Suspension - Do Springs Wear?

Sorry, same question: do springs actually degrade with age? If so, how?

Front springs known to rust out and snap with age on 997 so expect same on 996 with cars being so similar

Not so much wear, but "sag". Think about the rear springs, they're being compressed by the engine and rest of the back of the car 24/7, 365 days a year. They're simply a coiled metal rod. The shape of that spring will compress permanently very very slowly under that weight. These cars are approaching 20 years old now, so that's a lot of time for the springs to sag. Would be interesting if someone could post the brand new wheel arch clearance for your car, then you could compare it and see just how much your springs have sagged. In all honesty, I'd be much more worried about the bushes in the joints between the suspension components as well as the gearbox and engine mount bushings. These are much more likely to have noticeably perished/hardened/split over that time and will have a noticeable effect on how the car handles.

This is my understanding too. But does the "sag" have any effect other than ride height? I.e. does work hardening in the spring lead to a change in spring rate? I have just done an (almost) full suspension refresh - the only original parts are the springs, so I'm interested in anyones experience here !

I haven't read any scientific analysis that I can point you to, but I would expect the spring rate to change over time with the sagging. Think about it on a smaller scale, if you held a spring slightly compressed in your fingers for 20 years, you'd expect it to not be as "springy" as a fresh one, wouldn't you? :? I'm surprised you didn't change your springs with your suspension refresh - they're not a particularly expensive or difficult part to swap for new?

Yes, that's where this question comes from. I'm thinking of getting my dampers rebuilt by Bilstein and I was advised (not by Bilstein) that to get the most effective package I should replace the springs. I'm still not convinced that it would make that much difference.

Maybe not, but it's not expensive to replace the springs? I think most people find the best benefits come from replacing worn out bushes wherever you find them. It let's the standard suspension work like it was supposed to from the factory.

Interesting point, I just changed my front springs, in the interest of cost saving bodgery, I picked up some US spec genuine 996 mo30 from design911 for 60 quid a pair as I guess no one wants them. the US spec springs are the same on the rear but 10mm higher on the front. So the new springs are now 20mm higher on the front as the old springs had sagged 10mm. Anyhow it suits me as I dont ground my splitter on the way out any more. Anyway back to the original question, the new springs despite being mo30 whereas my car is standard, do feel more supple on the road, so I attribute this to less sag and possibly work hardening, of course you may change them and feel no difference, but I did last week. also I find its easy to change them without removing the strut, just loosen the coffin bolt, remove the droplink from the ARB and the wheel will drop enough to wiggle the spring out, once you have removed the top strut nut. Just put the new spring on and use a jack and an block under the disk to put the spring back in place. Remember to tighten up the coffin bolt with the wheel at normal road position .

I've done a little bit of spring life analysis in my working life (small springs for pneumatic valves/spag clutches, but the theory is the same). Most springs have a design life, a maximum number of cycles but the definitions (for safety critical parts) should always be conservative. At least an order of magnitude (10x) more cycles than you would expect the spring to see in normal operation. They are designed to operate comfortably within their elastic limit, so no plastic (permanent) deformation occurs. I guess if bump stops fail (which they do commonly on the 996) it's possible springs may get over compressed in use, that could be a cause? I'm also not ruling out the possibility there may be effects of settling over 20 years above zero, but I would suspect almost 100% of any settling seen in real springs would be due to corrosion, or surface cracking due to corrosion. If you measured the spring rate of a new spring in a lab and then again after cycling it with 10 years with of driving with no corrosion there should be no appreciable difference in the spring rate. This is similar to the kind of testing we did perform (only accelerated life where you cycle it as fast as you can without it getting hot). Springs don't tend 'wear' really.

Thanks for the analysis; I would have thought that eventually a spring will fatigue, or more likely fail due to corrosion. That being said, my original front springs cracked and failed at only around 30k km. Not sure what drove that issue, as I'm fairly sympathetic and didn't take speed bumps like the Yummy Mummy's in their Range Rover Sports-ah [(c) Smith and Sniff] which lasted about a year...

Just had my 986 done at Centre Gravity, at 20yo it had sagged circa 20mm IIRC. Springs most certainly do wear. I'd not get new dampers without springs for the sake of a few hundred squid. Get it all done now and it's done for the discernible lifetime of the car in your ownership IMO.

That's really interesting - but perhaps counter-intuitive? My simple mind struggles to understand how a spring can sag, but not change its spring rate?

Fair questions, i was trying to explain instead of just make statements but I missed the summary of what I was trying to clarify. Spirings do fatigue (wear), but their designed fatigue life will be way more than you're likely to do in 150,000miles driving (150,000 miles is a typical VW design life). Therefore, you should be able to decide on whether a spring can be re-used just from visual inspection when you remove them. Corrosion is the biggest factor affecting the change in performance you'll see between a new spring and an old. So if you take your springs off and there's little or no corrosion then there's little or no point replacing. If a spring has sagged 20mm then this is likely because it is badly corroded. The outer surface of the spring adds the most stiffness so not only does rust affect it in the worst area, but that now damaged surface sees the highest stresses. This can lead to cracks starting to form and that will definitely lead to a change in spring rate in the corroded/cracked area.

Extremely interesting, thanks for taking the trouble to explain! :thumb: