Engineer shares how to build an electric vehicle from the ground up: Build effort

First, the Fiero donor car was stripped of its front body parts. The entire front suspension and steering gear came out as a complete assembly. Thus, began the build effort. The coil springs were replaced by an air bag suspension because the ride height needed to be adjustable with different weight batteries as John Santini started with affordable lead acid batteries and later, in a planned effort, replaced these with lithium-ion batteries.


Figure 1: Top view of front suspension from the Fiero donor car weighing only 200 pounds.

The basic suspension parts were laid out on the basement floor. The 12in floor tiles formed a working grid for the assembly.


Figure 2: Suspension parts laid out on the basement floor using the existing 12in tiles as a working grid

Next came the realization that the decision to work in the comfortable, heated, cooled, and well-lit basement would present a problem when the vehicle was finished and ready to drive out to the street. Santini worked a month on the exit strategy.


Figure 3: Santini opened an exit from his basement shop and added a garage door to get the vehicle out

Chassis
Any good chassis must do several things:
1. Be structurally sound in every way over the expected life of the vehicle and beyond. This means nothing will ever break under normal conditions.
2. Maintain the suspension mounting locations so that handling is safe and consistent under high cornering and bump loads.
3. Support the body panels and other passenger components so that everything feels solid and has a long, reliable life.
4. Protect the occupants from external intrusions

Santini has met all criteria. He started with a balsa model of the planned frame (Figure 4). Pink parts are 1in-square tube; black ones are ¾in-square tube (.060in wall). As you glue in cross braces, you can feel the frame dramatically stiffen up. Santini said that this is mechanical engineering 101, but there's nothing like a mechanical model to get a "feel" for how the geometry stiffens the structure. He started with well-proven chassis construction techniques.


Figure 4: Balsa wood model of the frame to test the geometric design for stiffness.

The hollow metal tubing was welded by Santini, who purchased a welder from eBay (the same one he used at school to qualify for his AWS certification). The first side rail is shown assembled in Figure 5 (partially welded 1 side). With only the four verticals, but without the diagonals, a 250lb load in the center caused a 0.700in deflection. With the diagonals, it dropped to 0.020in (Figure 6) With it fully welded, deflection was barely measurable at perhaps 0.005in.


Figure 5: The first side rail assembled with only four verticals was not strong enough.


Figure 6: Side rail with added diagonals increased the strength by more than two orders of magnitude.

Structural stiffness is the basis of what you feel at the seat of your pants. It defines how a car handles, body integrity, and the overall feel of the car. Chassis stiffness is what separates a great car to drive from what is merely OK. Every chassis is a compromise between weight, component size, complexity, vehicle intent, and ultimate cost. When I drove the vehicle, the strength was apparent in the ride over rough road and bumps.

When the frame, suspension, steering, and alignment were completed, Santini towed the car to the top of a hill for a roll test. He reached 25mph coming down the hill. Coming around the turn the chassis handled pretty well. There was no wheel hop or chirp from the rear wheels, even in a full turn. The rear wheel spacing was a compromise and a guess as to how far apart the tires could be before tire scrub on turns became an issue. Only the front brakes were connected, but the chassis stopped pretty well, with moderate pedal pressure. He later changed the brake pads to a softer material and further reduced pedal effort, so power assist was definitely not needed.


Figure 7: A happy and triumphant Santini after the successful roll test down a hill.

The rear of the chassis was revised (cut and welded) several times during body construction to round off the rear and cut weight. Later the gas pedal assembly was added. A special 5 million revolution rated potentiometer was used in conjunction with the motor speed control.


Figure 8: The gas pedal assembly showing the 5 million revolution rated potentiometer.

Fuse block and wiring were added next. Santini used a fuse block from Painless Wiring, a leader in automotive wiring and electrical products for more than 20 years. It's very convenient and has all the fuses nicely grouped and marked. It also includes the flashers and horn relay.


Figure 9: Fuse block assembly and harness wiring, flashers and horn relay.

The dash assembly is interesting. Most of the analog meters are for battery voltage monitoring and controller and also speedometer.

The original idea was to put a forward-reverse switch on the dash, but after driving the car a bit, Santini decided it should be a more conventional "gear shift" lever on the center console. I can tell you that this was much more comfortable than a two-way dash switch when I put my hand on the center console gear shift and backed out of a parking space and started onto the road.


Figure 10: The front dashboard with assorted gauges and switches

The speedometer design required some thought. The VDO speedometer (VDO is a German-based, leading international supplier of automotive electronics and mechatronics) needed a pulse train from a wheel. It turns out the motor shaft was the most convenient place for it. Santini made an aluminum disk to hold four magnets, which gives 21,640 pulses per mile. A Hall Effect sensor switch picks up the pulses and creates a nice clean square wave.


Figure 11: Hall Effect sensor mounted to the motor shaft for the speedometer.

Probably the biggest challenge was designing and building the gull wing doors. They only contain a partial stub frame off the hinges; the rest is all fiberglass and foam. With the sliding glass "RV" windows, they weigh only 12lbs each. Side impact protection is contained in the side sills, which have a 5in wide truss at a height of 18in off the ground.

The original setup used gas struts with bottom-mounted bear-claw latches. Although this worked, Santini had problems with the doors twisting on the hinges due to constant strut pressure on the doors as they opened and closed. The hydraulic struts unfortunately applied pressure to the door and hinges even when the doors were closed. This issue was solved by changing to motorized struts to raise and lower doors, thus eliminating extra spring force and latches. They are adjusted to just close and hold door down snugly. Santini built a door controller to drive the motors up and down alternating each time a pulse is received. This pulse comes from the remote key fob transmitter or a switch on the dash if you are in the vehicle.


Figure 12: Motorized struts solved the warping problem on gull wing doors and hinges.


Related article
Building an electric vehicle from the ground up: Design Choices


About the Author
Stephen Taranovich is a freelance writer with 40 years of experience in the electronics industry. He received his MSEE from Polytechnic University, Brooklyn, New York, and his BEEE from New York University, Bronx, New York. Steve is also chairman of the Educational Activities Committee for IEEE Long Island.