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The tzero Electric Sports Car - Part 2

Getting that electric power to the ground.

By AN Brooks and T B Gage, AC Propulsion Inc

Click on pics to view larger images


This article was first published in 2001.

Chassis Systems

The underpinnings of the tzero evolved from the original Piontek Sportech chassis. However, many changes have been made to the structure and suspension for the tzero application.

  • Frame
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The tzero has a triangulated space frame built with square and rectangular 304 stainless steel tubes. The frame of the first prototype tzero was made of mild steel. Stainless steel was chosen for its higher strength (and hence ability to use thinner wall sections) and because painting or other corrosion protection would not be required. All frame joints are TIG-welded, with special attention paid to minimizing warpage and carbon embrittlement.





  • Suspension
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The suspension system is a classical double-wishbone arrangement front and rear. Hard polyurethane bushings are used all around except for the upper front arms, where rubber is used. Double-adjustable coil-over shocks are fitted for the development vehicle. The suspension arms are tzero specific and are fabricated in-house. Adjustments to camber and toe (front and rear) can be made without any disassembly. A tubular steel sway bar is fitted at the front.

  • Steering

The steering system is a conventional unboosted rack and pinion setup, with the rack ahead of the front axle centreline. The rack from the Mazda MX5 Miata is used and is shortened for the tzero application. The rack features an internal roller support, lessening friction and improving driving feel. The kingpins are also adapted from Miata kingpins.

  • Brakes
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The brakes in the tzero are normally used only for slowing the last 5 km/h or so before stopping. Wear is virtually non-existent. Even with so little use, the brakes must still be fully capable in times when regeneration is not present or when slowing is desired at a rate faster than regeneration can provide - such as in autocrossing or other hard driving. Additional requirements are that there should be little or no drag of the brake pads when they are not applied.

In order to save space and weight, and reduce complexity, the tzero uses unboosted brakes. In order to have acceptable pedal force, large-diameter rotors are desired, along with high mechanical advantage in the hydraulic system. The brakes are split into separate front and rear circuits, with an adjustable front/rear balance. The front calipers are of a fixed type, with four pistons. The rear calipers are of a floating type, with one piston as well as an integral mechanical parking brake mechanism. The brake rotors remain a development item. To save weight, metal-matrix-composite rotors were fitted - ventilated in front, and non-ventilated in the rear. While these are very suitable for daily use - since they are almost not used normally - there are concerns that MMC rotors may not have adequate capability to meet the testing required by the Federal Motor Vehicle Safety Standards. Both prototype tzeros are now fitted with vented and drilled MMC front rotors and conventional solid iron rear rotors.

  • Tyres

The choice of tyres is always very important for an electric vehicle. Rolling resistance can represent a substantial fraction of the total energy consumption. Tyre manufacturers have developed excellent special low rolling resistance tyres for electric vehicles. The main elements of a low rolling resistance tyre are: silica tread filler compound, thin sidewalls, and tall sidewalls (ie not low profile). Unfortunately, none of these tyres are suitable for the tzero (none even fit).

Because of the tzero's 57% rear weight bias and high-power, considerations other than optimising rolling resistance need to be considered. In order to provide safe and satisfying handling, the rear tyres must be substantially wider than the front tyres. The front tyres must fit 15-inch wheels (to clear the large brakes), but are very limited in outside diameter. The rear tyres also have a limited outside diameter, but can be larger than the front tyres. Taken together, these requirements are only met with low-profile, high performance tyres.

Click for larger image

Unfortunately, high performance tyres generally have the highest rolling resistance. Typical high performance tyres have about 2.5-times-higher rolling resistance than the best EV tyres. However, not all high performance tyres have the same rolling resistance, so an extensive search was performed to find performance tyres in the sizes needed with the lowest rolling resistance. The search ultimately led to tyres made by Continental General Tyre. For the front wheels, a nearly-ideal tyre was available - it is a moderately-low-rolling-resistance model from the EcoContact line, and is an OEM application on the rear wheels of the DaimlerChrysler Smart car.

For the rear, the choice is from the SportContact line. Two rear tyre sizes (16 and 17-inch) have been tested, with good handling results for both.

Rolling resistance data is defined and tested differently at different tyre companies; reported data is not directly comparable between companies. A very simple low-speed coast down test was developed to measure rolling resistance of a complete vehicle. The test is nothing more than measuring deceleration rates in a low-speed parking lot coast-down. Initial speed is typically 7 mph (11 km/h). The test is run in both directions to remove the effect of slope and in multiple repetitions to reduce random error. Representative rolling resistance test data from a number of different vehicles and tyres is given in this table.

Summary Of Tyre Rolling Resistance Coefficients

Vehicle Tyre Rolling Resistance
(as % of weight)
4 passenger EV
prototype
Michelin Proxima, 205/60 R15 0.54%
EV1 (worn tyres) Michelin Proxima 0.59%
EV1 (new tyres) Michelin Proxima 0.65%
Honda Insight BF Goodrich Potenza RE92 0.68%
Honda Evplus Dunlop 0.72%
tzero Conti EcoContact/SportContact 1.14%

The tzero has the highest rolling resistance of the vehicles measured (but it would have been much worse with other high performance tyres). For the tzero, rolling resistance typically accounts for 35 to 43 percent of the total energy consumption. If the tzero tyres were as good as the best EV tyres, range would increase by 22 to 30 percent.

  • Body

The tzero body is non-load bearing, and is made of fibreglass/epoxy composite material. The parts are built with hand layup in female moulds. Foam core and carbon fibre reinforcements are added for stiffening in the larger parts. The inner fenders and belly pans are made of Kevlar/epoxy composite. There are just six major pieces to the body - front body, rear body, two side pods and two doors. The front body is bolted on to the frame and does not open. The rear body is hinged for access to the trunk, suspension, electronics box, and rear batteries. Ingress and egress are aided by small doors and a tilt-up roof panel. The boot volume is the same as that of a Mazda MX5 Miata

Interior

Instrument Panel

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A complete set of driver displays is provided to communicate the status of the drivetrain components. The main elements of the instrumentation are listed below:

  • Speedometer - standard analog unit with trip odometer.
  • Volt/Amp meter - replaces tachometer. Shows instantaneous battery current (charge and discharge) and voltage.
  • Pack LED display - on centre in pod above steering column. One LED for each battery. Each LED lights with increasing brightness if module voltage is between 14V to 16V or between 9V and 11V. Above 16V or below 9V, the LED is on at maximum intensity. This display gives the driver a direct real-time visual indication of the status of the battery pack.
  • Motor and PEU temp - indicates temperature of motor and power electronics unit.
  • Energy status display - user configurable display. Can show Ah, Wh, regenerated Ah, Wh, average speed, trip distance, trip Wh/mile. There is no indication of miles remaining, as such an indication is an estimate based on future driving, which is strongly influenced by future hills and speed, both of which are not known to the system. In place of miles remaining, the energy status is indicated as simply net Amp-hours discharged. The driver becomes well aware of the capacity of the battery pack in the course of everyday driving, and uses the net Amp hour discharge vs. the capacity to remain informed.

Battery status display - user configurable display and battery and recharge control centre. Screens with multiple bar graphs for battery voltage or temperature, module detail, BatOpt 5A charger manual control, Recharge control for line current, battery current, battery voltage lid, battery heating temperature, cooling blower-on temperature, maximum battery temperature for charging.

The overall approach to instrumentation on the tzero is to provide detailed battery data in real engineering units, rather than the 'dumbed-down' displays found in most other electric vehicles. Many early-adopter electric vehicle drivers have expressed a desire for more detailed energy status information. Several EV1 drivers have incorporated supplementary PalmPilot or lap top computer displays of serial data available from the diagnostic port.

Seats

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The compact seat shells are of moulded fibreglass. The seats are not adjustable for and aft (instead the pedals move). They are thinly padded and upholstered in leather. An inflatable air bladder is fitted in the lumbar area of the driver's seat.

Pedals

The brake and accelerator pedals are mounted to a movable frame to allow for adjustment for different leg lengths. Pedal movement is accomplished with an electric motor and lead screw. The accelerator pedal has longer than normal travel to prevent the car from feeling too 'jumpy' - with so much power available it is not desirable to have a short throw pedal.

Heating and Defogging

The heating/defogging system has two sources of heat. The primary source is waste heat from the drive motor. If this is insufficient or the motor is cold, two PTC heaters are mounted in the air ducts near the windshield. Air is delivered through slots at the base of the windshield and through openings in the ducts aimed at the foot well area. The system has fresh and recirculate positions, and also a setting for recovering waste heat from the motor. In this mode, fresh air is drawn through the motor cooling fins and then passed through ducts behind the seat and forward along the side walls of the interior to the windshield area for discharge. Return air for the recirculate mode goes through openings on the forward edge of the seat. All fresh air for ventilation and powertrain cooling is drawn through two inlets near the front edge of the rear deck.

Performance

  • Acceleration
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The tzero exhibits stunning acceleration performance - very few street cars can match it. In testing at the Pomona, California drag strip using Road and Track magazine's radar timing system, the tzero accelerated from 0 to 60 mph (97 km/h) in 4.36 seconds (raw data). When corrected to remove the first foot of travel (for comparison to all auto magazine test data), the time is only 4.07 seconds.

This performance is substantially above that of the first prototype tzero, which registered 4.9 seconds to 60 mph. Reasons for the improvement are a 45kg weight reduction (now 1106kg), and a 25% boost in the motor drive inverter current rating. Corrected elapsed times to distance are as follows:

1/8 mile 8.51 sec 86.4 mph
1/4 mile 13.24 sec 90.0 mph

(Note that the tzero is speed-limited to 90 mph)

Equally significant is the accessibility of this performance. No special driving skill is needed to achieve these numbers. Response to the accelerator pedal is better than virtually all other street cars. Maximum torque or power is almost instantly available at any speed - no downshifting is needed. Top-gear rolling acceleration performance (as is often reported in Car and Driver magazine) is a good indicator of how responsive a car feels. A comparison with several other performance cars is listed below.

Top Gear Acceleration

Vehicle 30-50mph
(48 - 81 km/h)
(secs)
50-70 mph
(81 - 113 km/h)
(secs)
tzero 1.7 2.5
BMW Z8 7.2 5.1
Honda S2000 10.3 10.2
McLaren F1 7.0 3.7
Porsche Carrera 4 9.6 9.6
  • Maximum Speed

The maximum safe speed is limited by the 12,000 rpm motor maximum speed and the gearbox ratio. The gearbox ratio of 9:1 has been selected with the aim of optimising performance at normal speeds in the United States. The maximum speed of 90 mph (145 km/h) is more than enough for almost all normal freeway or highway driving in the US. If a lower numerical gear ratio were selected to achieve a higher maximum speed, efficiency and acceleration would suffer in everyday driving. If fitted with a multi-speed gearbox with appropriate ratios, the power-limited top speed would be in excess of 160 mph (258 km/h).

  • Handling

The tzero suspension and tyres have been tuned for good driving feel and safe handling at the limits. Ultimate cornering limits were not the goal. Considerable testing has been performed on a skid pad to develop the vehicle dynamics at the cornering limits. Limit behaviour is moderate understeer. Applications of regeneration or motoring torque while cornering at the limit produce benign results due to the traction control and limits on regeneration while cornering.

  • Energy Efficiency and Range

A real-world example of the tzero's range capability is a 135km trip. Composed of freeway and highway driving, this trip included climbing to the top of a local 1500m mountain (starting from 275m).

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