Sorry Doug,
I didn't intend to confuse you so much.
I wrote:
"That means the front wheels must tilt at the same rate as the rear wheel as the car corners. The original Beetle geometry is good in this respect. "
You reacted to that with:
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"Further interesting how you equate understeer stability characteristics with rear suspension design, and actually consider a lateral rear swing arm suspension a good thing" ... and you continued ranting.
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I would have thought it was
stunningly obvious that we were talking about front suspension geometry here, because with a rear trailing arm, you don't have anything to work with at the back: the wheel
must lean with the car's roll angle: as soon as the car goes into a turn, the rear wheel is disadvantageously positioned with the road. To avoid oversteer, the fronts need to also lean with the car, as provided by the
front suspension of a VW, the
front suspension of a 2CV, and the
front suspension of a DS (most Citroens, in fact) ... and, of course, the Morgan three wheeler.
With the Civic
front suspension, the geometry is designed to keep the outer wheel in a turn roughly perpendicular to the road, (so its top must lean into the body, called negative camber) to give it better traction than it would have if it simply leaned with the body. Likewise, the rear suspension on the Civic does the same thing. If instead, the Civic's rear suspension were straight trailing arms, then the front suspension would also need something very close to trailing arm geometry, to avoid oversteer.
Here is a pic of a 2cv cornering. You can see the wheels are far from perpendicular with the road, but are instead parallel to the cars rolled centerline plane.
From the Wikipedia article on camber:
"Camber angle alters the handling qualities of a particular suspension design; in particular, negative camber improves grip when cornering. This is because it places the tire at a better angle to the road, transmitting the forces through the vertical plane of the tire rather than through a shear force across it."
No other factor of suspension geometry has as large an effect on the cornering ability and cornering characteristic (oversteer, neutral, understeer) as camber management vs roll angle and the relation of front and rear camber. (Much of the rest has to do with feel and feedback, anti dive, etc. -- and if you screw that stuff up, at least you're not likely to swap ends while taking a bend.)
You rambled on about straight line stability, which I had not brought up. I am only concerned about safety aspects, and could care less if it wanders a little: there are plenty of factory fresh cars that wander a little, and almost everything from GM has crummy feel -- those are little things.
Roll cage issues:
You wrote:
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Your recommendations on rollcages are just silly. Spectacularly silly. I hope you mistyped your recommendation for a 1.75 cage of .125 for a 1000 pound car.
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Your roll cage is not a roll cage, because the spans are far too long and you have no triangulation. To make up for the long spans you need larger diameter tubing. If your car were to actually weigh 1000 lbs this would be less of an issue, but it will not weigh 1000 lbs, even empty.
Your structure is far less efficient that the Civic's structure, but also longer, which makes it whippier yet. So to to keep it at Civic weight would be hard, without sacrificing safety and torsional and beam stiffness. The X Prize Aptera was 2300 lbs, and it was at least a partial monocoque.
These are the Auto Power Industries specs for a Civic roll cage: 1.75 x .120 DOM. Their cages are engineered and tested, however, so you would do well to overbuild rather than underbuild, because your structure is different. Here's a link that shows a Civic roll cage built of 1.75 inch tube:
Roll Cages for the Honda Civic from HorsepowerFreaks
I did not recommend .125 wall. Nor was I
recommending a roll cage for a 1000 lb car. I have no reason to believe that your car will be 1000 lb, even unloaded. I am also not
recommending any roll cage dimension -- that requires engineering, which no one will do in casual conversation over the web. The safety cage for a passenger vehicle needs to be engineered. I am not saying that 1.75 inch tube is "enough." I am saying that anything less requires more extensive engineering and testing. At a minimum, you should do the testing for rollover strength spelled out in the Fed regs.
Your roll cage has no provision for accepting the load from the door guard beams, and even a 1.75" tube is probably inadequate for that, if it occupies the place where your existing downtube is. Read the regulations, apply the test loads.
Narrowness:
Read about rollover. This may be difficult to understand, but imagine moving the front wheels closer together, and eventually so close that they merge. You would then have no roll resistance at all. Now, as you move them apart again, the car does not go from having no roll stability at all, to having "plenty." One width is "way too narrow". Another is "pretty close", etc.
Think about Explorers flipping over, and why the online rollover threshold calculators ask for width.
I mention these things for your safety, and that of your wife. I see that all this falls on deaf ears, however. So I will trouble you no further.