Late Model Mustang Suspension Basics


Wheels

Wheels are characterized by their diameter, width, bolt pattern, bolt circle, offset, and back spacing.   For example, a rim might be referred to as 14x7 (14" diameter and 7" wide) with a 5 x 4.75 bolt pattern (5 bolts in a 4.75" diameter circle) and back spacing or rear spacing of 4".   Some vendors use back spacing (always a positive number), some use offset (may be positive or negative).

   

What is the size of the wheel on your car?   If it is a stock Ford wheel you should be able to find a size indicator cast or forged into the back of the wheel.   Above on the left is the indicator on a cast aluminum wheel that is 15"x7" with 24mm offset, and on the right a forged aluminum wheel that is 16"x7.5" with 30mm offset.

  

The above diagrams illustrate how offset can be measured even with a tire on the rim, and how the bolt circle is measured on 4- and 5-bolt axles and rims.


Wheel Alignment


Camber is the angle at which the tire is tilted in or out, as viewed from the front of the car.   Ideally the tire should be as near vertical (zero camber) as possible under all driving conditions.   However, some negative camber is desireable in hard turns to offset the tendancy of the inside tire to roll under.   As noted above,   the Mustang's MacPherson strut suspension doesn't do a very good job of producing this negative camber in turns.   That's why Mustangs used for autocross are often set up with about 3 degrees (or more) of static negative camber.   But if that same negative camber setting is used on the street, it will produce unstable handling and uneven tire wear.   Ford recommends degree negative camber on the 2000 Mustang.  

Caster is the angle (in degrees) between the steering axis (the strut) and true vertical.   In the illustration above, note that the tire contacts the ground at a point behind the point where the steering axis hits the ground.   When the car is moving this generates a self centering force, which causes the steering wheel to return to straight-ahead when you release it after a turn.   The greater the caster angle, the heavier the steering will be at low speeds, but the more stable the car will feel when traveling in a straight line at high speeds.   2000 Mustangs come from the factory with caster angle fixed at about 3.2 degrees.   At 4 degrees caster or less, you will gain positive camber when the wheel is turned (not good).   More than 4 degrees of caster will produce negative camber gain when the wheel is turned (good).   People often set up their cars with more than 4 degrees of positive caster for that reason.   Five or six degrees of caster is optimal on a street-driven 94+ Mustang.

Cars driven on highly crowned roads are often set up with cross caster or caster lead.   The left front wheel is given about 1/2 degree less positive caster than the right front wheel.   This makes the steering pull very slightly toward the side with less caster.   That slight pull compensates for the crown and helps keep the car going straight down the road.   Cross caster is also used on cars set up for oval track racing, so that they turn left with minimum tire scrub.   A car set up for open track racing or autocross (flat road surface & turns in both directions) would probably be set up with equal caster on both sides.

Toe-in & Toe-out. Toe-in keeps the car going straight, improving high-speed stability.   Toe-out produces wandering and high-speed instability.   Too much of either one will cause excessive tire wear.   When the car is in motion, drag forces create a natural tendency for tires to toe-out.   For that reason, you want to set up the front suspension with just enough static toe-in so that the tires do not go into toe-out at high speeds.   Ford recommends 1/4 degree of   toe-in on 2000 Mustangs.


Adjust Toe In/Out by changing the length of the front steering tie-rods, as illustrated above.

 

Adjust Caster & Camber by moving the top mounting point of the front strut, as illustrated above.   Camber is decreased by moving the top of the strut toward the center of the car (negative camber) and increased by moving it toward the outside of the car (positive camber).   Caster (always positive) is increased by moving the top of the strut toward the rear of the car.

Note that the Mustang's strut towers do not have slots to permit caster adjustment.   If you want to change the factory caster setting, you need to replace the stock plates on the top of your strut towers with aftermarket caster/camber plates (illustrated below) that permit adjustment in two directions.  These aftermarket plates also provide a wider range of camber adjustment than can be obtained using the factory setup.   The additional adjustment range can be used to add negative camber when setting up the car for autocross or open track racing, or to bring the suspension back into alignment after you have lowered the car with aftermarket springs.   Some caster/camber plates permit mounting the strut a bit higher in the strut tower than the stock plates would permit, thereby keeping the strut in its normal operating range on a lowered car.  Good quality caster/camber plates are usually double adjustable (two plates move at right angles to one another) and include a spherical bearing to replace the factory rubber bushing at the top of the strut tower.

             

If all you need to do is increase the range of camber adjustment and you can't handle the expense of aftermarket caster/camber plates ($150-$300), there is a cheaper alternative.   For about $30 you can buy and install a set of camber correcting bolts aka "cam bolts" or "crash bolts" (illustrated above) which have eccentric shafts or heads that can be turned to increase or decrease camber by moving the bottom of the strut in or out.

Other, even cheaper, approaches exist.   You can file a slot in the strut flange or use a bolt that is slightly smaller in diameter than the stock bolt to allow similar in/out strut adjustment.   Obviously, the disadvantage of these methods is that they reduce the strength of the strut/spindle mounting point.   Caster/camber plates do not weaken the strut mount.  

Wheel Alignment is usually offered in three different packages.

  1. Standard alignment - they align the front wheels to the vehicle's centerline, without reference to the rear wheels.
  2. Thrust alignment - they adjust the front wheels only, however the alignment equipment references the rear wheels and aligns the front wheels to the rear thrust line. The front wheels are adjusted to spec in the position they will be traveling straight down the road.
  3. Four wheel alignment - they adjust all four wheels to a common centerline. Most front wheel drive vehicles are designed to be four wheel aligned.

You want either a standard alignment or a thrust alignment. Your car cannot be four-wheel aligned because it has no adjustment in back.


Books


This article was written for new Mustang owners who want to gain a little knowledge about their car's suspension.  Some of the statements fail to reflect the true complexity of the issues discussed.   That was intentional.  I'm just trying to impart some basic information.   Comments and corrections are welcome.  AZ2KVert  (Bill Wenger) email: miracerros@comcast.net


Last Updated: 13 Jan 08   http://miracerros.com/mustang