Rear springs & shocks.

[MetBlue]

Rear suspension on my car can't be as Mr Chapman intended and designed it.
I know roads these days are bad, especially around Northampton, but if I hit a bad surface mid corner, the car seems to skip and jump. On straights that are poor surface, it rides very hard. I'd hate to drive it on cobbles.
I rebuilt both front (front rides fine) and rear suspension 2 years ago. New bushes and dampers to front, new dampers (Lotusbits remake supposedly to standard rates) and top spring seat to rear. Rear bushes all looked good, so I left them. Rear torqued up correctly at ride height great care taken to ensure springs sit correctly in the top polyurethane isolator (well known the inner dia is bigger than the ID of the springs, so chamfered first to guide spring in). In short, I'm confident all parts are assembled correctly, but ride is harsh.

I've read many items here where the SJ Protechs have received positive reports, so I'm going to try a pair.

My question is, is it worth changing springs for new at same time? Current fit are the originals, which I got powder coated 2 years ago. I know both springs are the same rate but can a spring Work Harden?

Thoughts from the wise please.

Tony

[rbgosling]

"...especially around Northampton..." - as I also know all too well!

"Work hardening" changes the strength of a metal, i.e. the load at which the material will start to yield. Yielding is the point where the material deforms enough that it does not return to its original length/shape once the load is removed. A road spring should not ever reach the yield point in normal use, you will hit the bump stops or even get the spring to fully compressed (i.e. each coil is contacting the one above) and it should not yield. So, even if you did shift the yield point through work hardening somehow, it would not have any effect on spring behaviour in normal use.

Spring stiffness depends on the stiffness of the metal, also known as Young's Modulus. It's pretty much impossible to change this value, it's fixed by the alloy composition and nothing you can do to the spring short of melting it down will change it. The only thing you can do to change the stiffness of a spring is to change its geometry. I did find that "changing the geometry" of my front springs (i.e breaking them!) did stiffen them up a bit!

Even if you did manage to overload the springs past their yield point, the effect of that would be to make the coils a little closer, so the spring would be shorter. But, with the same number and diameter of coils and the same thickness, the spring rate will still be the same, the car would just sit a little lower.

[Pete Boole]

Sounds like your ride height might be a bit low? The rear dampers have very little travel before you're on the bump stops.

Pete

[John]

Hi Tony

Like you I've wondered if road springs harden over time, thanks Richard for sharing your engineer's knowledge on that subject. Maybe the polyurethane top isolators have less yield than rubber? I've heard of people changing from OEM rubber bushes and they've wound up with a harsh riding suspension, and suppliers now offer different grades, race or road.

Was it riding well to start with, then gradually worse? If so, can only think that top bush may have split, faulty damper, or you've got a broken spring.

John

[MetBlue]

Was it riding well to start with, then gradually worse? If so, can only think that top bush may have split, faulty damper, or you've got a broken spring.

Its never really been right. Might even have been better 3 years ago before I did any work on it - I really can't remember. I do recall that the rear original shocks seemed to still be working OK ( when moving free after removal), but I'd had to destroy the lower eye on one side to get it off, so can't try putting them back on. .
I know the lower bushes on the new shocks I fitted are soft and extruding out, which can't be good.

Thanks Richard for your stress engineers / metallurgical insight. I've chosen for now to just order the SJ Protechs and stay with original springs. If I change just one thing at a time, I'll have a better idea where the real issue lies.

Tony

[John]

Makes sense, the skittish handling over rough surfaces could be faulty damping so thats a good place to start. The dampers could also be causing the hard ride.

From what Richard has said your original springs should still be fine. Modern road springs seem to break more often, probably thinner coils, cheaper steel and dreadful road surfaces. My first cars were Triumphs, and I never had or heard of MOT failure from a broken spring. When my modern daily driver failed with a bust spring I thought the garage were conning me! Turns out its become very common.

Cheers

John

[rbgosling]

...Modern road springs seem to break more often, probably thinner coils, cheaper steel and dreadful road surfaces.

I've managed to break both original front springs on my Excel. That said, it has covered over 200,000 miles in 25 years, the last 9 of which on the aforementioned Northamptonshire roads, so not entirely surprising.

To add to what I said earlier, while I'll stick to the stiffness not changing over time, repeated flexing could conceivably make the material more brittle. Also, while spring steel should be fairly immune to fatigue (failure at well below the yield strength, caused by repeated load reversals gradually enlarging what start out as minuscule cracks), no steel is completely immune, so age-related failure is definitely a thing.

[Marten]

Also, while spring steel should be fairly immune to fatigue (failure at well below the yield strength, caused by repeated load reversals gradually enlarging what start out as minuscule cracks), no steel is completely immune, so age-related failure is definitely a thing.

The crack probably starts with a surface defect caused by corrosion

[Pete Boole]

I'm currently replacing a broken front spring on my wife's Audi. Pig of a job.

Pete

[MetBlue]

Following consensus advise from site, I placed an order with SJ's just for the Protech Dampers.
Very impressed with their service. Order placed on the Monday and delivery driver knocking on door by 8:15 AM on Thursday. And that was with the standard delivery option.

Impressed by the fit and quality, although a couple of observations.

The spring was slightly smaller ID than the spring plate register. Resolved by clamping one side in place with a tool makers clamp, and the drawing the other side on with a G clamp.


The first washer between the damper and the lower rubber top mount is 13mm dia as supplied, but the rod is 11mm. It would have probably been OK, but I had some large thick washers from the original dampers with correct size hole, so used these. Just feel more comfortable without the potential for movement. Photo below shows both washers in place, unused one on top.


Lastly, just an observation that the Protech does not have the two dished washers isolating the rubber bushes from the chassis. Rubber sandwiches straight to the chassis. I pondered for a while whether to use the old dished washers, not least because I've never seen the rubber going straight to chassis, but in the end went with no washer ( as supplied). Main reason was that I never felt the register in the old solution was a size for size fit in the chassis hole anyway and I'd always been suspicious of movement. Any other thoughts on this? - Protech on the right in photo below.


Fitted Saturday and I can report ride and handling a significant improvement. Damper set on the 5th notch from softest currently - Not yet played around with others.

More to follow cos it's not yet 100%, but we're getting there.

[MetBlue]

Over 500 miles into the Protechs now and can report a significant improvement in ride, handling and knocking noises from the rear.
Damper settings notched up to around 7 during runs in the Peak last weekend. Ride has not suffered too much.

One reason driving this change was noises (Knocks) coming from the rear. I'm now convinced there are several sources for these knocks. Having removed the damping related ones, I could home in better on other sources.
Now convinced something was moving in the boot, took a look at the jack storage only to find the elastic strap broken and Jack able to bounce around. Now secured - Better but some noises still present.
I know the diff bushes are breaking up, so that's next. One way to generate a noise is to hammer into a roundabout in a spirited way, which would be consistent with side loading and diff movement.

Probably should retitle this thread "Knocks from the rear"

[Pete Boole]

Great progress Tony. Always satisfying to get rid of rattles!

Pete