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Looking for some roll center height for FD cars


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I thought I would ask hear I have been checking the Beetle on suspension upgrades . but have found not any real good information on it so far.

The best I can come up with is 3 inch front at race ready . With the why it is now and we are fast but for every 1 inch on downward movement we are 2.5 below ground.

Thank you for any help.

Bob Mann

www.DRVOLKS.com

bobtec@comcast.net

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Drop pins  fix the roll center issue. Still even without  drop pins,I have gone faster as I went lower ,. Just watch the dynamic camber . If you go into camber loss with compression raise it back up a little .

 These arms are made for a Mk 1 but the later cars canuse the same method . Marry a Coleman lower tube to the back half off the  stocker.  Fix the castor/camber/ axle plunge all at the same time.   Your chart shows a spring rate of 350 and wheel rate of 180. The wheel ratio sounds low to me. But I have to confess that I have never measured it correctly. I just add some until I like the F/R balance. At this level you have comber loss with compression of .42 per in?  That also sounds high per in  but is in the range. To runthis low you have to add static camber to about -3 . or more. 

 The later VWs start out with flat control arms @ stock alt. and get worse with lowering.  

Edited by flyinglizard
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Flyinglizard we talked on the upgrade lower ball joint  I did get a set of 1 inch rebuild-able stock car ones to machine to fit the VW 1.8 spindle.

I think the best way to raise the inner a frame points is to cut the top mounts of the K support 1 inch or so and take out the rubber rear spacer and and redo the floor mounts with a flat plate. we need to do some trimming for the left axle too.
 

Bob Mann

www.DRVOLKS.com

bobtec@comcast.net

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FL, I see that the orange control arm is also moving the ball joint forward to gain caster.   On the truck, we were able to move the steering rack forward to straighten the inner tie rod ends some.  Obviously with an engine in the front, this may be difficult enough to be not worth it.

We use a 3/4 Heim on the outer which will allow a 3/4 bolt (head cut off) to be used in the knuckle (non threaded section in the clamping area)   This lowers the outer ball joint (pivot) location relative to the wheel center and thus without cutting in the chassis allows easy movement of the roll center.
I will try to post a pic soon.

Edited by Superdave
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  • Technical Advisory Committee

M. Ogren and superdave. Some great ideas here. I envisioned doing something very similar for our Focus. Another problem I want to alleviate is that the Focus knuckle LBJ hole is tapered. Twice now, we have had the LBJ shaft loosen up in the clamp bolt. I thought engaging the LBJ shaft into the knuckle as far as possible, then torquing the clamp bolt, was the thing to do. But I've since found that I need to snug up the clamp bolt, then push the LCA down, then torque the clamp bolt fully down. 

Marginal design in my mind, that doesn't do well under high G loading endurance racing usage. So I intend to bore the knuckles to get rid of the taper, and use a drop pin and heim joint to help the roll center issue. 

Edited by mcoppola
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Thanks, gents.  Dave, The outer joint is a 3/4 in mono ball with a coleman long stud turned to fit the knuckle. The fixing nut is faced back a little to clear the wheel.   Yes on the movement foreward to add Castor.  The bump steer gets poor over about 1/2 in foreward and I can't  move the rack much legally .   This drops the  outer  pivot about 1.8 in from stock. I can get a little more with 15in wheels but this results in the same  overall geometry as we need to lower the car another 1in .   

   I work with these parts from Coleman as they are off the shelf . I could also use the double rod ends. The car has been wrecked and the outer joint still works as new. While the inner Rod end bent as did the stock piece of the VW arm   The folding of the arm in a hard hit surely saved the chassis mounting points.    

 Mike, if my front knuckle came loose more than one time I would straight bore the B. :) 

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This is on the Truck which would have to run EC last I knew if it ever runs CC.
It is now used for Track Day, show and street  with an occasional ICSCC race thrown in.
Ball joint ~ in forward, Rack ~4in forward
And you may even note strut mounted sway bar which preloads the sway bar with steering input as well as no loss due to control arm ratio.

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  • Technical Advisory Committee

^^^Nicely done!  Both guys' examples... 

Funny - from opposite corners of the states!

I'll be doing the same. Thanks for the pictures.

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54 minutes ago, Superdave said:

 

 

Dave.

 

Unrelated question.

 

Do you have room at your place to store a small car?  it runs and drives.  I picked it up at PM Auto at 3011 East Sprague.  They're doing the paperwork, I think they said the paperwork would be done Monday.  It's already paid for.  :)

 

Edited by Ham Sammich
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Crap Dave.  After looking at your stuff and my rules..  Then  doing the usual engineering preparations, drinking two beers and taking a shower..   I have to look at my arms again. My rules state control arms are free.   I can use an allen head bolt in the from the bottom just like you have done, this allows me another 1/2 in pin extension and  I can drop  my car another 1/2in.  While I'm at it, I can move the front wheels foreward about 2in and still get the rack to work. This would force me to move the rear wheels forward at least 1 in   , to stay inside my wheel base rule.   This car is already offset about 1 1/2 in , what a little more to move the wheels? 

 I'll get about 10# off of the front wheels and closer to my target front weight of 54%.  It is one of the fastest VWs in the country but the little CRX Honda is still faster.

Edited by flyinglizard
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It is not an allen bolt from the bottom. 
It is a regular bolt with the head cut off and installed with the removed head on the top so there are no threads in the clamping area.

Then the bottom nut is turned to prevent binding and it was a shorter nut.

Then it was tacked to make SURE it did not come loose.

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  • 1 month later...

Ok been working on it so here what I have so far. Machined the spindle to take these Stock car Ball joints. now running 275/40/17 on 10 inch rims.

as the car sits now roll center jumped to 6.2 inches if we lower it by one inch it will be 3.8 inch witch we feel is the best point to be. I will raise the sub frame one inches in to the body this will lower the center of gravity and body 2 inches over all and still keep the roll  center at 3.8 next is the bump steer  to check and adjust .As you can see a small body height change can screw up ever thing in a flash on the VW plate form.

Bob Mann

www.DRVOLKS.com

bobtec@comcast.net

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  • 3 months later...

Ok I thought to give a update on how it worked just did Thompson L**** race The changes work very well . Got the car down to 1.30.115 with over 100 cars on the track at any given time .fastest car time was 1.26.580 and we were 5th fastest B class and over all 22th fastest   out of 111 cars. And on diesel!! 

Bob Mann

www.DRVOLKS.com

bobtec@comcast.net

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