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Spring hanger failure

by

Mike Meakin

Our 7 week touring holiday in France this year came to an abrupt end in the middle lane of the M2 at 60 - 65mph. The outside arm of the offside spring hanger broke away, leaving the short stubweld still attached to the chassis rail, releasing the spring - instant collapse of right front suspension. Fortunately, we managed to get on to the hard shoulder, just a couple of hundred yards from the slip road to Cobham Services. Lifting the bodywork off the driveshaft allowed the car to be pushed in to the service station area to await the recovery truck.

Once home, with the car on axle stands, close examination showed that the hanger bracket had "torn" along the weld bead (only 1" long), spring action twisting the hanger inwards. Contact with Phil Gregory revealed that this had occurred with other early chassis, as a result of which, later frames were modified by increasing the hanger bracket material from 25 X 3mm to 30 X 3mm and adding short, straight strip braces to the outside arm (to the side rail) and from the straight top of the "crescent" infil in the bottom of the hanger to the frame crossrail, both at about 45 degrees. Short stub welds of 30mm was considered sufficiently strong to prevent detachment.

My first priority was to re-attach the sheared bracket temporarily, sufficient to be able to refit the spring and pull rod, so that the car could be moved around. Thanks to Tony (Weg3) for that!

sketch of reinforcement
sketch of reinforcement
angle brackets
finished reinforcement
finished reinforcement

Initially, I considered replacing all four spring hangers with 30 X 3mm strip, but the inner arm of each is seamwelded along one edge for the full depth of the chassis rail. The next notion was to make up a hanger bracket overlay, with a 90 degree "foot" allowing for a 30mm X 3 weld to the frame rail, instead of the 25mm stub weld. In the end, I made up 4 sets of two brackets from 30 X 30 X 3mm angle-iron to attach to the hangers, either side of the pull rod, to have 2 seam welds to the cross rail at the top and a vertical and curved seam weld at the bottom, plus a straight brace (at around 45 degrees) from the side of the hanger bracket to the frame rail. Now with welds tidied up and bare metal repainted, it's not the prettiest repair, but in my opinion, plenty strong enough. (Must get some fresh "Smoothrite"!)

During the conversations with Phil Gregory, it seems that the frame design has changed from my early ( chassis #188) version. From the PMC supplied images, it appears that the outer box section has been replaced with a 30 X 3mm braced, tension strip. I wasn't able to get details of what modifications had been incorporated from specific frame numbers.

Guy Gregory wrote an article with pictures showing the modifications later chassis have to reduce the problems that Mike and others have suffered. The Editor

I remain of the opinion that cold welding a short, stub weld strip to 16 swg sections without "tempering" the assembly runs a risk of fractures at weld beads when stressed - I have had so many instances of this on my car, repaired by the insertion of triangular braces, seam welded in 2 directions. I must however stress that this is my amateur, unproven opinion and remind readers that the described repairs are what I consider adequate for my own safety - anyone choosing to follow the same route, does so at their own risk.

Speculating on the cause of our hanger failure, we have been very aware of the proliferation of "speed humps" in France mostly in 30kph limits, outside Town Halls, schools, hospitals and for pedestrian crossings. Most, but not all have warning signs but of concern is that they can vary between a smooth raised section and a "sloping kerbstone". We took each one slowly and deliberately, to the obvious displeasure of French WVM (white van man) who invariably overtook us at speed over the ramps. The cause of the unbraced outer arm of the spring hanger is thought to be the springs being lightly loaded on full drop and as a result, out of alignment fore/aft. The twisting action when coming back in to tension overstresses the outer arm stub weld. The original hangers, formed from flat 25 X 3mm mild steel strip are unbraced. The PMC solution has been to increase the strip size to 30 X 3mm and to add side and forward braces to prevent the hanger twisting. Bracing can be added to the car without any dismantling. I would advise all early frame owners to fit bracing to their cars.


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