Ideally always replace in pairs, in most cases only replace in pairs
only in certain case replace singulary.
ask yourself, what is a spring? how does it work? what differences are there in different springs?
firstly, a spring is the shock absorber, it not the damper takes the shock of bumps as the suspension does the work. all the damper, (all too often called the shock absorber) does, is damps the oscillations the spring causes by virtue of is operation.
a spring, is basically a rod, that is twisted by the suspension movement, think torsion bars, thats all a coil springs is, but 'coiled' into a smaller design.
what the spring is trying to do, is return to its original shape, as it does that it overdoes the movement, causing the oscillations.
in time, the spring - as does all metal - suffers from the twisting, bending if you like, and becomes 'tired' less able to support the weight, or return as quickly to its original shape.
the difference in spring is in fact many fold, but basically comes down to the strength, and speed it will return to its original shape.
many vehicle manufacturers list springs according to the spec of individual cars, BMW and Merc for instance have charts of options that have to be taken into account, then added up, to arrive at the correctly rated spring, factors on the whole use a 'one fits all' approach.
IE, 2.0ltr engined saloon 100 points, air con, 5 points, factor radio 1 point, electric memory seats 4 points, total 110 points.
2.5ltr 120 points air con, 5 points, factor radio 1 point, electric memory seats 4 points, total 130 points = harder spring.
in conclusion newish car, I may replace one, but asking 'what caused the breakage? a bad batch?
but would definitely replace a pair on an older car, or if not using genuine springs.
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I have only ever replaced springs individually as well, hence why you can buy them singularly and never had any problems. Dampers and shockers yes I will agree should always be changed in matching pairs but springs no. Its the same as tyres in my opinion, ideal to be done in pairs but in reality does it make that much difference if you don't?
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I'd change fronts as pairs,rears singly.
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The important thing is to buy properly made ones with chamfered end-coils, so that there is a smooth and progressive seating inside the cone. Ones that look as if they have just been chopped out of a continuous production of longer springs will soon break again.
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>>Ones that look as if they have just been chopped out of a continuous production of longer springs will soon break again.
I don't think that's necessarily true, and it depends upon how the spring was designed in the first place - i.e. spring end detail matching the seat detail.
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NC - I thought it was generally accepted that modern springs break so often because the ends are not finished off properly, and do not seat in a moulded fitting?
When you look at a properly made spring assembly it has 3 features:
1) The final half turn or so gradually tapers, as if its outer surface had been ground down
2) the coils are closer together at the ends
3) the coil sits on moulded rubber seats
The result is that the weight is spread over a considerable portion of the final coil, and further compression gradually and progressively lengthens the area of contact.
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Hi Cliff,
>>I thought it was generally accepted....
I'm not so sure - suspensions with unfinished spring ends have been in use for a long long time, but coil spring snapping seems to be becoming more frequent in recent years. So, I think that if the end detail were the root cause of the problem, it would be an old problem, and there wouldn't be a sudden rise in the numbers of failures.
>>When you look at a properly made spring assembly...
There's nothing improper about designs without those features - coil springs with unfinished ends usually sit onto sculpted spring seats, and similar contact regimes are operating.
HJ mentions about the problem being prevalent among Western cars, and he has drawn the conclusion about the spring seats. I'm not so sure.
What we can't see are all the other factors which might be responsible;
- Spring material
- Spring anti-corrosion coating (possible a nasty material like Cadmium being made illegal?)
- Spring design stress (and margin against material limits)
- Spring heat treatment
- Spring wire surface finish (fatigue cracks can grow from machine handling marks or grinding marks)
An example of another possible cause might be that to obtain a lower bonnet line, the spring design is constrained to use a lower number of active coils, pushing up the stress.
At the moment, it seems to me that people have looked for the most visible difference between the springs, and have automatically ascribed the failures to that. They *may* be right, but I'm not sure.
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>>They *may* beright but I'm not sure.
Thanks NC - that's interesting, and you are right, I was just jumping to an obvious but as you say not the only conclusion.
I have only ever had cars with shaped spring ends as I described (apart from several with leaf springs) and I have never had one break. Some have been over 40 years old or covered many hundreds of thousands of miles.
Unlike cart springs they seem unbreakable. My only reason for ever replacing them has been when they eventually sagged, but obviously I then replaced them in pairs.
Edited by Cliff Pope on 28/02/2009 at 11:32
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Prompted by this and another thread on the subject, I've had a look in my copy of the SAE Spring Design Manual. In the section on the design of helical compression springs, I found a passage describing the effect of dynamic loading on the spring.
The sudden loading causes a stress wave to propagate in the spring, and this stress wave superimposes itself over what might might be termed the quasi-static stress in the spring - i.e. if you slowly compress the spring, the stress will rise to a given value, but, if you compress it quickly, the stress will, temporarily be higher.
For any steel compression spring, for each metre per second of loading velocity, the increases by 35.3MPa.
How fast might the suspension be loaded?
For the moment, assuming a solid tyre;
v_mph=70; % convert to m/s v_ms=v_mph*1609.3/3600; v_ms
v_ms =
31.2919
OK, at 70 mph, the car is travelling forwards at 31 metres per second or so. It's easy to imagine that if the car hits a pothole, that the suspension will move upwards at, say, 10% of that speed, giving something like an extra 100 MPa for that stress cycle.
Two effects which will make this loading more onerous - 1) the awful condition of our roads, and 2) the modern preponderance of stiff, low profile tyres, which would more closely mimic a solid tyre under a sudden loading than a tyre with a proper sidewall.
The SAE design guide goes on to say;
"The influence of the loading function v(t) on the life of springs under sinusoidal loading has been strikingly proven by fatigue tests: The average life of springs under sinusoidal loading may be reduced as much as 50:1 under impact loading, with the same stroke and frequency"
It seems possible that manufacturers spring design rules may not have properly kept up with changes in conditions of both the roads and the vehicles. Again, I only advance this as a possibility - I'm not in any way saying this is the definitive cause of the problem.
The SAE design manual does talk about the spring end types, but only as far as giving design guidance for how many of the coils may be considered as active during spring compression. No mention is made of any heirarchy of spring end types and fatigue resistance.
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I wouldn't consider replacing springs as a pair if I was paying. Dampers brakes and ideally tyres yes but springs no.
I'd agree with you Hawkeye. My Mk2 Mondeo had one replaced at 90,000 miles. Shortly after, it passed its MOT so there was obviously nothing wrong with the other side of the car.
Springs are either broke or they ain't.
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>> Springs are either broke or they ain't.
Or they sag, or lose their springiness.
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Not on my Mondy they hadn't. It drove as tight as a drum and obviously passed the bounce test at MOT.
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Im in the always in pairs band (unless its of very young age/use).
make your own mind up, its legal to purchase and fir singly.
But
". It drove as tight as a drum and obviously passed the bounce test at MOT."
Is very misleading. The mot test on suspension is so basic it is untrue. If it failed the mot test it probably ought to be with a "dangerous to drive" warning too.
Dont be fooled by thinking the mot is a good test of spring/shocks. Its not. It comprises of a check for leaks/ broken coil / equal (by eye) bounce!
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well I'm not sure what you're trying to say then. If a bounce test is to check for broken springs or leaking shocks, and neither is found then the car is ok.
Alright it won't be as good as it was when it rolled off the production line but for the average driver pootling around the UK its fine, perfectly legal and safe.
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At my last MOT, I had a cracked front spring (that I was completely unaware of incidentally).
It's common on the front of the Vectra C apparently. As the car was five years old with about 65k on it I replaced the pair. GM springs weren't particularly cheap.
I have the old 'good' one as a spare now.
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My gran's Focus spring snapped and I replacedjust the one spring, no difference at all (believe me when I say that I was pushing her little car to it's very limits whilst it was in my care)
My old E36 had the same problem and had one spring replaced, absolutely no problem.
I am a very picky driver in most cases and spot handling problems easliy but even I am in the one at a time group.
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Interesting thread. My car (Golf Mk4) has just failed MOT with a fractured rear spring and my (local, trusty) garage just replaced the one. No noticeable difference at all, but you've all got me thinking now...The one thing I struggle with is how I can have a fractured spring without that side "collapsing", my minds eye has the spring holding the car off the wheel, if you see what I mean - clearly an incorrect assumption!
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