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450knotOffice's Avatar
 
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Question Question for all of you suspension geometry experts out there

My front end is rather low. I just measured the angle of the A-arms relative to the horizontal plane and have noted that both of them have a negative slope. By that I mean that the A-arm slopes upward a bit.

I've been told that one wants to be sure that the A-arms have some positive slope and that negative slope is bad.

Can someone please explain this a bit further. I've got my theories which seem to indicate that as the A-arm moves upward past level the camber will actually start to decrease as opposed to increase, which would be bad obviously.

By the way, the car feels pretty good when I drive it but I'm wondering if it could be better by raising the front end a bit so that the A-arms are at least level.

What do you all think? Clay Grady, TRE Cup, Tyson Schmidt, Jack O? Anyone?
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Old 01-02-2006, 02:21 PM
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I'm insulted ....

I mention this in other threads.....

Slightly angled "down" ( viewed from centerline of car to balljoint) ..or flat..is preferred, to avoid goofy camber curve patterns during cornering, taking bumps...or even combined cornering/bumps. Pretty much the idea you're talking about.... this is oversimplified but gets the idea across...

- Wil
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Old 01-02-2006, 02:26 PM
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So,

If I raise the front end up so the A-arms are level, will I notice a change in handling or is it more theoretical and noticeable on a race track only?
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2009 997.2 Carrera 4S
2000 996 Carrera
1984 911 Carrera Cab
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Old 01-02-2006, 02:47 PM
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I think you will feel it on the street...a more confidence inspiring drive than before...

A lot may depend on how smooth your local streets are, and how aggressive you drive.... but I would say you can sense this.

The change is easy enough to do to find out for yourself. The 11 mm bolt raises/lowers the front by about 1/4" per 360 degree spin...unload the front end to avoid unnecessary wear on the adjuster. Toe changes a bit however, so do understand that this will influence what you feel , too.

- Wil
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Old 01-02-2006, 02:52 PM
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Thanks Wil. I appreciate your help here.

I rebuilt my suspension last may so I know about the adjuster bolts. If I raise the front end a bit (say 1/4 to 1/2 an inch), how will it affect the alignment settings? What'll happen to my toe, for example?
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2009 997.2 Carrera 4S
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Old 01-02-2006, 02:56 PM
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I believe going up will tend to toe-in....going down will tend to toe-out...

Can't tell you how much for every __ " drop.....

- Wil
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Old 01-02-2006, 03:07 PM
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Are there any negative effects in geometry if you lower the rear to much?
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Old 01-02-2006, 03:18 PM
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screws up the aerodynamics -- just set it at US ht. or Euro ht. or even lower if you lower the front & use lowered spindles etc. - track only for the last
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Old 01-02-2006, 03:53 PM
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As you compress the front, the wheel will continue to gain negative camber even after the control arm has passed horizontal, but the gain will be at a progressively decreasing rate.

It will gain negative camber until the angle between the strut and control arm is 90 degrees. Beyond that point, camber becomes more positive with further compression.

The general rule is don't let the arms go beyond horizontal, and even this is really too far.

Allowing these extreme control arm angles has several negative affects:

- lowers the front instant roll center. This is a bad thing, it will cause excessive roll in the turns.

- reduces suspension travel. You have insufficient travel before you hit the bump stops.

- magnifies the bump steer problem. The relationship between control arm and tie rod goes to hell and bump steer gets bad.

The correct way to lower the front aggressively is to raise the spindles. You must then make a compensating adjustment to the tie rod end to correct bump steer, which would otherwise be severe with a raised spindle.
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Old 01-02-2006, 04:15 PM
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To clarify Chuck Moreland's point about roll center, the downward movement of the roll center increases the roll couple. (The distance between the CG and the roll center). The larger roll couple decreases the effectiveness of the sway bar. This page provides some basic information about lowering MacPherson strut suspensions.
Old 01-02-2006, 04:43 PM
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