Broken Spring

[Alex 21]

I was thinking binding upper mount. The top end must rotate freely to allow steering, also the spring itself needs to wind up and down as it works. Too much resistance would load the spring with a force it really doesn't like much, and would have a distinct break pattern, different than torsional loading and looking more like a shear. Need a good picture of a fresh break.Edit: You would feel that quite strongly in the steering I'd think, and would get it fixed promptly before it would break a spring. Just an idea.

Edited February 28, 2012 by Alex

[MikeT 747]

That would have to be a hell of a seized steering swivel! The Peugeot 404 I drive in the eighties when I was a student had seized swivels and the lower part of the spring just slid along the lower cup mount, making a bit of a boinging noise on the way. I replaced the swivels after a few years and I still have the 600,000 km springs! I think on the smart, should the swivel seize, the lower part of the smart spring cup should do the same?

[tolsen 630]

The smart's springs are helically wound torsion bars, so I think shear is an unlikely failure mode in anything but a very severe collision in which the spring is impacted.

Let me rephrase my explanation:A coil spring is loaded in torsion causing shear stress in the round bar spring material. Shear stress is zero at centre and maximum at surface. Using Mohr's circle, one can resolve the shear stress into principal stresses. One principal stress will be tension and the other compression. Principal stress orientation is at a 45 degrees helical angle to the bar of the coil. A brittle fracture in the coil spring will start at the surface and propagate through the coil bar at a 45 degree helical (or twist) angle to the bar.Remember the practical demonstration of brittle fracture from our school days: Twist a piece of blackboard chalk until it breaks taking care not to apply any bending. The fracture is just as described above.

[MikeT 747]

Whew, I thought you might have been wrong for a second yesterday!

[tolsen 630]

The graph shows UK spring failures per months among AA members from Nov 09 to Jan 11. The failure rates are highest during cold weather months, however slightly offset to the right. Low temperature and high stress are prerequisites for brittle facture. Brittle fracture will also occur at normal temperatures provided stress is sufficiently high. Potholes appear after road surfaces have thawed out. These are often not repaired before end of summer hence explaining lowest spring failure rate in October.

Reduce risk of spring failures by ensuring the coating on the springs is in good condition and not damaged when being fitted. Hook type spring compressors should therefore not be used as they damage coating.

The reason for the high spring failure rate seen during recent years is manufacturers drive to reduce weight. The springs have been made lighter and therefore much higher stressed resulting in brittle fracture and spring failure when conditions are right.

[pinhead19 0]

I even bought the special clamp after struggling with the first one.

Do you have the number? I could not seem to order it.Or maybe you want to rent/sell yours?

[tolsen 630]

Do you have the number?

Two part aluminium clamp - 451 589 00 63 00.

46 mm open ended spanner - 450 589 01 01 00.

Dealers are not supposed to sell special tools to the general public.

It is cheaper making your own:

Material is oak, but any good hardwood ought to work. Finished tool dimensions are 20 mm thick x 50 mm wide x 340 mm long. I shaved off 0.5 mm from inboard side of each stick making the 18 mm hole oval thereby gripping better on shaft of shock absorber. Ideally material should be slightly thicker than 20 mm, perhaps 30 - 35 mm and use 8 mm bolts. I used some oak sticks I had in stock hence reason for making tool only 20 mm thick.