knightinblackar

Hey guys,

I've got a project on the go here and need some advice. I'm hacking the roof off my Trans Am for a very special project. The car will be a permanent convertible, don't ask why, its a secret!!

The question I have is, how can i reinforce the chassis to take the lack of roof as i'm not cutting ANYTHING until the chassis is ok.

Any help will be great,especially from anyone who works on race cars or serious power platforms.

Regards
Rich

91_5.7_TPI

iTrader: (2)

SFCs would be a minimum. Cutting the roof completely off would scare me, but I don't guess it's much different than the normal verts. Maybe some sort of mild and nicely done roll bar/cage?

knightinblackar

Yeah thats what i'm thinking. I have the original ASC convertible blueprints and plans so i think i'll follow those as well as modifying them for todays use!

87 formy

IIRC there was a guy on here who did a t-top to targa top conversion. I think he had a set of SFC's and a cage in the car when he did it and said even with them he still felt the car give in a little when he cut the T bar out. I'm not sure what the best way to prevent that "give" or if it's even possible to completely prevent, but I'd say at least a set (or two) of SFC's, and maybe a cage depending on what you plan on doing with the car.





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Shagwell

Personally I would jig the car up properly on a frame table, brace it, then cut the top off. A roll bar or cage will only help if you run solid door bars, which in turn creates a hassle to get in/out of the car. I would go a step beyond sfc's and do something like the "through-floor" frame rails used in backhalfed cars. - a great example of this in red406's prostreet build thread.

Yes, all ASC did to create the OE convertibles is chop the roof of a hardtop(ever notice the full length quarter molding for the spoiler on a vert? covers the cut to fill the body line). - The only difference in the convertible frame/bracing are the rocker boxes. Many people speak of some elusive "x-brace" but after owning 3 convertibles and being closely involved with 2 others I've never seen anything different other than the rocker boxes.

If you ever have the interior out, the top down and the doors open it's a bit scary looking....

knightinblackar

Sorry about this but in the UK i gues we have different terminology. What is a rocker box?!!! Or should i say where is a rocker box. Great advice though. So ASC never used the x-frame then?

customblackbird

iTrader: (15)

i would due a custom SFC with thru floors (thats the strongest type) one that connects the actual subframes(trans part) and then run a second set into a perimeter type that would connect the rockers etc as well as the first set u designed. i would look into a rear shock brace kinda like a front STB but welded in. i didnt something like that to stop twisting and to connect the cage i stuck in better. look in fabrication for "rear strut brace". U will need it to take the place of the roof.

i would also look into some type of cage. at least an 8pt, and look into reinforcing the front subframes. wonderbar, front STB and some custom reinforcing to the rear LCA/rear subframes etc

knightinblackar

Thanks but i still don't understand what the 'rockers' are!!!

customblackbird

iTrader: (15)

rockers are the area below the doors, its the lower perimeter of the car.

look at UMIs SFC they run along the rockers.

knightinblackar

Right, now i got you. Here they're called sills!! Like window sills but different!!

You gotta link for UMI's?

So i need to reinforce the area under the tank between the two suspension areas, the sils, sorry, rockers. Will i need any kind of frame between the two sides of the car (in the area where the back seats used to be) or will the bar under the tank do that job for me?

Also what is a wonderbar?

I already have a great big thick reinforcement bar between the front subframe rails in the engine bay around the power steering pump area from one side to the other, will this help?

customblackbird

iTrader: (15)

haha oh god. this is gona be along thread. u could do a search and save alot of time. im not following u with the "tank" area u speak of. the tank aka GAS tank is under the rear of the car and above the rear end. no supporting is really needed there depending on how stiff u want the car. the gas tank must be taken out inorder to weld in a ROLL cage since u will be welded right above the tank and will risk fire if it is left there. this BIG THICK BAR ur talking about is prob the front sway bar and has nothing to do with chassis reinforcement.

you want to stiffen the car as much as possible im guessing. so 2 sets of subframes like the UMI ones below and a set of TDS ones tie 2 different parts of the subframes together for a stronger car. also to further reinforce the car... the only other thing to do would be to install a roll cage. the more points on a cage the stronger the car. a minimum would be an 8pt cage.

subframe connectors, installed pics are at the bottom of the page.
http://umiperformance.com/2400?category_id=113

http://www.spohn.net/shop/1982-1992-...-Top-Cars.html

TDS subframe connectors
http://www.top-downsolutions.com/pro...products_id=90

wonder bar AKA steering brace
http://umiperformance.com/2030?category_id=112

this is the wonderbar that i have by TDS. i like it better than the spohn/UMI one because i have dual electric fans and just seems stronger to me.
http://www.top-downsolutions.com/pro...products_id=45

these are strut tower braces that fit above the engine and connect the front towers together for added stiffness in the front end.

http://umiperformance.com/2008?category_id=167

this is BMRs site. it has subframes at the top, as well are strut tower braces and wonderbars etc.

http://www.bmrfabrication.com/F3-chassis.htm

knightinblackar

Thanks for that information, that's really helpful. No the bar i was referring to in the bay is aftermarket and bright red!! It's bolted and welded between the two front chassis legs (the one the steering box is attached to and the opposite side) and its pretty thick, from your description I think its a wonderbar.

So do i need to strengthen the top of the suspension towners as well as that wonderbar or not?

Also do i need all 3 sets of subframe connectors or can i get away with the meatiest set along the rockers?

I was planning to reinforce the rear of the cars frame (where the rear seats would be) across the two 'b' pillars (under the speaker housings) straight into the frame as Autoforms did with the first ragtop 'birds. Would this be any advantage with the subframe connectors or just a waste of time?

Rich

customblackbird

iTrader: (15)

its prob the wonderbar but a pic would determine this.... ive never heard of anyone welding them in... that would not allow it to unbolt.

u dont have to reinforce the strut towers but if you where roadracing or putting alot of stress on the towers (autox, hard street driver etc) it would be a good idea.it will stiffen up the front end and connect the towers for better cornering etc. not needed tho.

as far as 3 sets u do not. 2 sets would be better than 1 and 1set is better than nothing. u hacking off the roof is gona really weaken the car. i would say 2 sets like the UMI and TDS. they connect 2 different parts of the subframes together and offer increased rigidy vs just one of the sets. you would really want a perimeter type SFC like the UMI/SPOHN/ also jester has a nice set of subframes that are comparable to the UMIs and are perimeter as well (bolt on ones) i would pic among those 3 and then the TDS set is the only one i kno of that connects the way it does. so those would be a good idea as well with one of the other sets i just stated.

these cars flex a good bit so 2 kits will make the car stronger than with just one SFC kit. u want it to be the stiffest. i would do both and then somehow do some fab work and connect them together and to the car in some more areas. then if needed u could throw in a Cage or roll bar and increase the rigidy of the car along with the subframes.

not really sure what u mean by connecting the Bpillars. pics of what the autoforms did would help.

knightinblackar

Well i already have the TDS wonderbar as I've found a receipt for it, so thats one item off the check list!!

With regard to the autoforms frame i have a pic :



They welded that square frame from one side of the car to the other in the area which houses the speakers in the pillars behind the doors. I was thinking of simplifying this arrangement (due to no hard top being required) to add extra strength to the car.

basiccamaro

A full rollcage would of coure give you the most ridgidity, but at too high of a cost utility- wise on anything but a total race car.

But there is another way that can help stiffen up th car, especially when used in conjunction with a roll cage or partial rollcage.

Why does the rollcage help so much? It's not just in the extra material added to the chassis. Your original frame/unibody basically has it's framerails/rocker boxes/sills pretty much on one plane.
But when you add a roll cage, the top of the rear main hoop, and the middle perimeter (at seatback and door bar/dashboard height) of the cage itself provide another parralell plane approximately 24" or so above and parralell to the original structure.
Now you can run bracing bars down from these higher areas at an angle to provide triangulation and therefore stiffness and strength into the structure, sort of like bridge suspension towers. So in this case, height is your freind!

Since a full cage is too cumbersome for a street car, another way of providing another plane of for add- on structural integrity is through the transmission/driveshaft tunnel.
You could square up, widen, and enlarge the tunnel to some degree to get some heighth into it. This will not be nearly as high as the rollcage position, and it will not span the width of the car like a rollcage would, but if you ran two round or square tubing bars on each side, lengthwise down the top of a higher, squared off tunnel, and then tied it into the floorboards (possibly with more pieces of lengthwise tubing along the floors), firewall and/or the front frame rails/wheelwells, and to the rear framerails/trunk floor, you would substantially stiffen the chassis structure. Kind of like building a beam right down the middle of the car.

That should help with the stiffness and strength. And the new tubing could be bent to flow with some of the original contours of the floorboards and driveshaft/transmission tunnel, you'd just be adding stiffening metal along the top and maybe the bottom of the tunnel area.

This kind of structure would somewhat mimic the "X" style frames of early Chevy Impalas and other convertible car models from the past.
And if tied in with some other roll cage- type add ons, could be quite effective.

customblackbird

iTrader: (15)

not totally understanding what hes talking about with the tunnel and raising it etc. but he has good info there.

cages are not needed. and usually are dead weight unless your building a full out race car. most street cars dont need cages/rollbars and i would advise not to get one unless u actually need it. say u race the car and NHRA requires u have a cage or u cant come back to the track. or if u autocross/road race and it is required as a saftey measure. i would have to argue tho that if your dropping the gas tank or taking out the rearend and have all of the interior out, u might as well install a 4pt or something since you will have to tear everything down again to install one. a 4pt or 6pt will stiffen the rear up substainsally but has no where near the affect of a 8pt-10pt-12pt and full cages.

he is correct on the roll cage siffening capabilites and how it triangulates the chassis. u cant beat the overall rigidy of a full out cage or a full framed car but u can definitly increase the unibodies rigidy. the SFC kits i posted are for ppl who dont want to bother/have the time/have the skills in fabbing up sometihng of there own. those kits will work and are basically just bolted/welded on. but there are better ways to strengthin a car. they are designed for easy of installation so that almost anyone can do it. to really stiffen these cars up u have to get creative, think outside of the box and with my ingenuity u could acheive alot more than aftermarket peices can offer.

cages are very annoying to get into and out of, i put in a 6pt with out door bars (i have them) just didnt want to put them in bc getting in an out will be a BlTCH! Cages are great but on a convertable they seem kinda pointless and will look pretty funny. You can def do some R&D and build some kind of weldon tube chassis for the car... build a Brace out of roll bar size tubing 1 5/8x.134" mild steel tubing and create some kind of Xbrace for the bottom of the car as well as something for the inside. incorportate it into the floor boards/stock subframes as much as possible, maybe a thru floor design(using the floor as extra rigidy) and follow the contours so u dont have this cumbersum looking thing on the car. keep it clean and keep it strong. come up with some designs and post it

knightinblackar

OK I'll try to get my head into the design of a system when i've finished my current customers car. Theres nothing like being prepared for an eventuality though which is why i'm posting all this here now so when i come to actually hacking the car apart i have a concrete and definate idea of what i'm going to be doing.

I'm not chopping the roof off until i've strengthened the car because it'll sag as soon as i take the roof away. Therefore i have to find a method of holding the car in such a away that its current structural integrity will remain the same once the roof has been removed.

Ths most important thing I can think of is the centre part of the car, because after driving and use, or in the event of a collision the floor will snap and collapse and i damn well don't want to be in it when that happens. These things are weak to start off with so i'm making absolutely sure that its tough enough first.

My original idea was a x frame tied to the subframes front and rear and two rocker reinforcing tubes or box sections running the length of the car. Once that is done i was to creat a x frame between the door pillars from side to side to stop scuttle shake within the car. The main reason is that it will flex and snap my other plans for the back which i'm not posting until i've done it....BIG SECRET!!!

If there are any other things i should know about before strengthening this car please let m e know as i don't want to put something in and find that it is useless in the overall structure of the car.

Rich

Street Lethal

iTrader: (16)

.... any updates Rich?

BPWilliams

Hummm sounds like my project.... NASA / SCCA Camaro Race car.....

customblackbird

iTrader: (15)

wats the rectangle tube stickin out of the floor for BPWilliams? attachment straight to the SFC?

BPWilliams

The square tube is where the main hoop was welded to my sub frame basically making a sandwich out of the flimsy floor pan. It is hard to see in these pictures but it is tied in in the front and back.

knightinblackar

Hey guys,

Sorry I've been away for such a long time, my business here in the UK has taken a right kicking and I'm on the bones of my *** right now! Project convertible is out of the window for a while while i get back on track but I'll be keeping this thread active so it doesn't disappear because I WILL do this!!

How are you guys?

By the way, thanks for those pics, they've helped out a hell of a lot...pictures speak a thousand words, right?!

Regards
RB