Actually, im extending this thread minus the bickering...

https://www.thirdgen.org/techbb2/sho...hreadid=276644


1.This is only with the T-56 crossmember?
2. That picture of the cracked frame..(I know it looks extreme..) Is this possible with the TH-350 crossmember under extreme horsepower? Is the damage from downshifting/gassing stress fracture? If the torque arm is lined right up with the bolts this would theoretically elimiate the extreme flexing (from what I gathered in the post)

As much as I love f-body's, this is one reason I do not like them. I know anything can be reinforced, but has anyone actually experienced this?

I hope to be running 10.30's to 10.50's in a few weeks and I actually thought about how weak this part of the subframe is. This post only concerned me more. Very informative post

 

Theoretically, since I am running solid motor/trans mounts, with the stock setup with a B&M TH350 adapter, the torque arm force should go directly to the crossmember through the solid mount with hardly any force on the tailshaft bolts.. Since the solid mount is right between the bolt holes, it shouldnt cause any cracking.

 

as long as the only thing that the x-member is supporting is pretty much inline with it's bolting points, there won't be any rotational forces applied to the x-member, just a direct up and down force.

all the stock x-members i've seen, are set up this way. the trans mount is just about directly between the bolt mount points, thus no twisting.

i think you should be fine with the stock torque arm setup.

the problem only occurs when a force is exerted on the x-member at a point not along the line drawn between the bolts, which will cause the x-member to try to rotate which isn't good for the weak frame.

 

What about the 4th gen T56 ones? They are setup like spohn t56 ones.

Kat

 

well, whadya guys think....

Spohn torque arm and crossmember (which I will modify a bit to reduce the flex problem)

OR Global west setup (although I'm not a fan of using the floor pan)

 

good point. see attached picture for 4thgen t56 crossmember.

i suspect that GM strengthened the frame rails, but i have no evidence or knowledge of the 4th gen design to back this up, so i could very well be wrong.

my personal opinion is that the problem is not nearly as bad with the torque arm mounted to the trans. if you push up (or down) on the tailshaft of the trans, it "pivots" on the front engine mounts...which, results in nearly verticle force on the crossmember, which doesn't give it much leverage to 'twist' or 'pry' on the frame rail.

when the TA is mounted to the x-member directly, the pivot point becomes the crossmember/framerail union, which allows more rotation of the crossmember and thus wear/fatigue/flexing of the framerails.

just my opinion, i'm certainly no engineer.

 

BINGO,
http://www.swracecars.com/pdf/stockerparts.pdf

I would like to see this crossmember and torque arm on something... This appears to be something I havnt seen on thirdgen...

 

i'd vote for the spohn setup if you want to avoid the floorpan stuff, with the following changes:

(1) brace the crossmember, somehow. dean made a good suggestion on how to do this in the thread that got killed. i personally like the idea of bracing the crossmember at (or behind, toward the rear of the car) the point where the torque arm mounts (perhaps by running a bar straight out to the subframe connector tubes, or some kind of extension to the frame rails, or something)

2) replace the shackle style connector IF it gives you problems. It seems to work for some people, not for others. my personal opinion is to go with a spherical rod end on the torque arm (so as to allow full rear end rotation/articulation for handling) and then a firm rubber bushing on the other end of the shackle (to kill the noise / vibration / harshness). I did the math in the other thread - even if you got a full inch of deflection (no way!), it's less than one degree of change in pinion angle.

that's exactly what I would do if I had the fabrication skills to brace the crossmember.

 

looks like a step in the right direction. too bad they are bolt ins, and double too bad that they neck down in the front. I bet they woudln't clear a twin cat car (like mine), and I bet they aren't as good for handling/chassis rigidity as spohn's are (because of how they neck down). the further outward the SFC's are, the more they resist chassis twisting from front to rear along the centerline.

but the torque arm mount is nice.

 

Still go with a spohn setup?

What do you mean by neck down?

 

notice how the SFC bars are all the way out towards the rear, but are closer together (near the center of the car) up front.

spohn's stay at the outer perimiter of the car the entire way.

with the bars all the way out at the edges of the car, the chassis is stiffer. with the bars closer to the center, it's easier for the chassis to twist (along an imaginary line drawn through the front license plate to the rear license plate). if you put all the strength in the center, there is less rigidity to resist twisting about that imaginary line.

despite the problem with the crossmember, spohns fabrication work is top notch and he still is willing to bring us thirdgenners new products, i see no reason not to give him the business; just keep in mind that you may want to improve things a bit after you install it. the whole point of bringing this stuff up is so that (hopefully) the vendors who supply us will take note on what is desired and if there is enough demand for it, make an improvement to the design, somehow.

if you have his SFC's, then as mentioned above, find a way to fab up a tube that goes from left SFC to right SFC and mounts to the rearmost point of the TA mount, and that should take care of any/all flexing problems.

 

Well, I think that torque arm setup bolts to the SFC's, which are seperate from that unit....

ANYWAY... I contacted spohn for some info and heres what I got...

am in the process of building a low 10 second 84 Camaro. I was planning
on running your torque arm and crossmember setup until I saw this.
http://www.ws6transam.org/video/torquearm.wmv
This car supposedly is under 300 horsepower, and the flex is pretty bad. If
I were to put 700 horses to that I don't know what would happen.
My question is...Is it possible to order a custom (slightly longer) arm and
crossmember (TH350) with the torque arm mount on the crossmember over, or
very close to the centerline of the bolts on the subframe to eliminate this
flex?

> Thanks in advance, Tom


Tom, The torque arm won't clear the transmission, it's as far front as it
can be. That video is basically an incorrectly set up torque arm. The
front mount is not set up to pivot, and thus it is binding up, which of
course causes the flex because something has to give.

We have customers with 1200+ horsepower running mid 8's and
pulling low 1.2 60' times, and I have NEVER had any type of
crossmember failure, bend, crack, nothing. They are built tough,
and they work.



I replied:

How is it set up wrong?

"The front mount is not set up to pivot, and thus it is binding up, which o fcourse causes the flex because something has to give."

What front mount?
The torque arm mount top or bottom bolt too tight?

Tom,
Correct. My guess would be that the bottom bolt was over torqued,
it's supposed to be set up at 30 ft./lbs. Some people hear a little
noise from it and then they tighten it down so it stays quiet. The top
bolt gets torqued tight. I've never had anyone call me and say the
subframe cracked. If it did, it was probably rotted beforehand and
the torque arm set up just finished it off. That subframe assembly is
extremely strong.



I guess that put my mind at ease...

 

Heh, good luck, I’m responding because you PM’ed me asking me to, and as always, I’ll be good unless someone starts prodding/being rude…



Mostly, but could be any setup that significantly cantilevers the tranny mount or TA mount out past the vertical plane of the cross member bolts.



No, I doubt that you could do it with something like the old B&M 350 conversion kit that bolts the TA to a new bracket that bolts to the tailshaft bosses. The stock length TA puts the force into a location that was able hold more then it originally saw.

the subframe moves more then it should for something like that. Remember that whole basic subframe/sliding link TA design was originally conceived by GM for an application that has ½ the hp that most of our f-bodies have stock, but strength <> rigidity

FWIW, as long as you put any force into it straight on (no twisting) it should be OK.



Not only is the subframe much stronger, thicker and a larger box section, but the crossember is much larger and makes contact with the subframe over a much larger area.



with the 4th gen design, yes, somewhat, the offset 3 dimensional shape on the crossmember and the causes the whole assembly to “bind” more if you try to twist it. that is not the case with a 3rd gen T56 swap crossmember.



It’s a 6/half dozen thing. Yes, wider is better, but mounting it to a stronger location on the car is better also. With the basic structure of a 3rd gen both end up being a compromise.



Spohn’s stuff tends to be overbuilt but under engineered. You have no reason to worry about his parts falling apart, but I would think twice about excess weight and putting stress into places that were never designed to see it. That being said, I think that BMR takes their design/engineering a bit more seriously, GW is probably the best designed stuff out there from the major vendors (OTOH, their customer service and delivery times leave something to be desired). UE and a few others make some nice stuff but be aware that a lot of it is designed and tested on 4th gens and then adapted to 3rd gens later…



the owner of that car is semi local and quite involved with the local list. He tried going back and forth with Spohn MANY times about that setup, and never got any satisfactory answer. What motivated him to make that video is that basically spohn is claiming that something is bound up, causing the flexing while it is at the same time pretty clear that what is supposedly bound is still moving enough that it is causing a clearly audible clunking. Seeing/finding the flexing was purely accidental and not his original complaint.



Um, all of the fast cars that I have seen using a stock type TA suspension are using a separate crossembmer to mount the front mount (for lack of a better word, it’s not really a mount but a sliding link if you intend to maintain the original function of the piece) of the TA. None of them are using a setup even remotely like what is in that video or in the pictures that we’ve discussed. For that matter, all of the 3rd gen cars that I know of that are running even close to that power/time are running either BMR or GW parts, as far as I know, the only spohn parts using that style TA mount that are going nearly that fast/that much HP (and someone correct me if I’m wrong, I’d love to see it) are in a 4th gen, which again, is significantly different.

My overrated and undervalued $.02: run either the B&M style conversion or make a bracket to mount the TA sliding pivot between the stock cross member bolt holes (look at GW’s piece, a bit clunky but accomplishes it well) and run a stock TA, possibly with some reinforcement added to the outside edges. You don’t really need more for most >10s cars. If you intend to go faster then look into something with a dedicated TA crossemember.

 

Thanks for the reply...I think I am going to reinforce the stock torque arm and run my b&m setup for now, with the solid trans mount, it shouldnt hurt my tailshaft housing. When I get a few more dollars I will look into a GW setup when I get a little money. If it breaks, it breaks. Nothing on my car can't be replaced. Thanks for the CONSTRUCTIVE criticism of products for our cars. If we didn't criticize, nothing would get better.

 

Ever see a Spohn x-member fail or break? I didn't think so

Its not the crossmember thats the problem, is a fatigued or rusted chassis.

Most applications that the Spohn t56 cerossmember goes onto do not suffer from any kind of flex. Those chassis' are in good shape still and are not suffering from corrision. Most of them are not putting out double the HP that support is engineered to hold.

So I say again, You need to stop blaming Spohn for "poor engineering" (Uhum Crossfire this means you). get off you allmighty highhorse and engineer something better if its so easy to do with a general fit-all bolt-on unit that works fine for everyoone with a reasonable application.

There is always the rare case installs with any aftermarket product. These people are either overpowering the intended use of the product, or suffer from fatigue or corrosion and then try and blame a product bought rather than their crappy condition chassis. Weld the chassis then install any x-member, and you be fine adding doulbe the cars HP.

This should have been a suject ended the last time, there is NO WAY to shortcut the situtation. If Spohn designs it with more bracket mount points then welding is required in EVERY installation- most cars do not need this so the rare cases it is needed they should learn to take their cars and have the chassis beefed up and not try and penalize a fabricator because it won't work in every senerio. Don't be naive people.

This thread should have never came back. Every part of the topic was covered already and even as cure for anyone with an application that is suffering from chassis fatigue. Anyone asking any further questions is in denile and needs to re read the first thread for cures. there will be know one size fits all cure without welding involved.