Pressurized nitrogen bumpstops (often called air bumps) are the ultimate protection against hard bottoming your suspension and slamming parts metal-to-metal. Rubber and polyurethane stops are effective for casual off-road driving and slow suspension action where you just need to limit up-travel. But when running at high speeds or consistently pushing the suspension near full compression points, nitrogen bumps rule.
A cylindrical nitrogen bumpstop is essentially a short-stroke pressurized shock. It uses oil flow through internal orifices to dampen, or slow, the suspension movement during its final few inches of travel. They typically exhibit some rebound damping as well, so the suspension moves back to extension faster than the air bump, and the bump does not apply active pressure back onto the suspension.
We consulted with Mike Leighton at King Shocks to better understand some of the technical details of choosing and using nitrogen bumpstops. King offers both 2- and 2½-inch diameter body bumpstops. Size and weight of the vehicle determine which diameter bump is needed to provide the appropriate level of bottoming protection to control the suspension.
Like most suspension components, choosing the right parts up front is important, but tuning the working package is where you really get the best performance out of the system. This holds true for nitrogen bumpstops as well, as they have a number of variables that can be adjusted to dial in the right behavior.
Generally, the bumpstop stroke length is determined by the amount of up-travel each specific vehicle has from ride height. The length is also relevant to how soon and for how long it starts to slow down your suspension before it reaches complete bottoming. Common stroke lengths are 2 to 4 inches. Longer bumps are not always better, especially on a vehicle with limited up-travel. If the bump is too long, you may find yourself using the action too early in the suspension travel. On a milder vehicle this can mean the bumps are often in use during short suspension movements, leading to a harsh ride. It may be best to run a firmer 2-inch stroke bump that only contacts at the very end of the limited travel.
King bumpstops are assembled with a set amount of oil volume and provide a compression ratio of 3:1, meaning for every 1 inch of compressed travel the rate increases three times the initial force. This provides the rising rate in the bumpstop. With the proper amount of oil this ratio can be changed to provide a higher rate. However, before adding oil, it’s often best to experiment with higher nitrogen pressure, or consider revalving the bumpstop. The hydraulic action of the valving and the nitrogen pressure provide a smooth, exponential rising rate as the bump is compressed.
Bumps mounted directly over a solid axle interact at a 1:1 ratio with upward axle travel. Bumps may also be mounted to limit a trailing arm, or, on this vehicle, these bumps halt upward IFS travel of the top A-arm. The 2-inch stroke bumps here may work over a 4-inch upward travel range out at the tire. In the same way IFS shock spring rates and valving differ from straight axle parameters, the bump valving and pressures here have to be chosen to account for the leverage ratio of the inboard bump.
The majority of bumpstop tuning is done with the nitrogen pressure and King specifies a working range of 5 to 400 psi. They recommend a starting pressure of 50 psi, but there are a few considerations. For example, which shocks are used on each corner, and is a bypass shock used? Longer travel bumpstops may run less pressure than shorter stops as the pressure will rise more over the longer stroke length. Fine-tuning the pressure is done per vehicle basis and to driver preference, using terrain test runs to help dial in the final settings.