By Craig Weeks
Oh big deal, a suspension is a suspension is a suspension you say. If that’s the case, why do some rigs ride like a Cadillac and others like a brick? Often it is because the specifications were not written with comfort in mind—whether it is because of lack of knowledge or an assumption that fire trucks just ride like that.
I recall receiving a telephone call from the fire chief of a large city fire department a couple of years ago asking me what happened with their truck. I guess I was just supposed to understand the question but after a little prodding I got it. This department had made a tag-on purchase for a tractor-drawn quint from one of my solicitations. The chief said, “We drove your rig and it was the best riding truck we have ever driven. We get ours, which is identical, and the tiller ride is horrible. It is so bad we are getting injury reports filed, and personnel are refusing to operate it. So, what is wrong?” Now although the chief said the rig is identical, I have yet to see a tag-on that doesn’t have changes, even if just cosmetic.
So in my typical suspicious nature but not meaning or wanting to be offensive, I just had to ask, “You made no changes whatsoever?” The answer was, “Nope, it’s your truck.” OK, so I start going through the process: tire pressure, equipment loading, weight distribution, and so on. But, then I circled back to the suspension and I sensed a little agitation in his response when the chief said, “No. It is specified exactly the same as your truck!” So, I told the chief I had no idea, that being 2,500 miles away I could not just come out and look at it, but that I would think on it and provide some suggestions.
I then contacted the apparatus manufacturer and discussed the conversation I had just had and was told, “Yeah, we are trying to figure it out too. But, we don’t understand it.” So I said to the project manager, “The chief said they made no changes to the specification,” and I was a little surprised by the answer. “Sure, there were some changes but not on the tiller.” So, I asked for a copy of the change order, which the manufacturer provided. Looking through it, the changes were insignificant to the issue for the most part until I saw, “leaf spring drive axle suspension in lieu of air ride.” Bingo. So, I called the chief back and asked him about the drive axle suspension change. His reply was, “The problem is in the tiller, not the tractor.” Hmmm…so the same truck reacts differently, but no changes were made except for the drive axle suspension. Let’s consider the obvious.
This whole issue was that of geometry. What happens at point “X” is compounded as the motion travels to point “Y,” in this case about 40 feet away. And, then compound that movement from the fulcrum (the tiller axle) back to the tiller seat, around 10 feet. So, the more rigid leaf springs with little dampening send nearly the full motion of the chassis through the hitch, 30 feet down to the fulcrum and 10 feet to the tiller seat. So after finally convincing the chief that the problem was the change, what was the solution? Well, I proposed two options: change out the leaf springs for air ride for $20,000 to $30,000 like it was originally specified or live with it and the potential injuries and ten times the cost—or more—that will come with them. It’s an easy decision from a risk management perspective.
So, what are the different types of suspensions available? Here are some of the most common in fire apparatus.
Multi or Stacked Leaf spring suspension systems typically use spring steel sections, or leafs, that are the same thickness and width but get shorter in length from the top down until the weight capacity is reached. These stacks can have eight or more leafs, all of which are bolted together with a common center bolt or pin and clamps on the longer sections and make direct contact with each other. These systems can be used in any axle position and configuration.
Parabolic Leaf Spring suspensions are similar to the stacked leaf except that each leaf is essentially the same length and typically equal in its capacity. So if each leaf is rated for 2,000 pounds, three leafs on each side will be rated for a 12,000-pound axle rating. These leafs may be tapered—thicker in the center and tapering to thinner to the outer ends. These spring stacks also do not directly contact each other by using nonmetallic friction reducing spacers, providing for a quieter and smoother ride. These systems are typically only used on a front axle.
Air Ride suspensions use a rubber air bag in place of a steel spring to support the vehicle and absorb impact. The size of the spring depends on the weight to be supported. Simply put, a 100-square-inch air spring will be capable of supporting 6,000 pounds at 60 pounds of air pressure. Adjustable, dynamic valves are used to set and maintain a constant ride height regardless of the load placed on them. These systems can be used in any axle position and configuration.
Independent suspensions are exactly that. Each wheel end is connected to the frame independently through a series of pivoting control arms. The spring component of these systems can vary from a torsion bar to a coil spring to an air bag. These systems are only used on a front axle.
Walking Beam, or bogie, in itself is not a suspension but rather a system of connecting tandem axles together while allowing some independent movement. The spring component of these systems can vary from leaf to air bag or even solid rubber cushions. These systems are only used on tandem rear axles.
So, which suspension is best? Well if you have read any of my previous articles, you will probably realize I don’t promote or demote any product but identify the pros and cons and leave it up to you to decide what is best for your application. So, let’s look at each one.
Multi or Stacked Leaf spring suspension systems are the simplest and most universal and thus the most cost-effective. They provide the best stability in turns and tilt testing but are also the roughest riding. However, getting the weight rating correct in the beginning will limit the abusive ride and the longer the spring, the better. Regular lubrication of the system is necessary but overall maintenance is minimal.
Parabolic Leaf spring suspensions are similar in simplicity and maintenance to the multi leaf spring but ride quality is typically much better. Cost is typically a bit higher and stability a bit less, but overall a good option for cost effectiveness with a good ride.
Air Ride suspensions provide for a great ride and are likely the least stable but most forgiving. Maintenance is minimal and in the event of an air bag failure, are the most field serviceable. The cost is significantly higher than a leaf spring but the versatility is unmatched. Single or tandem drive, front steer, tiller steer, it doesn’t matter—it can be air suspended. Air suspensions also have duty ratings identified by percentage of off highway operation. The ride height and ground clearance remains constant regardless of the weight load. That said, too much air ride reduces road feel so I would not recommend all axles and air suspension seating on the same vehicle.
Independent suspensions are undoubtedly unrivaled in ride quality. However they are the most expensive and highest maintenance systems. If not kept properly aligned, they will wipe out tires in short order.
Walking Beam, or bogie, suspensions are only used in tandem drive axle configurations and should only be considered in the heaviest of applications. Depending on the spring selected, they are typically very stiff but also very stable and the most cost-effective. Although they do allow for some independent axle movement, they do creek and groan and tend to leave tire marks on the ground during tight maneuvers.
All this said, there are many suspension manufacturers, and each has its own positive qualities. But, no one suspension is best for all applications. For example, if I am specifying a vehicle that will have a high center of gravity or go off highway, I will lean toward one of the more stable systems. Alternatively, if I know the fire apparatus will likely be deployed on an assignment many miles away, I will be considering a softer riding suspension. In the end, it is your choice but hopefully I was able to shed some light on the subject in order for you to make an informed decision.
CRAIG WEEKS is division chief/fleet manager for the Los Angeles County (CA) Fire Department and president of Specialty Fleet Consulting.