||12-15-2007 04:58 PM
The Importance of Toe-in or Toe-out
There has been quite a few questions about alignment lately. Alignment is most affected when a car is lowered. The more it is lowered, the more impact occurs. In general, camber and toe are the main settings that are impacted. Since camber is discussed so frequently, we all pretty much know what it is and the problems it can cause. The impacts of toe (whether toe in or out) are not nearly discussed as much. You've heard me indicate many times that excessive camber combined with too much toe can be the death to tires. This remains true. But we rarely discuss the impacts of toe in or toe out so I'm going to give this issue a shot. I will do it as simple as possible and hope not to offend any engineers.
Toe-in is when both tires on the same axle (front or rear) point toward a line drawn through the axis (center) of the car. Toe-out is when the tires point away from the centerline of the car. The amount of toe measured is tenths of a degree or hundredths of an inch. It's minute measurements that have major impacts!! Zero toe means the tires are perfectly parallel to the centerline of the car. In a perfect world, with perfectly flat roads, and the ability to have rigid suspensions, zero toe on the front and rear would be the ideal. But because we have crowned roads and enjoy nice compliant rides (meaning movement in the suspension) zero toe would lead to a car that tends to wander (wants to roll off the road in the direction of slope). To correct this, toe adjustment was built into cars. Caster is also an integral component of directional stability but we'll save that for a later discussion.
A Focus and almost all front wheel drive cars have TOE-OUT in the front. This is reflected by "negative" numbers on the alignment specs (TOE-IN are the "positive" numerals). Why toe out on the front? Its simple physics. As the tires pull themselves forward (aka torque) they pull themselves into the desired toe-in position. In essence it's a self compensating. Once at a consistent speed the resulting front toe ends up being a tiny bit inward. Just enough to allow that directional stability that is needed. If the toe is set to far out it never gets to the proper amount of toe-in when the car is moving. What are the downsides besides bad handling....tire wear. Too much "toe-out" causes inside edge tire wear (toe-in causes the outside edges of the tire to wear). Combine it with camber and it only amplifies the problem. Another thing to consider, the alignment specs were determined using factory bushings. When you install harder bushings, the wheels may not be able to pull themselves fully back into the desired toe-in position. Watch yours tires carefully for wear after installing bushings and be prepared to reduce the amount of initial toe-out if wear is occurring.
The rear of the Focus is just the opposite. Since the rear wheels have no power to them and they are not able to pull themselves into the correct or needed toe-in angles, it must be set with some initial toe-in. The ever going debate is how much toe-in. While Ford provides some specs we have learned that the rear tires are very temperamental to too much toe especially when combined with a lot of rear camber (even if its within the allowable spec range). In simple terms, you want to reduce toe-in if you want to increase camber. My philosophy is to run the minimum allowable rear toe-in to avoid wear problems.
What about toe out in the rear? First, Ford specs don't allow it. This should only be used for autocross and the track and not the street. It will make the car react very quickly to any steering input. Stability is drastically affected and 100% driver concentration is required if rear toe-out is used. A sneeze can literally cause you to change lanes. If you intend to use it for motorsports events, use VERY small initial toe-out adjustments until you understand the full impacts of the adjustment. And please don't use the freeway for testing as these changes are amplified by speed.