Question for Chassis Gurus

[Scottie-GNZ]

How critical is the inner fenderwell to chassis stiffness? I want to remove the equivalent of 6"x6" piece of the inner drivers side fenderwell. The metal would be removed where the fenderwell meets the frame rail just behind radiator support. I would also be removing a lot of the sheet metal from the radiator support, down low between the radiator and the HL.

 

TIA and would appreciate no guessing or speculation.

[74_5.0L_Z]

Hey Scottie,

 

I think you're OK removing some material ahead of the strut tower. I'd be more concerned about removing material between the towers and the firewall. If it would make you feel better, why not make up some strut tower braces.

 

Dan

[Jeff]

The structural courses that I have taken are related to building construction but I would think the same theories apply. If we consider the inner fender as a structural panel then typically towards the center of that panel the forces are the least. The location you are talking about is towards the center of the panel so you should be able to punch a hole with no effect.

 

I can't visualize what you are talking about on the core support but remember V8ZRacer260Z's trick core support on his car. He removed everything but the bottom cross member and built a tubular top cross member that was bolted in. I have that top cross member if you would like to copy it and I have pics of his engine bay.

[Drax240z]

I'd rethink it a bit. The beauty of a unibody is that it is light and stiff for its weight. This is achieved basically with stressed shear panels. These panels can be in either tension or compression depending where your loads are at a given time. Even though you are in front of the strut tower, the material in that area still has a vital role to play, in that it resists the deflection of your strut tower. (sometimes under tension, sometimes under compression loading) The rad cross member aids in this as well by transferring some of the loads to the opposite side.

 

Once you start weakening a chassis like that, it isn't so much that high stress will cause an instant catastrophic failure of the metal. More likely, the increased deflection and strain of the surrounding components will cause fatigue on any stress risors created by the removal of the panel. This fatigue will lead to failure much sooner than expected by a simple stress calculation. (which is how we normally would do it in our heads by looking at it, even if we don't cosider it a stress calculation/estimation)

 

For most Z's this might not be a huge issue, but you are putting down some serious power and torque, so I'd be careful with it. If you were to do this, a strut tower brace would be a must, but I would strongly consider triangulating your towers to the firewall as well if not allready done. Also, aim to remove any stress concentrations in your cut. (no sharp edges, file it smooth, nice radius corners, good surface finish, etc) Best bet would probably be to make a large circle, and then flare the edges to add rigidity in that manner. You'll probably have to make a die the same size as your circle to do it well, but a dolly and body hammer might suffice. You can likely regain ~50% of the lost stiffness by doing this.

 

Any material in front of the rad support does almost nothing structurally, as only the headlight bucket attaches up there. I'd say that is free game.

 

And yes this IS guessing and speculation, because I've never done such a thing and tested it. I do however have a degree behind it, so take it for what you will. I'd imagine that if you threw caution to the wind and did it, you'd probably be ok... but engineers don't really work in "probably's" and for good reason.

[Guest Thurem]

HHMMM sounds like a bigger turbo or a new intercooler. I say do it and make some tubular braces to make up for the strength lost.

Thure

[Michael]

I have a similar mod to my radiator support on my ’78 280Z. The lower crossmember remains, but the rest of the radiator support was removed with a sawsall and the spot-welded guts were ground away from the inner fenders. A 3/4" tube with clevices on each end supports the radiator and electric fan on top, while down below a similar tube is welded to the frame rails, supporting the bottom of the radiator and reinforcing the front sway bar mounting locations. The lower tube is also braced to the stock lower crossmember. The upper tube’s clevices connect to tabs welded to reinforcement pads, themselves welded to the inner fender sheet metal. There’s a strut tower crossbar and diagonals from the top of the strut towers going back through the firewall, meeting at a dash bar.

 

Also, I cut away the heavy-gauge reinforcements used on the 280Z to support the front bumper struts. While the weight savings claimed from bumper surgery is, in my view, somewhat exaggerated, removing the aforementioned metal saved about 5 pounds.

 

I second what was said regarding the sheet metal forward of the strut tower and sway bar mount. It’s not important, if the strut towers are themselves adequately braced, and the sway bar mounting hard point is not compromised. Of course, front-end collision crash protection suffers, but that’s probably not your primary concern.

 

Rigorous assessment of whether cutting sheet metal forward of strut towers is entirely harmless would require some controlled loading/deflection test. Has anyone done this? Short of that, my brief experiences driving the car showed no steering deadband, front end vibration/looseness or rattles.