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I don't know anything about the Leaf's steering specifically, but modern steering power assist systems usually have a controller which gets steering angle and road speed inputs (as well as possibly driving mode selections) over CAN and use them to determine the amount of boost, which is then sent to the boost motor controller by CAN as well. If you're really lucky it runs at some default level of assistance without any communication, but I wouldn't count on it.

There are now power-assist units designed to be added to a steering column and intended for aftermarket use, not requiring any integration with OEM electronic controls. I have no idea how convenient they are to install, or their cost.

One issue with using the original rack with any kind of power assist is that the manual rack will likely be slower (more turns of the steering wheel required) than would be desirable with power assist.
 

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That car has a rather unconventional assist on the steering column rather than down in the rack. Interestingly, Brian covered both 😂
While the first electric assist systems mounted the motor on the rack (sometimes on a second pinion gear and sometimes parallel to the rack and running a ball screw), mounting the motor on the column (usually about at the point where it pivots to tilt the wheel) has become common. ZF offers all three types (dual-pinion rack, ball screw rack, powered column). Some companies use different systems on different models, to suit the vehicle's packaging; for instance, a Mazda 3 has a powered column, but a Mazda MX-5 has a powered rack because the engine position makes it too cramped around the column. This variety is handy, because it means:
  • there are options,
  • electrically powered racks allow the use of original columns in older cars,
  • electrically powered columns allow the use of original racks (with removal of any hydraulic assist), and
  • due to the use of powered columns there are modern racks with fast ratios that don't have a motor on the rack (for applications where assist is not wanted or not practical - they're effectively abnormally quick manual racks for someone building a vehicle with a light front end.
 
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