Rear Wheel Steering: Why Not?
[ July 21st, 2008 ] By: Mark Ozimek Posted in » Ramblings
I was pushing a cart around at work today at a rather rapid rate, and no, I was not racing a co-worker, I swear! It was one of those carts that had fixed front wheels and rear wheels that were free to pivot. Being the forever analytical engineer I am, I noticed how much the cart liked to turn, and how much the front wheels slid. I didn’t give it much thought until I hopped into my car to go home. Then: “Why don’t we see many, if any cars with rear wheel steering?”.
Obviously, it must have some sort of major handing characteristic that is undesirable, or we would have seen it in racing a long time ago. It is kind of hard to predict what such a car would handle like without testing it out. I would say drive a normal car in reverse, but that’s a little different because the suspension is designed in such a way to make the wheels center when going forward. In reverse, the car wants to turn more into the direction it’s turning.
From an overall physics standpoint, it’s rather curious to think about. Normal cars turn by pulling the front end in towards the center of the turn, the rear wheels just follow along. With a car that has rear wheel steering, the back end is let loose, and follows the front tires that stay on the same line. This should sound vaguely familiar, as that is what happens when the car oversteers. To be precise, it’s very very similar to what is happening when the car oversteers just enough that the turn can be held by keeping the front wheel’s axis in line with the center of the turn.
Knowing that, it’s pretty easy to understand why we don’t see it in cars; The setup is highly unstable. However, this could be a very good thing for certain types of racing, if the driver is up to dealing with the demands. Rally is one where it’s benefits could be seen greatly. Going around hairpin turns would be a breeze, just keep the front wheels on the line you want to take, and pivot the rear out, just like what happens through careful use of throttle, steering and handbrake use on normal cars. If the front tires start sliding, simply turn a little harder to pivot the car in some more. If the back slides out, turn less. Very intuitive, as opposed to countersteering and managing throttle input to keep the back from spinning around, or juggling weight transfer through braking to manage understeer.
Someday I’ll have to try racing a rear wheel steering car to confirm my suspicions. Until then, just a thought to keep in the back of your mind, instead of taking for granted that cars should always use the front wheels for steering. Also, think about how easy parallel parking would be!
July 24th, 2008 at 10:16 pm
There have been a few cars that featured RWS, but they were never well received and usually had extremely high maintenance issues.
The one that comes to mind was a Honda Prelude from the 80’s, but I know there are others. The rear wheel steering is only to augment/compliment the main steering of the vehicle (in the traditional front).
GM (I think it was GM) had a pickup truck with RWS to a point, too - supposedly to give it a tighter turning radius as well as make backing a trailer easier.
There have been a few experiments with 3-wheeled cars that used RWS, but they were quite unstable, and basically widow makers in any sort of congested area.