If you stand in front of the car and look back at it, look at the two pivot points for one of the wheels. On older cars that's the upper and lower ball joints as shown in my sad drawing below. On yours it's the upper strut mount and lower ball joint. The upper pivot is inboard a lot compared to the lower pivot. That angle is called "steering axis inclination" and can be measured on an alignment computer. There is never a spec given for it. All that's important is it must be the same on both sides. If it is not, something is bent. (This is a big problem on GM front-wheel-drive cars because the cross member has to be removed for some service work and if it's not bolted back in exactly the same way, it will result in extremely miserable handling even after an alignment). The typical reading is 28 to 32 degrees plus or minus 0.2 degrees.
The RESULT of scrub radius is it causes the inside of the tire tread to try to make the tire turn toward the center of the car, and the outside part wants to make the tire pull away from the center of the car. Those two forces balance each other out and make both tires happy to go straight. Any mismatch becomes a bigger factor when braking. That's when friction becomes important and causes the tread to, ... Uhm, ... "Scrub" on the road surface.
We rarely concerned ourselves about scrub radius on our older muscle cars because the two front brakes were always on one hydraulic circuit and the two rear brakes were on their own circuit. If you popped a front flex hose, you still had about 30 percent of the stopping power from the rear brakes. If you popped the rear hose, the two front brakes still stopped evenly.
With almost all front-wheel-drive vehicles, there is such a high percentage of vehicle weight in the front that a brake hydraulic system split in the normal way, front and back, would present a braking problem if a front hose would rupture. You'd have less than 20 percent braking power from just the rear brakes. They would lock up and skid, and they'd find you in the next county before the car finally stopped! To address that, front-wheel-drive cars use a "split-diagonal" brake system. The left front and right rear are on the same hydraulic circuit. If a front hose pops, you will still have the other front brake and 50 percent of your normal braking power.
If the split-diagonal system had been used on older rear-wheel-drive cars and a hose had popped, even light braking would have tugged the steering wheel out of your hands. That happened to many of us in our younger and dumber days when we would plug the ruptured line instead of replace it. You don't want to drive a car like that very long. That was addressed on the split-diagonal systems by modifying scrub radius. Section "X" in my drawing got bigger and "Y" got smaller. When you brake, road force wants to pull the tire backward causing both of them to want to turn away from the center of the car. The steering linkage between them holds them straight and the forces counteract each other. When one front brake stops working, (lets say the right one), the left tire still wants to pull back during braking but the right one isn't there to counteract it, therefore, it's the tree, oncoming traffic, or the crowd of people. By modifying scrub radius, section "X" of the left tire will want to pull toward the center of the car, and since it has been increased in size, that pull to the right offsets the tire's tendency to want to pull back, and therefore to the left. The result is the steering system stays straight when braking with only one working front brake. Chrysler has had that perfected many years ago so you often don't even notice a braking problem other than the red warning light on the dash. On most other cars all you'll see is a tiny wobble in the steering wheel as the brakes are applied.
Given the importance scrub radius plays in braking, you don't want to do anything to change it. If you install wider wheels and tires, or wheels with a deeper offset, (they stick out further from the center of the car), section "Y" will get bigger. The two tires will offset each other until one brake doesn't work. That's when it will really be noticed. It can also have an effect due to road surface irregularities since both tires are rarely in equal contact with the road surface all the time. That's what the lawyers are referring to when they say you were "less able to avoid the crash".
Scrub radius also changes when the outer circumference of the tires is changed. Now sections "X" and "Y" are further down, but since lines "A" and "B" are at the same angles, they no longer intersect at the road surface. This creates an unstable condition that probably won't be noticed in city driving, but at highway speeds the car will have a miserable tendency to follow the irregularities in the road. Roads slant to the right so rain will run off, (we call that "road crown"), and we adjust the alignment slightly to overcome the resulting pull, but the wrong scrub radius will make the car sensitive to that slant. When the road flattens out at intersections, the car may shift from pulling to the right to going straight and you'll have to constantly correct your steering. Ask anyone with over-size wheels and tires how the handling has improved on the highway, and they'll either lie out of embarrassment or tell you they don't drive much on the highway. The car business is so extremely competitive that if a manufacturer thought they could sell more cars by offering over-size wheels and tires without risking lawsuits, they would do it.
One last note that I started to make reference to in my "Notes Page" has to do with the wheel bearings. The offset and width of the wheels places the vehicle's weight right over the larger bearing. That's line "A". Moving the center of the tire tread and wheel out moves that weight closer to the smaller bearing and used to often overheat them so badly the inner race would weld itself to the spindle. Your car doesn't use this larger and smaller bearing. It uses a rather large double roller bearing. They have to be so big because the two rows of marbles are so close together that the spindle has lots of leverage to put a lot of stress on them. Changing the wheel offset puts even more stress on them. Years ago it was very rare to not have the front bearings last the life of the car. In the last 25 years with the lighter cars AND the much larger bearings, it is real common for them to become noisy. Two or three replacement bearings in the life of the car is not uncommon on any brand, and changing the wheel offset is going to make that even worse.
Image (Click to make bigger)
Wednesday, November 14th, 2012 AT 12:48 AM
