Gary Morrell on Alignment
Thanks to Gary Morrell :
Jim makes some very good points about SHO alignment here, permit me to inject my experiences, some very recently learned:
Just a preface: I've found that just because I can get to certain numbers on MY car, doesn't mean that you'll be able to get them. Some of you have experienced difficulty just getting your alignment centered within the factory specifications. The tolerances in body assembly make it difficult to publish a set of "specs" that everybody can hit. The numbers that Jim provides from the Helm manual are basic factory specs, everybody (hopefully) should be able to hit the nominals. The specs from Doug Lewis are a bit more aggressive and some chassis' may not have enough adjustment range to achieve these numbers.
On Mar 31, 9:37, Jim Cesari wrote:
> Subject: Re: Alignment Specs
> At 05:14 PM 3/30/97 -0500, Tim Pavlik wrote:
> >Hi
> >
> >I was wondering if anyone knows the alignment specs four all four wheels of a 91.
>
> The '91 Helm manual recommends the following:
>
>
> Nominal Minimum Maximum Units
> FRONT
>
> Caster +3.8 +2.8 +4.8 Degrees
> Camber -0.5 -1.1 +0.1 Degrees
> Total Toe -0.2 -0.45 +0.03 Degrees
> -0.1" -0.22" +0.015" Inches
^^^^^^
My experience disagrees with any front toe-out, it tends to make the car darty on the highway, especially with wide tires. I'd opt for neutral or slightly toed-in, per the spec, -0.1" total. Front drive vehicles tend to generate some dynamic toe-in as the driveline forces work on the front suspension bushing compliance.
> REAR
>
> Camber -0.9 -1.6 -0.2 Degrees
> Total Toe +0.12 -0.12 +0.38 Degrees
> +0.062" -0.062" +0.188" Inches
^^^^^^^
Minimum rear toe should be ZERO. Any toe-out, combined with negative camber on the SHO's lightly loaded rear tires will wear the inside edges pronto. Tire wear due to over-camber will be different from toe-out: Too much negative camber will (generally) produce even wear on the tire's inside edge. Toe-out wear will have a scalloped appearance or feeling, with alternate tread blocks wearing high and low, especially noticable on the inside edge. Scalloping can be felt by running your hand over the tread - you'll feel sharp, raised edges on the leading faces of the tread blocks. Some of you will note that this conflicts with some of my earlier posts about alignment, I've since learned that rear toe-out is generally undesirable. Also note that the factory rear toe specs are not centered, they're biased towards toe-in.
A bit of rear toe-in will also counter the Taurus' tendency to toe-steer. (Jeff Sparkman got a real good education on this recently...) This is due to bushing compliance in the tension struts that laterally locate the rear suspension.
During cornering, the soft bushings stretch a bit, allowing the outside rear knuckle to move rearward and toe out. Rear toe-out, if excessive, can also lead to unpredictable oversteer. I have the TCE tubular rear control arms and have experimented with this on the road course and autocross: The addition of some rear toe-out has _dramatic_ and unpredictable effects on handling, the oversteer was very entertaining. At the track, I set up the rear for -2.0 camber and 0.1" toe-in per side.(-0.2" total) The poly-urethane rear tension strut bushings will also help to counter the toe-steer problem.
I talk alot about racing alignments, but the following is street car advice: The alignment of your vehicle needs to reflect the majority of its driving environment, which for most of us is driving straight down the road. Camber is designed to spread the vehicle corner load evenly over the width of the tire tread. Excess negative camber tends to concentrate the straight line load on the tire's inside edge, so unless all your driving time is carving up road courses, which heavily loads the tire's outside during cornering, excess negative camber is going to wear the tire's inside edges prematurely. For cars that are not being regularily driven at the limit, such as frequent autocross or road race, excess negative camber is probably unnecessary, especially on street tires.
Racers add static negative camber to counter McPherson strut camber gain and to help use more of the tread contact patch when the car is being cornered hard. As a car is cornered, weight transfers to the outside tires, tire loads are moving to the outside edge of the tread, and the body roll compresses (bumps) the outside suspension. Due to the geometry of McPherson strut suspensions, there is some camber gain (camber becomes less negative) as the suspension bumps, so the static negative camber that we're using to spread the tire load is being lost as the body roll compresses the suspension. That's why racers dial in as much static negative camber as possible - within limits - excess negative camber can also increase stopping distances as the tire tends to be up on its edge during braking, which can cause loss of braking traction.
> All of the above numbers are good except for the rear camber. If you want
> to get maximum wear out of the tires and you don't do a lot of mountain
> driving/hard cornering/road racing, the rear camber should be between -.25
> and -.5 degrees per wheel. On my '91 I had significant rear inner edge wear
> with the original settings of -.7 and -1.4, with the right being the -1.4
> and showing the most edge wear.
>
> Now the real problem of aligning my '91 SHO...
>
> There was not enough factory adjustment in either the front or the rear to
> bring camber close enough to the preferred nominal specifications. The rear
> had only factory toe eccentric adjuster bushings on the rear, inner control
> arms with not nearly enough range to make any significant adjustment even
> after installing adjustable eccentric bushings on the front, inner arms.
> The front also originally started at -1.3 and -1.6 degrees and after
> drilling the spot welds out, only provided about .5 degrees adjustment per
> side. This got me below the maximum point in the range, but not quite to
> the desired nominal specification of -0.5.
>
> Note:
> Make sure the left to right toe doesn't vary more than .5 degrees.
Very important point here, all the alignment parameters should be balanced side-to-side if possible. Camber imbalance from side-to-side (>0.5 degrees) may cause the vehicle to pull to the side with more negative camber. If caster is negative, which it shouldn't be, or not sufficiently positive, the vehicle may pull to the side with the lower negative camber. For example, if the left rear was toed-in at -0.1" and the right rear was toed-out at +0.1", you'd have a total toe of zero, but the car would pull right because the thrust angle would
be pointing right, instead of parallel to the car's long axis.
Viewed from the top:
/< Thrust angle
Front tires: || ||/
/
/
/
/
Rear tires: // / //
This is a positive thrust angle, as it points away from the steering wheel.
> Four months ago I did the following and so far everything rides and wears
> great:
>
> 1) Drilled the front spot welds out and moved the top of the struts to
> their outer-most points. About .5 degrees positive camber adjustment.
>
> 2) Installed 4 eccentric bushings ordered from Pep Boys ($27.75/pair,
> Northstar #46-131) in the inner position of both forward and rear control
> arms. Again this gave me a max. of about .5 degrees positive camber
> adjustment per side.
>
> If more adjustment is needed the following options are available:
>
> 1) Install the Specialty Products front camber kit #89625 to gain more
> positive camber adjustment.
>
> 2) Install more Pep Boys Northstar eccentric bushings on the outer ends of
> the rear control arms.
>
> 3) Replace the Northstar inner eccentric bushings with the Specialty
> Products #87300 (about $35 for four) bushings mentioned on SHOtimes, which
> would give slightly more adjustment range. Also use the Northstar on the
> outer ends for extra adjustment.
>
> 3) Fabricate some rear control arms with adjustable Heim joint ends similar
> to those that were in the picture on SHOtimes.
Why not just buy them from TCE Performance? What you do with the rear brake bias adjuster is your business.
> One of the most important points to remember when installing eccentric
> bushings in the rear control arms is to make sure the forward and rear
> control arms on each side remain as parallel as possible after adjustment.
> If the forward and rear arm pivot points are not close to the same distance
> from the ground (angle of rotation) at their respective ends, there will be
> a significant toe change as the suspension moves up and down through it's
> range. This will cause the rear of the car to move slightly sideways over
> bumps.
Correct. I've looked at the SHO suspension on an alignment rack as we pulled the suspension into bump by winching the chassis down with a come-along. The toe change is minimal if the control arm pivot points are in-line. This isn't a problem with the TCE arms as the bushings remain centered while the alignment is changed by making the link between the bushings shorter or longer.
Camber gain in bump is another matter, especially on lowered vehicles. As the suspension bumps due to body roll while cornering, the lower control arms force the knuckle's lower ball joint or bushing to move thru an arc that changes the camber curve of the suspension. On a strut suspension vehicle at stock ride
height, the outer ends of the lower control arms are lower then the body attachment points, so as the suspension bumps, the arms effectively get a bit longer, keeping the camber gain very low, or at zero, so we're not losing much, if any, negative camber.
When a vehicle is lowered by shortening the springs, the control arms may end up parallel to the road or even pointing upwards. Now, when the body rolls and bumps the suspension, the control arm arc effectively shortens, pulling in the knuckle and causing the camber to go positive. I measured about 0.3 degree of camber gain per inch of bump on my car. When the camber goes positive, the cornering load on the outside tires is shifted more and more to the already overworked outside of the contact patch, resulting in loss of cornering
traction. Camber gain, if excessive, may also lengthen stopping distances as the suspension dives. Strut suspension vehicles that are lowered excessively may need to relocate the control arm chassis pickup points up and out, (an expensive proposition) to regain a useful camber curve.
There are some benefits in lowering a vehicle to lower its center of gravity, which lowers the body roll center, but if taken to extremes, the negative effects on the suspension geometry will eventually outweigh any handling gains from lowering the Cg.
Gary Morrell
gmorrell@ford.com
