Selecting a Power Steering System

October 2004

Requirements

Power steering is required to assist with turning vehicle at low speeds.

Alternatives

• Accessory Motor to Drive Existing Power Steering Pump

  Use a seperate motor to directly drive the power steeing pump.

  Since the motor is large, drive the motor from the pack, so does not load DC-DC converter. A pack motor will also not require heavy gauge wiring.

  The drawbacks to such a system is that it is heavy and runs continuously - although a system can be developed to switch the system on only when actually steering.

  If a motor is used to drive the existing power steering pump, how powerful of a motor will be needed? Guesses are approx 500 Watts for power steering. 1 horsepower is 745 watts.

  What will be the operating RPM of the motor? 1500 RPM?

   

• Electric Power Steering (EPS)

  EPS uses 100% electricity to run the power steering. It is more common in Europe, but is used on one or two North American cars:

  • Honda Acura NSX
  • Honda S2000
  • Honda Hybrid Insight
  • Honda Civic Sedan
  • Saturn Vue SUV
  • Saturn Ion Sedan
• Electro-Hydraulic Power Steering (EHPS)

  EHPS was developed as a temporary solution while the automotive industry slowly migrates to complete Electric Power Steering (EPS).

  Since these units are 12v, they require a lot of current, especially at peak times. The accessory battery should be sized to handle these peak loads, or an ultra capacitor can be used to avoid loading the DC-DC system with this heavy load.

  The advantages to this system are that it is OEM quality in an integrated package.

  The drawbacks to this system is that it will require heavy wiring (60-70 Amps peak), loads the DC-DC system, and will require a system so that it only turns on when the steering turns.

  EHPS units are available from:

  • 1988 Fiero
  • 1991 Toyota MR2
  • New MINI
  • Other European, South American cars: (Vaxhaul Astria, Renault Clio, Citroen Saxo, Peugeot 307, VW Polo, VW Seat Ibiza)

  Jon (Sheer) Pullen has installed an MR2 unit on his EV. A few things he has done:

  • An optical encoder on the steering shaft is used to control a very large relay (100A DC) to switch the unit on and off only while turning.
  • The unit develops a huge amount of pressure (2000 psi) and requires custom fittings from the pump to the steering box.

Power Steering Cooling

Hydraulic power steering requires cooling. If the stock cooling system is not used, consider using a small automatic transmission cooler or a purposely built power steering pump cooler.

Selection Criteria

• Cost to construct
• Time to construct
• Ability to integrate with existing parts
• Durability
• Electrical Safety
• Effectiveness of power steering
• Weight
• Low power consumption
• High efficiency

Selection for the New Beetle EV Project

A used, but very good condition Toyota MR2 EHPS unit was acquired from MazToy Auto Recycling in Oregon for $65 USD.

The intention is to build a system similar to Sheer's, with the following:

• rather than the elaborate VW plumbing used to cool the power steering fluid, a compact lightweight cooler will be used.
• the stock reservoir will be used
• optical encoder on the steering column, triggering a 100 Amp relay to power the motor.
• the accessory battery will be used to handle the surge power of DC-DC current (Chris Z uses a 26Ah Hawker with 1kw inverter)

Toyota MR2 EHPS

Reference