I took out my carpet for replacement and noticed that in the rear floorpan area, just in front of the rear lower control arm mount, the pan is cracked and buckling. I have sub frame connectors but this seems to be a weak area still. I am considering adding some material to "beef up" this area and possibly reconnect it to the sub frame connectors. Sort of "bring the sub frame connectors into the interior". I notice there is an opportunity to triangulate the rockers and floor pan in this area right in the area where the wiring harness passes through. I am wondering if anyone else has had this issue and what was done to correct it.

 

Do you have a picture? It sounds like it could be some kind of jacking damage based on the description, it wouldn't be the first time I've seen a damaged floor pan from someone not knowing how to use a jack.

 

Lol yeah that certainly is a possibility. I've owned this car for over 10 years and it didn't sound like the original owner was a DIY guy but who knows. Funny they are symmetrical and positioned right in front of where the lower control arm mount reinforcement ends. 414 CI, 5 speed, nitrous, I'm going with the assumption it could use reinforcement.

The first pic is before. You can see the slight buckling and some cracking at the stress point. I straightened out the bends and welded the crack. Then I took some 18ga mild steel and formed a piece to "cap" the area. The bolt goes through the RCA mount stack. The new piece extends to the seat mount which goes up and over to the other side. The floor pan steel is pretty thin in this area so I plug welded as well as perimeter welded. I'm going to add some straps to the bottom which will bolt through to the new piece and weld to the sub frame connectors. Maybe overkill, but it's not too often I have access to this area and always felt this was a weak link in the chassis.






The fill piece looks flat in this image. It is more of a "J" or bowl shaped channel and is pretty stiff once formed. It is also heavier steel than the floorpan.

 

No matter how strong your driveline is, it didn't do that.

 

Why do you state that so emphatically? I'm not arguing as I hope you are right but still I'd like to know your thoughts.

 

The suspension mounts to compound bends in the body tub specifically designed to withstand great amounts of force or torque. Loads are distributed in this way over a larger surface area and throughout the tub. In any of these cases, the rubber on your tire will have the lowest coefficient of friction and will not be able to overcome the force required to buckle your body tub.

Unless of course you've changed your suspension & found some other mounting point to work with.

All that being said, I have in the past split the lead seams at the tops of the quarters on my 77 Formula and cracked the windshield on the passenger side of my 78 T-Top SE after repeated street racing, but that has more to do with body flex than it does to brute force.

I just read your car stats & that's pretty stout! How hard have you launched and have you launched with the Nitros or do you give it a shot down the track?

 

Thanks for the reply and info. I do understand how folded/layered sheet metal is a lot stronger than most people think. Our cars get down graded because they don't have a conventional, separate frame but they are still pretty strong. Still, the sub frame connectors help and do "shift" the load around somewhat. As always the next-weak point shows up and that's what I figured was going on here.

As far as my car I have only ran a few dozen passes total and probably only 5-6 with the current combo. I always launch pretty soft since I'm still running the 9 bolt rear & street tires. 1.8 60's come to mind but I'm not looking at slips. It's no wheel stander!

You've made me feel a little better. I'm glad I did the reinforcement detailed above but like I said it's probably overkill.

I searched before I posted this because I thought I had seen this very topic discussed before. I looked through the LCARB & sub frame threads but didn't see anything. I may look a little more.

 

Back when I was running a stock style suspension, my LCA were lower in the rear than in the front. After a season, I had much worse buckling that that.

 

That's on the passenger side?

 

That's where mine was also.

Chassis rotation. When launching, the left front of the car comes up. That's normal because of how the engine is rotating. At the same time, the right rear is coming up which causes the typical one wheel burnout. Since the diff rotation is forcing the right side of the diff to rise, it needs to push against something. The thin sheetmetal of the unibody can only take so much stress. Adding a rear sway bar (anti roll bar) can transfer some of that energy back to the right side of the chassis and help reduce flexing.

 

Thanks Alky, yeah mine isn't that bad but I didn't want to just let it go. Although the plate I added doesn't look like much the bend/curvature following the floor pan adds considerable stiffness to it. I don't know if you can tell but there is a bolt running through the new piece and the stack of sheet metal below it, which are the parts that make up the lower control arm mount. The front of my addition goes up to the front seat's rear mounting rib which is a boxed piece running from side to side. I had considered adding some straps underneath following the same basic route, but welding them to the sub frame connectors. It looks like there's an opportunity to also connect the sub frame connector's Y brace that goes to the trans xover, and weld tabs/bolt through to the front seat mount bracket as well. When you only consider sub frame connectors from the bottom view it obscures how it all connects up top. I know most people don't want to bother with going into the interior when adding sub connectors but it seems you could really improve stiffness that way. At least on the floor pan plane. Problem is no matter how "stiff" the floor pan/frame is, it still is flexible from the side view & rear view (twisting). You'd need a cage to really do something about that. The doors, and in my case T-tops, really weaken the structure.

 

What did you do? I'm presuming you have a cage since you're doing wheel-up launches and most likely NHRA or track legal. 8-9 ET's aren't usually just for street fun lol.

Oh BTW I do have all sway bars in place and with the exception of sub-frame connectors, boxed lower control arms w/LCARB, adjustable torque arm and urethane bushings my car is otherwise stock. I have my control arms set for about 1" rear drop at ride height. From judging drag launches I'd estimate they're about level at that moment. It doesn't leave real hard but does get great traction at the concrete strip with everyone else's soft rubber laid down. On the street 1st and most of 2nd have to be pedaled back or it's just a smoke feast.

 

I already had welded in SFC and a 6 point roll bar when I first saw it starting to buckle. I cut out the 6 point roll bar and installed a cage when I was getting close to the 9 second range. I just lived with it until I finally cut out the floor to back half the car and redesign the cage. My floor that's forward of the back half is still junk. No passenger seat and the driver's seat sits in a tubular frame that's welded to the cage so it's not even attached to the floor any more.

Never had an issue with my T-tops.

My car is track only and I race under IHRA rules. Other than a license change to say I can race in the 8's, my car is legal down to 8.50's. Needs more tubes in the cage to go lower than that but I can't see hitting 8.50 any time soon. I'm still trying to get an 8 second pass. Last time I tried, I put the car on the back bumper twice before calling it a day. Has wheelie bars installed now but haven't had a chance to get back to the track to test them.

 

Man that's awesome, I've always wanted a car with enough launch to wheel stand! I can see why you'd have issue with the stock frame. Thanks for the input. I suppose we'll see how my reinforcement holds up and if it was even a power issue.

 

Just need good traction and a lot of torque. My car is still nose heavy.

 

I think all of you guys are in abusive relationships....& your cars could press charges!

Some nice FAST cars you guys are building!

 

Nose heavy????????? The video scared the crap out of me! One second you see the full length of the track, and the next second theres clouds and birds and crap...LOL

 

Looks like a good fix

Just slightly bent my sills , couldnt obviously find the front jack point, used what I thought it was and well its pushed into the car a bit more now. Kicking myself

Im thinking I might weld in angle iron the full length of the sills to strengthen them up, does this sound like a good idea ?

 

I have no idea if its a good idea but I welded 1"x2" rectangular tubing from the front to the back of mine as part of my sub frame connectors and I havnt had any problem jacking anywhere i have wanted or needed to jack. I figured my car needed the additional support... The PO thought anywhere there was metal was a good jacking point. I had to beat 3 different parts of my floor pan back flat. The passenger side front floor pan had a 4 inch tall bulge in it...

 

I realize that sometimes I assume too much; but this just screams that it belongs in the "if you have to ask... " category.

 

That sounds good man, id like to see that. What thickness tubing did you use and howd the welding go ?

 

Thanks for the bash, ok so worded differently just wondering if its something others have done ?
Made sense to me, but just wondered.
Yes I am a t+#t for what happened but its not as obvious as the back end and any other car ive come accross also the manual sucks. I dont currently have a trolly jack for the front sub frame or sus arms mine went pop. But no excuse I know. Im just human.

 

I constructed my sub frame connectors out of 1 x 1 square tubing with 1/8" wall which basically follows the pinch weld just inside the rocker panel. It is tied into the rear suspension pivot point and the back end of the front sub frame area. Angled braces run inboard to the boxed in section the trans cross member attaches too. Still, I don't use it as a jack point. I jack just behind the front tire about 10" in where it's boxed in and just in front of the rear tire where the control arm attaches if approaching from the side of the vehicle. The stock jack locations may be on the rocker pinch weld but have never felt safe using them.

 

Sounds good, def what I need to do. Have you thought of using the same piping from the hatch hinge locations down to the boot lip, parallel? Might stiffen the rear up some what ?

 

1x1" square tubing is a little small to add any real flexing stiffness. My first set of custom made SFC were made from 2x2 and all welded in.

 

I think it was 3/16" maybe 1/4" I had to flatten out a few places along the pinch weld to get it to lay flat all the way, on the big 4 inch wide piece of boxed in subframe in the front i welded some 1/4" C channel I had that fit over top of it and connected that to my side pieces. Well one side so far, I havnt tied in my passenger side yet was waiting to see how my new exhaust was going to fit.

Besides getting it to lay flat and surface prep welding went pretty good there was a few wider gaps I had to fill in