Rear tower simple triangulation

[cygnusx1]

I had an idea over the weekend that might do a good job of tightening up the mid to rear end of the chassis. I wanted a simple bolt-on reinforcement to help minimize that pesky roofline crack at the rear quarters. I am considering running two diagonal bars inwards and forwards from the rear strut tops down to the tranny tunnel in between the two seat backs. I would connect to the tranny tunnel as far forward as possible without creating an interference with the seats and/or my elbow. I would use a backing plate inside the tunnel and bolt through. It probably wont do that much to prevent torsion between the floors and the rear deck but it will help bending loads on the rear half of the chassis.

 

The advantages are, easy install, unobtrusive, and light.

 

Any thoughts?

[Pop N Wood]

I think basic subframe connectors might be more better (ala Pete Paraska). They are a proven design that is suppose to go a long way toward minimizing chassis flex.

 

A long time ago someone posted about a hoop they welded to the back hatch area and claimed it eliminated the paint cracking. It was a long time ago and I can't remember if the hoop went up along the inside of the sheet metal from the strut towers and over the roof line, or if they went along the outlines of the rear hatch. I do remember the guy was from Austrailia. It sounded simple enough and was totally hidden by the stock interior panels.

 

Your idea sounds simple enough too, but you would most likely need to reinforce the trans tunnel first. The trans tunnel is rather flexible. I don't think it would properly anchor the new braces. I am also having a hard time visualizing how good or bad that type of brace might look. I think I would go for a basic 4 point roll bar and subframe braces first.

[JMortensen]

I don't think that's going to do enough to prevent the B pillars from cracking. After having been crawling in around and under the rear subframe to stitch the whole thing together I've got a couple suggestions. First is weld a straight bar between the two strut towers. Second is do a proper hoop that attaches to the roof support (map light area) and tie that back to the strut towers also. That really is the answer structurally in my not-a-structural-engineer opinion. The trans tunnel has no support in it at all really (at least my 70 doesn't), so I just don't think that it's the right place to run a brace to.

 

If you didn't want the whole roll bar you'd actually be better off just running the supports BACK and to plates welded over the rear subframe than to the tranny tunnel IMO. But really tying the roof to the strut towers is what you want to do and the best way to do that is with a roll bar IMO.

[cygnusx1]

I already have a home-made bar between the rear towers and I figured it would be simple to tie it down (fore-aft) to the trans tunnel with some bolt-in bars bent to fit. If I insert a steel hoop under the tunnel and bolt the forward bracing into it, sandwitching the tunnel metal, it should be pretty rigid. I don't have welding capabilities so I do everything with borrowed milling machine time and simple hand tools. It appears to me that the metal in the trans tunnel is a bit heavier gauge than the floor pans. It seems very strong but I have never seen a 280Z stripped down to bare metal so I really don't know for sure. A four point roll hoop for the rear would tie in at the rear towers and the floor pans just behind the seats. I have heard that the weekest point of a Z chassis (besides rust) is the weak connection where the floor structure meets the vertical section behind the seats. The subframe connectors that you mention span this weak section but they dont get alot of leverage being so close to the moment arm. I feel that a diagonal from the rear strut towers to the tunnel, or floor pans, for that matter, would have alot more leverage...the further forward on the floor structure, the better the leverage...of course the seats are in the way which is why I opt for the tran tunnel.

[JMortensen]

I have heard that the weekest point of a Z chassis (besides rust) is the weak connection where the floor structure meets the vertical section behind the seats.

I believe you heard wrong. It looks to me as if the area between the floor behind the seats and the deck between the strut towers is the strongest part of the car. There is more structure there than just anywhere else in the car that I'm aware of. I think the floor to deck area is the closest thing to a "torque box" structure in a Z. In addition underneath the doors connecting the firewall to the area right behind the seats are the biggest frame "rails" in a Z. I don't know what you call these parts, but they essentially look like very large frame rails and connect the front of the car to the back again in that spot behind the seats. So I think this is really the strongest area in the whole unibody, braced by the torque box in the rear and the "rails" in front. I think this is why the hoop is tied in right there.

 

Katman just posted this link again. If anyone knows, he does: http://www.izzyscustomcages.com/ZCageArticle.html

[Here comes trouble]

The Z roof is soldered on and seams could be welded after removing solder....I am not too sure about tunnel bracing and the connectors underneath may work better.. here is a scan of a roll bar design in my photos that may be of benefit http://pg.photos.yahoo.com/ph/larryjohnson97438/detail?.dir=/392a&.dnm=9bce.jpg

[cygnusx1]

I agree that nothing will make the Z rigid like a properly welded full cage but alot of us are not looking for that type of protection. The front and rear strut tower bars made a nice improvement in the dynamic steering response and they were a simple, reversible, addition to the chassis. With the fairly rigid suspensions that alot of us run on the street, we strain the chassis not only in the lateral loads from cornering but also from the loads due to potholes,bumps, and uneven pavement. Nothing makes me cringe more than when I have to pull into a steep driveway diagonally to keep from punching the nose into the ground and I feel a wheel lift off the ground as the chassis and interior creeks sweet mercy. Subframe connectors would help but there is alot more fabrication required to make those work on a 280Z than meets the eye...especially when the frame rails are already replacements and nothing is level and square under them anymore. The way I look at it is, if I take the roof off, so to speak, and try to fold the car, it's going to fold at bottom rear edges of the door openings. That's right about where the floor frame rails are ending. So besides the roof, there isn't much to keep the car from folding. That's where forward diagonals down from the rear towers would help...IMHO. Make sense or am I just making scent?

[Phantom]

I just went with a triangulated rear strut tower brace from http://www.racetep.com and an autopower 4-point roll bar. You can see them in my photo album. I had a very small crack on the passenger side B pillar before the LS1 was installed but it has not grown since then.

[Pop N Wood]

Before you would "take the roof off" you should weld temporary cross braces everywherre to keep things in place. The "proper" say to do it is to straighten the entire car then mount it to a rigid table before cutting off structural components. I have even seen plans for homemade "tables" that can be adapted to different vehicles by simply welding different braces or mounts as needed.

 

But not many guys do it that way.

 

From what I have read, the subframe connectors stiffen things up quite a bit. I don't want a visible roll cage either and have been considering solutions like Jon mentioned. But I don't don't know if the braces you are considering will accomplish much.