Monday, September 10, 2012

1.3.1 Rigid Axle

2010 Dodge Ram 1500.

Longitudinal leaf springs can absorb both forces in all directions and the drive-off, braking and lateral force moment.

There are many options for attaching a rigid axle rear suspension beneath the body or chassis
frame. Longitudinal leaf springs are often used as a single suspension control arm, which is both supporting and springing at the same time, as these can absorb forces in all three directions as well as drive-off and braking moments. 
Dodge Ram Rear Suspension.

Advantage :
  • They are simple and economical to manufacture;There are no changes to track width, toe-in and camber on full bumper/rebound travel, thus giving low tire wear and sure-footed road holding. There is no change to wheel camber when the body rolls during cornering, therefore there is constant lateral force transmission of tires.
  • The absorption of of lateral force moment by a transverse link,which can be placed at almost any height (Panhard Rod).
  •  Optimal force transfer due to large spring track width, the lateral force compliance steering can be tuned towards under or over steering
Disadvantage :
  • Mutual wheel influence.
  • The space requirement above the beam corresponding to the spring bump travel.
  • Limited potential for kinematic and elastokinematic fine-tuning.
  • Weight - if the differential is located in the axle casing, it produces a tendency for wheel hop to occur on bumpy roads.
Compare this " Rigid Axle Coil Spring Suspension to the above " Leaf Spring Suspension ". We can find that coil spring solid axle suspension needed the trailing arms for (longitudinally) and Panhard Rod (blue link paralleled to the axle) for (laterally).

1. Spring

Left hand axle spring seat provides a vertical spring centerline at full jounce, due to axle pivoting action as it moves vertically, due to the 4 trailing arm instant center, this provides a rising rate in the spring rate leading to a smoother ride.

Right hand axle spring shown provides anti-torque action due to the curved axis of the spring
(note how spring is curved to the rear, compared to the left.) This provides an anti-hop
geometry to the rear suspension.

2. Shock Absorber

Shock angle allows rising rate leverage ratio. This provides a smoother ride and the effort to
increase the jounce increases in proportion to the distance traveled.

3. Panhard Rod (Track Bar)

This allows the track bar to be smaller in diameter, lighter in weight, and more capable of
transferring the lateral loading to the frame.

4. Rear Stabilizer Bar

This is a bad position due to greater flex of frame rail end. The free ends of the bar
should have been " aimed " forward in the vehicle. Being less effective at minimizing roll,
the main bar must be larger in diameter, and thus heavier, than is needed for the loading
applied. By having the bar pointed forward, the suspension design engineer could have had less offset going over the axle pinion, a narrower spread than around the housing and a smaller diameter.

5. Stab Bar End Links

These links are excessively long (but needed for the packaging) for the compressive and
tension loading the bar will see in this position. As stated on the bar comments, these links
need to be attached to the forward axle position to shorten the links and provide a more efficient bar.

6. Control Arms

The UCA (Upper Control Arm) links appear to be splayed outward at the frame attachment points, providing a lateral stability to the system that, in conjunction with the track bar (or "panhard rod") keeps the lateral shift of the axle, between jounce and rebound (maximum travel up and maximum travel down- NOT "bounce" and "droop"), to the minimum arc possible. Due to the over-constrained system (more about this in a moment) the axle will travel laterally in the vehicle, through an arc of approximately 2" total, left to right. The positioning of the track bar ensures that the travel will also be split evenly from jounce to rebound, minimizing the dreaded "head toss" so prevalent in the Jeep XJ (Cherokee SUV), MJ (Comanche pickup), and early ZJs.

Panhard Rod 

While the purpose of the suspension of an automobile is to allow the wheels to move vertically
with respect to the body, it is undesirable to allow them to move forward and backwards (longitudinally), or side to side (laterally). The Panhard rod is designed to prevent lateral movement.[2] Not to be confused with a traction bar which controls axle wrap and suspension loading. The track bar or Panhard bar is a simple device, consisting of a rigid bar running sideways in the same plane as the axle, connecting one end of the axle to the car body or chassis on the opposite side of the vehicle. The bar is attached on either end with pivots that permit it to swivel upwards and downwards only, so that the axle is allowed to move in the vertical plane only. This does not effectively locate the axle longitudinally, therefore it is usually used in conjunction with trailing arms which stabilize the axle in the longitudinal direction.
This arrangement is not usually used with a leaf spring suspension, where the springs themselves supply enough lateral rigidity, but only with coil spring suspensions.

The blue linkage (paralleled to the solid axle) is the Panhard Rod using to control the
lateral movement.

Ford F-150 Frame and Chassis.
Thanks to highly developed suspension parts and the appropriate design of the springing
and damping, it has been possible to improve the behavior of rigid drive axles. Nevertheless,
they are no longer found in standard-design passenger cars, but only on four-wheel drive and special all-terrain vehicles.
Rear axle on the VW LT light commercial vehicle.

The long, parabola-shaped rolled-out, dual leaf springs cushion the frame well and are progressive. The rubber buffers of the support springs come into play when the vehicle is laden. Spring travel is limited by the compression stops located over the spring centers, which are supported on the side-members. The spring are prevented from shifting against one another by the spring clips located behind them, which open downwards.

The anti-roll bar is fixed outside the axle casing. The shock absorbers, however,
are fortunately located a long way to the inside and are also angled forwards so that
they can be fixed to the frame side-members.

Mutual influence of the two wheels of a rigid axle when traveling along a road with pot-holes, shown as mutually-opposed springing. One wheel extends along path S2 and the other compresses along path S1.

Jeep Wrangler Rubicon.

Because of its weight, the driven rigid axle is outperformed on uneven roads
(and especially on bends) by independent wheel suspension, although the deficiency in road-holding cab be partly overcome with pressurized mono-tube dampers. There are more expensive, but on the compressive stroke, the valve characteristic can be set to be harder without a perceptible loss of comfort.

The rear axle in a Ford Escort Express delivery vehicle.

Single leaf springs carry the axle support the body well at four points. The shock absorbers
(fitted vertically) are located close to the wheel, made possible by slim wheel-carrier/hub units.
The additional elastomer springs sit over the axle tube and act on the side members of the
body when at full bump.

Longitudinal rear leaf springs support the body of a car in two places-under the back seats and under the boot - with the advantage of reduced bodywork stress. This economical type of rear suspension also has the advantage that load area on trucks and the body of passenger cars can be supported in two places at the back: at the level of the rear seat and under the boot.
The longitudinal leaf springs can be fitted inclined, with the advantage that during cornering
the rigid rear axle (viewed from above) is at a small angle to the vehicle longitudinal axis. 
To be precise, the side of the wheel base on the outside of the bend shortens somewhat, while
the side on the inside of the bend lengthens by the same amount. The rear axle steers into the
bend and in other words, it is forced to self-steer towards " roll-understeering ".

This measure can, of course, have an adverse effect when the vehicle is traveling on bad roads,but it does prevent the standard passenger car's tendency to oversteer when cornering. Even driven rigid axles exhibit - more or less irrespective of the type of suspension - a tendency towards the load alternation (torque steering) effect, but not to the same extent as semi-trailing link suspensions. 

1 comment: