Its only natural for me to ponder certain problems and try to understand why things happen sometimes. (apologies in advance for my long post)

For example, I am one of those people that understands that potholes are not only possibly damaging to the wheel itself, but the suspension also takes a considerable beating as it absorbs the impact. In the same fashion, speed bumps are 6" of travel for a shock abosrber that most drivers (and I will go out on a limb here, and say that most women in particular) think that a car should be designed to take those beatings in stride and move on. I mean, if the car didn't feel like it drove over a dead body, surely the suspension parts won't fail or the shock won't or spring a leak, right? Its how these owners unfortunately think.

Ever drive behind someone in a grocery store parking lot and see them fly over those speed bumps? I cringe and see dollar signs in their future. My mom is bad about doing this (until recently). My girlfriend thought that since the jeep IS a jeep, well she could drive THRU the bumps. Sure, its apparently smoother for the driver and obviously quicker than driving really slow like myself, which I take each bump/large pavement crack as if I'm climbing over a fragile egg in the driving lane. People behind me...you can just wait!

Naturally, my progression of thought leads me to how a typical shock/strut works, with fluid moving thru chambers as it compresses and returns in its designed attempt to absorb the energy. The less energy to 'deal with' the more likely the shock will last. I've had leaking shocks before. And I've heard of these newer 2004 + XJ air shock seal failures. I started trying to consider maybe why this is happening.

Shocks eventually fail, we all know that. But if the life cycle, say...8-10 years or 75,000 miles is achieved, we all think that's pretty good, right? But these air shocks are failing much sooner; could the reason be as simple as the difference between the AIR and FLUID properties as it transfers energy in the shock travel?

This is where I start my theory. (1) Fluids are incompressible under normal conditions, where air and gases are. At first glance, that would lead me to first think that air is a better shock 'medium' than a fluid, because it would take some of that impact from a compressed shock and compress before moving another part, or moving into another chamber of the shock. But, there is something else to consider that fluidic properties have that air or a gas does not. Fluids have the natural ability, due to its density and mass, to not only transfer energy from the impact and/or action of the shock, but it can absorb it (dissipate) a small portion. The transfer of energy from the actual mechanical impact passes thru the fluid and will be converted to heat.

Not only that, but (2) the actual speed of transfer of energy thru a fluid versus a gas is much faster. Imagine sound waves above water...now submerge yourself, its muffled and not as loud. The waves (whether sound, energy, light, etc...all behave similarly) in fluid travel at lower amplitude and speed compared to air. Fluids inherently slow down the energy, and this very fact may be another variable why the air shocks are blowing seals.

We've got dozens of engineer's as members, others who know the complexities of science, and technical savvy guys who just flat out understand these car's systems. I'd like to know if you have any comments to either support or dispute my theory. Please, I may be mistaken, but If I am, I would sure like to know where my logic may be clouded or incorrect.

If there is a moral to this thread I'd like to relay, it is to not assume the cars suspension systems are bulletproof, and realize those speedbumps do what they are designed to do, slow you down...either before you drive over them now, or after you've blown a shock or two from the abuse they have taken in your rush to leave the parking lot. Please pass this advice over to anyone who drives your jag too, girlfriend, wife, children...knowledge is key. After two evenings of bushing replacements on her lincoln, my mom is now enlightened by my opinions about her driving, after hearing me swear like a sailor under her car while she was forced to hold a light or ratchet extension for me. My girlfriend - now an ex, I'm sure she's flying over every speedbump in my old Jeep, but hey...at least I don't have to sympathize with her after she gets the bad news from the shop for ANOTHER shock that is bleeding all over her axle.

 

Hey Matt, just an FYI...

Man, You're a Bad Driver
Good Men Project
August 17, 2010

A New study suggests that guys should consider riding shotgun.

In a striking blow to every man who has ever pulled the “women are bad drivers” card, a new study shows that male drivers are 77 percent more likely to die in a car accident than women (per miles driven). It’s even worse in NYC, where guys account for 80 percent of fatal or near-fatal car accidents.

This has been attributed to men’s tendency to drive faster than women and have less regard for traffic laws (not to mention a masculine affinity for speeding, driving drunk, running stop signs, and therefore crashing twice as often as women do, according to the research).

David Gerard, the Carnegie Mellon University researcher who co-authored the study, takes his own research dead seriously—no pun intended—and travels in the passenger seat with his wife on most trips. “I just put a mitt in my mouth and ride shotgun,” he said.

Need more proof? In his book, “Myths, Lies, and Downright Stupidity,” ABC News’ John Stossel quotes research by the Social Issues Resource Center:

“A 2002 report analyzed a stack of studies on male and female driving differences and came to a bold conclusion: In all studies and analyses, without exception, men have been shown to have a higher rate of crashes than women.”

We could chalk it up to hunter-gatherer instinct or testosterone, but at the end of it, guys—you may want to consider taking a back seat for your own safety.

—Lu Fong

 

Good post.

I'm not an engineer, but as I understand things, sound travels about 4 times faster in water than in air, but at lower amplitudes and intensity thus giving the muffled not as loud effect.

IMHO the term 'Shock Absorber' is a misnomer and they should be called 'Dampers'. They do not absorb the impact, the springs do that, they damp the oscillation in the spring after a small impact and help to keep the car from behaving like Zebedee.

If ever you get the chance, remove the dampers from an old banger and drive vigorously around a car park/track, Skippy will be put to shame.

Obviously a high energy large impact as you describe is not going to be good for any part of the car, but a really large impact would cause the car springs to bottom out and hit the bump stops and should protect the dampers a little.

I have had many years driving off road vehicles quickly and under 'normal' conditions ie not suddenly becoming airborn at high speed with the associated hard landing, the dampers give out more quickly when subjected to very rapid continuous movement.

So are air spring systems likely to have a shorter service life than traditional spring + damper, and what about the fully hydraulic systems used on such as Citroens?

I don't know.

Others with more knowledge than I, please chime in.

 

That said, I agree that it's better to drive slowly over speed bumps (not speed humps, which are designed to be crossed at a slightly higher speed) than whip across them, which I see plenty of people, both male and female and all ages/races doing on a neighborhood street in my community.

I dated a guy (who now owns his own racing team in FL) who never downshifted, claiming that the clutch has a maximum number of shifts before it's worn out, so why shorten its lifespan by downshifting? Er, yeah, but, um...??

Hard to argue against your theory and his, but IMO what it comes down to is if you don't mind paying for the repairs when they are needed, who cares how you drive your own car? If you don't like the way someone else drives your car, then DON'T LET THEM.

Problem solved!

 

H2OBoy, ready for a little bit in the way of science class?

Ok, your statement about liquids being non-compressible is somewhat inaccurate. A lot of fluids are compressible, they just don't compress all that much. In the nuclear industry, we use the compressibility of water to maintain plant pressure. Now, with that being said, a little bit of water will change pressure by a significant amount. But, think about all the hydraulic systems out there in use on tractors and whatnot. They are using a fluid and it has a semi-significant compression factor (Still not that of a gas, but it is definitely there). Also keep in mind that the gas inside of a shock is already under a fair amount of pressure (300-400 psi as I recall). So, with pressure like this, the gas will start to have fluid properties. Just to add more confusion to the subject, each type of gas also has different flow properties. So, the ability to get what you want is only limited by your choices.

Now, in addition to all of this, keep in mind that you normally have 2 sets of valving in a shock. You have essentially some small orificed holes and then you have a larger plug that can open and close. So, when you go slowly over a bump, the force seen on the shock is fairly low, so the plug can not be opened. This forces the gas (normally) through the small orifices. The restriction provided by the holes is what minimizes the rate at which the wheel will move (this function works in both directions). But, you hit a larger hole/bump, the forces seen by the shock increase. Normally this will be enough to open the plug in the center and allow a lot of the gas to flow through the big hole, allowing the majority of the shock movement to occur and then the final bit will be accomplished through the small holes. This combination allows for the handling of large bumps, yet keeping the car fairly flat during higher speed turns.

You start getting into some of the fancier shocks, you can actually adjust the size of the orifices, the spring load on the plug, the direction of the plug, etc.

As for why shocks fail, a lot of time it isn't so much that they have been moved through their limit of travel, they have been move beyond their limit of travel. This excessive movement drives the internals into the end of the shock. This obviously damages them, causing them not to function as they were designed to. Hit hard enough and you can rip a small hole in the shock. Needless to say, that lets out all the gas pressure. Now you have no dampening.

In addition, think about what all happens to the under side of the car. Think about all the sand, dirt, salt, rocks, and who knows what else that pelts the underside of the car. Some of this will strike the operating rod of the shocks. This can cause small pits to be formed in the rod or the material will stick to the rod and then be dragged down into the seal (normally an o-ring made of rubber). If you pit the rod, you are providing a little bubble for the gas to sit in so each time the rod moves in and out, you allow a very small amount of gas to escape. Over the 1,000's of cycles the shocks make, you can loose your pressure. With the o-ring, you start forcing the dirt into the o-ring, you will eventually cut the o-ring. Oh wait, now it can't seal. All the gas goes bye bye. But, in the case of air ride systems, they will be adding the air back in. But, adventually the leakage will become enough that even the system can't keep up (or leads to failure of the compressor because it isn't designed to run 100% of the time, actually only meant to run like 1 minute out of every 10 or something silly like that). Also keep in mind that most air ride system have what is called a kneeling position. This lowers the car to make getting in and out easier. When the car kneels, you are taking a part of the shock that isn't normally getting wiped off through normal use and taking that big wad of dirt and putting it up against the o-ring. Hmmm, looking like an o-ring trashing party to me.

Now, all of this can be solved/minimized through the use of a good rod boot for the shocks/struts. This will keep a majority of the stuff off of the rod, therefore leading to prolonged life of the o-ring. Being someone that is not afraid to dunk a truck in 4 feet of muddy water, this was something that I was definitely interested in. You seal the boots against the shocks and you will have something that will be well worth the money spent on the boots.

 

Thanks for the comments guys (and you too Diane, though more of a slap on the back of the head for the women remark, TBH, I didn't think you'd read my dribble...)

I understand what your saying thermo, and I was under the impression the air shocks DID have a boot type cover, remembering brutal say the rebuilt Arnott shocks do not. Do you think then, in your best guess, that the air shocks on the newer XJs are busting seals due to external factors, like debris and such? From overtravel? or a combination of such...and in what proportion? I don't doubt that the dirt and other abrasives can be drawn into the shock, but something is leaning me towards something more internal for a primary cause. What about the absorbtion and transfer of energy in a fluid based shock, did that have any merit at all in your opinion?

I appreciate the time to help explain it, I have chosen to educate myself on this one subject...till my brain hurts.

 

If anyone wants to feel what REAL suspension is like, see if you can get a ride in a Citroen DS (pronounced day-ess which in France means Goddess). Then you will know what a good 'ride' feels like. I believe some 1980s Rollers also had the same suspension. The later Citroens were good with their hydo-pneumatic suspension, but the DS beat them all. You couldn't exactly say it was 'soft' because body roll was absent. It kinda "sighed" along in an almost euphoric way.
Speedbumps? In a DS, what speedbumps?
Leedsman.

 

Re. bad drivers -- UK insurance statistics indicate that all drivers are much the same, but with one glaring exception. Male drivers between the ages of 17 and 25. This group is resposible for a disproportionate amount of SERIOUS accidents where big insurance payouts have to be made. See Jeremy Clarkeson's piece on this when he tried to get insured via the 'phone, claiming he was aged 17.
Leedsman.

 

I think you've misunderstood two basic functions.

Air shocks don't use the compressibility factor of gas to dampen the suspension movement, the air simply takes the place of a conventional coil or leaf spring. A hydraulic damper (shock absorber) is still required.

Hydraulic shocks don't rely on compressibility of liquids at all to perform their function. The liquid is forced from one chamber to another via calibrated orifices as the shock is compressed or extended, which dampens the velocity at which they react. So called 'gas shocks' still rely on the controlled flow of liquid from one chamber to another. The high pressure gas is in fact isolated from the liquid and perfoms no damping function. It's main purpose is to help prevent aeration of the liquid during rapid suspension movement.

Springs are springs, shocks are shocks, two very different functions but with complex and interrelated effects. The common terminology of air shock and gas shock just confuses things as neither is technically accurate.

 

thanks for the addition mikey, I guess I didn't understand the components of the 'air shock' as well as I do the traditional fluid shock. Coil functions are replaced by the air shock, so do you think overtravel is then more to blame then anything else in the air component?

 

I'm not all familiar with the Jaguar version of air suspension or apparently questionable reliability/durability. No idea what the failure mode is. (?)

I do have aftermarket air shocks on one of my motorcycles that so far, and touch wood, have given 28 years of perfect service. Many tractor trailers use air ride suspension, the bladders are visible under the trailer's rear axle sometimes.