Chassis Stiffening

[JGC403]

So I decided to build an autocross car. Been thinking about it for a while and decided to start planning the build. Think it would be fun to take out a classic car for some track time on a road course. I want to keep the stock type suspension, aftermarket control arms etc. but keeping torsion bars.

I thought it would be cool to us my Barracuda for this project. But I remember someone choosing a notchback Barracuda over a fastback; because the notchback chassis was a little more rigid because of the bracing behind the rear seat. So in my case since I have a fastback Barracuda and a Duster. Between the 2, the Duster would be the better platform to use because of the extra bracing.

But anyways, so my plan is to stiffen up the chassis but to minimize the weight increase. I'll make a list of what I was thinking of adding to stiffen up the chassis. If I missed something or if there is something is better please comment.

Welding up, gusseting, and reinforcing K-frame (almost no weight penalty)
Front and rear torque boxes
frame connectors
core support stiffener

Now I was thinking either a brace between the shock towers and the firewall, or US Car Tool inner fender brace, do they accomplish the same thing or 2 different areas and should use both.

Any other suggestions?

 

[273]

Don't forget to look up the rules and see what your allowed to do.

 

[abodyjoe]

have ye seen these kits? seem pretty nice..

http://store.uscartool.com/mopar-chassis-stiffening.html

 

[draginmopars]

Our Swinger

We decided to tie the frame connectors in multiple places
to prevent body twist in the middle

outriggers from connectors toward outside of uni body
cross members between frame connectors
with U pieces welded to the trans tunnel and cross members
and short pieces of boxed tubing welded to the floor at the cross members/connector intersections

The trans crossmember and forward frame rail
was also plated while the floor was out

due to having to widen and raise the floor for the trans shield and 518 transmission

the sides of the trans tunnel and the new floor sections are over lapped 7" and spot welded together, to give more "shear"
strength .
Maybe being double layer> a little more protection, in case of trans explosion ????

 

[72bluNblu]

If your intent is to build a competition autocross car, I would decide what organization you're going to race with and what class you want to be in before you do anything in the way of chassis stiffening. Some organizations (SCCA) are pretty strict about what they allow in their classes, and you don't want to end up in an "unlimited" type class if you want to be competitive. So, if that's your intent, get a rulebook before you get to work.

Now, if your goal is just to have fun, do whatever you like. There are a few places now to do autocrosses with fairly open classes, like at GoodGuy's or USCA events.

I really don't think there would be a huge difference in chassis rigidity between the fastback Barracuda and the Duster. The wheelbase is the same, and that's probably one of the biggest considerations for autocross. Keep in mind that while the fastback doesn't have cross bracing, it does have other bracing to make up for it (just like the fold-down seat Dusters have additional bracing to make up for the lack of the X brace). Once you've added subframe connectors and torque boxes its not going to be a make or break situation, I doubt you'd see much of a time difference on the track. I added a fold down seat to my Duster for utility, and after doing so I'm pretty certain the fold down cars weren't any less rigid than cars with the X-brace in the back. I have a whole thread on my fold down seat conversion, it details all of the factory bracing pretty well. http://www.forabodiesonly.com/mopar/showthread.php?t=258695

As for the rest of my car, I fully seam welded the K frame, reinforced the LCA pivot and steering box mounts, added a skid plate, boxed the LCA's, reinforced the torsion bar anchors, added subframe connectors, torque boxes, a tubular radiator support brace, and "J" bars that tie the firewall to the frame rails and support the shock mounts. It make a HUGE difference, the car seems very solid now.

 

[AJ/FormS]

What I discovered with my fb 68 barracuda is that the rear glass and support works seems to make the car quite heavy.It does, however help to get it to 47% rear weight bias It was hard to get my S down to 3256 # .I could lose a bit more if i tossed the GV, but that aint happening.
The Duster is likely 200 lbs or so lighter, with all same trim.Thats a lot of weight to have to get moving/stopped.

 

[72bluNblu]

What I discovered with my fb 68 barracuda is that the rear glass and support works oseems to make the car quite heavy.It does, however help to get it to 47% rear weight bias It was hard to get my S down to 3256 # .I could lose a bit more if i tossed the GV, but that aint happening.
The Duster is likely 200 lbs or so lighter, with all same trim.Thats a lot of weight to have to get moving/stopped.

I doubt the '68 fastback is heavier than a '76 Duster. The shock bumpers alone on that car add over 100 lbs. If it were an earlier Duster I'd believe it, but after pulling some of the later model parts off of my '74 and replacing them with the '71 versions I doubt it. Still, not too surprising that the glass and the extra support work weighs more, another reason the notch's are a little better suited. But unless you're going to get really competitive it won't be that big of an issue.

And again, it also depends on what you're going to be doing. The USCA, for example, has a class cut off at 3200 lbs. Get down under that and you're racing against some really light cars.

 

[Locomotion]

Are you planning on a roll bar? Properly located, that goes a long way in stiffening and tieing everything together.

 

[C130 Chief]

I think you're on the right path. I would do the inner fender braces AND the 73-76 fender to firewall braces.

The big advantage to the US Car Tool kit, aside from being precut and ready to go, is that it is pretty discreet. A coat of rattle can undercoating and most people wouldn't know it isn't factory.

As others have said, decide which class you want to race in first before going too nuts on chassis mods. Have you been to any of your local events? I would suggest checking out the local scene and asking questions. There may be some "old iron" guys with some advice, even if they drive Mustangs.

 

[AJ/FormS]

Blu , when I said in same trim, I meant when equally dieted down the same way that I dieted my S down. That is; No heater system, no bumpers or brackets, no exhaust, no swaybar, no rear seat,no passenger seat,stuff like that.

 

[autoxcuda]

Blu , when I said in same trim, I meant when equally dieted down the same way that I dieted my S down. That is; No heater system, no bumpers or brackets, no exhaust, no swaybar, no rear seat,no passenger seat,stuff like that.

I'm at 3209 on digital scales measure two weeks ago. I have steel bumpers, brackets, heavy interstate battery in front, full thick TTI headers-exhaust-tips, full interior, power steering, heavy console, thick 1.12 torsion bars...

I do have aluminum: heads-intake-water pump-radiator, one plastic horn, aftermarket steering wheel, pro car bucket seats.

Light in the back. Need help that with battery moved, heater removed, aluminum power steering pump, aluminum master cly to start.

 

[AJ/FormS]

I think my GV weighs in at 23 lbs,the factory S buckets are heavy,no console, Optima in trunk, Front skid-plate, al tailhouse,al heads,AG, A/C 26"rad,A ton of sound-deadener, P/S, P/B. Oh wait, I took the skid-plate off for racing. I cracked the Milodon pan in my first GRASS autocross. I couldnt believe how much fun that was. The last jump I hit, came down and buried the pan in the SOD. Man thats a sudden stop. So the plate went on, no more worries.

 

[autoxcuda]

I think my GV weighs in at 23 lbs,the factory S buckes are heavy,no console, Optima in trunk, Front skid-plate, al tailhouse,al heads,AG, A/C 26"rad,A ton of sound-deadener, P/S, P/B. Oh wait, I took the skid-plate off for racing. I cracked the Milodon pan in my first GRASS autocross. I couldnt believe how much fun that was. The last jump I hit, came down and buried the pan in the SOD. Man thats a sudden stop. So the plate went on, no more worries.

What pan are you running?

What's a GRASS autocross?

My headers and k-member are lower than my Milodon road race pan. I went through the desert brush/tumbleweeds/grass sideways off the long backstretch at Willow Springs.

 

[AJ/FormS]

Is that a wall to wall belly pan?
My pan is I think a 7 qt, with the xtra oil going sideways.Yes it hangs about an inch below the K. IIRC the pn ended in 40.
A grass autocross is just about the most fun you can have with your clothes on.Hillbilly yahooing at its best.Its all first gear, all twisty, a couple of speed-bumps thrown in, one on the straight dash to the finishline, set up mostly for short WB FWDs.A lot of people, some with prepped cars on trailers, were surprised to see me there. In full street trim no less.My car was fresh then.When I pulled up to the ticket booth and asked for two tickets, the attendant said 2 for the bleachers. I said no, 2 for the event. Her jaw dropped, and then asked; "you want to compete in that car?!"Well, you can imagine the rest. Cars were losing their struts on the one jump, or wheels or just generally destroying their beater FWDcars.I drove 25 miles there and 25 miles back,no worries. My best time was just 5 or 6 seconds behind the eventual prepped winner, who posted a time of 59 seconds as I recall.I just went on the spur of the moment, kind of on a lark. Man what a blast.I could do that every day of the week.One guy hit the jump,lost both frt struts. the LCAs and wheels folded up under the car and he landed on them. You know that guy tried to get that sorry pos to continue on? When it wouldnt go, he got out to see why I guess.What a surprise he got! Later that day I pulled into the shop for an inspection. Thats when I discovered the cracked pan. What a PITA to braze that up.
That day lives on in my memory. Good times.

 

[autoxcuda]

Sounds like fun. For something like that, you'd want to raise the ride height. Sorta like a rallycross setup.

I think you have the Milodon LoPro pan.

 

[JGC403]

autoxcuda, what have you done to your barracuda to stiffen it up?

What would benefit more lighten the car as much as possible or achieve as close to 50/50 weight distribution as possible?

Cause I could use fiberglass bumpers front and rear, hood and deck lid, etc. But would it be better to just use fiberglass components up front and keep steel in the back to try and get close to a 50/50 weight distribution?

 

[autoxcuda]

autoxcuda, what have you done to your barracuda to stiffen it up?

What would benefit more lighten the car as much as possible or achieve as close to 50/50 weight distribution as possible?

Cause I could use fiberglass bumpers front and rear, hood and deck lid, etc. But would it be better to just use fiberglass components up front and keep steel in the back to try and get close to a 50/50 weight distribution?

Just have subframe connectors and reinforced K-member.

Don't let your eyes get bigger than your stomach. There are plenty fast cars that are not 50/50. Walk before you run.

Realistically 50/50 is not going to have without major $$ and major cutting up of the car. And still most likely would come after you just add lead to the rear of it after you lighten it. I think the SCCA C-Prepared 70 Cuda of John Sandburg is close to 50/50 after he added lead weight to get to the class mandated minimum weight. That is a full on trailered race car with total fabricated front clip and suspension.

The red 68 Valiant Tim Werner built that LilCuda now owns is 55/45 F/R. It's will hang on the road course with a Z06 C6 Corvette. That car has a full cage and weighs 3150 lbs. It's got aluminum heads and aluminum cased 4spd.

To reduce front weigh percentage, of course you've got to reduce front weight. But you if reduce the same rear weight, the percentage stays the same. Moving battery to rear, glass hood, glass front bumper, aluminum heads, aluminum water pump, aluminum master cylinder, hollow front sway bar, etc gets weight of the front.

Remember you don't drive your weight percentage. And your weight percentage does not cross the finish line first. BUT you car does! Work on your whole car. Don't put all your efforts on weight percentage when you don't have good tires, shocks, springs, sway bars...