Chassis reinforcement

[CrazyZ]

Hey all,

 

I am current reinforcing my 260z chassis for 450-500 whp.

 

Atm, the frame rails have been upgraded with 18 gauge box steel and will be planning a half cage in the rear of the car.

 

I've also heard of another method of "tying" the front and rear chassis together by removing the rocker panels and welding in some heavier gauge steel to tie the front and back of the car together.

Any advice on this method or what would be the best method of tackling this job?

[260DET]

Heaps of info around, try a search. Seriously.

[johnc]

I've also heard of another method of "tying" the front and rear chassis together by removing the rocker panels and welding in some heavier gauge steel to tie the front and back of the car together.

 

 

Waste of time. The rockers are already pretty strong. Search please.

[z-ya]

I too am interested in how to strengthen the middle of the S30. Stiffening the front and rear sections are pretty straight forward. A street application. No interference with seat or door panels. Just make the chassis stiffer without it looking like a complete cage.

[JMortensen]

The rockers are where it is at. Dave Kipperman (I think his screen name is viperredls1z) put 2x2 frame rails inside the rockers. Once that is done it would be nice to try and tie the rockers into subframe connectors or into the front and rear subframes. I did this a little on my car but I didn't really strengthen the rocker itself, I ran a bar just over the rocker on my cage.

[Mikelly]

I'd Listen to Johnc on this one. I'd also plan my additional metal placement and amount of carefully. We had a substantial cage welded into my car and I now have a 2700 pound pig. Between the cage and the framerails, we probably added 200 pounds to the car.

 

Mike

[rags]

With all due respect to JohnC, I have done the 2x3 tubing inside the rockerboxes and tied those to the TC cups with 1.5" round tubing. I have done nothing to the original flimsy frame rails under the floor and feel they are now just along for the ride. The car is super tight.

 

I'm really interested in why the thought is that strengthening the rocker boxes is a waste of time as this is the location that holds the car together? Don't believe me? Cut through one of them whith a sawzall and see what happens

 

Joe

[z-ya]

Joe,

 

Got any pics? This is exactly what I was thinking. Reinforce the rockers and possibly the floor frame rails to minimize the twist in the middle that a stock S30 has. No cage, or roll bar (I'm not all that keen on driving a car with cage without a helmet). Keep it looking stock inside. Maximizing seat travel and interior space while making the chassis a lot more rigid. Front and rear strut tower bracing has been well documented. I'm looking at way to stiffen the middle of the car.

 

Thanks,

 

Pete

[rags]

Pete,

 

Unfortunately I didn't take any pictures when it was being done. The only thing that I can take pictures of now would be the round tube to the TC cups and how the round cut outs on the inner rocker panels are now blocked by the 2x3. This was done on the red 260 back in 85 by a guy that used to work for the old ZCenter in Clifton, NJ back in the late 70's. His name was Mike Janiak and I'd really love to find him today as he was a wizard with a modified Z car! The car is still as stiff as it ever was.

 

Joe

[johnc]

On the S30 chassis as it comes from the factory the rockers are the strongest part of the car. IMHO, I would spend time and effort strengthening the weaker parts of the chassis first - fix the weakest links.

[JMortensen]

No reason to guess at the weight increase. You can look up weight per linear foot at www.onlinemetals.com.

 

Here is 2x3 .083 wall: http://www.onlinemet...241&top_cat=849 This works out to 2.73 lbs/ft

Here is 2x2 .065 wall: http://www.onlinemet...845&top_cat=849 And this is 1.71 lbs/ft

 

EDIT--The rockers are 4'6" from the back to the tip in front, so 9' of metal does both sides. So 15.39 lbs for the square or 24.57lbs for the rectangle, before any plate or weld wire gets in there.

 

There are other thicknesses available, but you just don't need .250" in a Z car, especially when you're attaching it to stuff on the chassis that is much thinner.

 

The problem is that the ties from the rockers to the strut towers are weak. So you can stiffen the rockers, but they aren't attached to the frame rails very well, and the strut towers aren't attached to the rockers very well either. The main structures in front and back are the torque boxes and in the front the vertical area between the torque box and the rocker.

 

Here is a plan for improving the connections here, and gaining a lot of stiffness without a cage. In the front you can run a brace to the TC rod as Joe did, and one straight from the front corner of the rocker to the frame rail perpendicular to the center line. This attaches the frame rail to the TC rod to the rocker in a big triangle. In addition, run one to the upper frame rail outside of the strut (or punch a hole in the fender and run it right to the strut tower a la BRE). Now you have a vertical triangle from the rocker area and the stock vertical support to the front torque box and the upper frame rail/strut tower area.

 

There just isn't a similar simple way to tie the back end together because of the location of the wheels and the strut towers. I would run the tubing in the rockers and then I'd do the X in the back from the strut towers down to the frame rails that Cary, Steve Parmley and I have used. This X is a serious pain in the ass to weld to a cage, but I think it would be a lot easier to weld to the rocker without having to worry about a main hoop. To do this you would probably want to cut out the rear torque box. Probably good to check for seat clearance. I know I didn't lose any seat clearance, but mine attached to the hoop which is considerably higher than the rocker.

 

Then run a good set of strut tower bars front and back, and I think the car would be A LOT stiffer than stock. This is essentially what I did on my car, then I put the cage on top.

 

Some pics of Dave's car.

Edited September 5, 2011 by JMortensen

[tube80z]

Joe,

 

Got any pics? This is exactly what I was thinking. Reinforce the rockers and possibly the floor frame rails to minimize the twist in the middle that a stock S30 has. No cage, or roll bar (I'm not all that keen on driving a car with cage without a helmet). Keep it looking stock inside. Maximizing seat travel and interior space while making the chassis a lot more rigid. Front and rear strut tower bracing has been well documented. I'm looking at way to stiffen the middle of the car.

 

Thanks,

 

Pete

 

One option I haven't seen discussed is to put an X in the floor. Very common in the pro touring car categories (BTCC, V8 supers, etc.) For about the same weight as the subframe connectors a lot of people put in you can increase the stiffness a lot. Ideally this should tie into the cage completing the bottom.

 

The rocker reinforcement shown on Dave's car was at my prodding. We had a hillclimb accident where the driver was seriously injured because this area folded up and struck him. The door bars did their job but the rocker and floor gave up. He now has a limp and was off work for about a year. If you look at newer cars this area has been massively reinforced compared to the Z.

 

Cary

[rags]

I did do some additional reinforcements in the rear to stiffen up there as well. Again 2x3 was used. This ties the trans tunnel to the rocker reinforcement. From there I go up the rear wheel arc to the strut tower. Still do not have the rear towers connected but I wanted something without interfering with the interior of the car.

 

Joe

[JMortensen]

I looked hard at putting an X underneath but the problem was exhaust clearance. I bought the metal to do it, so if it can be worked out...

[EvilC]

I agree there are weaker points but if you are building from scratch why not do the rockers. Everyone has seen plenty of Z cars driving with no floors but ever without rockers? Joe and myself, cut a parts car up and when you slice through the rocker, the car starts the fold. The doors can't be opened and if you get it open that door can't be closed back. The rockers should be tied into the, frame rails and rear strut towers. From my own knowledge and I could be wrong but after all those points are tied together, a main hoop should tie the strut towers to one another and then into the roof (meaning the "rally" mounting points) that are by the quarter window. I didn't do all of that on my car, so I am waiting to see the paint crack by the pillars

[rags]

Hmmmm

[johnc]

I agree there are weaker points but if you are building from scratch why not do the rockers.

 

Efficiency. IMHO, I want a chassis that's strong enough to do all of its intended jobs and no stronger. Any additional strength is added weight and wasted time and money. Every one has different goals, mine almost always emphasizes efficiency.

[JMortensen]

But John, how are we determining efficiency? If the car in question is going to run big tires or really stiff springs as seems to be the trend amongst the faster autoxers and road racers lately, it would be nice to try and calculate what is actually needed and then try to hit that stiffness goal. With some testing it could actually be figured out and then maybe we could have an argument that was quantified in some way. There are guidelines for torsional rigidity and spring rates out there, if you google it you can find them. The problem is that right now we're all shooting from the hip, and that is largely because testing torsional rigidity is a real pain in the ass.

 

To continue the SWAGging though, I think you're probably right that reinforcing the rockers alone without any other tie ins to the chassis probably isn't the greatest expenditure of time and effort and just makes the car a bit heavier. However, if the strengthened rockers get tied to the strut towers or a cage then I think it's a lot harder to make the case that it's wasted weight.

[johnc]

The closest I have to any empirical data is some FEI analysis done by Bill Savage at T-Mag back when he did some work for me on the rusty old Datsun. The biggest weak areas on the model he and I made of the S30 was the connection of the firewall to the rockers and the floors. That area acted like and hinge. Within the limited rule set I was building to we built a triangulated strut tower brace and I (illegally) added some reinforcement to the upper frame horns and filled in the open box (my car was a 1970) on each side of the firewall connecting the rocker, fire wall, and cowl. Those changes, along with a welded in roll bar, let me run spring rates 350% higher then OEM. It stopped the chassis cracking that had regularly been occurring at the firewall/floorpan junction when I was running 225/50-15 Hoosiers - even after upgrading to 275/45-16 Hoosiers.

 

If I had a full roll cage with braces to the top of the firewall and through the firewall to the strut towers the little analysis we did says we could have gone 600% higher on spring rates. None of the analysis we did showed any strength issues with the rockers.

 

Now, its was a pretty rudimentary analysis done 11 years ago and FEI has been superseded by much more sophisticated tools and techniques. Maybe we screwed up the input parameters and got garbage results. There are a whole host of things that could (and maybe did) have gone wrong with that analysis. Unfortunately that's the best I got and on track results sorta confirmed what the software said.

 

One way to help figure this out is to cut up a 280Z. I've cut a few and the reinforcements are mostly in the roof (for rollover protection), front frame rail/firewall/cowl junction, rear bulkhead/floor pan junction, seat mounting, suspension mounting points, front crossm ember, front frame rails. I honestly don't remember if the rockers were reinforced so hopefully someone who has cut a 280Z up and remembers can chime in here.

[EvilC]

Hey that is something for me to keep in mind for the next few part cars. We can all start taking pictures of the structure of the s30 chassis. I was surprised to see rust in the inner fender box on the last 73 car we cut up. There is cancer forming everywhere on our cars. Plus while cutting up the car, you get a good sense of where the strengths and weaknesses are.