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Old 05-02-2004, 05:06 AM   #1
rex n effect
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Join Date: Feb 2002
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02 wrx pollen silv
& '11 TDi dadwagon

Default What is the biggest pro/con of more caster? How much is too much on a WRX suspension?

In my mind, the biggest benefit is more camber when the wheel is turned (when you need it the most). How much caster is too much for the street on a WRX suspension? 5 degrees? 6? If I install the kca375 bushings, I'd have over 5 degrees. What are the downsides? Thanks.
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Old 05-02-2004, 07:55 AM   #2
ellisnc
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Did someone say track day

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really the only downside to too much caster is the steering effort becomes a lot higher the farther from center you go.
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Old 05-02-2004, 01:50 PM   #3
WRX_Mundi
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I've been driving around with 5.8 on my STi. It's fine around town. I drove mine and a stock STi (mine had 4.0 stock) and the biggest difference was that mine turned in faster and required less wheel input. Of course mine also had a heck of a lot more camber and some toe out where his was aligned by the dealer (which probably means 0 toe and 0 camber). I'd be nervous about running more than 8 degrees of caster, but 6 doesn't seem to be excessive.
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Old 05-02-2004, 02:07 PM   #4
slowgo
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one thing is to be careful when doing any change in caster as you will be changing the height of the steering arm and can introduce unwanted bumpsteer,roll steer,and Ackermann changes.
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Old 06-20-2004, 04:13 AM   #5
BobbyD
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Quote:
Originally posted by slowgo
one thing is to be careful when doing any change in caster as you will be changing the height of the steering arm and can introduce unwanted bumpsteer,roll steer,and Ackermann changes.
english please?
--bobby
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Old 06-20-2004, 08:00 AM   #6
Patrick Olsen
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QuickSilver Metallic

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We generally adjust caster by tilting the top of the strut back. The strut is directly bolted to the spindle, so that means as we tilt the strut back we're also tilting the spindle back the same amount. The tie rods attach to the steering arm, which sticks off the front of the spindle. Hopefully it's not too hard to visualize that as you tilt the spindle back, the steering arm will come up. Meanwhile, we haven't adjusted the position of the steering rack at all, so the position of the steering arm has now changed relative to the steering rack, which means all those things slowgo said could be affected.

Bumpsteer is the tendency of the wheels to toe in or out as the suspension moves through its range of motion. As the control arm moves through its arc, if the tie rods aren't the right length they'll move through a slightly different arc, which means the spindle will be turned in our out. Does that make sense? This can be corrected by using spacers so that the control arm and tie rods scribe the same arc. Here's a picture of the Griggs Racing front suspension on my Mustang (also a MacPherson strut setup, like the Subie) - in front of the yellow strut you can see the end of the steering arm poking out from behind the rotor, and then about 2-1/2" below that you can see the tie rods, with a nice beefy -AN spec bolt and a spacer stack to line up the geometry correctly.
http://www.submariner.org/thepno95/P...0K-member5.jpg

Roll steer is similar, except now the suspension is moving through it's motion due to the roll of the car - one side goes into compression, the other side into rebound.

Ackermann, well, I've never really understood that one. I just know it's better on my Mustang with the newer '94/95 spindles and the Griggs stuff.

Pat Olsen
'97 Legacy 2.5GT sedan
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Old 06-20-2004, 11:05 AM   #7
trhoppe
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The best explanation for ackerman I've seen is nate whipple's (solo-x) response to my question

"ackerman is the term used to describe a difference in steering angle between the inside front tire and outside front tire. positive ackerman, what a wrx has, turns the inside front at a faster rate then the outside front. for example, you turn your steering wheel enough so the outside front turns 7 degrees. the inside front would turn 12 degrees.

toe-out could be considered "dynamic" ackerman. lets say you are using 1 degree of toe out on both front tires for 2 degrees total toe out(you never would use that much, but it works for the example). now you have to turn the wheel a bit more to get the same 7 degrees on the outside front. all that extra toe will result in the inside front being turned more then 14 degrees. (the 12 degrees from before, plus the combined toe out, which was an obscene 2 degrees total toe out, plus the ackerman effect on that additional degree you had to turn the wheel to get the outside front to the same place). "

-Tom
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