After putting close to 3,000 miles on our Allard K3 since rebuilding it from a 'basket case', we experienced a frightening event. This past May on a trip to Paso Robles (120 miles from home) for a car show. A few weeks prior to that trip I had the car up on jack stands for its annual service and inspection. All bolts for the running gear and suspension were tight at that time as observed by the safety wires or, in some cases, cotter pins.
About an hour into the trip we began to notice a vibration, and at one point pulled over to see if anything was loose. Since we could not see anything, we decided to push on. The private road going into the show is a rather rough washboard blacktop. Because of that we were only going about 15 mph when we suddenly heard a bang, and the rear end dropped down as the car came to a sudden stop.
Fortunately some guys involved with the aircraft museum came by and were able to loan us the proper equipment to get the rear jacked up so we could do some diagnosis. The basic problem was that the (2) ½” bolts that secured the right radius rod (trailing arm) to the DeDion tube and outer hub housing had sheared. These guys were able to provide us with a couple of temporary bolts so we could drive to one of the hangars where they re-build vintage aircraft. We must have looked pretty trust-worthy, for they gave us the run of their shop and parts bins. As we were doing those repairs we also checked the left hub, and found that the front bolt securing that radius rod to the mount housing was also missing. We were able to install some grade 8 bolts, drive the car into the show, and subsequently drive back home that evening. While grateful for all the help and good luck, one can also envision the consequences if that failure would have occurred at highway speeds.
After dismantling the rear suspension (DeDiontube and right outer hub), we discovered the primary cause of the failure. On the hub housing, there is a curved flange by the mounting eyes that prevents the radius rod mount plate from fitting flush, forcing a .030” gap between the two faces. Because of the radiused flange, it allows the user to tighten the bolts so they think the whole assembly is secure, but in fact there is a measurable gap that is not visible unless you look at the faces from an end view with a flash light. We deduced that this gap allowed the hub housing and radius rod mount plate to move in relation to each other enough over time that they caused the bolts to fail. Additionally, we found that one of the mounting eyes on the hub housing had broken. We have been fortunate to obtain a replacement hub housing and are proceeding to replace the damaged components with (1) washer (approximately 1/16” thick) for each bolt between the two faces along with grade 8 bolts and Nylock nuts (no castled nuts).
We have contacted a few other Allard owners with DeDion rear ends and they too have found that they have washers installed between the two mounting faces. We also contacted David Hooper and Dudley Hume (ex-Allard Engineers) to see if they recall hearing about this issue and they did not.
It’s not known if the washers were a factory solution or a field fix, but we recommend that you inspect your DeDion equipped car immediately to see if you have a gap between these two parts and if the mounting eyes are in good shape.