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Chassis stiffening for street car

Posted: Tue Feb 21, 2017 7:23 pm
by tomhorn1913

I'll apologize in advance if this was discussed last week. I DO use the search function, but perhaps I don't use the proper key words. At any rate, I didn't find as much info as I had hoped, so let's rehash it.

The platform is a '64-5 Falcon hardtop. I've read these are second only to convertible in terms of lack of rigidity, because of the absence of a "B" pillar. This is just a street car, so even if it were open-tracked, it would be once-in-a-lifetime, & just fun, not competitive. I plan to stick to stock type suspension & pick-up points, so no concern about trying to fit an F1 chassis underneath. I'm trying to turn a 50 year-old chassis into a modern daily driver, since we can do that here in the Garden of Eden, in southern AZ.

So, given these milquetoast parameters, what do the you guys (& gals) recommend to make this uni-body sufficiently rigid? I'm willing to add as much weight as needed to accomplish a stiffer platform, but I would prefer to avoid redundancy & unnecessary tonnage. Since it IS a street car, I don't figure I need the 99th percentile of stiffness, but 75-80% sounds pretty good.

Are subframe connectors enough? Any preference on type or manufacturer? Is some sort of roll bar or cage in order? I don't mind climbing over bars. If so, how much bar/cage? Rear pick-ups behind rear shackles? Through the firewall to the front rails, or is behind the dash sufficient? Please tell me about your experiences from the good old days.

And THEN...

I can't help but think about things like the geometry of the Fox rear suspension. Built-in bind. Is there any reason NOT to work toward a reasonably rigid platform for the mid-60s Falcon? Are there issues that will be exacerbated by reducing the platform flex?

It never ends, does it?

Much obliged,

Re: Chassis stiffening for street car

Posted: Wed Feb 22, 2017 9:33 am
by panic
But, mechanically, it's a Mustang, yes?

Re: Chassis stiffening for street car

Posted: Wed Feb 22, 2017 1:04 pm
by PackardV8
Seeing a '63-65 Falcon or '65-68 Mustang stripped to the tub is scary. There's nothing there but bent tin and not much of that.

Much as it will pain you, spend time on the early Mustang and Shelby sites. They've been doing it since day one and will have all the parts and procedures to make your Falcon track-worthy.

Re: Chassis stiffening for street car

Posted: Wed Feb 22, 2017 6:23 pm
by tomhorn1913

Even though the mechanicals were derived from the Falcon, the Mustang IS different. I've been doing so much reading lately
that I can't keep the sources in order, but at least two accounts I saw spoke of how the Mustang chassis was pretty darned
stiff compared to other uni-bodies of the era. I can see where the Falcon hardtop chassis might be a bit more cavernous
than the Mustang, & therefore less rigid.

I will spend a few days poring over the Mustang sites, but again, I am not trying to build a race car. I would like to transform
a mid-60s sedan into something that handles like a modern street sedan, but without the cheap-out features like McPherson
strut front end, transverse-mounted engine/fwd, etc. I will upgrade the suspension & driveline, but I prefer not to
re-configure it, unless there is good reason.


Re: Chassis stiffening for street car

Posted: Wed Feb 22, 2017 7:07 pm
by Brian P
How much engine are you planning to put in this?

The Falcons came with pretty weak sixes and small V8s, not big-block power. Are you staying there, or going big?

The leaf-sprung rear end won't have the Fox-body quadrabind issues - and even that wasn't an issue with the stock parts, which are designed to flex. It's only if you remove the designed-in ability of the arms and bushings to deflect in the ways that they NEED to deflect, that it starts binding.

Subframe connectors are a darn good idea and it looks like they're available. Do that.

Achieving something that handles anywhere close to a modern car will require changing everything. Steering, suspension geometry, spring rates, dampers. The stock stuff is all wrong for anything but going slowly in a somewhat straight line. Understand that a live rear axle can be made to function on smooth surfaces but the unsprung weight and left-to-right connection are bad on the good/bad scale when it gets bumpy.

It looks like suspension kits are available, including kits to retrofit the front end to coil-over-shocks and retrofitting the awful stock steering mechanism to rack-and-pinion. ... ersion-kit (Retrofits to MacPherson strut) (Upgrades the existing upper and lower arm setup but I'm not sure what they do about the steering, if anything)

I presume you're fitting power disk brakes.

I wouldn't put a cage in a street car.

P.S. I have a soft spot for Falcons. Pics as you go, please!

Re: Chassis stiffening for street car

Posted: Wed Feb 22, 2017 8:35 pm
by tomhorn1913
Hi Brian,

I've got a bunch of SBFs awaiting assembly, the top-of-the-heap being a AFR-headed 327, built on one of the new FRPP Boss 302 blocks (still trying to resolve potential compatibility issues over on the engine forum). 400-ish HP/Tq. Stout, but no BB numbers or weight.

Rear suspension is a big question mark. Still studying springs, staggered shocks, over-riders, Watts linkages, & on & on. Obviously, a suspension that was "improved upon" by changing to the Fox 4-link had to be awfully primitive. Roads aren't too bad in SE AZ, but it's still gonna need work.

The subframes seemed like a no-brainer, but I have read a couple of opinions that they are unnecessary (redundant) weight, if a cage is to be installed. I don't know which side of the threshold I need to be on - hence this thread.

I'm avoiding strut or Mustang II front suspension, but I'm not averse to the aftermarket components with bearings replacing a lot of the bushings.

Brakes will be discs all around. I am toying with a vintage-type set-up similar to the Trans Am Mustang arrangement. However, that includes fixed calipers on the rear axle, which might necessitate a floater rear. So, as usual, one thing leads to another.

I think I understand your concern, relative to the cage. I've heard the point made that if passengers aren't fully restrained (like a race car), they can sustain WORSE injuries in a crash, due to impacts with cage tubes. Makes perfect sense. Going back to Fox bodies, I am thinking about the "street cages" that were popular for chassis stiffening, more than intrusion protection. Dunno if anyone has ever tried the concept on earlier platforms, but I'm looking...


Re: Chassis stiffening for street car

Posted: Sat Feb 25, 2017 4:12 am
by pdq67
Way back when they were new, my friend Paul Posey put a new 221" V-8 out of a fresh wreck in a Falcon H/T. He had to make the dual exhaust for it and that was the pain if I remember right.

He basically contracted the conversion for a guy.

I had a 221" and a 260" V-8 and had forgotten just how small they are.. I torched a piston in one of them because the rings were stuck and I ran her wide open for like 50 miles trying to blow the cobs out of it. I think I got another piston and did a $150, "Krylon", overhaul to her and ran it for years in my '64 Ford Fairlane 500 S/W. Boy, I still miss that car. I could slide a 4x8 sheet of plywood in the back by putting the rear seat flat.

My Nephew has a fresh '68 or '69, 302" V-8 in his shop for like 40 years or so. He oils it and turns it over once a year. I found a F4B type dual plane intake and sold it to him and it is still sitting on the floor.

My B-I-L gave him the H/T car after painting it up all pretty, BUT the front-end was toast so my Nephew ended up rolling it on a country gravel road, DAMNED SHAME!!

It was a hoot because my B-I-L overhauled it, but got the rear main seal in backwards. My Nephew told me that it leaked oil so I told him that his Dad needed to turn the lip of the rear main seal around. As*hole B-I-L never thanked me or said sh*t about it. My Nephew just said, "You know how my Dad is", and we let it slide...

He is the guy that stomped a '67 'Bird because he got mad at it because he wouldn't leave the Q-Jet alone once I adjusted the back butterflies. Army Green 4-speed 400" engine car. And I mean he got right up on top of it and stomped the top, hood, and trunk!! He was nuts.. Two 6-packs a beer and-------

Traded it for a new 1/2 ton Ford P/U.. Go figure..

Funny thing was that '67 400 'Bird had it all over my '67350SS/RS Camaro and he didn't know it...................


Re: Chassis stiffening for street car

Posted: Sat Feb 25, 2017 9:17 am
by tomhorn1913

Sounds like some of your in-laws might be related to some of MY in-laws... :lol:

Posted: Mon Feb 27, 2017 7:17 pm
by dwilliams
The Falcon has all the rigidity of a wet Kleenex box.

You can try to patch it up with subframe connectors, shock tower braces, and the like, but the gain in stiffness will be fairly small. There's just too much flat sheet metal stuck together with spot welds.

In the end, your best bang for the bucks would be a cage. Even a 'street' cage with the door bars kinked down for easy access would be way better than you could get from welding odd bits of bracketry in. Some bolt-in sidebars would also let you play on the track with the big dogs, assuming the cage meets the specs of your sanctioning body(ies).

Of course, a cage is the most expensive/least comfortable option. TANSTAAFL and all that.

Re: Chassis stiffening for street car

Posted: Wed Mar 15, 2017 12:05 am
by tomhorn1913

I'll throw in another twist. After a couple weeks of internet research, I've found a few folks who are completely unapologetic in their condemnation of roll bars/cages in street-driven vehicles. They point out that a roll bar or cage is unsafe without a helmet, due to the risk of severe head injury in ANY vehicular mishap. I must admit to having bounced around in a few minor crashes, & discovering mystery bruises a couple days later, from striking parts of my body on who-knows-what in the cabin. The prospect of kissing a steel tube (even one wrapped with approved roll bar padding) with my melon, strikes me (pun intended) as, wellllllll - "bad"...

So, if a bar or cage is a bad idea for a street car, due to potential cranial damage, what then to safely stiffen up a platform? Has anyone ever seen or heard of what I'll call a "chopped" cage? Would a bar or cage serve any purpose if the main hoop was only as tall as the back of the driver's seat? Side bars could be run diagonally in a conventional manner, but, "Where to position the rear bracing?" might be the better trick. The shoulder belts could be attached to the top hoop, since it would at the same height as the crossbar in a normal cage. Obviously, all rollover protection is lost, but that was not the intent of this structure. Would chassis stiffness be enhanced to a significant degree?


Re: Chassis stiffening for street car

Posted: Wed Mar 15, 2017 11:57 am
by Brian P
I am glad you did your own research concerning the risks of using a cage in a street car.

As for stiffening your unibody ... given that what you have is a car with a roof and not a convertible, mostly what's needed is front-to-rear torsional stiffness. Bracing the door pillars (half-pillars?) from bending in or out by installing a "half cage" does not accomplish that goal. What's torsionally stiff is a tubular structure as opposed to an open cross section. This is the theory behind subframe connectors. In end-on view you need to stop the firewall from twisting relative to the rear axle mountings - that's torsional stiffness. Putting in some sort of tubular frame structure between the front of the car and the back of the car is what's needed - whether that's in the form of subframe connectors or some sort of additional bracing in the rocker panels or in the center driveshaft tunnel.

If my memory is correct the Falcon's floor pan doesn't really have a trunk floor ... the top of the gas tank is basically the trunk floor. (Ford saving money) If that's the case, that is a whole load of not good. You need torsional stiffness - closed-cross-section tubular longitudinal members - between the front leaf spring hangers and the rear leaf spring hangers.

And now I'm going to explain where I'm coming from and an obvious limitation to how specific I can get: I've never worked on one of those cars. My experience with unibodies relates to how modern cars are built because my work involves customers who stamp out sheet metal and weld them together into new modern unibodies. So I can explain things about how a modern car is built but I can't tell you how that relates specifically to a Falcon.

The driveshaft tunnel in a BMW X5 has three sheet metal layers to it. That's an indication of how stiff they want to make the driveshaft tunnel.

The door sills in that car (and others nowadays) are welded together into a massive tubular section. More indication of both torsional stiffness and side-impact rigidity.

Post some photos of what the underside of the car looks like, with particular attention to the sills, the driveshaft tunnel, how the front leaf spring hangers tie into the structure, and what connects the front leaf spring hanger to the rear leaf spring hanger.

If you can possibly get your hands on a convertible, take a close look underneath to see if there are differences in how the floor is built. Lots of convertibles have additional reinforcements in the sills and the floor pan, because in a convertible, the floor has to supply ALL of the stiffness in both torsion (between front and rear) and bending (in side view).

Re: Chassis stiffening for street car

Posted: Wed Mar 15, 2017 1:30 pm
by Dogwater
What about Aron's Falcon on Gas Monkey Garage. But then he striped it down to nothing. But who knows might be some info on what was done that could be helpful.

Re: Chassis stiffening for street car

Posted: Tue Mar 28, 2017 7:29 am
by german4inline
Matt, I was involved for a long time in German DTM and Rally developments and on my privat activities I tried to improve the torsional stiffness on my cars anyway.
The best way to improve it is to install a roll cage which is – best case – welded to the car on the most important areas as possible, mainly to the A / B-pillar roof and from the roof tubing of the b pillar directly to the front fixing point of the rear leafs. It´s very difficult and inappropriate to connect the roof tubing with the rear leaf spring fix point what is not even necessary to my opinion. I would support the bent area of the frame above the rear axle with two tubes leading to the roof tubing, maybe in the rear damper fixing area. All longitudinal tubes should be reinforced by a diagonal cross welded in. Have a look to all serious race or rallye cars and you can increase the expense stepwise. On the front, you should apply the loads preferable at the front support of the springs directly to the A-pillar tubing to the roof, breaking through the firewall and the dashboard. That means that you have to knock down your car to the naked shell – might be you wouldn´t do this due to the stock inside view without cancelling the rearseats and make your dashboard not looking as a Nascar…
About an accident with the rollcage: I guess it´s better to prevent the roof from collapsing completely causing a neck fracture….. Anyway, you should wrap your rollcage areas which might touch your head with a foam tubing to avoid headaches…..
To improve the torsional stiffness with less expense you can put a transversal crosstube behind the frontseats in the region of the B-pillar. If you take a shoebox without the cover and try to warp it, then glueing a transversal carton sheet in the middle of it, you will feel immediately what improvement in torsional stiffness it makes. This might simulate as if the car would have real B-pillars. Additional, you could install two tubes from this roofpoints behind the seats directly to the front leaf fixpoints. Probably, you can´t keep the rearseats afterwards….
I wouldn´t reinforce the car with the same arrangements as they will be done on convertibles. Strengthening the door sills will be the best way if you have no roof, but torsional and bending loads should be applied by increasing of the cross section area of the shell, door sills are just a very small improvement in this way of torsional loads. Just glue two small carton bars on your shoebox simulating the door sill reinforcement and find it out – small improvement.
I don´t know if the falcon has a frame or an integral body. If it´s frame, you could replace the rubber cushion bushes by solid aluminium ones to improve the stiffness.
Good luck !

Re: Chassis stiffening for street car

Posted: Tue Mar 28, 2017 9:29 am
by Brian P
Falcon is unibody - that much I know. It has a partial front frame extending forward of the firewall and which supports the front suspension and engine mounts etc and it extends rearward under the floor a little bit and it's welded into the unibody. The means by which that front partial frame transfers loads to the firewall/A-pillars/sills/driveshaft tunnel is not great by modern standards. This was pretty much the way 1960s-1970s unibodies were built before they started integrating the front inner fenders into the structural design.

Tubes exposed in the interior of a car in which the driver will not be wearing a helmet and restrained by a harness = no bueno. Other than the original post mentioning that this is going to be a "street car" we still don't know much about what the original poster wants to do with it. By 1960s standards the Falcon was a functional street car the way they came off the assembly line. If the intent is to end up with something that drives like a modern car then the best course of action is to buy a modern car. There is a spectrum of possibilities in between those two.

Re: Chassis stiffening for street car

Posted: Wed Mar 29, 2017 8:06 pm
by tomhorn1913
Thanks for all the replies -

Lots of thought-provoking feedback. I kinda figured the answer to an early unibody would ultimately be a cage. I'm not an engineer, nor do I play one on TV, so re-designing the floorpan as a backbone chassis is a bit outta my league. I hadn't considered the threat to one's noggin, but after discussing that, I'm thinking (& hoping) a cage might be employed safely. The problem may not be a big as I imagined (a bold statement, if you have ever shopped for size 8 headwear). If the main hoop is moved, or angled rearward a bit, it might be eliminated as a threat to the cranium. The hardtop has no B-pillar, so there is no apparent penalty for not having the main hoop tied to it. The halo would need to be wedged in as tightly as possible above the door openings, & the front hoop would need to be tied to the A-pillar, a la racecar, to give maximum effectiveness to the front tubes, going through the firewall. Real seats & harnesses will be used, so the risk of bouncing off other cage members is no more dangerous than the events of an impact in most modern road cars. Rear seating will need to be sacrificed to achieve proper bracing in the rear, but my kids are grown up, & have kids of their own, & this is MY car. Door bars will be attached to the main hoop low enough to avoid head strikes, & angled down as much as possible to avoid arm injuries in the event of a crash. I'll need to seek a cage fabricating pro to calculate efficiency, & do a lot of measuring to see if it fits.

Far from being a certain resolution, please continue to post. I'm always open to ideas, as long as they make a little bit of sense.