[DeTomaso] Pantera Steering

[SOBill at aol.com]

Julian,
 
 
To me, an objectionable aspect of driving a Pantera is the  steering effort 
required and the severe steering wheel kickback from potholes,  bumps, and 
train tracks.
 
The steering rack has nothing to do with these problems.
 
The cause of both of these problems is the excessive scrub radius of the  
front suspension geometry.
 
The line connecting the front upper ball joint and the  front lower ball 
joint defines the steering axis of the front suspension.  The front wheel 
rotates about this line when the steering wheel is turned. If  you extend the 
steering axis to ground level and then measure the  sideways distance from the 
steering axis at ground level to the center  of the tire contact patch you 
will have the "scrub radius" of the steering.  If the center of the tire 
contact patch is at the spot where the steering axis  meets the ground, you 
have zero scrub radius. If the tire contact patch is not  coincident with the 
ground level steering axis point, a lever arm is created  between the tire 
contact patch and the steering axis. This lever arm has two  effects: 1) When 
turning the steering wheel the tire will be dragged side ways  around the 
steering axis; 2) Any road impact at the tire contact patch will,  thru the 
scrub radius lever arm, be transmitted as a twist to the steering  wheel.
 
Effect 1) causes "heavy" steering because you are actually dragging the  
tire sideways when you are steering and effect 2) causes kickback in the  
steering wheel from road irregularities.
 
Most rear wheel drive cars have around 1 inch of scrub radius for  "road 
feel." Front wheel drive cars have nearly zero scrub radius to prevent  
"torque steer" caused by the traction of the driving wheels acting on the scrub  
radius lever arm when the car is accelerating. The Pantera has 3.5 inches  of 
scrub radius. 
 
If you doubt that the scrub radius is the cause of these problems, install  
the 8 inch Campys, which have a greater inboard offset than the stock 7 
inch  Campys, on the front. You will notice a very real reduction in steering 
effort  even though the scrub radius has been reduced by less than an inch. A 
one inch  reduction in scrub radius would be around a 30% improvement.
 
The only true cure for this problem is to reduce the Pantera scrub radius  
by moving the tire contact patch inboard with respect to the steering axis.  
Unfortunately, there is no simple way to do this. Installing wheels with  
greater inboard offset (which must still clear the brake mechanism) will move 
 the tire contact patch inboard. The stock Pantera chassis is already  very 
close to the inboard edge of the tire at full steering lock. Moving the  
tire inboard a couple of inches will cause the tires to  rub before full 
steering lock and will look peculiar with the tires  recessed into the fender 
wells. 
 
The solution to the reduced steering lock and the tires being recessed  
into the fender well when reducing the scrub radius is to move the wheel and  
steering axis outboard by installing longer upper and lower a-arms. Longer 
upper  and lower a-arms will then require longer steering tie rods and a 
longer front  roll bar.
 
Another solution is to mask the problem by installing power steering.
 
 
The main  thing is to keep the main thing the main thing.
Do the best you can with what  your have where you are.
Have fun  today!
SOBill
-------------- next part --------------
   Julian,





   To me, an objectionable aspect of driving a Pantera is the steering
   effort required and the severe steering wheel kickback from potholes,
   bumps, and train tracks.



   The steering rack has nothing to do with these problems.



   The cause of both of these problems is the excessive scrub radius of
   the front suspension geometry.



   The line connecting the front upper ball joint and the front lower ball
   joint defines the steering axis of the front suspension. The front
   wheel rotates about this line when the steering wheel is turned. If
   you extend the steering axis to ground level and then measure the
   sideways distance from the steering axis at ground level to the center
   of the tire contact patch you will have the "scrub radius" of the
   steering. If the center of the tire contact patch is at the spot where
   the steering axis meets the ground, you have zero scrub radius. If the
   tire contact patch is not coincident with the ground level steering
   axis point, a lever arm is created between the tire contact patch and
   the steering axis. This lever arm has two effects: 1) When turning the
   steering wheel the tire will be dragged side ways around the steering
   axis; 2) Any road impact at the tire contact patch will, thru the scrub
   radius lever arm, be transmitted as a twist to the steering wheel.



   Effect 1) causes "heavy" steering because you are actually dragging the
   tire sideways when you are steering and effect 2) causes kickback in
   the steering wheel from road irregularities.



   Most rear wheel drive cars have around 1 inch of scrub radius for "road
   feel." Front wheel drive cars have nearly zero scrub radius to prevent
   "torque steer" caused by the traction of the driving wheels acting on
   the scrub radius lever arm when the car is accelerating. The Pantera
   has 3.5 inches of scrub radius.



   If you doubt that the scrub radius is the cause of these problems,
   install the 8 inch Campys, which have a greater inboard offset than the
   stock 7 inch Campys, on the front. You will notice a very real
   reduction in steering effort even though the scrub radius has been
   reduced by less than an inch. A one inch reduction in scrub radius
   would be around a 30% improvement.



   The only true cure for this problem is to reduce the Pantera scrub
   radius by moving the tire contact patch inboard with respect to the
   steering axis. Unfortunately, there is no simple way to do this.
   Installing wheels with greater inboard offset (which must still clear
   the brake mechanism) will move the tire contact patch inboard. The
   stock Pantera chassis is already very close to the inboard edge of the
   tire at full steering lock. Moving the tire inboard a couple of inches
   will cause the tires to rub before full steering lock and will look
   peculiar with the tires recessed into the fender wells.



   The solution to the reduced steering lock and the tires being recessed
   into the fender well when reducing the scrub radius is to move the
   wheel and steering axis outboard by installing longer upper and lower
   a-arms. Longer upper and lower a-arms will then require longer steering
   tie rods and a longer front roll bar.



   Another solution is to mask the problem by installing power steering.





   The main thing is to keep the main thing the main thing.
   Do the best you can with what your have where you are.
   Have fun today!
   SOBill