Clank! ... if that sound is coming from your suspension, that's not a good thing. If you just bottomed your springs and heard that metallic clank, it probably means you banged metal on metal and are at risk of bending or breaking some part of your suspension. And that means you probably need a reminder on the value of bump stops. Let's take a look at the function of a bump stop and some of the various applications for them.
The main purpose of a bump stop is to serve as some final suspension cushion to keep the metal parts from coming together or from traveling too far, which might in some way damage the suspension or chassis. In the case of a coil-sprung suspension, you'll want the bump stop to stop movement before a condition know as "coil bind," in which the coil spring is completely collapsed and can compress no further.
Consider also that a bump stop used in the final bit of travel in a suspension behaves much like a variable (or rising rate) spring. This allows a vehicle to use a softer spring rate than would normally be needed if there were no bump-stop control. Using a bump stop allows the suspension to soak up large bumps while still allowing use of that soft spring for normal suspension travel conditions.
There are several types of bump stops and we'll discuss each in some detail here. We'll consider fixed-element stops and valved stops that use a compressed-gas cylinder.
Rubber & Urethane Bump Stops
The most common type of bump stop found on production vehicles is that of the rubber variety. These are simple to employ and low in cost. A common aftermarket step up from rubber stops are those made from urethane.
Both of these varieties supplement the final portion of spring travel with a high rate spring constant inherent to the bump stop. Additionally, their tapered shape provides somewhat of a progressive increase in spring rate as the bump stop is compressed.
One downside to a rubber or urethane bump stop is that they dissipate energy into the suspension rebound, which can cause your shocks to work harder. These bumps offer little damping and often behave much like a pure coil spring.
Urethane offers benefits in that it is easy to form and can be shaped with common machining processes. One downside to the material is that its durability can sometimes suffer as the urethane can begin to tear apart if it is compressed with excessive force.
Some newer model trucks use micro-cellular urethane bumps which can be recognized by their yellowish color. This type of material does offer some minimal damping affect as the material is compressed and then released under rebound. It also offers improved durability over basic urethane.
All fixed bump stops are designed to be short and come into effect late in the final distance of suspension travel. As such, they have a drastically increasing compression rate. That is to say that they stop the suspension movement very quickly and typically in a harsh manner.
Nitrogen Bump Stops
One form of stop that's become much more popular in recent years has been the gas pressurized nitrogen bump stop (often called air bumps or hydraulic bump stops). These cylindrical units consist of a short stroke shock mechanism that is velocity sensitive. Oil is used inside and moves through orifices much like a standard shock. This allows the bump to effectively dampen, or slow, the suspension movement through its final inches of travel. In contrast with an air or gas shock, they have large shaft diameters and are much shorter. Air bumps also exhibit some rebound dampening as well.
We consulted with Mike Labbe from Light Racing about the use of hydraulic bump stops. As mentioned, a suspension using air bumps can often run a softer spring rate so the ride is smoother under normal driving conditions. The air bumps provide the progressive end of travel function to supplement the spring rate of the suspension. A suspension that uses a fixed bump often becomes much stiffer earlier in the travel cycle to reduce the chance of a hard bottoming on the rubber or urethane bump stop. As of late, several OEM car manufacturers are exploring the idea of adding hydraulic bumps to allow them to run softer primary suspension spring rates for improved on-road ride quality.
Air bump reliability has increased in recent years with improved materials, seals and surface finishes to help prevent leaks that can quickly disable their function. Stop ends are typically made from aluminum or nylon, and longer travel air bumps can sometimes be converted to shorter travel versions with the addition of an internal spacer to reduce the expanded length.
Dirt-bike riders talk of a condition called suspension packing, where the rebound dampening has been set to high. As the vehicle hits successive bumps, the too-slow rebound keeps the shock from expanding before the next bump compresses the shock further. As a result, the suspension height drops with each successive rapid bump. A bump shock should ideally be set to lag the rebound speed of the suspension but not much slower than that rate.