src=”elements/100656.gif” width=”180″ height=”131″ alt=”” border=”0″ align=”baseline” /> When a vehicle is involved in an accident, extreme forces are placed upon the design characteristics of the structure, as well as all related and attached system components. This energy management of the collision forces is what began the need for technical training and the formation of I-CAR. And the importance of this training over the years has set the U.S. collision industry apart from most other countries in the world.
Since frontal collisions are the most common and since a high percentage of today’s front suspensions are struts , it makes sense for us to concentrate on strut systems.
Repair or Replace?
Since suspension systems must be able to absorb road shock and handle the forces of braking, acceleration and turning, the question is often whether parts of this system should be repaired or replaced.
In other countries, it’s common to repair some of the parts. Two factors have made this acceptable: the availability of the part, and the lack of liability and concern regarding safety.
You see, many suspension parts can be produced in a casting process rather than a stamping (forging) process. The casting process is normally used for spindles, tie rod ends and steering knuckles. But it’s not just cast suspension parts that shouldn’t be repaired. Other suspension parts that aren’t cast also aren’t candidates for repair.
What you need to recognize here is the stress that these parts take on a regular basis. If we repair them and they’ve been weakened, we’ve compromised the safety of the people in that vehicle. And we are accountable.
It’s All In the Angles
Diagnostic angles are integral to today’s collision structural repair and wheel alignment. Steering Axis Inclination (SAI) – SAI is defined as the inward tilt of the upper pivot point compared to the lower pivot point referenced to true vertical when viewed from the front or back. This is the angle created by a line drawn from point A through C and compared to true vertical. It’s the primary directional control angle for today’s vehicles. It’s measured in degrees, and there’s no positive or negative because the angle must lean in toward the center of the vehicle.
It’s critical to look at the SAI readings for every collision-related alignment. SAI is the key factor in validating proper structural dimensions in these suspension mounting areas during collision-related wheel alignments.
It’s also very important that side-to-side (left-to-right) comparisons of the camber angles are balanced or within specifications given by manufacturers. Generally, the collision repair industry should ensure that this balance is within 1/4 of a degree side to side and that each reading is on the same side of zero (both negative or both positive). This is referred to as the “split” or “cross camber.” Imbalance of camber readings and/or out-of-specification settings can easily cause the vehicle to pull to the side of most positive.
Included Angle (IA) – IA is a calculated angle created by adding the SAI to the camber angle. IA determines if there are bent or misadjusted parts between the upper and lower pivot points of the suspension.
MacPherson Strut Systems and SAI
If structural repairs are completed properly, the SAI on MacPherson strut systems will be correct unless a lower control arm is bent, a ball joint stud is bent, the engine cradle has shifted side to side or the strut tower is mislocated.
If the technician, using even a tape or tram gauge, verifies the relative lower ball joint position from side to side and makes sure that the ball joint isn’t bent on either side and that the upper strut tower dimensions are the same left and right, then it’s reasonable to assume the SAI readings will be the same left and right. On a symmetrical vehicle, this also means that the structural dimensions for the lower and upper front structural relative to centerline are likely correct.
This is definitely the time to find out that a possible alignment problem exists relative to the structural repairs – not once the job has been completed and sent down to the alignment shop. If SAI angles are the same left and right, don’t go thinking that the frame is still bent. It isn’t. The problem is most likely a bent spindle or strut.
Now if the readings aren’t the same, it is remotely possible that the vehicle is non-symmetrical as well. Verify this through your structural dimension guides. This is a remote possibility for only very few vehicles, but just make sure before you jump to conclusions.
If SAI is lower than specification, look for damage low. If SAI is greater than specification, look for damage high. Also, in both cases, check that the lower ball joint isn’t bent. Left unchecked, this can misdirect many technicians into thinking damages exist in other areas. To check this, raise the wheels off the ground and manually grab the tire and move each wheel through its turning radius. Pay close attention to any abnormal (not smooth) rotation of the lower ball joint.
You also need to look carefully at whether one side is less than specification and the other is greater than specification. In these cases, look first at the engine cradle or crossmember to ensure it’s properly positioned. These opposite-type readings usually indicate that it’s not. Once t corrected, re-check the SAI angles.
If no specifications for SAI are given, begin by assuming that the undamaged side (or least damaged) SAI reading provided is correct and use it as a guide for the other side. This can, however, be misleading so use caution before jumping to conclusions. Once you’ve verified the correct reading, write it down.
In this area during a collision repair, there’s a tendency to replace anything that might be damaged, rather than finding what’s really causing the problem. This is usually caused when only the camber reading is used to diagnose the problem.
Improper camber angle readings are only a symptom of the real problem. It’s unfortunate the camber is what most alignment technicians “fix” at all costs, even if it dramatically affects other alignment angles. One of my main truths that I attempt to convey to any technician is, “No one should ever sacrifice one alignment angle to fix another. Don’t react to only the symptom. Fix the real problem.”
For example, one reason camber readings may be out of specification is because the SAI angles are incorrect. And this would be the real problem, as opposed to the symptom — the camber being incorrect.
Another reason camber will be out of specification is because the IA is out of spec. As already noted, this calculated angle determines if there are bent or misadjusted parts between the upper and lower pivot points of the suspension. So, if the IA and camber are out of spec, then it’s likely that the strut and/or spindle is bent. (If there’s an adjustment for camber, check that first.)
As for special kits to “fix” camber problems, they should never be used to mask a problem with SAI or to mask a bent part. Wheel alignment is the fine-tuning of wheels for optimum driving.
