Friday, January 14, 2011

Leaf Springs

Some folks new to RV'ing and driving a vehicle that is essentially a truck have a few, or even sometimes a lot, of complaints about the ride of their Lazy Daze. Occasionally owners will choose to make some modifications to the suspension system in an effort to improve the ride. Before doing that, you might want to consider the following information.

There have been a few companies that offer a system that completely does away with the leaf-springs of the original Ford system. Be aware that the leaf springs accomplish several things:

1. They locate (keep in alignment) the rear axle so that the vehicle travels straight

2. They support the weight

3. They offer progressive rates (notice how some of the leafs go
full-length, others just partial, each is an individual spring rate, as the
longer springs compress first, then load is transfered to the next longest,
as it flexes, load is then transfered to the next longest, and so on). And
example could be: 1,000lbs compresses by 1", 2,000lbs compresses by 1.5",
etc. This gets very complex when you add cornering forces, and then a bump
in mid-corner

4. They offer friction to reduce bounciness (both a plus and a minus). All
leaf and coild springs have internal friction (coils are actually twisting,
leafs are actually bending), and most modern leafs have glides that help
reduce the friction between the leafs.

5. They are inexpensive.

Many leaf sprung vehicles also have an additional piece to help locate the position of the rear axle, usually a single bar, properly known as a Panhard rod (also called a Track Bar by many) which is a single bar that usually is just behind the rear axle, mounts to the axle on one side, and the chassis on the opposite side. This takes much of the side-load off of the leafs, allowing them to be designed more for vertical load control. However, such a single bar by nature (geometry) scribes an arc, so the axle no longer moves perfectly up and down, but slightly to the side also ( A two bar system calls Watts Linkage addresses this, but I do not remember seeing it in light truck usage)

A 4 (or 5) link rear suspension actually addresses and improves on many of
these areas as below:

1. They can locate the position of the rear axle, and address side loads while scribing a much better arc

2. They are easily adjusted to alter ride height and can easily level the chassis to any angles desired (within reason of course)

3. They offer linear rates, simpler to calculate and predict. As in: 1,000lbs will compress by 1", and 2,000lbs will compress by 2", etc.

4. They offer very little internal friction, so all bounciness must be dealt with separately, actually using a shock absorber for this ride control. So typically the shocks must be changed to shocks valved specifically for these systems. This is one of the biggest improvements in the ride.

5. If a system is both front and back, it can actually offer air-levelling for camping. This is what most busses and many high-end motorhomes also. No additional levelers needed, saving weight, complexity, and space.

6. The biggest drawbacks are that they are not designed and manufactured to the same qualities as original systems, and there are the potential for leaks and failures (plus, when the bushings wear (and they will), you need to find someone that knows how and where to get the repair parts.

Overall, an aftermarket rear air suspension can offer a measurable benefit in ride comfort and control, but there are serious factors to consider. Weighing into that decision should be how you plan to use the coach, such as, are you a road-warrior driving many miles, then spending little time camped, or are you someone that spends a lot of time camped, and only connects these camp time with short drives?

Contributor: Ken Sann

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