A technician is performing a four-wheel alignment on a vehicle with a MacPherson strut front suspension. The alignment machine shows that the front left caster is +1.5 degrees and the front right caster is +3.0 degrees. The specification for caster is +2.5 degrees ± 0.5 degrees. The vehicle exhibits a strong pull to the right. Which of the following adjustments would be the MOST effective to correct the pull and bring caster into specification?
- AAdjust the front left caster to +2.5 degrees and the front right caster to +2.5 degrees.
- BDecrease the front left caster to +1.0 degrees and increase the front right caster to +4.0 degrees.
- CIncrease the front left caster to +3.0 degrees and decrease the front right caster to +1.5 degrees.
- DIncrease the front left caster to +2.5 degrees and decrease the front right caster to +2.5 degrees.
Show answer & explanationAnswer & explanation
Correct answer: D. Increase the front left caster to +2.5 degrees and decrease the front right caster to +2.5 degrees.
To correct a pull to the right, the caster on the right side should be less positive (or more negative) than the left, or both should be equal and within spec. The specification is +2.0 to +3.0 degrees. The left (+1.5) is low, and the right (+3.0) is at the high end. Increasing the left to +2.5 and decreasing the right to +2.5 would equalize them within specification, eliminating the caster split that causes the pull.
Why the other options are wrong
- A. While equal, the initial left caster cannot be adjusted to +2.5 degrees without also considering the right side's adjustment to correct the pull. This option focuses only on the target value, not the split correction.
- B. This would create a very large caster split with the right caster significantly higher than the left, causing an even stronger pull to the left, which is opposite the current problem.
- C. This would exacerbate the caster split and the pull to the right, as the right caster would become significantly lower than the left, causing a pull to the left or an even stronger right pull if interpreted as increasing the difference.
Caster Split Correction
Caster split correction involves adjusting the caster angle of each front wheel to be equal or nearly equal, and within specification, to eliminate vehicle pull.
- Higher positive caster on one side causes a pull to the side with lower positive caster.
- Adjustments aim to equalize caster values within the specified range.
- Often involves moving the top of the strut or lower control arm position.
Memory trick: To fix a pull, make caster 'equal and spec-tacular'.