Laying Up the Floor Panels

Before this post, the car had no floor underneath the occupants’s feet. An aluminum underbody will be attached, but it will be too thin to support the weight of heels resting on the floor. So, two floor panels were constructed from fiberglass and epoxy resin and secured in the vehicle. To start, two mirrored mold plugs were made from lumber and eighth-inch particleboard. Since the A-side is the inner face of the part (facing up towards the occupant), only a male mold needed to be made. The floor panels have three sides, intersecting at one vertex. So three separate sheets of particle board were glued to a two-by-four base.

After a rough sanding on the corners, body filler was mixed and applied to fill in the small gaps between the boards. This was again rough sanded with 120-grit to remove excess material. More filler was applied and sanded until the gaps were completely filled and the transitions were smooth.

Then, finer 240-grit sandpaper was used to smooth the fibers and prepare the surface for painting. Two coats of gray primer were applied. Then, the entire molds were sanded again with 240 and then 360. Finally, the molds were painted with two coats of black enamel to prepare them for waxing.

Mold-release wax was applied over the entire molds, allowed to haze, and then buffed off with a rag. This was repeated four more times to completely seal the molds and prevent adhesion of the resin. Then, two thin coats of PVA mold-release film were sprayed onto the molds using an HVLP gun. This water-soluble liquid dries into a thin, green film with weak internal attractions, allowing it to tear easily when attempting to part the workpiece and mold. From here, the layup process began. Three plies of midweight fiberglass cloth were used, with a schedule of 0-45-0 degrees. Between the second and third layers, eighth-inch plywood pieces were added to act as a core material. Using plywood adds a lot of strength to the part without adding much weight and lessens the plies of fiberglass required, bringing the total cost down. After an overnight cure, the floor panels were removed from the molds. One parted easily while the other brought along some chunks of fiberboard. A little acetone dissolved it away easily.

The angle grinder was used to trim the perimeter of the parts and to make cutouts for the seat belt bracket and part of the frame. The panels were test-fit into the car and hoels were drilled for mounting. After a thorough washing and brief sanding, two coats of black epoxy enamel were applied to the panels.

 

 

Installing the Fuel System and Engine

A 4-gallon aluminum fuel cell was purchased and adapted to fit in the car. No mount points were included on the fuel cell, so four brackets were waterjet from 5052 aluminum and welded onto the tank.

Corresponding mounts were welded onto the vehicle frame. The tank also didn’t include a fuel level sensor, so a universal unit was installed. The stem of the sensor’s float was trimmed to work with the height of the fuel tank and a hole was cut into the top of the tank with a hole saw. Five holes were drilled and tapped, and the level sensor was screwed into the tank. Finally, the tank was installed into the vehicle with four bolts.

Two fuel pumps were used in the car. One is attached on the engine itself, pressurizing the fuel rail. The second, the lift pump, was mounted just after the fuel tank. A simple steel bracket was cut out and welded to the frame. The pump was bolted on and a wiring harness was made to connect it to the power supply from the engine’s circuitry. Quarter-inch rubber hose and a number of fittings were used to connect the tank to the lift pump and the pump to the engine.

The engine mount, being interconnected with the intermediate shaft mounts, was already tacked into the rear of the car. After test fitting, the mount was welded fully into the car. The engine was seated on the mount and four bolts were installed. The primary clutch of the CVT was slid onto the engine’s shaft and the end bolt was torqued on. The V belt for the CVT was installed over the two clutches.

The throttle for the engine was mechanical, so a system was made to take electronic inputs from the pedal and actuate the throttle. Of course, eventually, this system will be controlled by the hybrid control algorithms, rather than via a direct connection to the pedal sensor. A high-quality, titanium-gear servo was chosen as the actuator. An aluminum mount was cut and welded to hold the servo near the throttle. A tiny set of ball joints and a pushrod connects the servo arm and throttle.

Finally, a simple circuit board was made by lasercutting a pattern and etching away copper with ferric chloride solution. This board together with an Arduino read the pedal signal and transmit a PWM command to the throttle servo. At this point the car is driveable in gas-only mode.

 

Fabricating the Drivetrain

The power from the gas engine and electric motor is delivered to the wheels through an intermediate shaft and a Miata differential. The mounts for the longitudinal intermediate shaft were already tacked to the car during frame fabrication.

Aluminum bearing mounts were turned on a semi-CNC lathe. These parts allow a press fit for the bearings and adapt to the frame mounts via a three-bolt flange. Into a 6-inch diameter aluminum billet, a pilot hole was drilled and then a boring operation was performed to bring the diameter to an interference fit with the bearing.

Once the hole was completed, a step-down was cut to assist in locating the adapter to the mount hole on the frame. Then, using a center-finder, the part was positioned on a semi-CNC mill. A three-hole bolt circle was drilled through the part. This process was repeated for both intermediate shaft bearing adapters and for one differential bearing adapter.

Finally, the bearings were pressed into the adapters with an arbor press. A small amount of epoxy was used to ensure the press fit wouldn’t release with vibration.

The intermediate shaft was made from a 3-foot long 1-1/2-inch chrome-plated steel shaft. The shaft passes through two of the bearing assemblies and interfaces with three different components. It receives torque from the engine through a belt-drive CVT. The secondary clutch of the CVT has a 1-inch bore with a 1/4-inch keyway. The last 120 mm of the intermediate shaft was turned down on the lathe to allow a slide fit for the clutch. Also, it was cut to length, faced, and chamfered.

The shaft was then fixed in the mill and the 1/4-inch keyway was cut using a programmed path. The shaft also receives torque from the electric motor and transmits it to the differential through two chain drives. The purchased sprockets already had 1-1/2-inch bores and 3/8-inch keyways, so two more keyways were cut in the opposite end of the intermediate shaft.

To allow for geometric imperfections in the frame’s mounts and to absorb vibrations from the rotating components, bushings were lasercut from a sheet of Buna rubber. The three bearing assemblies were bolted to the frame, sandwiching the bushings, and the shaft installed.

The differential was mounted by its carrier bushings in the rear and by two 3/4-inch bolts in the front. The mounts were already tacked onto the frame. So once the positioning was confirmed, these were welded fully. The differential was lifted onto a dolly and bolted into the vehicle.

The differential receives all the torque from the intermediate shaft through a flat sprocket bolted to its four-bolt flange. The four holes were drilled on the mill and the center bore was turned to 3/4-inch. A steel hub was also turned to reinforce the connection. This was bolted to the differential and a 3/4-inch stub shaft was inserted through the bearing and through the hub.

All bolts were then treated with red thread locker and torqued. The half shafts were installed into the differential and into the rear hubs and the wheel nuts torqued. Finally, a quart of synthetic gear oil was added to the differential.

Installing the Brake System

The brake system consists of a single pedal, two master cylinders, a bias valve, steel hardlines, PTFE flexlines, and four 4-piston calipers.

First the calipers were installed onto the uprights using adapters from Flyin Miata. The rotors were installed on the hubs.

Before the hydraulic system was fabricated, the mechanical handbrake was installed. An off-the-shelf parking brake handle was used and a steel mounting bracket was waterjet and welded to the frame. A steel bulkhead plate was made from a quarter-inch and 12-gauge steel. The ends of the parking brake cables slide through two holes in the plate and are held in by a couple c-clips.

After bolting the parking brake handle in and welding in the cable bulkhead, a simple bracket was designed to attach the cable ends to the handle. Finally, the cables were installed and attached to the rear calipers.

Then, the aluminum swing-mount pedal from Wilwood was installed. A tube was cut and bent and a number of steel eighth-inch brackets were cut and tacked in. After checking the pedal position, the mount was welded completely.

From there, the rest of the brake system fabrication was plumbing. A bias valve was installed on the front brake lines, since the master cylinders were chosen to favor the rear-heavy vehicle. Also, a tee was installed in the rear line to connect to a brake light switch and a brake pressure transducer, to be used later for motor regen. The front hardlines were bent, flared, and tightened. Thin steel brackets were welded onto the frame to mount the flexlines with c-clips.

The rear brake lines were constructed similarly, although with a little more difficulty. Since these were longer, it was tricky to get the bends correct enough to keep from pulling on the flare connections when tightened. With those installed, the brake system was complete.

Assembling the Front Suspension

Once the rear suspension was complete, the front suspension was assembled in a similar fashion with the fabrication of control arms, preparing the front uprights, welding on brackets, and fastening all the parts.

Both the upper and lower front control arms are fabricated from 1-inch tubes and were made in the same way as the upper and lower rear control arms. Tubes were placed on printed out layout sheets and then welded once the tubes had been cut to the correct length. Tube-end weld nuts were welded on and rod-ends were screwed on. Mounting brackets were bolted on to the ends of the both the control arms.

Unlike the rear lower control arm, both the front lower and front upper control arm use high-angle spherical rod ends to connect to the spindles rather than urethane bushings. The front uprights, like the rears, were pulled from an NA Miata. Again, they were disassembled, sandblasted, and cleaned. Since the rod ends used 1/2″ bolts, rather than the tapers originally in a Miata, the uprights required slight adjustment. The tapered holes were bored out oversize and bushings were press fit in, leaving 1/2″ holes for the control arm attachments and steering rod end. The picture below shows the boring of a tapered hole.

After all three holes had bushings inserted and the excess trimmed down, the two uprights were masked and painted.

The front uprights were then prepared to be attached to the control arms in a similar fashion. With the brackets bolted to the rod-ends, they were tacked to the frame. After tacking the brackets for the rocker arms and dampers, the pushrods were bolted on, confirming that the components fit correctly.

All components were removed and the welds finished. After reconfirming fit, the sway bar brackets were welded to the frame and to the lower control arms. The sway bar (an aftermarket Miata rear) was then bolted on. Below is a finished picture of the front suspension.

Assembling the Rear Suspension

With the frame completed, the first dynamic parts could be constructed. Assembling the rear suspension requires welding the upper and lower control arms, welding the rockers, preparing the used Miata uprights, welding on brackets, and bolting it all together.

The rear upper control arms consist of two curved 1-inch tubes, one gusset, and one bent bracket. After over-cutting and bending the tubes, they were placed on a 1-1 scale printout.

Using the layout, the tubes were cut to the right size and angle. The gusset piece and bent bracket were jigged, tacked, and welded. Tube-end weld nuts were welded on and rod-ends were screwed on. Finally, the bolt that interfaces with the upper bushing on the rear upright was added on.

The rear lower control arm was fabricated similarly: bending tubes, cutting to size, and welding according to a printed layout.

The polyurethane bushings and brass sleeves were left in the freezer overnight to make install easier. A C-clamp was used to slide them into the 1.5″ ID tubes on the lower control arms. Rod-ends and jam nuts were screwed into the weld nuts. To link the control arm to the upright utilizes a long 9/16″ steel rod instead of a bolt. After cutting the blank rod to length, a 9’16”-18 die was used to thread the last 3/4″ of each side. The upright’s 14 mm holes were bored out up to 9/16″ and a couple jam nuts were added on each side, as to allow adjustment tightness against the bushings.

With both control arms done (with exception of the brackets for the pushrod and sway bar), the old Miata uprights were to be prepared. They started dirty and rusted with unnecessary dust covers and grease-oozing bearings. After pressing out the old hubs and bearings with an arbor press, the dust covers and bearing seals were removed. The uprights and hubs were sandblasted to remove the dirt and heavy rust and then wire-wheeled to clean off any further corrosion.

After a washing with soap and water and then acetone, the uprights were painted with a coat of enamel primer and two coats of metallic silver, making sure to mask off the inside face into which the new bearing will slide.

When pressing in the new bearings, a steel tube with a diameter a little smaller than that of the bearing was used to keep the load on the outer race and prevent pitting of the ball bearings. After replacing the internal C-ring and adding a new inner seal, the sandblasted hubs were also pressed on, using a bit of oil to prevent seizing.

The pushrod and rocker arm were fabricated next. The pushrod consists of a short piece of tubing capped on both ends by a weld nut and rod-end. The rocker arm was constructed from 4 waterjet-cut pieces. The parallel plates were bolted together before welding to ensure that the bolt holes would line up. The completed rocker arm can be seen below.

Using measurements from the CAD model, the waterjet-cut suspension pickup brackets were tacked and welded to the frame. Also, the brackets for the rocker arm and damper pivot were tacked on. Once the control arms were bolted on and attached to the upright, the pushrod, rocker arm, and damper were installed. The wheel was installed to check clearances. At this point, all components of the rear left suspension are tacked in place, with the exception of the sway bar.

The same process was repeated for the rear right components. After a measurement and symmetry check, all brackets were welded into place.

Constructing the Rear Subframe

The structure behind the roll hoop plays many rolls. It supports the engine, motor, differential, and intermediate shaft as well as the rear suspension. Getting this area accurate was important, so many waterjetted pieces were used to help line tubes up.

This small section, to be located at the far rear of the vehicle, was constructed first. The tubes were measured, cut, and coped with the saw and tube coping rig. Using the 2D jigging method from before, the tubes were lined up and welded.

Since the differential is bottom-loaded into the vehicle, no permanent tubes can exist on the bottom of the rear subframe. To avoid compromising strength against buckling, a subrame brace was added to the design. After waterjetting the two parallel plates, two tubes were cut and welded into place. Corresponding brackets were also cut to allow the subframe brace to interface with the frame.

Brackets that mount the motor, engine, differential, and intermediate shaft were also cut. These were designed to help align tubes while welding. These plates were tacked in place, but will not be welded until the components can be fit checked into their mounts. After bolting the subframe brace into position, the remaining tubes were tacked into place.

After a few measurement checks, the rear subframe welds were completed. The main structure of the car is now complete, save for a few gussets and mounting brackets.

Attaching the Driver Compartment Trusses

The vehicle’s frame is essentially a span between the front and rear contact patches, so design features from bridges were borrowed. A set of triangular trusses connects the lower plane to the lower hoops. A second set connects the lower and upper hoops.

The lower set was constructed first, cutting the tubes with the miter saw and coping jig were necessary. The tubes were tacked in place in pairs, ensuring that the left and right sides were symmetric.

The top set of trusses were tacked in place in the same way.

After a few measurement checks, the seats were removed and the welds were finished. Since the upper hoops were cut a few inches long, they were to be trimmed down after welding. At this point, the structural sections of the frame ahead of the roll hoop were completed.

Adding the Upper Hoops

Two pairs of curved members surround the driver compartment. The lower hoops are part of the second plane, already welded in the car. The upper hoops won’t be as easy. The tubes aren’t in plane; they’re swung up 30 degrees on either side. A cross piece was designed, both for structure and for fabrication simplification, holding the two members in place during positioning.

As before, the tubes were bent using the manual tune roller. The radii were checked by drawing a line on the concrete with chalk on a string.

Coping the ends of the curved members was tricky, since the cut was dihedral. The cope was 20 degrees from perpendicular and twisted 30 degrees from the plane of the bend. A smartphone app that simulates a level was used to set these angles before cutting. An adjustable height desk chair was used to support the opposite end of the tube.

A wooden jig was constructed to hold the two bent tubes and the cross piece in place. Measurements were taken to ensure everything was in the right place before and after welding. The joints were tacked, but won’t be finished until the truss pieces are added.

Combining the Three Planes

So, after completing the first three 2-dimensional planes, it’s time to put them all together. The three sections make up the majority of the frame’s shell. As usual, we used the tube coping drill press rig and the cutoff saw to prepare the tubes that connect the two planes.

Jigging the three planes together was tricky. We started by rolling out the grid paper and screwing in supports for the lower plane. We measured out locations for the supports for the roll hoop and locked those in as well. After some checking and rechecking of critical distances and angles, we tacked the lower plane and roll hoop together, with a few connecting members.

Adding the upper plane required preparing a number of new wooden supports to hold everything at the right height. Getting everything right was difficult. We used the tack, measure, bend, tack again method to get the measurements just correct. After tacking everything in place, we rechecked everything before starting to finish weld.

Also added in this step was the first sheet metal piece. The 1/4-inch steel plate in the center of the car will mount two bearings that support the intermediate shaft and tail shaft. The shape of the plate, with round cutouts, aided in positioning the tubes. Finishing the welds took a couple hours and some contortion, as the frame is starting to get hard to work around. We placed the seats in their positions in the frame. It’s starting to look like a car!