Alignment for rocksteady street tires?

It's geometry, stiffness, bearings/bushings, and sprigs/roll-bars.

C&D or R&T did an article on 3 series BMW suspension a dozen-odd years ago. There are a host of interacting components; but basically it comes down to complete freedom of motion in the directionis you want things to move, and complete lack of motion in thoes you do not.

So the suspension bushings are low friction in the movements required as tires step over bumps, or the car rolls, dive and squats, but the bushing strongly resist forces in the 5 other degrees of freedom.

The springs and shocks are pointed towards the center of the contact patch laterally, and there is a small but stable amount of pneuumatic trail longitudinally. {The difference between pneumatic trail and caster is that one is an angular measurement the other is the linear measurement on the surface of the road.}

The kingpin inclination also points near the center of the contact patch, but you want "just that bit" of pneumatic offset where on the king pin as opposed to the springs and shocks.

The pair of geometric points causes road surface iregularities to create understeering effects, which comforts the driver. Thus, wind gusts, bumps, potholes, and other effects
are absorbed with aplomb.

In the steering plane, you want the kingping inclination to be near the center of the contact patch so the steering effort has to pull the outside of the tire and push on the inside of the tire with the least amount of force. For example; conside the situation where the kingpin inclination intersects with the inside of the front tires. The steering effort has to scrub the whole width of the contact patch forward. This has 4 times the effort as when the KPI points at the center of the contact patch.

All the bearings in the steering need to be perfectly stiff in the non-rotation directions, and perfectly free in the rotation direction. Wheel, shaft, steering box/rack, tie rods, hubs,...

Up front, one wants just enough toe-in to obtain the proper front tire temperature (so they don't run too cold compared to the rears), just enough camber for a nice rectangular/oval contact patch, such that the pneumatic trail is well within the contact patch, but significantly forward of the center. This generates the self centering effects so the car drives straight when there is no hands on the steering wheel.

Out back, one want just enough camber to generate long tire milage and proper tire temperature, and a hair of toe-in to prevent toe-out under any circumstances of brake, power, corner. Having the rear toe-in just a bit more than the front (in a force sense) creates aerodyamic understeer response to wind gusts. That is: the rear toe-in in times the rear tires width is just slightly greater than the fron toe-in times the front tires width.

Spring rate and sway bar selection: Consider a car under maximum deceleration (brakes). The front end dives, the rear end raises from weight transfer. Now consider the transition from maximum braking to maximum latteral accelration. Herre is the trick, you want the outside front tire and the inside rear tire not to move (up/down) durring the transition.

A typical wheelbase (100-odd inches) is about 50% bigger than the typical tract (66 inches). Thus, to have the same (up/down) movement on the suspension at the contact patch for both maximum braking and maximum lateral, one needs the anti-roll bars to generate 1/3rd of the force of the spring on the contact patch. If you do this then the outside front and inside rear tires "do not move" durring the transistion.

The suspension engineer can then transfer some of the rear anti-roll bars force to the front antirool bar by making the rear bar smaller in diameter and increasing the front bar's diameter. This transfer of force from the rear to the front creates understeer.

Summarizing: low friction on [parts you want to move, no movement in directions you don't want movement, and then making the geometry of the suspension components "point" at teh right places, and fianlly, setting up the suspension so that all external inducements of force crate understeering responses from the car. BMW then goes the extra mile (so do a few others) so that the driver can command oversteer under power (i.e. understeer never becomes excessive.)

 

On an a-arm car (Ferrari) the kingpin is a virtual line that passes through the pivot point at the top of the front suspension where the hub rotates, passes through the bottom of the hub where it rotates, and intersects the roat inside the contact patch.

On a strut car (BMW 3 series) the king pin is defined by the way the front hub rotates around the shock (i.e. strut). Here, the forces on teh movement are minimized when the srping is coaxial with the strut, but tire clearance problems make the spring pitch inwards for tire clearance.

If you lower a car with struts, you need to raise the chassis pickup points the same amount the car goes down to put the control arms are the desired (nearly flat) angle with respect to the road surface.

This effect is called 'tramlining' and wider tires tramline worse and are harder to control than taller or narrower tires. Stiffer sidewalls help, so do stiffer bushings and basically stiffer stuff anywhere. Here the tall BMW tires simply do not generate the effect, so there was nothing to get rid of. Race cars fromthe lat 20's understood the effect, and until steel belts and radial belts came on the scene, you simply couldnot make tires wide with out suffering.

 

When you lower a strut car without raising the pick up points, the roll center is lowered. THis causes the car to roll excessively, and thereby requires significantly heavier springs (compared to a strut car lowered but the pickpu pointsraised properly.

The only thing preventing you from raising the pickup points is access to welding equiptment.

Also note when one lowers a car, one upsets the bump steer on the steering arms--perfectly OK for racing,not so OK for street driving (or mindless cruising down interstates.)

One can change the pickup point at the bottom of the hub and not have to raise the chassis pick up points. That is the factory/near factory guys may have access to a part that is little known (and the ricers basically don't care.) The real trick is to leave the a-arm angle between the chassis pick up point and the hub pickup point the same before and after lowering.