You have probably heard this term applied to various newer model cars, as in: "Yeah, well mine's got NSRR." Owners of all water-cooled VWs can boast it, as well as owners of later Superbugs. But what is it?
Negative Steering Roll Radius (I'll call it NSRR from here on to save my typing fingers), is an arrangement of the front suspension components that places each front wheel's pivot point outside the centre of the tyre's axis. Huh?
Consider your front wheels pointing in a straight line (we'll assume no toe-in for simplicity). The central axis of the wheels, the one they revolve around, passes through both wheels and should be exactly 90 degrees to the direction of the car's motion. just as if they were joined by a solid axle all the way across..
Now think about what happens when you turn the steering wheel. Traditionally the wheels pivot around the steering knuckle as they start to point left or right, and of course the steering knuckle is closer to the centre of the vehicle than the centre of the tyre is. The effect is that the tyre will actually follow an arc as it moves from left to right
You can visualise this by thinking of the little circlip on the end of the speedo cable on a link-pin Beetle's left front wheel. As you turn left, the circlip (and the wheel too, of course) will move backwards and in towards the inner mudguard. Centre the steering wheel and turn right, and the circlip comes out and forward, reaches maximum outer position at the straight ahead, then continues forward and in towards the headlight bucket as you turn right. In effect, our circlip is the very outer marker of the arc the wheel is describing.
NSRR is exactly the opposite. The suspension components are arranged in such a way that the circlip now becomes the CENTRE of the wheel's turning arc - is outside the tyre's axis rather than inside at the steering knuckle.
Now imagine turning left This time, the circlip stays where it is and the wheel moves around it the front of the wheel comes out and the back goes in.
What does all this achieve? NSRR helps the driver maintain control when one front tyre has significantly poorer traction than the other. This might occur when braking hard while turning a corner, when one wheel drops off on the road, or when one front tyre blows out
When a conventional car encounters such a situation where one front wheel has much greater traction, that wheel will also have the greater stopping power. If this occurs, the car will tend to pivot around that tyre. NSRR makes the front wheels want to return to centre to oppose the pivoting force helping the driver maintain directional control , even though one wheel has much greater traction than the other. The result is a tendency to stop in a straighter line.
One other clever design trick used on front-drive VWs is a special type of "uniaxle" unidirectional front lower bushing. This gismo is designed to absorb road bumps in the fore and-aft planes, but without permitting changes in wheel angle that would affect steering. Like most good ideas, the bushing is simple: a disc with strategically placed openings that control its motion.