Possible Alignment Trial Outcomes

The axle alignment process is based on the results of running the car down a slight incline four times with a different starting configuration each time. The four configurations are:

Trial 1 - car facing forward with the wheels as far out on their axles as possible.
Trial 2 - car facing forward with the wheels as far in on their axles as possible.
Trial 3 - car facing backward with the wheels as far out on their axles as possible.
Trial 4 - car facing backward with the wheels as far in on their axles as possible.

After each run the position of each wheel is noted and entered in the alignment computer. From these results the computer calculates the adjustments necessary for each axle.

The table below describes the logic for making this calculation. It is based on whether or not the wheel moves from its initial position on the axle to the opposite position during the run. Columns 2-4 of the top row of the table describe the starting positions. The cells below them describe whether or not the wheel moved from that position during the trial. For example a Yes in the Forward & Out column indicates that the wheel moved in when the car was rolling forward.

The four trials result in a total of 16 possible combinations of outcomes for each axle. One of these combinations (#1) indicates that no alignment is necessary, i.e. the wheel remains in its starting position during all four trials. Three of these combinations (#4, #13, #16) indicate conflicting data from which no conclusion can be drawn. For example, if the wheel moves during every run. Four of the combinations (#6, #7, #10, #11) indicate that the axle needs to be moved predominately in one direction, i.e. up, down, forward, or back. Four other combinations (#2, #3, #5, #9) indicate that the axle needs to be moved roughly the same amount in two directions, i.e. up & back, up & forward, down & back, or down & forward. Finally their are four combinations (#8, #12, #14, #15) which have conflicting data from one pair of trials but some information can be drawn from the other pair.

Forward
& Out Forward
& In Backward
& Out Backward
& In Action: Justification

1 No No No No Do nothing: Wheel always stays where it started. no alignment is necessary.

2 Yes No No No Shim UP & BACK: Wheel moves outward when rolling forward but doesn't move during either backward trial. Assumption is that the axle is canted down with toe-in. The forces on the wheel therefore reinforce each other when rolling forward, moving the wheel in, but cancel each other out when rolling backward, resulting in no motion.

3 No Yes No No Shim DOWN & FORWARD: The opposite of combination 2. Assumption is that the axle is canted up with toe-out.

4 Yes Yes No No INVALID!: The wheel moves in both directions when rolling forward but not at all when rolling backward. This behavior is not explained by any axle position.

5 No No Yes No Shim UP & FORWARD: Wheel moves inward when rolling backward but doesn't move during either forward trial. Assumption is that the axle is canted down with toe-out. The forces on the wheel therefore reinforce each other when rolling backward, moving the wheel in, but cancel each other out when rolling forward, resulting in no motion.

6 Yes No Yes No Shim UP: The wheel moves in when rolling in either direction. The axle is canted down.

7 No Yes Yes No Shim FORWARD: The wheel moves out when rolling forward but in when rolling backward. The axle has toe-out.

8 Yes Yes Yes No Up/Forward???: The backward trials indicate that the wheel is canted down and/or has toe-out but the forward trials are contradictory. You may want to shim up and/or forward but try re-running the test first.

9 No No No Yes Shim DOWN & BACK: The opposite of combination 5. Assumption is that the axle is canted up with toe-in.

10 Yes No No Yes Shim BACK: The wheel moves in when rolling forward and out when rolling backward. The axlehas toe-in.

11 No Yes No Yes Shim DOWN: The wheel moves out when rolling in either direction. The axle is canted up.

12 Yes Yes No Yes Down/Back???: The backward trials indicate that the wheel is canted up and/or has toe-in but the forward trials are contradictory. You may want to shim down and/or back but try re-running the test first.

13 No No Yes Yes INVALID!: The wheel moves in both directions when rolling backward but not at all when rolling forward. This behavior is not explained by any axle position.

14 Yes No Yes Yes Up/Back???: The forward trials indicate that the axle is canted down and/or has toe-in but the backward trials are contradictory. You may want to shim up and/or back but try re-running the test first.

15 No Yes Yes Yes Down/Forward???: The forward trials indicate that the axle is canted up and/or has toe-out but the backward trials are contradictory. You may want to shim down and/or forward but try re-running the test first.

16 Yes Yes Yes Yes INVALID!: The wheel moves in both directions rolling forward and backward. This behavior is not explained by any axle position.

	Forward & Out	Forward & In	Backward & Out	Backward & In	Action: Justification
1	No	No	No	No	Do nothing: Wheel always stays where it started. no alignment is necessary.
2	Yes	No	No	No	Shim UP & BACK: Wheel moves outward when rolling forward but doesn't move during either backward trial. Assumption is that the axle is canted down with toe-in. The forces on the wheel therefore reinforce each other when rolling forward, moving the wheel in, but cancel each other out when rolling backward, resulting in no motion.
3	No	Yes	No	No	Shim DOWN & FORWARD: The opposite of combination 2. Assumption is that the axle is canted up with toe-out.
4	Yes	Yes	No	No	INVALID!: The wheel moves in both directions when rolling forward but not at all when rolling backward. This behavior is not explained by any axle position.
5	No	No	Yes	No	Shim UP & FORWARD: Wheel moves inward when rolling backward but doesn't move during either forward trial. Assumption is that the axle is canted down with toe-out. The forces on the wheel therefore reinforce each other when rolling backward, moving the wheel in, but cancel each other out when rolling forward, resulting in no motion.
6	Yes	No	Yes	No	Shim UP: The wheel moves in when rolling in either direction. The axle is canted down.
7	No	Yes	Yes	No	Shim FORWARD: The wheel moves out when rolling forward but in when rolling backward. The axle has toe-out.
8	Yes	Yes	Yes	No	Up/Forward???: The backward trials indicate that the wheel is canted down and/or has toe-out but the forward trials are contradictory. You may want to shim up and/or forward but try re-running the test first.
9	No	No	No	Yes	Shim DOWN & BACK: The opposite of combination 5. Assumption is that the axle is canted up with toe-in.
10	Yes	No	No	Yes	Shim BACK: The wheel moves in when rolling forward and out when rolling backward. The axlehas toe-in.
11	No	Yes	No	Yes	Shim DOWN: The wheel moves out when rolling in either direction. The axle is canted up.
12	Yes	Yes	No	Yes	Down/Back???: The backward trials indicate that the wheel is canted up and/or has toe-in but the forward trials are contradictory. You may want to shim down and/or back but try re-running the test first.
13	No	No	Yes	Yes	INVALID!: The wheel moves in both directions when rolling backward but not at all when rolling forward. This behavior is not explained by any axle position.
14	Yes	No	Yes	Yes	Up/Back???: The forward trials indicate that the axle is canted down and/or has toe-in but the backward trials are contradictory. You may want to shim up and/or back but try re-running the test first.
15	No	Yes	Yes	Yes	Down/Forward???: The forward trials indicate that the axle is canted up and/or has toe-out but the backward trials are contradictory. You may want to shim down and/or forward but try re-running the test first.
16	Yes	Yes	Yes	Yes	INVALID!: The wheel moves in both directions rolling forward and backward. This behavior is not explained by any axle position.