Hood holders: in the hood, holding it down.

In stock form, the hood is held down by a cable actuated latch that bolts to the core support–just like any other car. That mechanism has clear advantages on a production vehicle, however my priorities differ from those of the OEM engineers. I am after security, lightness, simplicity, and serviceability.

-The snap-ring pins or cotter pins that aftermarket hood pin setups come with have been tested and proven. So have OEM hood latches. With regard to this facet, neither the OEM latch nor a set of hood pins has a significant advantage.

-The OEM mechanism is heavy and complex compared to a set of hood pins. The core support that the OEM latch mounts to also has mass.

-Having the OEM core support hinders my ability to remove and reinstall the drivetrain. The core support would also make servicing the engine, while it’s in the car, more difficult. To streamline the installation and removal of the drivetrain, I removed the core support and anything else that was in the way via a cutoff wheel and sawmill.

I elected to use hood pins, and I made custom mounts to support them. The mounts that I made are light weight and are bolted in as opposed to welded so that they can be removed when they are in the way. When I say in the way, I mean preventing the removal of the radiator(I may be able to revise the sway bar mounts to overcome this), and the removal of the drivetrain.

The mounts are relatively simple, so without further ado, here are some photos:

The mounts themselves consist of: 1/8″ steel plate for the bases, 1/16″ tubing for the arms, and formed sheet metal for the pin attachment points.

Note the gap between the hood and the bumper. I found that such a gap can be made by loosening the hood-hinge bolts and sliding the hood back. This gap is desirable, for me at least, because air will be evacuated from the engine bay via the gap. Air accelerates as it passes over that curved region of the car, resulting in lower pressure–essentially an application of Bernoulli’s principle. The air in the engine bay(relatively high pressure) will want to reach equilibrium with the air over the gap(relatively low pressure), so air will be flow out through the gap, and the pressure differential across the radiator will be greater than it otherwise would be. Radiators work when air is flowing through the fins, and air is forced through the fins when a pressure differential is present. The effectiveness of this is dependent on how well the radiator is ducted to the “mouth” of the front bumper.

Chassis bracing abound

Considerable progress has been made since my last update, and the focus of this particular post is chassis bracing. In stock form, gussets in the engine bay help transfer loads from the frame rails to the transmission tunnel, which is the most rigid structure that connects the front of the car to the back. Without those gussets, the frame rails are more free to move when forces from the road act on them via the wheels and suspension. My engine is large and sits far enough back in the bay to require the removal of those gussets–I’m okay with that because my car, in present form, should be considerably stiffer than it previously was.

An intuitive breakdown of the logic behind my bracing:

The factory engine bay has gussets in the corners that help put the forces that are applied to the frame rails though the transmission tunnel to prevent movement of the rails. The gussets are circled in red, and the green illustrates what kinds of movements the gussets reduce. Essentially, they reduce all of the movements.

Under braking, the mass of the car will transfer forward. Grip permitting, up to about 80% of the mass of a Miata could end up over the front axle during hard braking. The frame rails will push into the front subframe, and will want to move up relative to the rest of the car. The magenta arrows represent hypothetical upward movement of the frame rails. The green arrows represent the forces that that movement would generate and where they would go.

This depicts what happens during a right turn. The frame rails, liked together by the front subframe(drawn in white) will want to move in the direction of the magenta arrows. That movement will generate a force(green) though the gusset, and into the trans tunnel. The teal represents a tension that pulls on the gusset and on the trans tunnel. Note that the trans tunnel is pretty solid, so those forces will not cause it to budge much.

On the inside, the forces that are transferred though the gussets work their way into the trans tunnel. Forces also travel through the gussets in the corners of the interior. Note how the path through the trans tunnel is the most direct path for the forces to take.

So yeah, those gussets are important. What did I do with them? Well, I chopped them out to clear a big engine…man shit. Now the forces that went though the engine bay gussets don’t have a sold path to the back. They’ll go into bending the firewall, and through other braces such as the stamped steel brace that connect the dash bar to the trans tunnel, through the frame rails themselves, and through the rockers. That isn’t the end of the world–it’s also not ideal.

With my braces, the forces incurred during braking pass through the green lines. Tensions are generated along the teal lines. The teal areas have been braced as well.

During a right turn, the forces will pass through the green lines and tensions will be generated along the teal lines. White represents the subframe, which ties the frame rails together.

This diagram uses the same conventions. Green lines are force paths, and teal lines are tensions. This drawing represents what’ll happen during braking. When turning, things will be slightly different. Note the use of triangles in some places as opposed to boxes. Those triangles prevent lateral movement, which if left unmitigated, would result in chassis twist and cowl shake.

My braces provide the forces that act on the front of the car with a good path to a relatively solid structure(trans tunnel) to reduce movement. Less movement in the chassis means that the suspension geometries stay consistent, and that the chassis won’t act as a large undampened spring. The suspension will also be easier to tune when chassis movement is a less significant variable in the equation.

An excerpt from a post that I made in a thread on stitch welding. This post is also relevant to my new bracing.

My personal experience, for what its worth, is that my car felt fine at lower speeds(auto-x) and quite numb at higher speeds(track time). I like to have feedback when I drive, so anything that firms up the structure that separates me from the surface I’m driving on, within reason, is welcome. I’ll be faster in that sense because I’ll be able to react more quickly as a driver.

As for the inherent capabilities of the car, the differences may be less profound for the average enthusiast. That said, flex in the chassis could result is dynamic fluctuations in the alignment, and there’s the whole big un-dampened spring thing. From that perspective, I think that the biggest advantage of stitch welding and bracing a chassis pertains to suspension tune-ability. Imagine the equation that you’d use to represent the handling characteristics of a car. Things like dynamic camber curves, spring rates, compression and rebound characteristics, caster, roll centers, toe, moments of inertia, etc….and chassis deformation. Most of those variables are consistent and predictable. Chassis deformation is not. Since you’re going to have a hell of a time trying to model how the chassis will respond to infinitely many permutations of input, tuning the suspension becomes less of a science and more an act of trial and error. If you can reduce chassis flex, you have less noise in your equation, and subsequently, changes to the variables that you can control are more apt to have a tangible and predictable effect. This applies more to racing teams and enthusiasts who take the time to actually develop their cars.

Gratuitous photos of my bracing.

Sheet metal braces were added to help solidify the span where the upper frame rails that the suspension bolts to connect to the lower frame rails that pass beneath the car. They provide an element of triangulation.

The frame rails that pass beneath the chassis have been bolstered via the addition of bolt in frame rail braces that slip over the factory frame rails. These come as a part of the V8Roadsters swap kit, and are included in the kit because they provide a solid attachment point for the new transmission crossmember. Flyin’ Miata sells frame rail braces that are suitable for those who are not performing such a swap.

The missing element now is a set of door bars. I plan to have those and a weld in roll bar fabricated by a professional at a later date.