Leveling the Load: The Physics, Dangers, and Fixes of Truck Squat
We’ve all seen it: a truck with a trailer on the bumper, bed almost touching the wheels. Some drivers think “we are just going down the road” or “one time won’t hurt”. And, for many, they get lucky towing loads that max out the capacity of the truck. However, after success with leveling out customers trucks we think the risks are not worth the costs of beefing up the suspension to handle heavier loads. We’ll cover how the leaf suspensions work on most trucks (the physics), safety implications of towing heavy without proper weight distribution, and how we’ve reliably addressed this in customers vehicles.
How Leaf Spring Suspensions Work
Basic Design
Leaf suspensions are the basis of most light-duty trucks on the road today. What is a leaf spring? A leaf spring is a half-moon shaped piece of steel that usually sits over the rear axle–held down by large u-bolts that secure one, or multiple leaves. Usually there are a few core leaves in the pack and then overload springs that sit on top of the main spring pack. Leaves are attached to the frame with brackets called shackles. These help distribute the load on the spring and provide some amount of dampening as well as ride height. Some drivers will install taller shackles to increase ride height in the rear. as well.
How Leaf Springs Work
Leaf springs are almost self-explanatory, but worth expounding upon because there has been significant advances in suspensions even though leaves continue to be extremely effective. Leaf spring function is governed by the following functions:
Hooke’s Law (Deflection)
The elasticity of a spring is expressed by Hooke’s law which says that the amount a spring is stretched or compressed beyond its relaxed length is proportional to the force acting on it. If a spring of relaxed length L is anchored at one end and pulled by some external mechanism at the other with a force F, its length is increased to L+x. Hooke’s law states that there is proportionality between force and extension.
It is important to note a gap in applying this classic definition directly to vehicle suspension. While the textbook definition of Hooke’s Law focuses on pulling or stretching a linear coil spring, automotive leaf springs operate primarily through bending. The downward force of the truck frame bends the arched spring flatter. Despite this physical difference in how the stress is applied, the proportional mathematical relationship of F=kx remains applicable for calculating the vertical deflection of the axle.
Pack Stiffness (Parallel Springs)
A single steel leaf can only resist a limited amount of force before deflecting severely. Trucks like a Chevrolet Silverado utilize a leaf pack consisting of multiple steel bands stacked together. This creates a mechanical system of parallel springs.
In physics, when springs are arranged in parallel, their individual stiffness constants (k) add together to create a total equivalent spring constant (keq). The formula for this relationship is keq=k1+k2+k3+⋯+kn, where n represents the total number of leaves in the pack. Because each additional leaf acts as a parallel structural support, the total stiffness of the suspension pack increases directly with every leaf added.
This increase in stiffness fundamentally alters how the truck handles leveraged weight. Returning to Hooke’s Law, we can rearrange the formula to solve for displacement, resulting in x=keqF. When you add a leaf to the pack, the denominator (keq) becomes larger. Therefore, when the exact same heavy trailer load (F) is applied to the hitch, the resulting vertical squat distance (x) is mathematically reduced.
For engineers calculating the physical deflection of a leaf spring, the specific formula for the spring rate is k=6L3Enbt3. In this equation, E represents the modulus of elasticity of the steel, b is the width of the leaf, t is the thickness, L is the length, and n is the number of leaves. Because n is located in the numerator, this mathematical model confirms that adding leaves yields a linear increase in capacity. In an ideal system, doubling the number of identical leaves in a pack will exactly double the overall spring rate, effectively cutting the rear suspension sag in half under a constant load.
Static Equilibrium
For a truck sitting stationary with a trailer attached, the suspension system must reach a state of static equilibrium where all upward and downward forces balance to zero. The trailer hitch acts as a Class 1 lever with the rear axle serving as the fulcrum. Because the tongue weight is applied far behind the rear axle, it multiplies the downward force acting on the rear suspension while simultaneously lifting the front axle. This leveraged force, combined with the weight of the truck chassis, pushes down on the leaf spring mounting points. To achieve equilibrium, the axle pushes upward against the center of the leaf spring with an equal and opposite force. If the total applied load exceeds the inherent spring constant, the suspension compresses until the forces finally equalize at a lower ride height, resulting in visible squat.
Kinematics (Shackle Extension)
The visual sagging of the truck bed is tied directly to the kinematics of the leaf spring. A leaf spring is mounted to the truck frame at two points. The front eye is fixed securely to a stationary bracket, while the rear eye is attached to a swinging shackle. As the downward force flattens the arch of the spring, the linear distance between the two eyes increases. Because the front mount cannot move forward, the elongating spring pushes backward against the shackle, forcing it to swing rearward. This physical lengthening of the spring pack is the mechanical mechanism that allows the axle to move closer to the vehicle frame.
Safety Implications
Vehicles are already dangerous machines. Make them longer and heavier, risks become far greater–especially if not loaded properly. But, trucks are supposed to be capable of heavy loads, right? Sure, some squat is normal under certain conditions. To assess whether you have an ok amount of lean, it is best to measure the truck before and after the load is applied. Simply measure the distance between the top of the wheel arch and the ground in the rear and front. Rake is the difference between the movement down in the rear and up in the front (which goes from the inverse as well).
Different levels of rake are acceptable. A drop in the rear of 1-1.5″ inches is tolerable. The shift in center of gravity is negligible from the front to back. Brakes and steering function normally. 2–2.5″ inches is quite noticeable. Vehicles in this gray zone are beginning to display signs of stress depending on how the weight is distributed. Some vehicles operate in this gray zone better than others depending on their length, height, brake size, power, torque, and driver capability. Any squat greater than 2.5″ inches is going to be unsafe
Once the rake of the vehicle gets too large, safety becomes compromised. Too much weight on the rear end makes steering lighter and can compromise effectiveness altogether. If you can’t steer properly due to no contact from the front tires, then turning becomes much more difficult–especially if there are emergency maneuver required at speed. Secondly, braking performance suffers. Because a majority of braking occurs at the front tires, any time contact patch is compromised braking gets less effective.
Fixing the Squat
Aside from lightening the load, fixing sag is fairly simple and not terribly expensive. And, the results is a much safer towing experience and less wear on other components.
Add-a-Leaf
We think one of the best ways to level out the load to improve handling the weight is adding structural components like leaves is the best way to permanently improve weight distribution of heavy loads. Yes, there are downsides, like increased ride stiffness, which can make this less of an attractive option; however, adding a leaf is excellent and you get a bit of lift in the back if the front has been lifted as well. Our preferred manufacturer is BDS Suspension products here in MI.
Our customers F350 is typically towing 20,000-23,000 lbs. It typically had a pretty significant rake and now remains almost completely level. Even the ride compliance remained more than we expected.
Road Active Suspension
Road Active Suspension (RAS) provides an awesome product that has a similar outcome as the add-a-leaf, but much easier to install and, in theory, could transfer onto another vehicle. RAS can improve ride quality even unloaded as well.
RAS helps to maintain its proper arch by applying force at the shackle. A secondary spring assists in counteracting the downward forces from the load on the spring. A bit of pre-load is applied when unloaded (which probably accounts for some of the reports of ride improvement).