[DeTomaso] Pantera Steering
Will Kooiman
will.kooiman at gmail.com
Sun Feb 23 22:07:24 EST 2014
For what it's worth, I did the 7" to 8" Campy exchange on my car several
years ago.
The 7" Campys jerked the steering wheel all over the place. After I went
to 8" Campys (in the front), it was gone. It was a completely different
car.
On 2/23/14 9:43 PM, "jderyke at aol.com" <jderyke at aol.com> wrote:
> 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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>
> 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.
>[1]mailto:detomaso%40poca.com?Subject=Re%3A%20%5BDeTomaso%5D%20Pantera%20S
>teering&In-Reply-To=%3C89cfe.6068fc2.403b9f14%40aol.com%3E
> 2.
>[2]http://poca.com/pipermail/detomaso_poca.com/2014-February/224636.html
> 3.
>[3]http://poca.com/pipermail/detomaso_poca.com/2014-February/224644.html
> 4.
>[4]http://poca.com/pipermail/detomaso_poca.com/2014-February/date.html#224
>643
> 5.
>[5]http://poca.com/pipermail/detomaso_poca.com/2014-February/thread.html#2
>24643
> 6.
>[6]http://poca.com/pipermail/detomaso_poca.com/2014-February/subject.html#
>224643
> 7.
>[7]http://poca.com/pipermail/detomaso_poca.com/2014-February/author.html#2
>24643
>
>_______________________________________________
>
>Detomaso Forum Managed by POCA
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>
>References
>
> 1.
>mailto:detomaso%40poca.com?Subject=Re%3A%20%5BDeTomaso%5D%20Pantera%20Stee
>ring&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#2246
>43
> 6.
>http://poca.com/pipermail/detomaso_poca.com/2014-February/subject.html#224
>643
> 7.
>http://poca.com/pipermail/detomaso_poca.com/2014-February/author.html#2246
>43
> 8. mailto:DeTomaso at poca.com
> 9. http://poca.com/mailman/listinfo/detomaso_poca.com
>_______________________________________________
>
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