Ok, a little suspension query for the Gurus

[rabbidmk1]

I realize that this is not the Prod forum, however there are a lot of knowledgeable people here.

Now that the MK3 is in the process of converting to F-Prod I had some control arms made with a spacer and rod end in place of the ball joint.

Here is a sample pic for views

My question is now that I have a 2" spacer, should I run the coils all the way down to level out the control arms or should I go with a shorter spring? I am running a Bilstein PSS9 setup with 850# springs so there is not a ton of travel to start out with.

I know this is a set up that many have used before, I just am trying to grasp the basic theories of why and how the spacer affects the car.



Finally when dealing with track and increasing it. I am allowed 103% of 62.4 plus 2.5" inches. (a little less in the rear) That allows me a total of about 4.37" that I can increase total. With an inch wider wheel from my IT legal ones, and with less offset plus a 25mm spacer I can achieve 2" on each side, but when does it become too much and affect my handling in a negative manner? Are there any mathmatical calculations that I should be aware of here?


Thanks for dealing with my silly questions.

 

[Sandro]

by going with a 2" shorter spring you would loose 1.6 lbs per corner so that might be something to think about if you are looking for places to cut weight

 

[seckerich]

The reason for the spacer is to change the roll center on the front of the car. It will also change the arc of the lower control arm and therefore change the camber curve. Slicks like a much different setup than radials and do not like drastic camber change. The spacer is fine but concentrates all the bending force to the intersection of the spacer and lower section of the upright. Check it on a regular basis.

When you lower the car to production ride height you will have already used up most of your strut travel and are close to bottoming the insert. Good check is to set the car on blocks under the rockers at intended ride height with no springs. Then raise the wheels and see if you have proper travel. I would suggest raising your upper strut mounts before just dropping the spring perches if you are even close. Good time to check your bump steer on the front and space it down to match your new pivot point.

Good luck and the advice is worth what you paid for it.:026:

 

[Flyinglizard]

You have a sway bar/lowerlink conflict. All 4 ends are fixed,but have different pivot points. The weakest link will break. Also ,the sway bar look like it might hit the arm, if it moves. Short end links might fix this.IMHO.
The extension bolt is very long and will take 125 % of the lateral load. Have you done the engineering math?? .
Is this bolt, spacer, race proven??
The lower Arm should point down for some camber gain. And roll center control.
MM

 

[Chip42]

I second the swaybar conflict - end links will go a long way to allowing the lateral travel of the sway bar arm as it moves through its arc.

also, remember that the "control arm" in geometric terms is the plane that passes through the inner pivots and the ball join. your spacers do what is required of them in that sense, mvoing the outer point back down to help level out the LCA plane relative to the ground. but as has been mentioned, the bending loads will be very high and will concentrate at the small clamp aat the bottom of the knuckle. this IS your future failure point.

keep in mind, also, that if you should raise your upper perch and/or adjust the strut angle that will also move the roll center about. look up roll center illustrations and make sure you are doing somethign that makes sense when you make these adjustments. given the nature of production cars, not everything is going to be indipendently adjustable so compromises will need to be struck.

but check strengths and add a link for that stabilizer ASAP!

 

[shwah]

That looks like a reference picture off a website, not a picture of Aaron's car. I expect he will get the swaybar mounts right - or leave them off and save weight at the front...

 

[rabbidmk1]

Chris is correct, that is not my car, just a photo of a VR6 gti on the manufacturer's web site. The front sway bar is going to be removed and I will make up for that with a little extra front spring. To help compensate for the large stress loads, the manufacurer (Southern California Cylinder Heads) mandated that the spacer is welded to the upright and bolted with higher quality than stock bolt.

 

[rabbidmk1]

Ok, I am by no means an engineer but from the photos that I have looked at, it appears that if I add two inches to the height of the strut tower, as long as my control arm is straight, the instantaneous center remains the same (provided the strut angle remains the same as before) and thus will keep my roll center right at or above the ground. Please correct me if I am way off here...

 

[boxedfox]

Ok, I am by no means an engineer but from the photos that I have looked at, it appears that if I add two inches to the height of the strut tower, as long as my control arm is straight, the instantaneous center remains the same (provided the strut angle remains the same as before) and thus will keep my roll center right at or above the ground. Please correct me if I am way off here...

Yes, when the control arm is perfectly level, your instant center will remain at the same height. However, since the planes of the control arm and strut mount will intersect further out than before, any any changes in instant center height as a result of a change in the angle of the control arms will be greater than before.

 

[Flyinglizard]

FWIW,If the same guy designed the sway bar, as the A arm, Doing your own load estimates, might be a good idea..
ALSO I am surprised that the Prod and the GTL guys have not figured a way to invert the knuckle, yet.
MM

 

[shwah]

At times like this, it is good to remember that a bad suspension geometry can't hurt you as much if you don't let it move. In other words - heavy springs are your friend.

 

[Chip42]

At times like this, it is good to remember that a bad suspension geometry can't hurt you as much if you don't let it move. In other words - heavy springs are your friend.

like carol smith says, any suspension design will work, so long as you don't let it.