Comment by userbinator | Hacker News Reader

Comment by userbinator | Hacker News Reader

For comparison, here's the front suspension of a 1956 Buick:

http://buick.oldcarmanualproject.com/manuals/1956/1956%20Bui...

Observe that the kingpin axis is entirely inboard of the tire, almost vertical, and the scrub radius is such that the axis meets the ground inboard of the contact patch of the tire (in other words, it doesn't really "scrub" the tire when turning).

What stops the suspension from actually steering is the steering system itself and the hands of the driver holding the wheel. You can visualize that in the picture above or below; if the ground pushes the tire rearwards at the contact patch, which is a few millimeters outboard of where the kingpin axis intersects with the ground, that tire will have a tendency to rotate, which will pull on the steering tie rod and send forces to the steering wheel.

What isn't mentioned is that if you're braking, then the wheel on the other side also has the same force applied to it; only the difference between the two forces is what attempts to back-drive the steering system.

Note that it is harder with rack-and-pinion to approximate https://en.wikipedia.org/wiki/Ackermann_steering_geometry which is advantageous for tire wear.

Personally, I prefer the smoother and more isolated steering feel of a traditional recirculating-ball system, but then again, I'm not the type of driver who likes fast cornering.

You'd loved DIRAVI like on old Citroën CXes then. Instead of a big spring in the Danfoss valve between the steering column and rack it had a little gearbox with a kind of "dogbone link" joining the two, that got pushed from side to side if there was a difference between the rack position and steering column position to push the valve. Then there was a kind of heart-shaped cam that pushed the steering wheel back to the centre, with a spring pressing against it controlled by hydraulic pressure from a speed sensor on the gearbox. The faster you went the harder it pressed the spring until at 70mph you could barely move the steering wheel but the slightest pressure would steer quite quickly.

Until you got used to its incredibly sensitive response (about 1.5 turns lock-to-lock) you tended to drive in zig-zags like you were tacking a dinghy up the road, but once you did get used to it nothing ever felt as precise or comfortable. I've driven some incredibly "sporty" cars, "cost as much as a house" end of the market cars, and none of them got close to the CX for steering feel.

The earlier GS and GSA had inboard front disc brakes on the side of the gearbox, allowing them to be massive while also allowing the tall skinny tyres (145SR80, about 150mm wide) to be absolutely centred on the kingpin, boresighted on the middle of the contact patch. They had incredible handling, so sharp and responsive, and the non-powered steering was as light as any modern car at parking speeds.

The equalizing brake force from the other side sounds good until you factor in things like ride height differences from worn suspension or imperfect roads. Those and other transient forces disturb that left to right balance. Weird things like bump steer start happening when left to right measurements are off.

I've never driven a car with that kind of suspension, does the design still use caster to self-center the steering, or will the tires stay roughly where you point them even if you let go?

I thought this was the result of front wheel drive.

Rear wheel drive cars also do it. The caster on a car mimics that of the wheel of a shopping cart. Having the pivot axis be at a slight angle relative to the direction your travelling makes the wheels trail behind that axis.

It does tend to be more apparent in front wheel drive cars, perhaps it's the weight of the motor? It's really important to get right in motorsport, as it denotes the characteristics of the car when you've oversteered. A car with correctly set caster will "self steer" itself back to straight without any driver input, and it will guide the car while oversteered in a way that's balanced with the direction the car's actually heading rather than where it's pointing (if you're intending to be sideways, like in drifting).

Yes, you have to have caster or the car would be very difficult to drive. I had a VW bug with a worm gear steering box that was going bad. It would almost keep the wheels wherever you put them and it was scary to drive. Think about how you let the steering wheel slip as the wheels return to straight when you turn onto a cross road, you had to return the wheels to straight yourself.

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