Need input from the mechanical engineers (was: roll-bars (gettinglong))

[jtoomasson at yahoo.com (Jeff Toomasson)]

(refer to text below for thread history) 
 
Let me also add that if I could get away with not using a roll cage, I would. I would love to find a similar, but unobtrusive option. Maybe some kind of reinforceing crown around the roof with ties to the side spans?? Can any of the mechanical engineers out there suggest possible solutions in terms of potential materials and configurations? I also plan to contact the guys at I/O Port racing and see what they could come up with...I doubt my entry-level physics knowledge could help me find my way through this one...  :)

My car's not going to be as "all-out" as Brian's monster from a power standpoint; and my suspension goal is to be stiff, but streetable. The first installment will be Tokico Illuminas, w/ either H&R Sports or Neuspeed Progressives, (haven't decided to go poly or not), 25/28 Sways, upper/lower front ties and rear tie-bar all contingent on potential installment of roll cage and its ultimate configuration
 
All input is welcome and appreciated - Jeff
BoxOParts <bhonnold@pac-cap.com> wrote:
I bought my Ragtop in 1990 with no signs of cracking and the car barely
ran. Over the next ten years, it got a RD motor transplant along with
cam and exhaust, full suspension and brake upgrades and full
stress/swaybars. I noticed the cracking in about 1998 soon after I
installed the Autotech front and rear sway bars and the subframe.

It's my belief that by taking the flex out of the unibody by using the
big swaybars, and the extra stiffness added by the upper and lower front
stressbars, that it caused the roof to come under more stress than
designed to take. I first noticed the supports for the ragtop beginning
to sheer off their rivets. Then I removed the ragtop to re-cover it and
saw the beginnings of cracking at all four corners. Not having the time
or money to fix the cracks, I continued to drive the car until 2000.

By then there were 2-3 inch cracks from all four corners. 

I now have a plan to strengthen the roof using a better brace design
than the original riveted one. And I plan to watch the corners much
more carefully.

Like Jeff, I plan to end up with a full cage in the car, however the
design is such that it will be easy to get in and out of and will
probably incorporate upper bars welded to the roof supports. 

BoxOParts
78 Ragtop ABA 16vT

-----Original Message-----
From: scirocco-l-bounces@scirocco.org
[mailto:scirocco-l-bounces@scirocco.org] On Behalf Of Jeff Toomasson
Sent: Saturday, July 12, 2003 6:34 PM
To: L F; scirocco-l@scirocco.org
Subject: Re: How are roll-bars typically made/purchased?


The roof I have that came with the ragtop didn't show any signs of
cracking, but I have no idea how much of a beating it took before I got
a hold of it...

But Mr. BoxOParts says his had some cracking issues (maybe I can get him
to get a pic).

This car will not be a Daily Driver. It is receiving a near-ground-up
restoration that I would like to be able to handle mountain driving
anywhere from 3-10 times a year. I'm going to drop signficant $$$ on a
paint job and down want to have it ruined because I was too careless to
not reinforce the car properly. Here's what I currently have:

http://www.validpath.com/images/ragtop/back%20reinforcement.JPG

http://www.validpath.com/images/ragtop/reinf%mount.JPG

http://www.validpath.com/images/ragtop/front%20rt%20corner.JPG

http://www.validpath.com/images/ragtop/back%20rt%20corner.JPG

http://www.validpath.com/images/ragtop/driver%20span.JPG

TTYS, JT
L F wrote:
Jeff
Ah, you are speaking more of a roll "cage". If you intend to tie into
front and rear strut towers it gets REAL complex. Not that it can't be
done, not at all, it just eliminates some day-to-day usefulness of the
car since a bar will have to go through the door openings (and the
higher the better). 
Simplest:
1) roll bar (RB) behind seats, with cross-brace going from base of one
leg to bend above driver's head. (easy part)
2) tube going from midpoint of passenger-side leg of RB to right rear
strut tower (attach to tower by whichever method you feel most
comfortable with...weld or mechanically fasten. BUT, you might as well
weld, since this thing won't be coming out without a Sawzall anyway)
3)tube going from midpoint of driver-side leg of RB to left rear strut
tower
4) (FUN PART) tube going from same midpoints of RB to front tower...both
sides. These will go through the firewall and possibly the dash,
including fuse blocks, HVAC ducting, etc.
5) cross bar from one rear strut tower/bar intersection to opposite
upper corner of RB.

This is not ideal (ideal would take lots more fabrication) but is about
the simplest structure that would be of any worthwhile
chassis-stiffening benefit.

Do rag-top roofs have a history of cracking?

Larry
----- Original Message ----- 
From: Jeff Toomasson 
To: L F ; scirocco-l@scirocco.org 
Sent: Friday, July 11, 2003 8:24 PM
Subject: Re: How are roll-bars typically made/purchased?


I guess maybe I shouldn't call it "simple", but I was looking for
something that would cinch the four strut towers in such a way to keep
chassis flexion to a minimum. Any suggestions on a pattern? :)

Going custom is not an obstacle - I'll need my rag to tackle the
twisties 5-10 times a year. And I don't want have the roof cracking if
I'm going to drop $$$$ on a paint job... :)

L F wrote:
Jeff,
A simple four-point roll bar will not stiffen the chassis to any
significant degree. 
As far as minimum gage, this would apply only if you are building to a
particular race sanctioning body's rule book. if you aren't racing,
don't worry about it. I wouldn't go under .062" wall thickness, .080"
would be better/heavier/more costly. 
I'd have one custom built, if it were me.
And I'd have a good welder install it. (although with a hardtop, you
could get away with bolting and bolting WOULD let you remove it more
easily) Mostly personal opinions here, but I've built race cages before.

----- Original Message ----- 
From: Jeff Toomasson 
To: scirocco-l@scirocco.org 
Sent: Friday, July 11, 2003 1:08 PM
Subject: How are roll-bars typically made/purchased?


Most likely I'm going to have to have a simple roll-bar setup when I do
my ragtop to prevent undue stress to the roof from chassis flexion.

Honnold mentioned that he got a "Autopower" setup, but I don't get
anything when search for that in Yahoo. Went to the .org for various
links to racing sites and got nothing (BSI, Shine...). I don't see any
at some of the tuners' sites I've checked either...

Are they typically custom-made? Is there a minimum tubing gauge that
must be observed? I'm assuming it's better to have them bolted in as
opposed to welded...

Also, should I have it installed when I have the car painted? Or do it
after?

Thanks in advance! Jeff _______________________________________________
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