Off Panhard bar

Gary Derian gderian at oh.verio.com
Sat Mar 13 21:20:53 GMT 1999


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.

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.

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.

A four link rear suspension, where the links are 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.  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>

>
>>Or go scrounging in a boneyard that has some old Alfas. They used an upper
>>triangle, with two pivot points on the chassis, and a BALL joint attached
>>just to the left side of the pumpkin, plus two lower trailing arms.
>>Particularly if you replaced the chassis pivots of the triangle (rubber
>>stock) with bronze on steel, also if you replaced the lower rod ends with
>>real rod ends, they were as NICE a live rear axle setup as you could want.
>
>(Lurk mode off)
>
>It's nice to have something to say !
>
>I disagree.
>
>Any assymmetry of the linkage will cause the car to react differently in
>right hand turns than it does in left hand turns. Secondly, because the
>A-arm is above the axle, and it determines the roll center heighth, the
roll
>center is high. In addition, since the a-arm components must absorb all
>cornering forces they are subjected to extreme stress, hence the ball
>joint.
>
>This could be improved by placing the joint in the center below the
pumpkin,
>providing symmetry and lowering the roll center, or better  yet , replacing
>the a-arm with two angled arms and eliminating the need for a high strength
>joint. 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.
>
>>Another very nice set-up was the old Rover 2000 Di Dion setup--instead of
a
>>lateral link, they used half shafts with no slip joint, upper and lower
>>arms on each side, and a slip joint in the Di Dion tube.
>
>Huh?
>
>Am I missing something here? I've read this three times and it still
doesn't
>make sense.
>
>It sounds like they added an upper arm to a Jaguar set up to keep the De
>Dion tube from pulling the side gears out of the differential on a hard
>turn. With no plunge in the half shaft, the half shaft and the upper and
>lower
>arms would have to be parrallel and equal legnth. This would cause the
tires
>to move laterally with jounce, hence the slip joint in the De Dion tube.
I'm
>sure that this scrubing of the tires can't be all that good to begin with,
>but imagine that we want to turn, and the body rolls to the outside as it
is
>wont to do. Because of the equal legnth control arms the tires lose camber
>angle, and cornering force. All the while the De Dion is trying to further
>unload the inside tire, and prolly binding up in the process.
>
>I don't mean this to be an attack on these two examples. I think the point
>is that cars roll as well as bounce. Even a Watts linkage with the
bellcrank
>attached to the axle will cause the axle, or the car, to move laterally if
>the car is in roll when it bounces (but it won't if you attach the
bellcrank
>to the chassis). It is prolly more important to worry about roll steer than
>lateral movement of the axle. Try driving your car backwards at any speed
>above a crawl.
>
>Another good source of info on four links is Dave Morgan. He wrote a series
>of articles for National Dragster named Pitside with (you guessed it) Dave
>Morgan and at least one book titled Doorslammers IIRC. Lots of
illustrations
>of instant center, anti-squat, center of gravity, etc. calculations.
Written
>for the drag racer, but applicable to a road car also.
>
>HTH
>
>Joe
>
>(lurk mode on)
>
>
>
>




More information about the Diy_efi mailing list