Two Guys Rally

Rear Wheel Steering: Why Not? [ July 21st, 2008 ] By:Mark Ozimek

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!

Aerodynamics: An Introductory Rant [ June 24th, 2008 ] By:Mark Ozimek

I’m going to take a brief break from the turbocharger! series to make a little segue into what I hope to by my next topic that I will take a close technical look at: Aerodynamics.

Really, we only care about two things here:

1. Downforce/Lift
2. Drag

Hopefully I’ll explain how these two things come about in a pretty simple fashion that explains it a little bit better than “Oh, the body pushes air out of the way.” However, before that, I have noticed some very disturbing trends among the modding community.

The most incomprehensible to me is the addition of only a rear wing to a front wheel drive car. This is really a double negative. The wing creates downforce in the rear of the car, behind the rear wheels. The wheels act as a fulcrum, and this downforce that is generated in the rear actually lifts the front of the car up at speed. Totally counter-productive, since it will decrease traction on the drive wheels, and increase the amount of understeer. Keep in mind that front wheel drive cars already tend to understeer a lot, a wing in the back will just make it worse.

The other negative of the rear wing is one you will have to deal with almost every time you try to generate downforce by pushing air up: Lots of drag. The engine has to put out a bit more power to overcome the extra drag, which increases with the square of velocity. So now we have a situation where when the wing is putting down the most downforce is when the front tires are applying the most force to the ground to propel the car forward through the air. If this downforce was applied up front, or if it was a rear wheel drive car, this would be a great thing. Since it’s not, the drag slows the car down, and increases understeer through turns even more!

The other are body kits that add ‘features’ to the body that don’t actually do anything. Some things are useful, like a front bumper that allows less air under the body of the car should reduce drag, however, many are just as counter-productive as a rear wing on a FWD car. A classic example is the intake vent on the hood and before the rear wheels on the some of the more recent generations of Mustangs. The protrusions do exactly that, protrude into the air stream. This adds extra turbulence to the airflow over the body, which is almost always a bad thing since turbulence usually increases the amount of drag on the car. It’s possible to use turbulence to your advantage, but that’s a complex topic to cover, and requires some pretty precise placement of fins to make the air go where you want it to, think F1 for an example here.

One thing that Charles said to me while we were discussing this article is that we have a pretty utilitarian view on what looks good on a car. If it improves performance somehow, we’re almost always for it. If it does nothing, is counter-productive, or just adds weight, we typically hate it and immediately reject the idea. The things that make the car ‘go fast’ also look/sound good to most people, since we associate that with performance race cars. Maybe what we should do when we get our car to modify into a rally car is mask the things that make it look fast. Sleeper race cars, ready GO!