stiffening frame rails- yay? or nay?

[Guest dirtbike daron]

Hello. I've been hanging around HybridZ for a about a year now and have been reading up most everyones Z builds and have come across some outstanding jobs done to some and so on doing research for a 240z build of my own.

 

My plan is to build a pretty tough l28et with sds, gt35r, forged internals, etc. and i realize that puting this hp to the ground causes the body to twist quite a bit. I've seen some of the tube work people have been getting themselves into to keep the flex down with the torque their RB's, 7M's and even L6's put out, but who are some that havent built frame rails/ and or cages and arent having problems to this day?

 

I dont plan on building a monster race car or a pure show car or anything but am just curious about what could happen in the long run if i didnt go through with this step in building my car.

[BillZ260]

I've read where other guys w/ pretty much purpose build drag cars end up getting wrinkles in the rear quarter behind the rear windows.

 

If your are digging in to your car much at all, it's not that difficult to add the subframe connectors. If your worried about it and have a welder, get to it

[alexideways]

If you have an 240/60 you can graft in fraim rails from an 280 as they go further down the back of the car thus making the car stiffer.

[2126]

If I remember correctly, the weakest point in the early Z chassis is dirtectly in front of the rear wheel wells. A good four point rollber, with the main hoop securely attached to the beefy portion of the chassis just behind the seat wells and the back stays tied to the rear strut towers will certainly tighten up the chassis. Strut cross bars add some degree of stiffness too. Of course a nice cage with supports running forward to the front strut towers and down to the area where the front sway bar attaches will be even better. Just some food for thought!

 

Tom

[JMortensen]

If I remember correctly, the weakest point in the early Z chassis is dirtectly in front of the rear wheel wells. A good four point rollber, with the main hoop securely attached to the beefy portion of the chassis just behind the seat wells and the back stays tied to the rear strut towers will certainly tighten up the chassis. Strut cross bars add some degree of stiffness too. Of course a nice cage with supports running forward to the front strut towers and down to the area where the front sway bar attaches will be even better. Just some food for thought!

The area where the rockers tie into the wheel wells behind the seat is actually the strongest part of the unibody. There's a lot going on there. The rockers meet the wheel wells and the doglegs, just up the way is the deck which has a triangular shaped structural member that goes all the way across the body, the roof comes down and hits near the strut tower. All that stuff is connected. I do agree that a good roll bar is really good for stiffening up this area, but I think it is one of the better areas in the chassis as a whole.

 

I don't know this for sure, but if I had to guess at the weakest part of the unibody I'd say it's right in the middle of the floor. The frame rails don't connect all the way from the front to the rear subframes even on the 280s IIRC, and the roof while a structural part of the car is really far away from the other source of strength, which is the rockers. The front has the lower frame rails and the uppers and the rockers. The rear has its frame rails and the rocker boxes and the roof. The middle... that's where it appears to be lacking.

 

I'd say the subframe connectors are worth it. I put the Bad Dog connectors on my car and I feel that was a good decision. Probably won't get a chance to find out any time soon, since I have about a million things to do yet. Check the TC rod relocation thread which is a sticky in this forum for pics on the subframe connectors if you're interested. I modified them a lot, but there are some good pics in there if you wanted to use them as they come from Bad Dog.

[JMortensen]

Just as a side note John Coffey loves to point out that you'll put a hell of a lot more twist into the chassis by cornering hard on a good suspension then you'll ever put into it with a powerful engine. I've chewed up 2 radiators with my NA L28, but I'm running slicks...

[tube80z]

Just as a side note John Coffey loves to point out that you'll put a hell of a lot more twist into the chassis by cornering hard on a good suspension then you'll ever put into it with a powerful engine. I've chewed up 2 radiators with my NA L28, but I'm running slicks

 

I wonder if putting an X in the core support would work. I've seen that on a few rally cars. I also wonder if a car setup this way is doing this because the front and rear roll angles are vastly different, causing the chassis to take more of the load.

 

Cary

[JMortensen]

I was going to do something like that Cary. Haven't figured out exactly where the X brace will connect to, but that was the plan. Thinking in front of the core support, not sure if it will connect to the support or the upper frame rail though. Then I was going to soft mount the radiator, just to be sure.

 

I wouldn't know how to address the question about the roll angles. I think my old setup was pretty standard for the most part.

[johnc]

A structure that ties the front strut towers into the rear strut towers while also connecting to the top corners of the firewall, the front and rear of the rockers, and some diagonals connecting each of these points across the car would be perfect. Unfortunately the engine and the driver get in the way of this ideal structure.

 

Think triangles and diagonals across the car to help with torsional stiffness. Lateral bracing primarily helps with bending. The firewall/front frame junction can benefit from bending support as does the rear bulkhead/frame rail area.

 

Some ideas:

 

1. Box the trans tunnel.

2. Reinforce and extend the transmission mount across the car to each rocker (cross chassis trans mount).

3. Fully box the seat mounts.

4. Run a lateral boxed member on the upper side of the floor pan (in the interior) from the firewall/frame rail junction all the way to the rear bulkhead. Make sure it ties into the fully boxed seat mounts and the cross chassis transmission mount.

5. Connect the roll bar or cage to the hatch hinge mounting points in the roof.

6. Connect the roll bar or cage to the body sides right under the 1/4 windows.

7. Connect the front hoops of the roll cage to the A pillars.

8. Connect the front and rear hoops of the roll bar/cage to the boxed trans tunnel.

[Clifton]

I've read where other guys w/ pretty much purpose build drag cars end up getting wrinkles in the rear quarter behind the rear windows.

 

There is a solder joint there. A local body shop that does alot of S30 resto's removes the solder and welds it to keep it from wrinkling again. There is also a solder joint on the front that cracks. My 73, when I got it had the wrinkle behind the rear quater window, as alot of them do. It had a stock L24 in it too at the time.

 

I have done nothing to keep the flex down on my street car.

[Jackhammer]

These are the latest addition to my quest to strengthen and connect the sub frame. These are bolt in instead of welded due to the fact I have already finished the underside of the car completely and did not want to have to re-do the burned areas that welding would cause.

 

These tie the re-enforced floor pan frame rails and strengthened engine bay frame rails to the main rear sub frame behind the seats. This will work together with the Top End Performance 3 way front and rear strut braces.

 

I opted to go this route instead of the roll cage approach because the car is built to be show and go versus a regular track driven car.

 

It will be interesting to see how this all holds up considering the fact the SBC is going be generating somewhere around 450 HP.

 

Just another approach to beefing up the sub frame.

[bjhines]

Here are the more conventional subframe conector type that I made...

 

They are 1"x3"... 14 gauge with a weld seam... They tie the entire length of the floor pan from the bottom of the TC buckets to the differential front mount... I placed the seam up and cut lightening holes into the upper surface...