[DeTomaso] Pantera Steering
David Nunn
dnunn at telus.net
Sun Feb 23 20:04:00 EST 2014
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 ½” 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
----------------------------------------------------------------------------
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[DeTomaso] Pantera Steering
SOBill at aol.com SOBill at aol.com
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ing&In-Reply-To=%3C89cfe.6068fc2.403b9f14%40aol.com%3E>
Sun Feb 23 11:59:33 MST 2014
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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 --------------
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;
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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