Lets talk Chassis stiffening and body flex

BlackGMC · May 15, 2007

Well I found a link from a sponser on another board about chassis stiffening and body flex on Uni-body cars. Really interesting and shows how the affect of body flex and what not. Anyways here are their suggestions. All good ideas which I had all ready planed on sorta except the front crossmember Idea, gotta add that one to my plan. Anyways check this stuff out, OH BTW this is for a mopar, but it is still a Unibody, so I think we can all learn something here.

Front crossmember

Inner Fender braces, I plan on building somehthing similar

Sub-Frame Connectors

Engine Compartment Brace, I plan on tying my inner braces into this.

This is a video demostrating chassis flex and what it looks like when it is fixed.
http://www.xvmotorsports.com/media/Chassis%20Stiffening.wmv

The only problem I see with this whole setup is that the inner braces and engine compartment brace to hooked to a piece of 16-18 sheet metal (the firewall), not very strong, but I guess in total it evens out. I planned on running a piece of square tubing behind the firewall and joining these braces together. Another problem, since I will not have shock towers, I plan on running a diagonal peice of tubing from the inner fender/engine compartment mounting spot to the front crossmember to make up for the lack of shock towers.

Now my main question, why is nothing ever stiffened in the back? I mean most Uni-Body cars have very thin rear frame rails (usually half the thickness as the front), but I have never seen anyone do anything back there..... What do yall think???

The reason I am asking this is because I will be fiinished with my floors in a few weeks and I am starting to plan ahead for the next stage.

jason@gmachine · May 15, 2007

i like the sub-frame connectors

JAson

HistoricMustang · May 16, 2007

Pretty nice stuff and pretty nice job on the early model!

HistoricMustang

RogerC62 · May 16, 2007

Now my main question, why is nothing ever stiffened in the back? I mean most Uni-Body cars have very thin rear frame rails (usually half the thickness as the front), but I have never seen anyone do anything back there..... What do yall think???

Black,

As I understand it, I think most feel a roll bar main hoop with diagonal bars to the rearmost suspension mounting points is sufficient stiffening at the rear. It triangulates and braces the rear frame rails. Along with the wheel houses and rear package tray bracing I think the consences is a fairly rigid structure. That said, I'm not certain its strong enough for me. I was questioning the strength of the factory metal which led me to designing a tube cage structure of sorts for mine. It keeps evolving as I learn what's more important to me and is evident by looking at the picture on my SN garage. I like the ideas you've posted and considered similar bracing of the body as a whole before deciding on a tube structure. I think the key to going with braces, subframe connectors and additional crossmembers is to tie into the strong parts of the unibody and not merely into a sheetmetal panel alone.

rbohm · May 16, 2007

BlackGMC said:
Inner Fender braces, I plan on building somehthing similar

The only problem I see with this whole setup is that the inner braces and engine compartment brace to hooked to a piece of 16-18 sheet metal (the firewall), not very strong, but I guess in total it evens out. I planned on running a piece of square tubing behind the firewall and joining these braces together. Another problem, since I will not have shock towers, I plan on running a diagonal peice of tubing from the inner fender/engine compartment mounting spot to the front crossmember to make up for the lack of shock towers.

Now my main question, why is nothing ever stiffened in the back? I mean most Uni-Body cars have very thin rear frame rails (usually half the thickness as the front), but I have never seen anyone do anything back there..... What do yall think???

The reason I am asking this is because I will be fiinished with my floors in a few weeks and I am starting to plan ahead for the next stage.

actually if you look closely, you will see the doubler plates that were welded to the firewall, as well as the torque type box welded to the old upper shock mount area.

as to the rear section, that area isnt as high stressed as the front and center sections are, and thus need little if any stiffening for most applications.

bnickel · May 16, 2007

jason@gmachine said:
i like the sub-frame connectors

JAson

that style of SFC is already made for the early stangs. they are made by spintech and were actually designed to provide more clearance for a side exit exhaust system but they are pretty much identical. i think the XV motorsports units for the mopars were also designed to allow side exit exhaust clearance as well and to keep it looking more stock too.

http://mustangdepot.com/OnLineCatalog/Suspension/subframe-connectors.htm

scroll all the way to the bottom of this page and you'll see the SFC's and they also have a jacking rail kit that connects the rails to the rockers to make the car even stiffer.

i plan on using the spin tech SFC's and their jacking rails as well and probably some kind of cross brace like the TCP style too.

Rusty67 · May 16, 2007

IMO sub-frame connectors should be tubular.

rbohm · May 16, 2007

Rusty67 said:
IMO sub-frame connectors should be tubular.

and the reason for that is?? the spintech subframe conectors weld to the floor which will make everything stiffer, than if you use the heidt's subframe connectors without the center cage.

bnickel · May 16, 2007

rbohm said:
and the reason for that is?? the spintech subframe conectors weld to the floor which will make everything stiffer, than if you use the heidt's subframe connectors without the center cage.

agreed. round tube SFC's may be stronger that square tube SFC's but in this case i don't think there is going to much if any difference in the strength of these vs. tubular of any kind. the frame rails on a mustang are exactly the same as the spin tech SFC's so basically what you're doing is giving the car a full frame, essentially.

BornInAFord · May 17, 2007

True, and tubular or not, the SFCs are only as strong and rigid as the places they tie into, in this case boxed channel 14 gauge steel.

Rusty67 · May 17, 2007

Tubular connectors have less chance of twisting. Like anything, a subframe connector is only going to be as strong as the weakest part of it, including installation. Those subframe connectors are interesting, I've never seen something like that before. I'd like to hear about the results of those vs some good tubular ones on a simillarly setup car. It would be interesting to see what the results are.

Hack · May 17, 2007

When I was researching what a strong but light frame should look like I Googled Corvette frame. Have a look at the photos on the internet of the recent generations of Corvettes. There are plenty of cut-away views online. Very deep rectangular tubing around the perimeter of the car. I'm sure that some of the design is for safety as well, but I plan on emulating some of the Corvette frame design features in my future build.

Edit: to clarify I should say that taller frame members are a lot more effective in stiffening a chassis than multiple shallow reinforcements.

Marshall · May 17, 2007

You asked about stiffening the rear. I added extra subframes to my 66 vert. They are made from thicker material than origional and I stitch welded the whole under body, wherever sheetmetal was joined it now has a weld ever 2" or so.

The cutouts are for the exhaust. The other plating in the pic is for the jaguar IRS.

Marshall

brianj5600 · May 17, 2007

Round tube is stronger, but it is hard to weld to flat surfaces. When round tube goes unsupported, not welded to the floorpan, the advantage goes away. XV is not a cosmetic company. Their parts are designed with the help of computers and tested on equipment that usually reserved only for OEM's. I doubt they chose form over function.
I think mine are going to be square tubes that run through the rear footwells and the ribs in the forward part of the pan will be trimmed to allow the square tubing to sit flush and to be welded to the pan the entire length.
The issue at the top of the firewall has been tackled be SN65 by welding a piece of 1x1 tubing the length of the firewall, even extending through the aprons. I think there are some pics here http://sn65.com/Fire & Ice unibody reinforcement.htm.

BlackGMC · May 17, 2007

brianj5600 said:
Round tube is stronger, but it is hard to weld to flat surfaces. When round tube goes unsupported, not welded to the floorpan, the advantage goes away. XV is not a cosmetic company. Their parts are designed with the help of computers and tested on equipment that usually reserved only for OEM's. I doubt they chose form over function.
I think mine are going to be square tubes that run through the rear footwells and the ribs in the forward part of the pan will be trimmed to allow the square tubing to sit flush and to be welded to the pan the entire length.
The issue at the top of the firewall has been tackled be SN65 by welding a piece of 1x1 tubing the length of the firewall, even extending through the aprons. I think there are some pics here http://sn65.com/Fire & Ice unibody reinforcement.htm.

That is pretty much what I have done and am going to do (firewall bracing)....

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brianj5600 · May 17, 2007

Are the arched pieces in the tunnel area just flat steel? It seems like there is an opportunity there for more strength.

BlackGMC · May 17, 2007

brianj5600 said:
Are the arched pieces in the tunnel area just flat steel? It seems like there is an opportunity there for more strength.

Yep they are just 1/8 X 2 flat steel. As far as an opportunity for more strenght, it is a little to late for that I have allready made the tunnel sheet metal and welded it into place, I think it is strong enough as it is. I still have not added the tranny crossmember which will tie in the front of the subframes, the rears are allready tied in with the rear crossmember I added. Oh and it will have a roll cage installed, soon...

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