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Discussion Starter · #1 ·
Been killing time waiting for my custom rack to show up (four months!), so I've been messing around with the frame trying to convert it from an auto frame to a manual frame, and trying to figure out a few ways to stiffen it up. The first picture shows the rear kick-up braces I tacked in. The kick-ups were a bit too flexible for my tastes. After putting these in, there's a noticeable reduction in how much the kick-up flexes when pushing up and down on the far rear crossmember.



It's well known that our frames leave something to be desired when it comes to torsional stiffness. And, during the rear kick-up flexing examination, I noticed that the frame middle area will bend up and down when I pressed up and down on the rear crossmember. I've had something I wanted to try for a while to see if I can stiffen up the frame from a torsional strength standpoint. Ideally I would love to put a large torsion bar down the middle of the frame, and connect it to the suspension attachment points. Unfortunately, the engine and transmission get first dibs on a good chunk of that real estate. So, I'm trying to see if I can improve the frame torsion rate by stiffening the available real estate. I put a 4 1/2 inch .120 wall tube between the trans crossmember and the trailing arm crossmember. This gives me a thick torsion bar between two torque arms (the crossmembers). My long term goal is to clamp the frame down and measure the difference in torsion rate with and without this strut. Meanwhile, I did notice a slight improvement in "beam" strength (if that is the right word) when I pushed up and down on the rear crossmember. Makes sense, as I now have three, rather than two tubes connecting the trans crossmember and the t/a crossmember. Down the road I'll be adding some struts to tie the front suspension to this torsion tube area to stiffen up the front half of the frame.



Now that the rack is finally here, I can start trying to package it. I left the original front crossmember in the frame, as I was hoping that the frame would look original when looking over the fender into the crank pulley area. But, it looks like I'll have to cut so much away to clear the rack that I'll probably just pull the engine back off the frame and cut the whole crossmember out, install the rack, and then weld in any needed bracing. It's not obvious, and I might look stupid in the next ten minutes, but I haven't seen a direct way to see where exactly left/right to install the rack. The inner pivot points are hidden under the dust boots. Before I call the factory to get some specs, does anyone have an easy way to do this so I end up with equal length tie rods?

thanks,
Mike

 

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It's well known that our frames leave something to be desired when it comes to torsional stiffness. And, during the rear kick-up flexing examination, I noticed that the frame middle area will bend up and down when I pressed up and down on the rear crossmember.
Mike, When you say the "frame middle area" do you mean the perimeter frame box that the #2 and #3 body mounts are connected to? Also when you are pushing down on the rear crossmember, where is the frame supported. Last, how much do you weigh:laughing: (if I didn't ask, someone else would have).
 

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Discussion Starter · #3 ·
My tests aren't terribly scientific at this moment, as I'm trying to do several tasks at the same time, and am just looking for potentially value added concepts. Right now, while I'm working on stuff, the frame is supported under the new front crossmember, and the trailing arm crossmember. When I stiffened up the kick-up area I then noticed I could easily get the center of the frame (I believe it is between #2 and #3 body mounts) to flex up and down, making a small arc. (The two crossmember points obviously just stayed planted on the wooden blocks.) I did also notice a definite reduction in the arc magnitude when I added the center stiffening tube. This was a freebie, as the original function of the tube was to add torsional strength. I'm 190#, and I confess I haven't done the measurements and math to figure out the moment arms and torques of leaning on the (bumper) crossmember.
OBTW, I still haven't welded the frame rail seams yet, so I'm hoping that will eventually help stiffen things up a touch. Once things are all tack welded, I'm planning on sandblasting the frame and finishing the welding on all the modifications.
I appreciate your interest in this.
Mike
 

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When I made the crossmember removable for th ROD 6 speed instal I also added a complete structure to brace up the frame from the trans cross member to the rear cross member.

This served a couple of purposes. first and foremost It was a complete subframe of 2x2 and 1x2 0.125 wall square tube to mount the seats and seat harnesses to. It also acts a a big X brace to tie the two xmembers together and to the frame rails. Lastly it ties together and provides a mount for the driveshaft loop.... :thud:

I kind of like your kickup braces I may be able to tie them to the back of my roll cage....

:thumbsup: :huh::cheers:
 

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Mike, What if you supported the frame at the two front crossmember suspension pickup points and put one support on the torque tube you are adding. You could load the rear crossmmber at the corners and measure twist. That would be good information to have (and to benchmark your improvements).

The arcing of the perimeter frame channels you describe is kind of a surprise to me. I would would have thought that these members would be stiff enough that the bending would have occurred at the joints.

Thanks for sharing the results of these tests.
 

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Been killing time waiting for my custom rack to show up (four months!), so I've been messing around with the frame trying to convert it from an auto frame to a manual frame, and trying to figure out a few ways to stiffen it up. The first picture shows the rear kick-up braces I tacked in. The kick-ups were a bit too flexible for my tastes. After putting these in, there's a noticeable reduction in how much the kick-up flexes when pushing up and down on the far rear crossmember.



It's well known that our frames leave something to be desired when it comes to torsional stiffness. And, during the rear kick-up flexing examination, I noticed that the frame middle area will bend up and down when I pressed up and down on the rear crossmember. I've had something I wanted to try for a while to see if I can stiffen up the frame from a torsional strength standpoint. Ideally I would love to put a large torsion bar down the middle of the frame, and connect it to the suspension attachment points. Unfortunately, the engine and transmission get first dibs on a good chunk of that real estate. So, I'm trying to see if I can improve the frame torsion rate by stiffening the available real estate. I put a 4 1/2 inch .120 wall tube between the trans crossmember and the trailing arm crossmember. This gives me a thick torsion bar between two torque arms (the crossmembers). My long term goal is to clamp the frame down and measure the difference in torsion rate with and without this strut. Meanwhile, I did notice a slight improvement in "beam" strength (if that is the right word) when I pushed up and down on the rear crossmember. Makes sense, as I now have three, rather than two tubes connecting the trans crossmember and the t/a crossmember. Down the road I'll be adding some struts to tie the front suspension to this torsion tube area to stiffen up the front half of the frame.



Now that the rack is finally here, I can start trying to package it. I left the original front crossmember in the frame, as I was hoping that the frame would look original when looking over the fender into the crank pulley area. But, it looks like I'll have to cut so much away to clear the rack that I'll probably just pull the engine back off the frame and cut the whole crossmember out, install the rack, and then weld in any needed bracing. It's not obvious, and I might look stupid in the next ten minutes, but I haven't seen a direct way to see where exactly left/right to install the rack. The inner pivot points are hidden under the dust boots. Before I call the factory to get some specs, does anyone have an easy way to do this so I end up with equal length tie rods?

thanks,
Mike

I went throught the same thoughts but I did'nt think that I could make the old x-member look good enough to keep and it seemed so heavy.It looks better than I thought it would.(nice work)I removed the motor mounts from the c-4 cradle and slid the whole thing up between new straight boxed rails.I was able to keep all the mounts and in place and it came out clean.(and drives great!)I suppose I could clean it up some more.(cut down)Please keep us informed on your progress I will be redoing the frame this winter.(Possible tubeframe)and would like to avoid as many obsticals as possible.:thumbsup: LOOKS GREAT!
 

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Discussion Starter · #9 ·
I'll post bigger pictures in the future. I was always annoyed at having to scroll left and right to see the whole picture on the screen. I assumed everybody shared that annoyance, but I guess I was wrong on that assumption.
Update pics will be bigger. I appreciate your interest.
thanks,
Mike
 

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I have 2 screens running @ 1680x1050 I don't have to scroll so bring on them pics ;) :D

Seriously, please make them at least 800x600 or post the small pics w/ a link to the huge ones so they don't automatically open and screw up the forum display.
 

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Discussion Starter · #12 ·
I'll try with the bigger pics, but I'm not a desktop computer guru, so I'll apologize in advance if my future picture posts don't work out as requested. Please bear with me.:laughing:
I'm off to the garage right now to pull the engine so I can cut up the (stock)front crossmember to make room for the rack. I'm real curious with what and where the steering (column to rack) shaft is going to end up interfering.
 

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does anyone have an easy way to do this so I end up with equal length tie rods?QUOTE]

lightly lock a pair of vice-grips on the rack input shaft, turn full travel from side to side, count turns, divide by two to center the rack before installing.

best chassis stiffener is a roll cage...next best is a big pipe from the engine compartment firewall to behind the seat, at shoulder height (shifter will be below this), with diagonal bars from firewall end of ''big pipe'' to front frame suspension points and from ''behind seat'' end of ''big pipe'' diagonally to rear side frame rails near rear x-member...''big pipe'' should be well padded and unfortunately also will function as a birth control device.
 

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Discussion Starter · #14 ·
does anyone have an easy way to do this so I end up with equal length tie rods?QUOTE]

lightly lock a pair of vice-grips on the rack input shaft, turn full travel from side to side, count turns, divide by two to center the rack before installing.

That won't do it. It merely puts the pinion in the center of the rack travel (which I did). It doesn't tell me where the inner pivot points are (they're hidden under the boots) so I can accurately measure the actual tie rod length, or enable me to figure out accurately where (vertically and horizontally) to place the rack.

best chassis stiffener is a roll cage..Not an option. .next best is a big pipe from the engine compartment firewall to behind the seat, at shoulder height (shifter will be below this), with diagonal bars from firewall end of ''big pipe'' to front frame suspension points and from ''behind seat'' end of ''big pipe'' diagonally to rear side frame rails near rear x-member... That is what I am trying to do with the tube I placed in the frame. Your big pipe, and my tube, both will experience a torsional twist if the frame tries to twist. The diagonals function similar to moment arms or torque arms. ''big pipe'' should be well padded and unfortunately also will function as a birth control device.
I may just mark the arc of the tie rods, and then use some geometry to determine the inner pivot points of the rack. I pulled the engine out this afternoon, and cut out the (unneeded) original crossmember to give me more room to place the rack. I'm trying to figure out an adjustable mounting system that will let me tweak the rack position so I can correct any geometry issues I might run into. So far it's looking like the steering shaft will run right through the left motor mount. I'm hoping like the dickens that I can connect everything with only two u-joints. Oh, I did slide the front sway bar up there, and the tie rods are consuming the space that the bar wants to sit in. So, at this moment I don't have a clue what I'm going to do about that.
 

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I may just mark the arc of the tie rods, and then use some geometry to determine the inner pivot points of the rack. I pulled the engine out this afternoon, and cut out the (unneeded) original crossmember to give me more room to place the rack. I'm trying to figure out an adjustable mounting system that will let me tweak the rack position so I can correct any geometry issues I might run into. So far it's looking like the steering shaft will run right through the left motor mount. I'm hoping like the dickens that I can connect everything with only two u-joints. Oh, I did slide the front sway bar up there, and the tie rods are consuming the space that the bar wants to sit in. So, at this moment I don't have a clue what I'm going to do about that.
I had to put a small clearance notch in the motor mount,(very small,just a rub)after that there was no problems and only two U-joints no bind or header clearance problems.
 

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Are you going to put a roll bar in? That's where you have the opportunity to stiffen the frame. All these little braces are a waste of time and weight and get in the way during maintenance.

Everybody mounts the hoop after the kickup which is a waste in my opinion.
Sure it provides adequate rollover protection but you want to design the roll bar system to provide as much structural support and frame stiffening as possible. You can add all the steel you want in the horizontal direction but it adds a lot of weight for the stiffening it provides.

Take a look at the way i did my rollbar. I've tried to explain this before but i don't think people are getting it so i drew a picture in paint.



I didn't cut down the hoop (trimmed it a little to fit). I welded the bottom to the frame just before the kickup area. My supports are not what you typically see. Instead of going straight back they angle towards the center of the top of the hoop. This provides a triangulation between the bottom of the frame and the kickup area providing much more stiffening.



There is a little bit of reduced rear vision but i don't find it to be a problem and the state inspector has never said anything. It's as much view as a vert has. And i think it looks muscular. The bars do crowd the window so i probably will never be able to put a removable one in. Just give yourself a good inch or two between the supports and window if you do it this way.

I shitcanned all the plastic at the top of the birdcage so the bar would butt right up against the steel. You have to get the main hoop as high as possible. The top of the hoop must be 2" above your helmet. Most roll bars do not meet this requirement so tech can keep you off the track if they want to be anal about it.
 

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Discussion Starter · #17 · (Edited)
Are you going to put a roll bar in? No plans at the moment. That's where you have the opportunity to stiffen the frame. All these little braces are a waste of time and weight and get in the way during maintenance. I disagree. Everything I've done so far clears anything that needs regular maintenance, and initial tests show that the frame is stiffer, definitely in beam, and hopefully in torsion. I plan on anchoring the frame and doing some torsion measurements down the road.Everybody mounts the hoop after the kickup which is a waste in my opinion.
Sure it provides adequate rollover protection but you want to design the roll bar system to provide as much structural support and frame stiffening as possible. You can add all the steel you want in the horizontal direction but it adds a lot of weight for the stiffening it provides. You'll find few people who are more (vehicle) weight conscious than I am currently. I weigh everything I take off, and everything I add. I've also done a bunch of calculations before I added this stuff. (I also had to go back and clear the cobwebs out of that portion of my brain that holds the stuff I learned in Statics class.:laughing: )Take a look at the way i did my rollbar. I've tried to explain this before but i don't think people are getting it so i drew a picture in paint. I definitely agree your rollbar is an improvement over other pure rollbars I've seen on the forums. On the other hand, and no disrespect intended, I do not see where you have gained any noticeable torsional improvement over the stock frame configuration. I believe lack of torsional rigidity in these frames contributes to poor handling more than the lack of beam strength. I'm trying for both beam and torsion improvements.



I didn't cut down the hoop (trimmed it a little to fit). I welded the bottom to the frame just before the kickup area. My supports are not what you typically see. Instead of going straight back they angle towards the center of the top of the hoop. This provides a triangulation between the bottom of the frame and the kickup area providing much more stiffening. .
I like the looks of your hoop. And, with the diagonals I have on the rear kick-ups, I think that would provide the desired third leg (from a side view) of the hoop triangle.
 

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if you're not going to put a bar or cage in, then you are just amusing yourself. That car will never see a track. And you can't push that car to its limits on the street. Unless you have ever been on a track you won't understand.

i disagree with your disagreement.:laughing: How are you ever going to get that driveshaft out with that huge tube around it. In the newer vettes its an integral part of the frame. Just not necessary and of dubious improvement. I think the kickup braces will interfere with getting wide tires on with adequate backspace. My roll bar attaches much further back on the rear frame providing much better support than that 1" wide flimsey channel iron.

If you don't understand how my bar stiffens the frame, better get out the statics book. And actually read it this time.:laughing:

I'm beating you up, i know. Because you really need to put a roll cage in and quit pissing around.
 

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Discussion Starter · #19 · (Edited)
if you're not going to put a bar or cage in, then you are just amusing yourself. That car will never see a track. And you can't push that car to its limits on the street. Unless you have ever been on a track you won't understand.
C'mon, Turtle, I've been polite and respectful to you in all my posts. Please give me the same courtesy.
This car has seen thousands of miles of track time (and no, not autocross stuff). I know exactly what its shortcomings are. Not trying to be a prick here, but I also take no backseat to you regarding professional driving school training (oval, road course, and dirt), nor my skill behind the wheel.
i disagree with your disagreement.:laughing: How are you ever going to get that driveshaft out with that huge tube around it. Easy. I just undo the bolts on the front and rear flanges. The tube is below the shaft, not around it. In the newer vettes its an integral part of the frame. Yes, I just wish Chevrolet had done the same thing 45 years ago! [/COLOR]Just not necessary and of dubious improvement. I disagree. [/COLOR]I think the kickup braces will interfere with getting wide tires on with adequate backspace. These braces are flush with the frame. No change to any backspace real estate.[/COLOR] My roll bar attaches much further back on the rear frame providing much better support than that 1" wide flimsey channel iron. It is square tubing, not channel. I only have to get back to the shock/batwing mount area. That's where the suspension forces are inputted. My earlier comment was that from the side view, you have an inefficient triangle. The bottom leg is an arc, not a straight line. [/COLOR]
If you don't understand how my bar stiffens the frame, better get out the statics book. And actually read it this time.:laughing: [I don't recall saying I didn't understand your setup. I do agree that it stiffens part of the frame, but the front 75% of your wheelbase is still the same flexiflyer that came out of St. Louis.[/COLOR]I'm beating you up, i know. Because you really need to put a roll cage in and quit pissing around.
I'm a career design engineer. I don't piss around. I look at all the pro's and cons before I start a design, or change somebody else's design. Like I said earlier, I'm trying to be friendly and courteous here, but it's a two way street, ya know. [/COLOR]
 

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Turtle, Nice bar for show, it's missing something, a very important something. It will not pass tech at a SCCA or NASA event. If you roll and land with the weight of the car on the left roof top,,,, your bar would most likely fail due to the lateral distortion on the the main hoop.

What your bar needs is diagonal lateral bracing to prevent lateral distortion of the main hoop.

I do the tech at SCCA racing events and assign a number stamp to any new roll cage or bars, that stays with the car for the rest of its racing days. Also have to drill them to check the thickness of the material.

I've seen failures in other racing venues with a bar like yours, two to be exact.

I bet when you are on the track, you leave nothing on the table..:laughing:
You might want to consider adding that. very easy to add. Would be much safer.

Just scares me to see a bar that is being used on a track without it.. Understand it will pass at a drag race without it.

Can provide drawings... if anyone is interested with specs on tube size and design.

**** happens...........
 
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