[DeTomaso] Pantera Steering

Sun Feb 23 21:43:44 EST 2014

David, all that and more can be found on Ted Mitchell's web site <www.tmich.com> under 'Pantera Info'. Decades ago, Ted took a '71 Pantera apart and measured everything you could think of. Then he entered the data into a pro racing suspension program that calculates things which cannot be directly measured.

Most people concerned with bump-steer and scrub radius are running low-profile near-max tire sizes. The scrub radius is the difference between the wheel's pivot point at the ground and a line drawn thru the upper and lower ball joint pivots at the ground. The ideal scrub radius for any car is zero. An 8" Campy mounted on a Pantera front spindle gives a central pivot-point almost exactly in the center of its wheel, thus close to or at zero.  A 7" Campy is offset outboard by the 1" difference in backspace between 7" & 8" Campy wheels (4.25" vs 5.25"). So to your question: yes, an 8" x 15 Campy reduces the scrub radius of a car with 7"x 15"  Campys by 1".

To use longer a-arms such as the Pantera 90's approx. 4" extra, you'd probably have to first adjust the wheel offsets to keep the tires withiin the body lines, then somehow change the front & rear spindle inclinations, the ball joint mounts or the inner a-arm attach points to the chassis. I have no measurements on how DeTomaso did this for the last few Panteras made, but the 17" factory wheels used obviously have very large negative offsets (toward the outside). Whether this was enough or other adjustments were needed is unknown. Chassis geometry is a fascinating study. 

 J DeRyke

-----Original Message-----
From: David Nunn <dnunn at telus.net>
To: detomaso <detomaso at poca.com>
Cc: SOBill <SOBill at aol.com>
Sent: Sun, Feb 23, 2014 5:04 pm
Subject: Re: [DeTomaso] Pantera Steering

   Bill,

   I've never heard the actual specification for the Pantera's scrub
   radius (3.5" with a stock 7.0" Campy). What tire is this with? My
   understanding is, the height of the tire has an effect on scrub radius.

   Would a 1" increase in wheel offset result in an equal reduction in
   scrub radius? If so, going from a 7" Campy to an 8" Campy would reduce
   the scrub radius by 1/2" because that's the difference in offset
   between the two wheels.

   What would the ideal scrub radius be for a Pantera? Are there any
   suspension geometry related drawbacks to fitting longer upper and lower
   control arms? I assume it would change the height of the roll center
   but would that change be significant? If you fitted longer control arms
   at the front would you need to make a similar modification at the rear?

   Dave

   -----------------------------------------------------------------------
   --------------

   [DeTomaso] Pantera Steering

   SOBill at aol.com [1]SOBill at aol.com
   Sun Feb 23 11:59:33 MST 2014
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       author ]

   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

References

   1. mailto:detomaso%40poca.com?Subject=Re%3A%20%5BDeTomaso%5D%20Pantera%20Steering&In-Reply-To=%3C89cfe.6068fc2.403b9f14%40aol.com%3E
   2. http://poca.com/pipermail/detomaso_poca.com/2014-February/224636.html
   3. http://poca.com/pipermail/detomaso_poca.com/2014-February/224644.html
   4. http://poca.com/pipermail/detomaso_poca.com/2014-February/date.html#224643
   5. http://poca.com/pipermail/detomaso_poca.com/2014-February/thread.html#224643
   6. http://poca.com/pipermail/detomaso_poca.com/2014-February/subject.html#224643
   7. http://poca.com/pipermail/detomaso_poca.com/2014-February/author.html#224643

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-------------- next part --------------
   David, all that and more can be found on Ted Mitchell's web site
   <www.tmich.com> under 'Pantera Info'. Decades ago, Ted took a '71
   Pantera apart and measured everything you could think of. Then he
   entered the data into a pro racing suspension program that calculates
   things which cannot be directly measured.
   Most people concerned with bump-steer and scrub radius are running
   low-profile near-max tire sizes. The scrub radius is the difference
   between the wheel's pivot point at the ground and a line drawn thru the
   upper and lower ball joint pivots at the ground. The ideal scrub radius
   for any car is zero. An 8" Campy mounted on a Pantera front spindle
   gives a central pivot-point almost exactly in the center of its wheel,
   thus close to or at zero.  A 7" Campy is offset outboard by the 1"
   difference in backspace between 7" & 8" Campy wheels (4.25" vs 5.25").
   So to your question: yes, an 8" x 15 Campy reduces the scrub radius of
   a car with 7"x 15"  Campys by 1".
   To use longer a-arms such as the Pantera 90's approx. 4" extra, you'd
   probably have to first adjust the wheel offsets to keep the tires
   withiin the body lines, then somehow change the front & rear spindle
   inclinations, the ball joint mounts or the inner a-arm attach points to
   the chassis. I have no measurements on how DeTomaso did this for the
   last few Panteras made, but the 17" factory wheels used obviously have
   very large negative offsets (toward the outside). Whether this was
   enough or other adjustments were needed is unknown. Chassis geometry is
   a fascinating study.

   J DeRyke

   -----Original Message-----
   From: David Nunn <dnunn at telus.net>
   To: detomaso <detomaso at poca.com>
   Cc: SOBill <SOBill at aol.com>
   Sent: Sun, Feb 23, 2014 5:04 pm
   Subject: Re: [DeTomaso] Pantera Steering
   Bill,

   I've never heard the actual specification for the Pantera's scrub
   radius (3.5" with a stock 7.0" Campy). What tire is this with? My
   understanding is, the height of the tire has an effect on scrub radius.

   Would a 1" increase in wheel offset result in an equal reduction in
   scrub radius? If so, going from a 7" Campy to an 8" Campy would reduce
   the scrub radius by 1/2" because that's the difference in offset
   between the two wheels.

   What would the ideal scrub radius be for a Pantera? Are there any
   suspension geometry related drawbacks to fitting longer upper and lower
   control arms? I assume it would change the height of the roll center
   but would that change be significant? If you fitted longer control arms
   at the front would you need to make a similar modification at the rear?

   Dave

   -----------------------------------------------------------------------
   --------------

   [DeTomaso] Pantera Steering

   SOBill at aol.com [1]SOBill at aol.com
   Sun Feb 23 11:59:33 MST 2014
     * Previous message: [2][DeTomaso] Value of rebuilt shocks
     * Next message: [3][DeTomaso] Flex-a-lite question
     * Messages sorted by: [4][ date ] [5][ thread ] [6][ subject ] [7][
       author ]

   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;