.

cody · 2006-01-10, 02:34 PM

Quote:

Originally Posted by sperry

But as a thought exercise, what would rear caster adjustment gain you?

Considering the fact that the rear wheels don't turn, adding/removing castor won't gain you anything.

AtomicLabMonkey · 2006-01-10, 03:29 PM

Am I talking to myself here? I already said there is a camber change effect from steering due to caster, just like on the front wheels, but it is very very small.

Dean · 2006-01-10, 03:45 PM

Yes. Yes you are...

What Austin said. But with our lower geometry, god knows what sort of binding you might get if you try and throw any real amount of rear caster at it. I'd use it to even the two sides up and leave it.

The real gotcha is that you can't rotate the wheel enough for any of the alignment machines I know of to measeure it. Plumb bob, and a level, I guess. Or camber guage on the front/rear of the strut if there is room below the spring.

sperry · 2006-01-10, 03:51 PM

Quote:

Originally Posted by AtomicLabMonkey

Am I talking to myself here? I already said there is a camber change effect from steering due to caster, just like on the front wheels, but it is very very small.

I had a nasty reply flaming the people that keep chiming in with less intellegent responses after your answer, but decided to take the high road and not post it.

Dean, I'm guessing that once they're installed, there won't actually be any adjustability. The holes are just slotted to allow for Subaru's manufacturing tollerences. When I install 'em, if there's any play, I'm just going to push it all the way to one side or the other and tighten 'em down.

ScottyS · 2006-01-10, 04:44 PM

Quote:

Originally Posted by sperry

I had a nasty reply flaming the people that keep chiming in with less intellegent responses after your answer, but decided to take the high road and not post it.

Haha --- I'd hate to have to write the Admin and request that they give you the permanent sig line: "I own this Forum just so I can post questions and flame the responders."

sperry · 2006-01-10, 05:10 PM

Quote:

Originally Posted by ScottyS

Haha --- I'd hate to have to write the Admin and request that they give you the permanent sig line: "I own this Forum just so I can post questions and flame the responders."

Done, and done.

cody · 2006-01-10, 05:11 PM

Quote:

[i]"I own this Forum just so I can post questions and flame the responders."[i]

You need a fwd slash in there Pal.

khail19 · 2006-01-10, 06:56 PM

Quote:

Originally Posted by sperry

Done, and done.

See, this isn't a total dictatorship.

2006-01-10, 01:42 PM	#1
sperry The Doink Real Name: Scott Join Date: Nov 2002 Location: Portland, OR Posts: 20,335 Car: '09 OBXT, '02 WRX, '96 Miata Class: PDX/TT-6 The way out is through	Rear Caster?!? So I got my new Ground Control rear top mounts in the mail yesterday. Get this, they're listed as camber/caster adjustable! The tops are slotted, which might just be for fitment purposes, but I'll certainly be able to finagle some fore/aft adjustment out of them if I so desire. What I can't figure out is how changing the caster in the rear does anything... since you don't steer with those wheels, positive caster isn't going to add (well, subtract) dynamic camber. Does rear caster have some sort of effect on the camber of the tire as the strut compresses? Now I'm guessing that the invoice is just a generic printing, and the slots are actually just to handle tollerance issues in the car. But as a thought exercise, what would rear caster adjustment gain you? __________________ Is you is, or is you ain't, my con-stit-u-ints?

2006-01-10, 01:50 PM	#2
MattR El Matador Real Name: Matt Join Date: Dec 2002 Location: Reno, NV Posts: 10,660 Car: 2012 Toyota Tacoma Class: ?	weird. There are sizing application challenges that would need a huge change in shock angle, but rear castor ( on our cars at least) seems useless. __________________ "Dallas..We have a problem.”

2006-01-10, 01:59 PM	#3
AtomicLabMonkey Nightwalker Real Name: Austin Join Date: Dec 2002 Location: Oshkosh, WI Posts: 4,063 Car: '13 WRX YGBSM	It has a small effect on dynamic camber, since there is some designed-in compliance and roll steer. It is not a large effect like on the front wheels though. As I recall the rear response will be in the neighborhood of like <0.1 degree camber change over 2-3" of bump travel, whereas the front response can be 1+ degrees at a typical steering angle during steady-state cornering. __________________ "None of you seem to understand. I'm not locked in here with you.. you're locked in here with me."

2006-01-10, 02:01 PM	#4
MPREZIV Token Real Name: Le Stig Afrique? Join Date: Jun 2005 Location: sitting next to a big yellow box Posts: 3,589 Car: 2001 Impreza 2.5 RS Class: 05 TDSP No, I won't work on your car. F your car*	Most cars that are 4 wheel align-able, only have toe and camber adjustments. As these are tire wearing and handling angles, and caster is a steering and handling angle, I can't see any caster adjustment in the rear making any actual driving difference. __________________ "...these condoms have a topical anesthetic to reduce sensitivity, so you can last longer. What a paradox. You can't feel a thing, but you can f*ck for HOURS..."

2006-01-10, 03:29 PM	#6
AtomicLabMonkey Nightwalker Real Name: Austin Join Date: Dec 2002 Location: Oshkosh, WI Posts: 4,063 Car: '13 WRX YGBSM	Am I talking to myself here? I already said there is a camber change effect from steering due to caster, just like on the front wheels, but it is very very small. __________________ "None of you seem to understand. I'm not locked in here with you.. you're locked in here with me."

2006-01-10, 03:45 PM	#7
Dean Señor Cheap Bastarde Real Name: Dean Join Date: May 2003 Location: $99 Tire Store Posts: 9,294 Car: $.04 STI Class: Fast,Cheap & Reliable=STI Deal, did somebody say Deal? Oh, Dean, yeah that's me.	Yes. Yes you are... What Austin said. But with our lower geometry, god knows what sort of binding you might get if you try and throw any real amount of rear caster at it. I'd use it to even the two sides up and leave it. The real gotcha is that you can't rotate the wheel enough for any of the alignment machines I know of to measeure it. Plumb bob, and a level, I guess. Or camber guage on the front/rear of the strut if there is room below the spring. __________________ I am a Commodore PET --- Now get off my lawn you kids... Last edited by Dean; 2006-01-10 at 03:48 PM.

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