Micro Cellular Jounce Bumperby Roland Graef
Attention - calling all bumpstops.
Well, we call them bumpstops because we always have. It is what we are used to calling them. But in the age of specialized tech-talk they are “bumpstops” no more; now they are micro-cellular progressive jounce bumpers. - What? ... That was a double decafe Mocha, skinny, no whip!
Most people think of a bumpstop as a black rubber snubber hanging down from the frame above the axle to protect against metal-to-metal contact. Just like my dad’s 1974 Chevrolet Caprice station wagon. You know, the one with the big block 454 and the hide - away tailgate. The Caprice was the cruising wagon of choice when highway 5 between L.A. and San Francisco first opened and gas stations were few and far between. The song, “Head out on the Highway, look’in for adventure” keeps ringing in my head. Where was I, oh yeah, trying to avoid the dreaded bumpstop (dumpstop). The thing was, no matter where you went in that wagon; you rarely came into contact with the bumpstop.
The micro-cellular progressive jounce bumper, or simply put, the foamy bumpstop, is actually a supplemental spring. It works to make the suspension progressive, allowing a smooth transition to full compression. What most people don’t realize is that the foamy bumpstop is an active part of the suspension. Let’s say you’re sitting in your M3, 996, Accord, Golf, S4, or Civic. (Hmm I can’t decide). They all use the foamy. Ok, you’re in your brand new Integra, just drove it off the showroom floor and are sitting at the stoplight waiting for the green. The foamy is actively engaged even while waiting at the stoplight.
Car manufacturers use this brilliant piece of high-tech engineering for many reasons. By using an almost zero-weight supplemental spring, you can build the main (steel) spring with a reduced spring rate for a comfy ride. The softer spring uses less weight, and, saving weight is a number one priority of new car manufacturers. Cars have thousands of parts. A little weight saved here and there really adds up. When the manufactures save weight they also save costs, less material=less cost. So we now know why these foamy gems are used. But how do they really work?
As mentioned earlier, the foamy bumpstops are an active working part of the suspension. They work in harmony with the springs and the rest of the suspension components allowing the ride to be comfortable while cruising. When the car is in a handling situation, such as in a turn, they make the car feel firmer for better handling. Think of the foamy as a little progressive spring over the shock rod. All car manufacturer bumpstops have a specifically calculated spring rate and compressed height. (block height).
When a spring is designed the spring rate must be calculated with the rate of the progressive bumpstop in mind. Otherwise the spring will not work in harmony with the bumpstop and the frequency of the suspension will be out sync. The human body interprets a certain range of suspension frequency as comfortable or uncomfortable. Frequency in an automotive chassis is like chords in music. If two or more notes are combined in a pleasing combination you have harmony, otherwise you have uncomfortable dissonance. The same thing happens in suspension tuning. A balanced suspension is important for correct function. This is why it so critical to understand the true effect of the pro-active foamy and the spring. Everything must work together in harmony!
The compressed height is what limits the wheel travel and controls how far the wheel and tire move up into the fender well. Aftermarket suspension designers must consider that enthusiasts will be upgrading the tires and wheels. Therefore, it is vital that these designers take into consideration the needs of tire and wheel fitment. By trimming the bumpstop you allow the tire and wheel to move further into the fender. This can cause the tire to rub where it did not before. If your application does not require bumpstop trimming and you are concerned about tires rubbing, just load up your car with your buddies and drive over some dips and speed bumps. If your tires do not rub with a full load in the car, they should not rub with lowering springs installed.
So, what has little Johnny learned today? There is no such thing as a bumpstop in a modern suspension. The bumpstop has been replaced by a supplemental, active foam rubber spring, which is one of the essential components in the suspension.