Off Panhard bar

[H. J. Zivnak]

Hello Gary,

Gosh, I post one time in a year and I start a discussion.

>There is nothing wrong with a high roll center on a live axle.  Of course
>the suspension tuning must take this into account.  With independent
>suspension, a high roll center usually also means high camber change and
>track change with wheel travel.  This results in jacking in a corner which
>is really bad, witness early Corvair rear, early VW Beetle rear, or Bronco
>II front.  A solid axle has no jacking and is usable with high roll center.


I agree a solid axle has no jacking. However, a lower rear roll center will
make a car handle more consistently even with a solid axle. Given a choice,
I would opt for a low roll center.

>A DeDion suspension is a dead axle connecting each rear hub.  This axle is
>located by various rods as is a live axle.  The final drive is mounted to
>the frame and halfshafts connect to the hubs.  The advantage is the
>kinematics of a live axle with low unsprung weight.  Usually the half
shafts
>have to change in length to accommodate suspension travel.  In the Rover,
>the dead axle is allowed to change in length while the halfshafts take the
>cornering loads.  I guess they thought one plunge joint is cheaper than
two.

If the dead axle is allowed to change length, then the axle track dimension
(centerline of tire to centerline of other tire) changes with it. If the
axle track dimension changes then the tires scrub laterally.

Remember, this thread started when Shannen asked about lateral movement of
the axle with a Panhard bar. With a Panhard the axle is not likely to move
laterally, the body will. In the Rover, at least one tire must scrub.
>
>Having an A arm locating the rear axle like the old Alfas works pretty
well.
>Having it offset a little actually compensates for the torque reaction of
>the axle.  The first generation Mazda RX-7 had an offset Watts linkage
which
>worked pretty well.

???
An a arm is not a Watts linkage. A Watts linkage resembles a Panhard bar
with a bellcrank in the center. The bellcrank should be attached to the
chassis and the two rods attached to either end of the axle. The rods are
parallel to the axle in their normal position.

>
>A four link rear suspension, where [two of] the links are [at] angles to
provide lateral
>location can only work if the links have rubber joints. Also the links have
>to locate the axle as well as control the torque.

How is this different from the a arm?

For high powered cars,
>you cannot get sufficient torque control (resistance to wheel hop) and bump
>isolation at the same time.  This is OK for race cars but makes for a poor
>road car.  A very good solution is a torque arm for torque, a trailing arm
>on each side for bump isolation and bump steer control, and a panhard rod
or
>watts link for lateral location.  Just like a 1976 Vega and used since 1982
>on the Camaro and Firebird.  Volvo also had a good solution on the 700
>series sedan.  The torque arm was replaced by two rods (half a 4 link
>mounted in the middle) which mounted to a subframe.
>
>Gary Derian <gderian at oh.verio.com>
>
You haven't thought this out. With a four link you can vary the angle of the
upper and lower links realitive to each other in side view to vary the
location of the instant center. The distance from the instant center to axle
is the length of the effective arm the axle is attached to. This arm is
infinitely long when the links are parallel. Your trailing arm length is
12"? 18"?

If I choose, I can place the instant center in side view on an imaginary
line from the contact point of the rear tire to the intersection of the
height of the center of gravity and the centerline of the front axle and
have 100% anti-squat. I will have more rear wheel traction for leaving at a
light or coming out of a turn and this means I can put more power to the
road. The only wheel hop problem I have with the four link is brake hop if I
make the effective arm length too short chasing too much anti-squat.



Come to think of it, you could reverse the angle of the arms,
>>attaching at the ends of the axle housing instead of the pumpkin and near
>to
>>the longitudinal center line at the chassis. Now you have a four link
which
>>provides lateral control and eliminates the need for a Panhard  or Watts.

By the way, looked it up, this is called a Satchell Link. According to Herb
Adams in Chassis Engineering, the advantages of a Satchell are that it
provides considerable anti-squat with roll understeer and a low roll center.


Regards,

Joe