Comment 5820

By the_experience03 (anonymous) | Posted March 11, 2007 at 19:40:25

You are CLEARLY not an offroader. Live axles front and rear reign supreme offroad. This is not anecdotal or opinion. It is fact.

I know you're going to come back at me with the HUMVEE example so let me dispel that one right away.

The Humvee uses a very unique independent suspension design. It uses heavy planetary gear boxes at the wheels which multiply the torque downstream of vulnerably weak constant velocity joints. These joints are necessary on independent front suspensions while the relative lack of compound angles allows regular carden style shafts to be used in the rear.

The HUMVEE suspension is heavy. Everything about it is overbuilt (yet it still has many failures). This is to give it enough weight to sink in water and it's down low to make it more stable (read: easier for the untrained to drive). Indeed, the US Military has made great strides towards dumbing down their equipment over the last 25 or so years. Automatic transmission, torsen planetary gear style limited slip differentials, etc are meant so that less training is required.

Still, the HUMVEE suffers from many of the offroading shortcomings of independent style suspensions.

Shortcomings of independent suspensions offroad:

-Changing ground clearance

Sure, differentials hang low on solid axle rigs, but they are built stronger. Most are housed in cast iron while independent suspensions typically use an aluminum housing. The Jeep KJ Liberty, for example, has a Dana 35 based front differential in aluminum while the TJ Wrangler has a cast iron center section Dana 30 front axle. The cast iron is stronger and less prone to deflection from impact than aluminum.

You also know where the differentials are in a solid axle rig. You learn this early on. You simple drive the tires over the highest point such as a rock jutting up on the trail. The ground clearance remains relatively unchanged. However, with independent suspension, the differential bottom is not constant with the centerline of the wheel. As one side of the suspension is compressed, the differential's location relative to the center of the wheel drops, thus reducing ground clearance. It is difficult to determine what your ground clearance actually is at any given suspension compression point.

Not to worry, though. The differentials are cradled in a weak stamped steel cradle that will likely break long before the differential itself is damaged by a rock, right?

-Lack of wheel travel and articulation

Wheel travel is just what it sounds is the amount a wheel can travel up or down. It is determined by any number of factors, but the limiting factor is often the length of the locating system for the axle. With coil springs this means the control arms and leaf springs, the springs themselves are the locating members.

Control arms on independent sprung vehicles are limited in length by the engine. Unless you want an engine that sits WAY up, the arms must be short enough to clear the block and oil pan. This means reduced travel. The longitudinal arms of a solid axle sprung vehicle are limited only by wheelbase.

The typical independently sprung rig is limited to about 7 inches of actual wheel travel while only typically utilizing 5 inches of this. That is up and down travel combined. Without wheel travel, you will not have traction because tires will not stay in contact with the road.

Articulation is the amount of height that a front right tire, for example, can vary from a right rear tire. In our previous example with IFS having limited wheel travel of 7 inches, this means with the front tire fully stuffed and the rear tire fully drooped we have a whole 7 inches of articulation. This will lead to picking up a tire, which in turn causes a loss of traction and stability. Not cool.

With a solid axle, for every inch a tire on one side moves up, the tire on the opposite side of the axle will move down an amount. this gives far better articulation.

I suggest you look up "RTI ramp." RTI stands for ramp travel index and is a measure of articulation. A score of 1000 on a 20 degree ramp means that a vehicle can drive a front tire on one side of the vehicle up the ramp all the way until the rear tire on the same side touches the ramp without picking up any other tires. Please show me an independently sprung vehicle that can score a 1000 or even a 700 (70% of the wheelbase before picking up a tire).

CV joints are necessary on independently sprung vehicles. It is unavoidable. In straightline travel they are generally stronger than a carden style joint as you would find with a solid axle style vehicle because they have more points of contact. A carden joint has two while a typical ball bearing style CV will have 6. However, when you turn, a carden joint will still have 2 points of contact while a CV joint will often be reduced to a singular point of conact (on a single ball) with less surface area. The result is breakage.

Another point about strength is in frame attachment points. With an independent suspension your contact points are all close together and towards the front and rear extremes of a frame. With leaf springs the frame contact points are well spread apart and spaced at the ends and the middle of the frame. Control arms with a solid axle coil sprung rig put the frame contact points on the bottom of the frame rather than the sides of the webbing. The moment of inertia is greater on this plane making the mounting points stronger. The brackets are also closer to the center of the frame, thus reducing the torque moment on the frame. With a parallel 4 link with panhard bar such as the system in the rear of the TJ Wrangler, the lateral and longitudinal forces are even directed to different points on the frame to reduce stresses.

Many independently spring vehicles actually use combination motor mounts and suspension hard points. My Toyota truck prior to the solid axle swap is a fine example. How foolish! All the stresses from both the suspension and engine are directed to a single node. If independent suspensions and unibodies are so strong, I have to wonder why subframe connectors between front and rear suspension hard points and across the strut towers in the front are so popular. There must be a reason why there is an aftermarket for parts to spread the load of the suspension across more points.

I could go on and on, but I think you're getting my point.

Your comment about unibodies being superior offroad blew me away. The only unibody rig I can think of that is even remotely popular in the offroad community is the Jeep XJ Cherokee and there are a host of aftermarket goodies to strength its weak unibody. Please, set me straight and give me some examples of superior unibody offroad vehicles because I am at a loss.

I do not wish to contend your points about the foolishness of the vast majority of truck and SUV owners out there. However, I cannot stand by idly while you contend points that are simply wrong about the engineering of the vehicles for the purpose they were intended.

I can't help but wonder if your opinions of these vehicles in heavy duty towing and offroad applications, the very setting they were intended for, is simply based on theory. I have experience both with IFS and solid axle vehicles both on and offroad. They both have their place. The unibody, independently sprung vehicle does not belong offroad, atleast not in its current form.

I implore you to do some research. Look at various offroading websites. Open the pages of a magazine dedicated to the hobby. You will quickly see that what I am saying is not simply the opinion of one man. For real kicks, research Walker Evans' offroad buddy. He tried an experimental rig with swing arm style independent suspension, but finished the season with Dynatrac Dana 60's to try and salvage some points in his professional rockcrawling season.

I look forward to your rebuttal as I'd truly like to understand where you are coming from. I respect many of your points, but fail to see the basis for opinion in others. Please let me know what your background is in the field. It will better help me understand where you are coming from. If you have desert raced, for example, I can see where you're coming from on your independent suspension arguments.

I will go first...
I am a 22 year old male who has been offroading since I got my license at age 16. I have owned cars, trucks, SUVs, and even a bus. My offroad rigs have had both IFS and a solid axle. I am also a junior in the field of mechanical engineering with an emphasis on materials sciences and machine component design.

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