To follow L'escargot's post which I agree with
The spring and the damper are fitted in parallel, and while they share the same relative displacements and velocities, they react differently to them, and the total force acting on the body is the sum of these two reactions.
The spring, as L'escargot says cannot deflect beyond its bump stops - it must be operating within its design limits.
A sudden bump does, however, produce a large relative velocity across the suspension, and, so, it's actually the damper that produces most of the reaction, most of the bump you feel inside the car.
If people were complaining that their dampers were being ruined by speed bumps, it would be more credible than the springs.
As I've mentioned before, the more I see and hear of spring breakage, the more I am convinced about the corrosion protection and progressesive repeated making and breaking of contact near the ends of the spring being at the root of the problem.
Yes, the bumps will be subjecting the spring to fatigue cycles which they are designed for, but, fatigue is much accelerated by corrosion, and I suspect these corrosive conditions and the springs' corrosion protection are not being properly dealt with during design and development.
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............ progressesive repeated making and breaking of contact near the ends of the spring being at the root of the problem.
Interesting. Contact between which surfaces ~ between adjacent coils or between the end face of the spring and the abutment/support face? Is it something which would occur during normal driving, and not just specifically when the car goes over a speed cushion?
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Contact between which surfaces ~ between adjacent coils or between the end face of the spring and the abutment/support face?
In my case (3 springs in a month) it was where the square cut end of the coil contacted the next "ring" up. This certainly seemed to place a large localised stress point and is also where the coils broke.
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Answer in a word or two "Metal fatigue".
Springs will break sooner rather than later the more force & stress that is applied to them over any given period of time. Smaller movements in a spring cause less molecular movement within and throughout the metal, large movements in a spring (e.g. speed bumps, curbs etc.) cause a higher degree of molecular movement within/throughout the metal.
Edited by MikeTorque on 01/09/2009 at 14:47
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Any steel being repeatedly bent will degrade, if you exasperate this by regular, fast and large deflections, this will inevitably accelerate the degradation.
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OK what about valve springs in an engine? Thousands of extensions and compressions every minute but they very seldom break (I've been driving for over 40 years and have NEVER had a broken valve spring). I've only had one broken road spring as far as I can remember but that was on a Woolworths quality car. Now it seems to me that in recent years, accountants have been ordering the steel! As with everything one gets what one pays for so in the end the punter pays. The lowest quality cars are in trouble by the time 50,000 miles or less have been covered so the term "consumer durable" doesn't really seem appropriate. As to speed humps these are bad news especially for commercial vehicles. I might add that a few years ago I saw a smart looking white van in an HGV service station. When I enquired as to what its problem was I was told that it was a write-off as its rear axle (the drive axle) was bent! The military used to have a similar problem with 4x4s that were heavily off-roaded on rocky tracks but they had a solution to the problem. The axles were straightened with a hydraulic press and eight angle-irons were then welded on to the axle casing. Its a poor do when similar problems happen in civvy street but perhaps the van wasn't up to the job in the first place. Should have bought an LDV!
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I was thinking of the contact between the ends of the spring and the abutment - on most modern springs, this contact length is constantly changing with more of the coil in contact as the suspension moves towards full bump.
The idea is that as the suspension moves towards bump, some of the uppermost and lowermost coils become "shorted out" against the abutments, leaving fewer active coils bearing the load, leading to a higher effective spring rate.
This allows the spring rate at normal ride height to be softer, giving good ride, but, firming up when required.
However, once there's some grit sitting in the abutment, and the corrosion protection on the spring is abraded away, the metal fatigue as described on other posts is then accelerated by the corrosion.
I suspect that the development testing will definitely subject the springs to fatigue testing far beyond what most motorists will ever subject their cars to, speed bumps or not, but, if the fatigue testing is only done under salt spray conditions with the plastic coating on the spring not breached, then, the tests will not capture this possible failure mechanism.
I'm a bit out of touch with the materials which are no longer allowed - when I was last involved in civil aerospace parts, we were having difficulty in keeping cadmium plating as one of our corrosion protection processes, and I suspect that all the coatings which actually work are now banned by the EU (alas, the nastier coating materials tended to work the best!). Such matters don't aspply in my current work - we can even specify Beryllium!
A few moments spent looking at the SN curves in BS7608 [The British Standard for fatigue in welded structures - much more mundane steels than those used in springs] are enough to demonstrate that fatigue is much accelerated in a corrosive environment.
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driving over a speed bump is akin to driving up a kerb but with a more rounded top
it therefore follows that its not good for anything especially ball joints and tyre walls or springs,thing is most of these springs are made in eastern europe out of in my opinion poor quality steel
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............. most of these springs are made in eastern europe out of in my opinion poor quality steel
The car manufacturer's drawing for the spring will specify the grade of steel, and regular quality control tests will ensure that an inferior steel is not used in place of that specified on the drawing.
Edited by L'escargot on 01/09/2009 at 17:27
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Logic tells me that driving over these humps will reduce your springs
I drive over them at greatly reduced speed, and crawl over some at 5mph but still my car scrapes over many of them,
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The proof is the scattering of 1/2 turns of springs in the road just after the speed bump. Regardfs Peter
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the scattering of 1/2 turns of springs in the road just after the speed bump
I take the view that the speed bump in this case is simply the straw which breaks the camel's back, and isn't the root cause of the problem.
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I take the view that the speed bump in this case is simply the straw which breaks the camel's back and isn't the root cause of the problem.
I agree. If a spring breaks when going over a speed cushion it's because it's been weakened (progressively or instantaneously) by some other factor during it's lifetime. If the car in question had never been over speed cushions the spring would probably eventually have failed anyway.
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Logic tells me that driving over these humps will reduce your springs I drive over them at greatly reduced speed and crawl over some at 5mph but still my car scrapes over many of them
There's no connection between spring failures and whether some part of your car scrapes the hump when going over it. Scraping is primarily caused by not having sufficient ground clearance.
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The idea is that as the suspension moves towards bump some of the uppermost and lowermost coils become "shorted out" against the abutments leaving fewer active coils bearing the load leading to a higher effective spring rate. .
SQ
That could well be. I've been looking at the design of some older springs I have.
Both Volvo 240 and Triumph 2000 springs thin and flatten out over the last 4 inches of coil at each end, so that there is a single but wide area of contact. They then sit in specially shaped rubber end stops, which have to be positioned correctly when replacing a spring. At no time does one coil close up on or contact its neighbour. The original plastic protection has long since flaked off, but I have never had a breakage. The Triumph's did sag however after 45 years of daily use.
Edited by Dynamic Dave on 02/09/2009 at 11:26
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>>>They then sit in specially shaped rubber end stops<<<
Ah! you've just answered a question for me - some time ago I had the shockers replaced on a Volvo 240 GLT (fast fit!) and forever afterwards there was a knocking noise over rough surfaces, I took it back but they couldn't find anything, I even had it stripped down again by a garage but they couldn't find the prob ... obviously the rubber end stops hadn't been positioned properly.
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I think the number of spring failures needs to be put into perspective. The subject only seems to get an airing in motoring forums where even the smallest failure gets publicity. I suspect that in reality the number of failures is small in relation to the total number of cars on the road. Most items in cars (or in any other man-made object) suffer a small proportion of failures ~ it would cost too much to make them perfect.
If the characteristics of modern suspension designs ............
The idea is that as the suspension moves towards bump, some of the uppermost >> and lowermost coils become "shorted out" against the abutments, leaving fewer >> active coils bearing the load, leading to a higher effective spring rate.
........... benefits the majority of owners who don't experience spring failures then the manufacturers are justified in their design. In any case, I've not heard of a spring failure having lead to anything catastrophic. In fact some owners admit to having had a spring fail and not realised until it was found during a routine service or MOT.
In all walks of life the small number of dissatisfied people like to publicise their dissatisfaction, but the large number of satisfied people don't feel the need to say anything.
In over 50 years of motoring I've not experienced a single spring failure, but I'm not going to say anything about it! ;-)
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>>I've not heard of a spring failure having lead to anything catastrophic.
On some cars, the sharp end dislodges and bursts the tyre - I don't know if this has caused any crashes, or has just caused inconvenience.
It's surprising how many spring failures go completely un-noticed until the car is inspected. With all the recent emphasis on car handling from the motoring press, one could be excused for thinking that the motoring public, if they have the skills to tell subtle handling differences apart would also be able to detect grossly damaged suspensions - clearly not!
Are Jeremy Clarkson's car tests therefore the motoring equivalent of Jilly Goulden's wine tastings, telling us about things that few of us could ever discern?
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That would confirm the impression given by the fragments found in the road that breakages occur near the ends, not in the middle of the spring. Presumably a fracture in the first coil just results in the suspension dropping perhaps an inch, but a break in the middle would surely let the two halves overlap and cause a very noticeable drop, if not a complete loss of the spring on the rebound.
Even so, I'd have thought the resulting misalignment would cause a twanging noise of some sort, like getting a stone stuck in the seating.
PS Why are they called cushions, since their purpose seems to be the exact opposite of cushioning?
Edited by Cliff Pope on 02/09/2009 at 12:54
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PS Why are they called cushions since their purpose seems to be the exact opposite of cushioning?
It's my fault ~ I copied the word from a previous thread. I think the government calls them road humps. tinyurl.com/l6yg96
Edited by L'escargot on 02/09/2009 at 13:06
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No, it's the real name:
www.dft.gov.uk/pgr/roads/tpm/tal/trafficmanagement...0
It's a sort of Orwell-speak
cushion = bump
calming= getting you annoyed
consultation = telling you
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Why are they called cushions ..
I presume purely because of their shape, roughly like a square pillow. I think Hump refers more often to a narrow mound stretching almost the full width of the carriageway? A properly made Cushion can be (almost) avoided by careful steering.
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As a related follow on question, is it better to drive over speed cushions one wheel each side or is it better to drive so that one side is on flat road and the other goes over the full height cushion? Or is there no real difference from a wear / damage perspective?
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