Yay or Nay on bracing to remove front rails on 240z?

[DC871F]

All right, been looking around, and no definite answers on bracing to R and R from frame rails.

Are there any worries on frame twisting? Upper and lower radiator supports are solid.

[Patcon]

If the upper structure is good and the car is supported properly under the rockers, I dont think I would add any bracing to replace the frame rails

[26th-Z]

Oh by all means, yes, alignment matters.  Make sure your transmission mount and engine cross brace are aligned properly.  String and align your sway-bar mounts.

[DC871F]

8 minutes ago, 26th-Z said:

Oh by all means, yes, alignment matters.  Make sure your transmission mount and engine cross brace are aligned properly.  String and align your sway-bar mounts.

Hey great point about the cross brace. So I guess I would tack it in in a just a couple locations and bring the cross brace up to make sure it fits correctly. I'm really hoping not to do any front frame rail work at all, the passenger side looks like it should be fine, the drivers side looks a lot more crunchy. Wont know until I get in there.

[Zed Head]

58 minutes ago, DC871F said:

All right, been looking around, and no definite answers on bracing to R and R from frame rails.

Are there any worries on frame twisting? Upper and lower radiator supports are solid.

Sounds like what you're asking about is whether you should temporarily brace things before you remove the old body reinforcements, AKA "frame rails".  The body is a unibody, or monocoque, chassis, basically just a steel box with strategically placed reinforcement, like the frame rails.  If you want to get a better feel for it, just do a Google on unibody or monocoque and you'll find a bunch.  There are several "boxed" sections around the body that are important, mostly on the edges, like the rocker panels and the door and windshield openings.  The frame rails are kind of secondary to the major boxed sections, like the rocker panels.  The Body chapter shows cross sections of all of the boxed areas.

I wrote all of that stuff because you used the word frame.  There's no frame.

Anyway, I've read that removing the floors in order to put new ones in can lead to flexing.  The floors are a critical part of the steel box.  So just think of it as a steel box and support areas so that the box can't twist.  Probably just well-placed jack stands are enough.

[DC871F]

3 minutes ago, Zed Head said:

Sounds like what you're asking about is whether you should temporarily brace things before you remove the old body reinforcements, AKA "frame rails".  The body is a unibody, or monocoque, chassis, basically just a steel box with strategically placed reinforcement, like the frame rails.  If you want to get a better feel for it, just do a Google on unibody or monocoque and you'll find a bunch.  There are several "boxed" sections around the body that are important, mostly on the edges, like the rocker panels and the door and windshield openings.  The frame rails are kind of secondary to the major boxed sections, like the rocker panels.  The Body chapter shows cross sections of all of the boxed areas.

I wrote all of that stuff because you used the word frame.  There's no frame.

Anyway, I've read that removing the floors in order to put new ones in can lead to flexing.  The floors are a critical part of the steel box.  So just think of it as a steel box and support areas so that the box can't twist.  Probably just well-placed jack stands are enough.

I get all that, I wasnt sure about removing the front rail because thats all the structure there is firewall forward as as far as any boxed beams longitudinal. I figured since the upper and lower radiator supports are solid not much flexing can occur. 

New floors will be in before I do any front end cutting. I've seen where jack stand sufficed for support, so I did plan on that.

[texasz]

In short yes, brace/support/align!  Like 26th-Z said, and he's got some experience with metal work on these cars.  😉 

[DC871F]

1 minute ago, texasz said:

In short yes, brace/support/align!  Like 26th-Z said, and he's got some experience with metal work on these cars.  😉 

So weld in braces, or just use the cross mount and trans mount?

Thank for the replies.

[texasz]

1 hour ago, DC871F said:

So weld in braces, or just use the cross mount and trans mount?

Thank for the replies.

I'll have to defer to someone like @26th-Z with experience, my knowledge in this area is based upon what I have learned/read not what I've done.

[Namerow]

(Stating the obvious) A structure won't deflect unless it's subjected to a load.  If the centre and rear sections of the unibody are supported (axle stands under the pinch welds at the front), then everything from the firewall forward will 'droop' because of its weight (i.e. load = force of gravity).  However, there's not much weight left up front after you remove the engine/transmission, the hood and fenders, the bumper, the front suspension/steering and the front crossmember.  As a start-off guess, I'd say maybe 150 lbs, tops. 

In vertical bending, each side of the front part of the unibody gets its strength from a ~ vertical panel (the inner fender) that's reinforced by an upper and a lower boxed section.  The upper box section is sometimes called the 'horn' and it's made from very thick (0.050" steel).  The lower box section (aka the 'frame rail') is also made from 0.050" steel.  The inner fender panel is the usual 0.032" (20 gauge).  Both box sections are pretty deep vertically (which is what you want for vertical stiffness).  The inner fender isn't really a 'vertical' panel, but it serves the purpose and it ties the upper and lower box sections together.

The center part of the unibody is boxed by the firewall  (which is, in turn, helped out by the transmission mount frame that helps to reinforce the open trans tunnel area).  The front frame rails aren't needed to box the center section of the unibody (as I think you would intuitively expect), so cutting one out isn't going to cause the centre section of the unibody to twist.

Ahead of the firewall, when you cut out the frame rail on one side you weaken that side of the front unibody structure somewhat, but the upper boxed section and the inner fender panel are still intact.  I expect they're plenty strong (see next paragraph) to support their respective shares of the weight of the front section of the unibody (150 lb?) without any any droop, remembering too that the 'cantilever' effect of the structural weight steadily diminishes the closer you get to the firewall.  The boxing of the front unibody structure at the front by the rad support frame will further serve to resist any differential droop (i.e. twist).  Let's break it down...

I have one of ZeddFinding's replacement frame rails in my shop.  It's made from 0.062" steel and weighs 8 lb.  Let's say the upper frame rail weighs 10 lb.  Then let's say the inner fender panel adds another 10 lb.  Add 2 lb for the T/C Rod bracket.  Add 5 lb for the shock tower blister.  Add another 5 lb for the upper ledge panel.  We're up to 40 lb per side, 80 lb total.  Let's add another 15 lb for the front rad support bulkhead.  That's 95 lb.  Add another 5 lb for miscellaneous.  Hard to make the whole front structure as being much over 100 lb.  That's just 50 lb per side.  And it's distributed over a length of about 4 feet (from firewall to front of structure), so it's not like a 50-lb point load hanging off each front bumper bracket.

When you cut out the lower frame rail on one side, the 50 lb on that side reduces by 10 lb.  So 50 lb on the intact side, 40 lb on the cut side.  10 lb difference.  Same as two 5-lb bags of sugar.  Not much of a twisting load.  If you're still worried, put an axle stand under the corner of the rad support bulkhead and shim it up to take out all the clearance. 

If you're still feeling uncertain, put a dial indicator on the underside of the rad support on the cut side before you cut out the rail and measure the droop after the rail comes out.  If you don't like what you see, use the axle stand and shims to push things back up into a zero-droop state before you weld in the new rail.  If you do this, I'll be curious to learn what you measure.  My guess is 4 or 5 thousandths of an inch (but I'm ready to be shown wrong).  Maybe someone else has already made this measurement?

Everything I've said depends on your car having good structural integrity where the front unibody elements join the firewall (and, to a lesser extent, where the rad bulkhead joins the inner fenders).

You may find the attached article, 'Analysis of Early Z Chassis' informative.  Credit to the author, James Lux.  It appeared in an old edition of a unknown club publication called 'NewZletter'.

 

Analysis of Early Z Chassis - NewZletter - James Lux - 19xx.pdf

[Patcon]

I agree with Namerow, and his excellent explanation. If I am understanding the OP correctly, he is talking about removing the lower sections on each side of the engine bay (that we call frame rails). If the car is stripped and not rotten as swiss cheese, there just isn't a lot of weight up there to cause distortion.

Floor pans and rocker panels is a different thing. With different tactics...

[DC871F]

19 hours ago, Namerow said:

(Stating the obvious) A structure won't deflect unless it's subjected to a load.  If the centre and rear sections of the unibody are supported (axle stands under the pinch welds at the front), then everything from the firewall forward will 'droop' because of its weight (i.e. load = force of gravity).  However, there's not much weight left up front after you remove the engine/transmission, the hood and fenders, the bumper, the front suspension/steering and the front crossmember.  As a start-off guess, I'd say maybe 150 lbs, tops. 

In vertical bending, each side of the front part of the unibody gets its strength from a ~ vertical panel (the inner fender) that's reinforced by an upper and a lower boxed section.  The upper box section is sometimes called the 'horn' and it's made from very thick (0.050" steel).  The lower box section (aka the 'frame rail') is also made from 0.050" steel.  The inner fender panel is the usual 0.032" (20 gauge).  Both box sections are pretty deep vertically (which is what you want for vertical stiffness).  The inner fender isn't really a 'vertical' panel, but it serves the purpose and it ties the upper and lower box sections together.

The center part of the unibody is boxed by the firewall  (which is, in turn, helped out by the transmission mount frame that helps to reinforce the open trans tunnel area).  The front frame rails aren't needed to box the center section of the unibody (as I think you would intuitively expect), so cutting one out isn't going to cause the centre section of the unibody to twist.

Ahead of the firewall, when you cut out the frame rail on one side you weaken that side of the front unibody structure somewhat, but the upper boxed section and the inner fender panel are still intact.  I expect they're plenty strong (see next paragraph) to support their respective shares of the weight of the front section of the unibody (150 lb?) without any any droop, remembering too that the 'cantilever' effect of the structural weight steadily diminishes the closer you get to the firewall.  The boxing of the front unibody structure at the front by the rad support frame will further serve to resist any differential droop (i.e. twist).  Let's break it down...

I have one of ZeddFinding's replacement frame rails in my shop.  It's made from 0.062" steel and weighs 8 lb.  Let's say the upper frame rail weighs 10 lb.  Then let's say the inner fender panel adds another 10 lb.  Add 2 lb for the T/C Rod bracket.  Add 5 lb for the shock tower blister.  Add another 5 lb for the upper ledge panel.  We're up to 40 lb per side, 80 lb total.  Let's add another 15 lb for the front rad support bulkhead.  That's 95 lb.  Add another 5 lb for miscellaneous.  Hard to make the whole front structure as being much over 100 lb.  That's just 50 lb per side.  And it's distributed over a length of about 4 feet (from firewall to front of structure), so it's not like a 50-lb point load hanging off each front bumper bracket.

When you cut out the lower frame rail on one side, the 50 lb on that side reduces by 10 lb.  So 50 lb on the intact side, 40 lb on the cut side.  10 lb difference.  Same as two 5-lb bags of sugar.  Not much of a twisting load.  If you're still worried, put an axle stand under the corner of the rad support bulkhead and shim it up to take out all the clearance. 

If you're still feeling uncertain, put a dial indicator on the underside of the rad support on the cut side before you cut out the rail and measure the droop after the rail comes out.  If you don't like what you see, use the axle stand and shims to push things back up into a zero-droop state before you weld in the new rail.  If you do this, I'll be curious to learn what you measure.  My guess is 4 or 5 thousandths of an inch (but I'm ready to be shown wrong).  Maybe someone else has already made this measurement?

Everything I've said depends on your car having good structural integrity where the front unibody elements join the firewall (and, to a lesser extent, where the rad bulkhead joins the inner fenders).

You may find the attached article, 'Analysis of Early Z Chassis' informative.  Credit to the author, James Lux.  It appeared in an old edition of a unknown club publication called 'NewZletter'.

 

Analysis of Early Z Chassis - NewZletter - James Lux - 19xx.pdf 624.8 kB · 5 downloads

Fantastic, thanks so much. I'm not a fabricator by any stretch of the imagination, I know how to use tools, and I'm willing to take my time and learn. Its all OJT.

I have Charlies rails and pans I'm putting in.

The passenger front rail looks like will I will be able to keep in, waiting to see how the driver side turns out when I do a little digging into it.