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Unread 06-01-2009, 09:11
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Re: 4WD Turning Difficulties

Quote:
Originally Posted by ZakuAce View Post
Would it be possible to do the car steering with half a swerve drive (the front wheels), then have the rear wheels powered normally?

Also if you wanted to put more weight in the back to reduce the moment the trailer creates, would it not then be a good idea to use rear wheel steering?
If you're referring to what I thought was "swerve drive", but is appearantly known as "ackermann", then

http://en.wikipedia.org/wiki/Ackerma...ering_geometry

The image on that page should give you a good idea of how steering in a car works. The yellow bar moves back and forth using a rack and pinion (hence, steering rack)

http://en.wikipedia.org/wiki/Rack_and_pinion

If you want to have a 4WD drivetrain with car-style steering, you're going to have to have a way for the shafts to the front transmit power while steering, probably through a simple CV joint

http://en.wikipedia.org/wiki/CV_joint

Alternately, you package a wheel and motor assembly together, and steer the whole thing. You'd need 3 motors for that type of drive, though.

Whatever drivetrain you pick, if you put the steering where more weight is, you'll have more available traction to steer with, and have less of a tendancy to understeer. Be careful if you put a steering rack on the rear wheels though, I believe the steering won't try and center it's self like it does in a front wheel steering system, although if it's electronicly controlled, it shouldn't matter.

An interesting way to power a robot with wheels that can steer would be to have the motor in the center of the car, with shafts going to the front and rear which then split to the left and right sides. To distribute the torque at 90* angles, you could have simple locked ring and pinion setups and live with a little wheelspin on cornering, or go with a differential setup, either electrically or mechanically controlled.

edit: seriously consider a traditional 4WD skid steer setup first, though. if your COG is going to be shifted significantly to the rear, then a conventional drivetrain will likely be perfectly adequate. run the numbers through that spreadsheet with coefficents of friction being .12 in every direction, and you'll find that a square wheelbase with a rear-based COG is going to turn easily, without resorting to complex steering systems

Last edited by SWIM : 06-01-2009 at 09:35.