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Also looking for roll bar advice

 
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vroom_vroom17  



Joined: 07 May 2005
Posts: 12
Location: Wisconsin

PostPosted: Wed May 17, 2006 12:43 am    Post subject: Also looking for roll bar advice Reply with quote

I am having a roll cage built by one of the local stock car cage fabricators in my area (LaCrosse, WI) and as he is normally a full framed stock car builder, we are looking for some advice on cage design and attachment points. The car is at this time only going to be used on the street and for autocross, but if I'm going to build a cage, I might as well make it legal for any future whims that I might decide to persue. I want to make sure we use the correct tubing and also would like to know which attachment points are the best for the 924. My interior is completely gutted including dash, so this should not be of any concern. We were thinking of a main hoop with cross bar, halo hoop, windshield down posts, dash bar, angled door bars, supports from the main hoop to the rear someplace and possibly two bars from the dash bar, through the firewall to the front strut towers. Where would be the best places to attach the bars to the car and what is the best option for tubing with regards to price, ease of working with and regulations?

Thanks for your help,
Brad
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John Brown  



Joined: 07 Nov 2002
Posts: 903
Location: Leesburg VA

PostPosted: Wed May 17, 2006 4:28 am    Post subject: Re: Also looking for roll bar advice Reply with quote

vroom_vroom17 wrote:
I am having a roll cage built by one of the local stock car cage fabricators in my area (LaCrosse, WI) and as he is normally a full framed stock car builder, we are looking for some advice on cage design and attachment points. The car is at this time only going to be used on the street and for autocross, but if I'm going to build a cage, I might as well make it legal for any future whims that I might decide to persue. I want to make sure we use the correct tubing and also would like to know which attachment points are the best for the 924. My interior is completely gutted including dash, so this should not be of any concern. We were thinking of a main hoop with cross bar, halo hoop, windshield down posts, dash bar, angled door bars, supports from the main hoop to the rear someplace and possibly two bars from the dash bar, through the firewall to the front strut towers. Where would be the best places to attach the bars to the car and what is the best option for tubing with regards to price, ease of working with and regulations?

Thanks for your help,
Brad


To my knowledge the places you might run a 924 the series specs would not allow the forward braces to the towers.

Check the SCCA and NASA cage specs for tubing size etc. Note that erw tube is no longer allowed. Must be dom.

Front down tubes go to the top of the sills. Main hoop goes to the top of the wide sill which slopes up to the rear. Rear braces go either to the top of the wheel arches (not very strong but good enough) or the the top of the frame rails next in the rear compartment. I prefer going to a point roughly over (or near) the top suspension attachment point as versus some who like to run them WAY back. If you run too far back you may excede the allowable angles per SCCA or NASA.
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80 931 - #931 44Cup
99 Escalade - tows track cars
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924RACR  



Joined: 29 Jul 2001
Posts: 8236
Location: Royal Oak, MI, USA

PostPosted: Wed May 17, 2006 11:12 pm    Post subject: Reply with quote

NASA's GTS doesn't have limits on number of attachment points or mounting locations, but I believe that's about the only one; SCCA does have some, depending, and I believe 944Cup or 944Spec (not sure if you have a 2.0L or a 2.5L S) do as well.

Check the rulebook for where you might run - only way to be sure. Each of those series offer a completely different racing experience.

There's another reason you should look at the rulebooks; they give diagrams of the proper basic layout for cages, which tubes should be where. Put the main hoop diagonal in the right place, etc.

Normal cage setups (most typical) run 6 or 8 attachment points. Two for the bottom of the main hoop, two for the rear braces for the main hoop, two for the a-pillar bars at the front base of the doors, and two optional ones at the firewall. This is a basic SCCA Improved Touring or Showroom Stock cage.

You'll have the main hoop w/ diagonal cross-brace and horizontal bar for harnesses. On a 924/944, the best place to mount these is on the sill either side of the rear seat (as seen in my OG cage -- check my webpage on the Construction section).

Two bars angled back, min 30 degrees off the vertical as I recall, going to the rear to brace the main hoop. In the 924/944, you can (and should) attach these to the frame rails running under the side edges of the trunk floor - again, the OG cage does this. Of course, you do have to remove the gas tank for this, requiring removal of the trans. If your main hoop is far enough forward, you may be able to run these to the very front edge/corners of the trunk (while still meeting the 30-deg rule); these are the upper shock mounting points and will improve rigidity again, if you're going to use coilovers.

Two bars proceeding forward from the main hoop, over the top of the doors and down to the front base of the doors - the a-pillar posts. Again, attach to the sills, not the floorpan, in the 924/944. We don't tend to use the halo construction in road-racing. One reason may be rollover; I've seen far too many cars flip, land on the front edge of the roof, and snap welds at the top of the door. Not sure if halo construction would be better or worse here, but your cage builder should know. Of course, without using halo-type construction, then you have a cross-bar across the top of the windshield.

The last two attachment points would be foreward to the firewall; in this case, you'll want to make sure they meet the cage where the door bars come in, I believe - have all the loads converge at that point, also where the bend for the a-pillar to go straight down is. Your cage builder should know all that, though.

Final requirement, door bars. Many implementations here, including single bars stretched across the opening, an X constructed of two crossing straight bars (which I have), and NASCAR bars (multiple bars running parallel front to rear, bowed out into the door cavity, and tied together vertically with short lengths. Straight bars can leave little arm-room for the driver or passenger, and I do have this issue; I've seen an X built as a flattened spear point outwards, with 4 straight bars intersecting outside the driver with gussets to strengthen, to address this concern.

It'd be a good time to point out these detailed photos of a killer IT CRX build:
http://www.improvedtouring.com/images/special/crx_build/

Definitely an awesome, first-class cage.

So the above would satisfy the basic requirements of most racing classes, assuming good construction, proper materials, etc. Here now are additional things you might wish to add.

The biggest concern of the above style cage is strength of the a-pillars for impact; they're virtually unsupported without additional bracing. Of course, you also have to be able to get your feet in there, and it's not usually an option in our cars to put any bar across the bottom to connect. Thankfully, if you've ever been under a 924, you've seen that the factory did try to stiffen quite a bit across the tunnel with additional bolt-on braces.

However it's wise to have a dash bar. Exact routing varies depending on your cage construction, but don't put it so low that it's gonna take your kneecaps off in a head-on! Mine is just above or below (can't remember) the steering column. Note that if you have additional braces to the firewall, at this point you can do a nice job of triangulating in the dash area to really add much strength here (the CRX build above shows this well).

A cross-brace on the halo strikes me as a good idea too; I've got one.

I've also pondered additional braces elsewhere, but eventually gave them up as unnecessary; the chassis is pretty rigid as it is, really - not that bad.

Hope that helps give some ideas...
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