# How do cars turn on a dime

1. Feb 10, 2014

### geoduck

How do you get car to have a small turning radius?

It should seem that putting center of mass to be in the center of the car is a good start, but is this true? Assuming that the engine is the heaviest part of the car, then should the engine be in the middle of the car? Do large buses have their engine close to the center?

Small moment of inertia helps?

2. Feb 10, 2014

### phinds

Turning radius has nothing to do with center of mass, just with the distance between front and back wheels and how far the front tires can be rotated.

3. Feb 10, 2014

### geoduck

Assume for a moment there are no net forces on the car, just a couple that produces a torque. Then the car would rotate about its center of mass. It would seem that if the center of mass were at the geometric center of the car, then the circle it sweeps out would be half the length of the car. If the center of mass were at the rear, then it would sweep out a circle equal to the length of the car?

Also would front wheel wheel drive make turns easier, since friction would be in the direction of the turned tire?

4. Feb 10, 2014

### SteamKing

Staff Emeritus
Most large buses have their engines mounted in the rear. You can tell because there are openings for radiators and panels which open to provide access for maintenance. Buses are not known for their tight handling.

If you put the engine of a car in the center, where will the passengers sit?

Good handling and turning performance in a vehicle involve a number of factors. Due to limitations imposed by size and cost, most of the handling performance will result from some sort of compromise in the design of the car. The best handling cars, irrespective of size and layout, are the ones which keep all four tires on the ground during the turn and where the tire design itself allows for high cornering forces to develop before a skid occurs.

5. Feb 10, 2014

### xxChrisxx

At low speed the answer is simple. Lots of lock.

At higher speed you dont travel in the same direction that the wheels are pointing. You turn due to lateral force arising due to slip angle.

If you want to get into it, rear wheel drive can spin up the wheels and turn on the spot.

6. Feb 10, 2014

### phinds

Huh? That sounds ridiculous. Have you ever actually driven a car? If you had a vehicle where you could turn the back wheels at the same, but opposite, angle as the front wheels then you could do what you suggest, but real cars don't work like that. And come to think of it, even then it would not rotate about its center of mass but rather its geometric center. I just don't get what you are talking about.

7. Feb 10, 2014

### geoduck

Well here's a video of a jeep hurricane that can spin in place by having the left side of the car's wheels spin forward while the right side of the car's wheels spin backwards:

But yeah, most cars only one set of wheels turn. I thought if something is spinning about the center rather than about the end, it turns in a smaller radius, so this would contribute some sort of effect. Guess not then.

Also, if there are no net forces, the center of mass doesn't move, so everything rotates about the center of mass. That's why I chose the center of mass to be at the geometric center to minimize the radius. But there are net forces so this isn't real, but I thought it could have some influence in the real world.

Last edited by a moderator: Sep 25, 2014
8. Feb 11, 2014

### jnnx

unless we are talking about some absurd 10/90 weight distribution, center of gravity has nothing to do with steering at low speeds.
as long as we are talking low speeds and open differentials, all that is important is steering angle of wheels.
low traction, locking/limited slip diffs or higher speeds complicates things because we have to include slipping wheels

if I understand your example right, it will work that way in outer space

9. Feb 12, 2014

### Ranger Mike

geoduc welcome
turning radius is a function of the steering linkage and the wheel base. Ackermann linkage improves the steering and provides ability of the inside tire to run sharper radius than the out side tire.

Mass only impacts the weight transfer when cornering via where the weight shifts. A car with lower center of mass will have a longer moment between the Center of Gravity and the Roll Center and thus stronger springs will be required to counter this. But Lower CG means less camber build and better tire contact patch, thus better cornering ability.
Engine placement can be compensated for with weight shift of other heavy vehicle components like battery, fuel cell, so you ultimately get 50-50 weight distribution front to rear.
Who cares about buses...

How exactly are you figuring small moment of inertia?