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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / January 2008What wears out in a shock absorber?
What causes a shock to go bad over time? Apart from physical damage causing the oil to leak out or the seals to go, I'm having a hard time thinking of why a shock absorber shouldn't last the life of a car. The only other things I can think of is the heat slowly kills the oil and effectively "messes up" the valving, which is why all old shocks go soft, and/or dirt and stuff gets past the seals and ruins the oil. I guess I really do have too much time on my hands, it just seems that after 100 years of shock absorbers, they should last more than 20,000 miles before needing replacing. And, why do trailers not come with shocks? (I'm referring to a typical utility trailer or car hauler.) Ray news <rollingviolation@domain.invalid.com> wrote in article <ZWkfj.1036$EA5.583@pd7urf2no>... > What causes a shock to go bad over time? > [quoted text clipped - 13 lines] > > Ray The internal piston seal will wear out. In most OEM-type shocks, it is usually only an O-ring or a teflon band. Sometimes a valve disk-spring - on the disk-spring-based shocks - will break. Valve seats will wear out. Coil springs that close the valves will weaken. Similar problems may develop with the base valve at the bottom of the inner tube. There are usually two spring-controlled valves - a high-pressure compression valve and a low-pressure refill valve - in the base valve assembly. Sometimes the shaft - either pressed onto or screwed into the piston - will become disconnected from the piston. Of course, external leaks resulting in loss of fluid are a major cause of shock failure. Shocks operate in an unfriendly environment, and the seals must wipe away water, sand, salt, road grit and grime, etc. resulting in wear. Etc., etc. There are many more moving parts to a shock absorber than are readily visible. We have taken apart, literally, hundreds of new and used shock absorbers and find what we would consider expected, normal wear among the above moving parts in shocks of varying age. We have also discovered through the use of our shock dyno, that new shocks actually improve somewhat after installation before heading inexorably downhill. We suspect it is similar to an engine's new piston rings sealing to a freshly-honed cylinder. > news <rollingviolation@domain.invalid.com> wrote in article > <ZWkfj.1036$EA5.583@pd7urf2no>... [quoted text clipped - 52 lines] > downhill. We suspect it is similar to an engine's new piston rings sealing > to a freshly-honed cylinder. Thanks for all that info. I know how shocks work from car magazines and high school auto class, but have never bothered to actually cut any open myself... didn't think I'd learn much from that. Didn't realize that there were springs involved in the valving - I could definitely see those getting weaker with age like a valvespring does. I am surprised that no one has come out with a sealed unit that lasts 100,000 miles. Although, considering how fun it usually is to get 5 year old shocks off a car that's been driven in a rust belt winter, I can imagine 15 year old shock bolts.... Ray On Jan 4, 12:42 pm, Ray <ray@!rollingviolation.example.com.invalid> wrote: > Thanks for all that info. I know how shocks work from car magazines and > high school auto class, but have never bothered to actually cut any open > myself... didn't think I'd learn much from that. Didn't realize that > there were springs involved in the valving - I could definitely see > those getting weaker with age like a valvespring does. Yeah, you should look at some of the more technical descriptions from some of the manufacturers. They can get pretty fancy with the valving in order to tailor the response of the shock with different resistances to different speeds and at different extensions. Otherwise, they'd all just cost $50. > On Jan 4, 12:42 pm, Ray <ray@!rollingviolation.example.com.invalid> > wrote: [quoted text clipped - 10 lines] > resistances to different speeds and at different extensions. > Otherwise, they'd all just cost $50. Either NASCAR doesn't use springs in their shocks, or the articles I've read in Circle Track magazine where they go into detail about valving never show any springs. But hey, learn something new every day. :) news <rollingviolation@domain.invalid.com> wrote in article <lvxfj.4630$uV6.2673@pd7urf1no>... > > On Jan 4, 12:42 pm, Ray <ray@!rollingviolation.example.com.invalid> > > wrote: [quoted text clipped - 16 lines] > > But hey, learn something new every day. :) The Penske and Ohlins monotube shocks that are common in NASCAR Cup and other oval-track racing series around the country use stainless steel disc springs of varying diameters and thickness assembled in stacks. They also use a floating piston to separate the oil from the nitrogen pressure charge - something that many Koni shocks also do. IMHO, Circle Track isn't the best source for technical information. Their Tech Editor's engineering background is on the Alaska Pipeline!!!.....NOT in the automotive field. > IMHO, Circle Track isn't the best source for technical information. Their > Tech Editor's engineering background is on the Alaska Pipeline!!!.....NOT > in the automotive field. I read car magazines for what they are - magazines. If I'm rebuilding the small block in my race car, I may refer to ideas gleamed from Car Craft, but I sure as heck won't be using the 2 page "rebuild your small block" article as the reference guide, instead I'll be using a proper manual, but you can gleam a lot of potentially useful ideas from car magazines for stuff you might want to do to your car(s). Reading an article on how shocks work helps when deciding what type of shock to buy (and where to mount them) for my race car, but it's not going to cut it for designing and building said shocks. :) Ray >news <rollingviolation@domain.invalid.com> wrote in article ><lvxfj.4630$uV6.2673@pd7urf1no>... [quoted text clipped - 31 lines] >Tech Editor's engineering background is on the Alaska Pipeline!!!.....NOT >in the automotive field. Also most of their "tech" articles are co-written by their advertisers. Don www.donsautomotive.com Don <don@NO-SPAMdonsautomotive.com> wrote in article <el5fo3tv44hjjlml5689e71b9g3d6t2qmd@4ax.com>... > >news <rollingviolation@domain.invalid.com> wrote in article > ><lvxfj.4630$uV6.2673@pd7urf1no>... [quoted text clipped - 37 lines] > Don > www.donsautomotive.com My guess is that the manufacturer/advertiser knows his product a LOT better than the guy who passes for a "Technical Editor." I'd rather read product specs and how a product works from the manufacturer than the magazine's in-house idiots. > I am surprised that no one has come out with a sealed unit that lasts > 100,000 miles. Well.... haven't they? I certainly don't change shocks much more often than that, and the struts on my wife's car went something like 150k before I noticed enough degradation to warrant replacing them. The first failure I've tended to notice is that gas-charged shocks will lose their gas charge, which lets the oil froth too much (cavitation) during operation. The gas seal around the shock stem is the weakest link in the whole assembly. >I am surprised that no one has come out with a sealed unit that lasts >100,000 miles. Although, considering how fun it usually is to get 5 >year old shocks off a car that's been driven in a rust belt winter, I >can imagine 15 year old shock bolts.... I have got more than twice that from Bilsteins before. Admittedly, it helps to drive on smooth highways. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >> news <rollingviolation@domain.invalid.com> wrote in article >> <ZWkfj.1036$EA5.583@pd7urf2no>... [quoted text clipped - 61 lines] >I am surprised that no one has come out with a sealed unit that lasts >100,000 miles. Most people would not want to pay the cost for one that lasts that long since most people only keep a car about 3 years. I've noticed that it does seem like they put more durable "shocks" in the struts then the shocks they would put on an otherwise similar car that didn't use struts. Most OEM shocks only last 20K miles but most OEM struts will last 50K to 100K miles. I think it goes back to the cost of repairs under warranty. For the times someone complains of about their shocks under warranty it's cheap for the auto maker to fix them. If they had to replace a strut it would cost them a lot more money for that repair. So it seems they make them more durable. >What causes a shock to go bad over time? The rubber seals fail. Rubber and viscoelastic parts are usually the first thing to fail on anything. >I guess I really do have too much time on my hands, it just seems that >after 100 years of shock absorbers, they should last more than 20,000 >miles before needing replacing. Why? Car manufacturers would never make any money selling aftermarket parts, if they made the original parts last too long. But you'll note that Bilstein shocks will last a whole lot longer than the ones you get at Midas. >And, why do trailers not come with shocks? (I'm referring to a typical >utility trailer or car hauler.) Because there is nobody riding in them, so the quality of the ride is not considered important. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >> And, why do trailers not come with shocks? (I'm referring to a typical >> utility trailer or car hauler.) > > Because there is nobody riding in them, so the quality of the ride is > not considered important. > --scott Ahh. That makes sense. Still, when I tow my race car, I have some rough roads to travel (pretty much any road in Manitoba) and I swear I can get trailer tire airtime if I try and go close to the speed limit. I guess there's nothing stopping me from adding shocks, it just was one of those things I wonder about late at night. :) Ray > What causes a shock to go bad over time? > [quoted text clipped - 13 lines] > > Ray Let me guess, you bought Boges, didn't you? nate (never, ever buy Boge shocks or struts.) >> What causes a shock to go bad over time? >> [quoted text clipped - 19 lines] > > (never, ever buy Boge shocks or struts.) lol, nope. Actually, the last shocks I bought were for my dirt track Camaro. Those were killed. Generally I buy whatever shocks for whatever cars, but for my Trans Am I'm going to buy good ones. Not yet sure what brand that is... > >> What causes a shock to go bad over time? > [quoted text clipped - 24 lines] > whatever cars, but for my Trans Am I'm going to buy good ones. Not yet > sure what brand that is... In my experience, Bilstein or Koni. nate > > >> What causes a shock to go bad over time? > [quoted text clipped - 28 lines] > > nate I just had the front struts replaced on my 944 last week, the old Sachs/Boge units were installed not long before I bought my car maybe 3-4 years ago, and I haven't put anywhere near any significant amount of mileage on the car - maybe 20K at most. They had no gas pressure and the compression damping was almost nonexistent. The new Konis while expensive make the car ride like it should again. I chose Konis simply because the only two mfgrs. making struts for my application are Koni and Sachs/Boge - apparently Bilstein discontinued their 87-up 944 front struts a while back. nate |
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