Trans tunnel backbone frame?

[boodlefoof]

Hi folks! (Sorry, long post)



Due to the births of my four kids in very rapid succession (and other time-consuming life events), I kind of fell out of the car hobby for the last 5 years or so. At the behest of my wife (she's a good woman!), I have picked-up a new project car to get back into the swing of things - a 1963 Pontiac Tempest. Now that I'm getting back into it with a new project, I feel like I've gotten extremely rusty on the knowledge I had previously acquired. I certainly would not consider myself to have achieved anything approaching genius status before, but now I feel as though I need remedial classes...

The question I have concerns stiffening the car.

First, some build background:

My plan for the Tempest is to make it generally appear to the casual observer to be something that could have come off of the factory floor and that was mildly modified in the early 60s - a "sleeper car," so to speak. I don't care if a real afficionado can spot that the car has a different factory engine from the same general period. Nor do I care if someone realizes the car is modified when they stick their head under the car to look at the undercarriage. What I want is a car that has an engine bay, body and interior that don't scream "highly modified." I plan for this car to be primarily a street car that will see some dragstrip action and which I also want to handle well on the windy country roads.

The car is a unibody car. I've designed a Satchell link rear suspension for it and put together the Ford 9''. Up front a Mustang II-based front suspension hides nicely under the inner fenderwells. It will have a stroked Pontiac 400 backed by a 4L80e. I will also put a simple 4- or 6-point roll bar in the car.

On to the question:

I want to add stiffness to the frame without intruding into the interior such that it looks out of place (aside from the roll bar). I first plan to fab-up some subframe connectors as a baseline measure to tie the front and rear rails together. To better resist twisting forces, I'm considering trying to make a backbone frame of some sort that can be tucked into the trans tunnel/driveshaft hump - allowing me to retain front and rear bench seats.

Anybody done something similar and have any ideas? With how small the backbone would have to be I'm not sure I'd see much improvement, but wanted to get some ideas/opinions.

Thanks in advance guys!

Edit: my signature block is really out of date...

[Rick Dorion]

Welcome back, John. I was thinking about your Z the other day. I, too, am getting re-involved with a 67 Bel Air I picked up with similiar thoughts to yours. I can't add to this post of yours but will follow with interest. Rick.

[astroracer]

A structure like that, even if it were larger, wouldn't add any real torsional stiffness to the chassis. It would have to be tied into the rocker panels and cage with triangulated bars to do any real work. All by itself it may add a little longitudinal stiffness but that will be lost when the body starts to "roll" around the centroid created by the backbone. If you were building from scratch it would be an excellant anchor structure for the cage but, the structure in your current floor and tunnel "almost" emulates what you are proposing to build. Add the requisite 6 x 6 floor plates to your current floor and your are there.
I won't argue that it wouldn't do "anything" as it may, but the extra weight you will add (and complexity), may not make it worth the effort and cost.
Mark

[boodlefoof]

Thanks guys. As further evidence of how long I've been out of the loop, I just spent three minutes looking for the "reply" button before I realized I wasn't logged-in... D'OH!

Rick - Happy to see that I'm not the only one in this boat. I haven't finished the Z yet. There is still a ton of fabrication work to be done on that project and I convinced myself that to keep my motivation up and get back in the swing of things I needed a "refresher" project that would be easier, and for which I could see faster progress. After I finish this one, completing the Z will probably be the next on my list.

Mark - I find myself coming to the same conclusion. I just wouldn't be able to get enough vertical placement difference from the frame rails to the trans tunnel to really be making much more than a glorified 3-rail ladder frame. The weight gain probably would not be worth it. I do think, however, that I could see some gain from building a structure around the transmission that would effectively stiffen-up the front half and shorten the long span of easily-twistable under-floor frame rail.

Still cogitating on that, but the general concept is to run a pair of short diagonal rails from the main rail in the engine bay up to the firewall (angling both in and back, coming together towards the top of the bellhousing, mostly hidden by the inner fenderwells). At the firewall there would be a rectangular perimeter of tubing hiding up against the firewall, with diagonals running from the top of the rectangle down to the rocker. This, perhaps would stiffen things up a bit forward of the firewall. Then, behind this another pair of longitudinal diagonals will come down from that top-of-bellhousing point and run along the transmission down to the trans crossmember.

On the plus side, I'm very pleased with the numbers I'm getting from the Satchell link design I put together. And, it will package under the car without having to get into the floorpan (rear seat) at all.

[MrQuick]

Welcome back John.

Sounds like a great plan so far. Adding an internal cage type system can help a lot. Doesn't have to be your traditional cage too. Under dash the atop the rockers and down to a cross bar under the package tray area.

63 Tempast ahyaa, It wouldn't be mint green metallic with a white top would it?

[boodlefoof]

MrQuick,

Thank you. No, it is a white car that I bought as a rolling body (well, it had a rear axle welded under there with no suspension so it could roll) with about 12 boxes of trim and other parts.

If I'm doing this right, here are a couple of pics for those interested.

As purchased:



Rear axle all done up with Satchell link brackets:



Frame upper link mount tacked-in:



When I get a chance, I'll pop a build thread into the other forum.

[astroracer]

You have a nice start on your Tempest. The floors look real clean.
On your "firewall cage" thoughts, run a cross tube between the "A" pillars, right over the the column. Tie it into the pillars and the front hoop for the cage. If you make a bar that would glove over the trans hump right at the firewall and drop down under the floor (picture an upside down "U") you could tie the bottom legs of the "U" into the rockers and front sub.
I did a quick paint render...

This would all be on the inside of the firewall.
If you can get the two diagonals positioned so you could tie the forward bar to the sub into their junction with the crossbar, that would be strong...

[boodlefoof]

Mark,

I was very pleased with how rust free this car is!

Thanks for the thoughts. Here is an MS Paint (my awesome Paint skills...) of kind of what I had in mind (without cage to make it easier to see).



The black is the existing frame with subframes tied together. The blue is the rear framing I'm going to put in to support the Satchell link bracketry. The purple is the trans crossmember. Red is the firewall frame structure that will go just behind the firewall. The brown bars might be a little hard to see, but the idea is two bars coming from the frame rails (in the engine bay) up to the firewall and another pair running from that point along the top of the trans down to the trans crossmember. Another pair of rails could run along the floor from firewall to trans crossmember, making kind of a backbone frame around the trans. Though it sort of dead-ends at that point... not sure if that really adds much other than weight.

[astroracer]

Yea, I can see what you are trying to do. Take my thoughts for the firewall bars and combine them with yours for the diagonals. You really need to have a straight bar tying the "A" pillars together towards the top (or as high as you can go essentially) This builds good structure for the rest of your cage and doesn't create a natural "hinge point" for the other bars to work against. In your sketch the bent bar above the trans tunnel will act just like a turkey "wishbone" and will breath in and out as the outer structure pushes against it.
You will definitely have to have the engine, headers, brake booster and steering in place before you build any diagonal bars. Those will ba a pisser to get in there and you may end up running straight forward bars on either side of the engine with a "Monte Carlo" type bar to give the diagonals an anchor point.
Mark

[exwestracer]

Sounds like a really cool project. I'd suggest that you give a lot of thought to what you already HAVE in terms of structure. The firewall design provides a great torsional shear plate for the front of the car.

I really believe you could simplify the firewall mods down to a "K" member. I agree with Mark 100% about tying the A pillars together with a straight bar. I'd mount it as far back and low to the tunnel as practical. If you can, get it right between the door hinges and just inside the lower dash. From the center of that, run your diagonals out and down to pick up the front horns as close to the upper spring hats as possible. Don't worry if you have to bend the diagonals at the firewall to get them around the engine and parallel to the front horns. I'd use doubler plates and weld the tubes into the existing firewall.

I'd also eliminate the lower "tunnel" tubes. Run the uppers all the way back to tie into your lower Satchell link mounts if possible. Again these can be bent to follow the tunnel and should be gusseted to the floor with sheetmetal (basically creating "rocker boxes" along the tunnel with the new tube as the top of the box). Running cross boxes from there over to the rocker sills (possibly used as front seat mounts as well) will add a lot of structure to the floor without much weight.

Tie the rear coilover mounts up into the C pillar structure. If you can box in the door arches without distorting the roof, it will make a huge difference in tying the front loads to the rear without a roll cage. Of course if you do go with a full cage, much of this is academic. I'm still thinking along your lines of the car not looking highly modified.

Anyway, just a few things to think about...

[parsonsj]

It wouldn't be mint green metallic with a white top would it?

Does it have Michelin Model XGV tires, size 75-R-14?

[exwestracer]

Does it have Michelin Model XGV tires, size 75-R-14?

...and a posi-traction rear end???

[boodlefoof]

Well, I managed to get a few hours out in the garage over the past few days and I fabbed and welded-in my subframe connectors. Other than the floorpan, the only thing that had been between the front and rear rails was a small 1'' high x 2'' wide U-channel that is tack welded to the underside of the floor... So, I sectioned the floor above those channels and made some rails of 2x2x.120 that slide right inside the factory reinforcement. The frame connectors are welded along the full length of the U-channel and the ends are welded around their perimeter as they slide inside of the front and rear frame rails (I had to cut and bend back the floor around the ends to get access). These will not interfere with the factory seat mounts.

Showing the U-channel existing in the floor, with floorboard above cut open:



Showing the rail in the car (it has a "Z" shape to follow the U-channel):



Stitching the floor back together where I had to spread it apart for welding access (ran out of shielding gas at the end today so still need to finish this up):



Just having done this much already makes me feel quite a bit better.

[boodlefoof]

Sounds like a really cool project.

... [snip]

Thank you for the thoughts, Ray. I appreciate it. In addition to trying to keep it from looking highly modified, I'm certainly trying to do this as simply and as weight-consciously as possible!

A quick question on your reply:

On the crossbar under the dash to tie the A-pillars together, you suggest putting it as low as possible. Other than from a packaging perspective, is there a "too low" point? I'm thinking that if it is low enough, the diagonals out to the upper spring mount area will not have much vertical angle on them (they may end up fairly close to parralel to the ground). I would think that stablizing the frame horns would be best improved by having more inclination of those diagonals up from the horn to the crossbar.

Again, appreciate the thoughts.

[exwestracer]

Thank you for the thoughts, Ray. I appreciate it. In addition to trying to keep it from looking highly modified, I'm certainly trying to do this as simply and as weight-consciously as possible!

A quick question on your reply:

On the crossbar under the dash to tie the A-pillars together, you suggest putting it as low as possible. Other than from a packaging perspective, is there a "too low" point? I'm thinking that if it is low enough, the diagonals out to the upper spring mount area will not have much vertical angle on them (they may end up fairly close to parralel to the ground). I would think that stablizing the frame horns would be best improved by having more inclination of those diagonals up from the horn to the crossbar.

Again, appreciate the thoughts.

John, I was looking at this photo when I made that suggestion.

My thinking was to have the bar pass just under or just over the steering column. Ideally, your diagonals off of that would run right over the top of the throttle pedal (on the left), get welded into the firewall near the edges of the tunnel, and continue out to a point near the upper a-arm mounts. Looking at it, there should still be significant down angle to those tubes. You will want to tie the rear tubes into that crossbar as well, so raising it means you will have to make up the height from the rear somehow. That either means a lot of bend, or running the tubes right up between the seats.

Keep in mind that there is already good structure at the firewall and base of the windshield (heater plenum box), so the idea is to spread the torsional load throughout MORE of the structure. If you were cutting it all out and starting over it would be a little different story.

Over the years, I've given a lot of thought (and effort) to "hiding" reinforcement in unibody chassis. You can't really look at it like a tube frame car, because that's what you'll end up building.... Hope that makes sense.

[boodlefoof]

Thanks again, Ray. I wasn't out in the garage looking at the car when the thought occurred to me on the slope of the bars. Hadn't been thinking about the trans tunnel bars either, where a lower cross bar would definitely make for a better fit.

I built a tube frame for my last project and think I am still kind of stuck in that mindset.