Urba Electric...

A Classic EV Design With Advanced Features

Urba Electric was introduced on the cover of Mechanix Illustrated magazine in February, 1977, long before the automotive world began thinking about manufacturing electric cars. Urba Electric's introduction was a landmark in EV development for a number of reasons. First, the car was ahead of its time simply by having been built at a time when EVs were considered non-starters by car makers. Second, MI readers collectively purchased over 20,000 sets of plans, which sent a strong message about public enthusiasm for EVs. Third, it was designed and built outside the automotive industry, and its development was entirely financed with private funds. And finally, although the car's 48-volt battery pack was little more than a copy of those used in golf cars, Urba Electric utilized cutting-edge composite technology in its construction, and it pioneered an innovative new concept in EV speed control and power system strategy. The car's technical innovations inspired engineers at Delco to purchase a set of plans and build one for testing. The Delco car later became known as GM's Drive I.

Urba Electric's main technology breakthrough was embodied in the Electromatic Drive Transmission. This ingenious and patented electronically controlled continuously variable transmission (CVT) let the motor run at a steady speed while the transmission's shift-position provided control over the car's speed from zero through 60 mph. The accelerator pedal was hooked to a device that varied a low-voltage shift-position signal to the CVT. This caused the shift-position to track the degree of depression of the accelerator pedal. As the accelerator pedal was depressed, the CVT smoothly upshifted causing the car to accelerate in unison. Meanwhile, the compound-wound dc motor ran at a constant speed - its most efficient speed. For regenerative braking, the driver simply let up on the accelerator pedal, causing the CVT to downshift to a lower ratio. Downshifting at cruising speeds forced the motor to spin faster, which reversed the direction of current flow and delivered a charge to the battery (due to counter emf) as it slowed the car. No traditional electronic speed control was necessary.

Urba Electric's chassis is not from an existing car. Instead, it was designed from scratch in order to keep weight down and efficiency up. The battery pack is arranged in a reverse "T" much like the battery pack of GM's EV1. Half of the car's eight 6-volt batteries occupy the space in a tunnel that runs between driver and passenger, and the other half runs transversely just behind the passenger compartment. The frame is made of rectangular steel tubing. A jackshaft coupled to a differential with a simple chain drive makes up the final drive, which is contained in a sealed oil-bath housing and suspended from the frame at the rear. The body is made of FRP/foam composite.

Today, the Electromatic Drive Transmission is no longer available, and plans include a substitute voltage-stepping speed control system and timing belt drive to the jackshaft, along with drawings of the original setup. With the low cost of today's power electronics, it is best to discard the voltage-stepping speed control in favor of a modern transistor chopper controller. Also, the trend today is toward much higher system voltages. Urba Electric's performance and efficiency would be enhanced by upgrading to a higher voltage. The most simple way is to switch to12-volt batteries. But in reality, anyone building this classic design should consider adding more batteries as well.

Urba Electric plans provide a technical insight into one of the most innovative and cutting-edge EV designs of the period. But building her today is not quite the paint-by-number project that it was when plans were first introduced - mainly because of the need to substitute new components where the ones specified in the plans have been phased out or replaced by newer hardware.