This topic is a sore point with me, I have been trying to get my views across for years, but several attempts to get them published in the relevant magazines have failed. Guy Worrall's problems with his drag link are another instance of many similar ones I have seen over the last 50 years. From the photographs I could not believe that the link would have been manufactured like this, It is clearly a bad design which was always going to fail because of the bend. This would cause continual stress at that point and the thread would not have helped. It looks as if somebody who had little real engineering knowledge made a bodged replacement.
When I had my 6hp Rover, I did try to bring some safety issues to other owners attention, in particular the stub axles being prone to breaking without warning. Unfortunately, in some cases, owners were swayed by cost issues to ignore this advice. I know of one particular example where the stubs were x-rayed and one was found to have inclusions in the metal. This was a clear sign that something was going on, but it was put back on the car and did fail after a while, luckily this happened in a pub car park rather than on the road. Another example was my Alvis Firefly, which had a Marles-Wheeler steering box. These came in all different sizes and were fitted to many makes of car. One of the components was machined with a very sharp radius, and this was a stress point at which a creeping crack would start so anyone with one of these steering boxes ought to replace it before ever taking the car on the road. I would always put a grinder through the flawed part and keep it with the car as part of its history. I cannot understand some peoples attitude in these cases, I heard last year of a car where the steering component was replaced with a modified part but the old part was sold off as being usable! I could write a lengthy article on examples like this that I have helped to sort out, in most cases the car was potentially dangerous to be on the road. Fortunately, all but one of the incidents were at low speed and no one was hurt.
I think that a register of know risk factors should be compiled for each make, and insurers should insist that any that apply to a car should be properly addressed before it goes on the road. Now there is no MOT requirement it is more important than ever that owners and dealers take full responsibility for ensuing a car is safe to be on the road. All kit cars have to go through an SVT test, so why not do the same for any car that has undergone major restoration?
Safety issues
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David Bliss
- Posts: 20
- Joined: Sun Oct 14, 2012 8:54 am
Further thoughts on the drag link failure
I think there is need to elaborate on the misgivings I had on the replacement drag link. There is more than one fault that could accelerate and/or cause this failure:
• The standard ball joint used probably increases the distance between the bend in the rod and the position where the rubber bushed eye end had been formerly.
• The rod might have been bent slightly sharper to allow clearance, adding these two things would put extra stain concentrated on the rod adjacent to the lock nut.
• A one-piece rod can flex over its entire length but fitting the screwed-on ball joint then stiffens the rod and again concentrates the strain to where it fractured.
• In an overloaded case, a sharply cut thread can start a stress fracture and cause a premature failure and, given that a solid rod has a central fulcrum, with the bend further out, was it of sufficient diameter for the modified part?
• Finally, if this rod had been heated in order to bend it, what metal was used? If somebody did the wrong thing with heating and quenching the metal could become brittle.
Many years ago I experienced an occurrence where a manufacturer made a slight change in the design of a component and also changed the weld type, this combination of changes caused problems. Luckily I picked up on some minor cracks before any failure had occurred and I contacted them. They sent me the design loading, they couldn’t see that two long gussets were weaker than four short, they had calculated loading in one direction as being several times the expected load in use, but there was a twisting force as well that they hadn’t taken into consideration. An incorrectly placed gusset moved the stresses to a weld with poor penetration, where a creeping crack began. They promptly started to look into the problem and found that I wasn’t the only one to experience it. As a result they supplied a brand-new front axle that had been welded differently, alleviating the problem. A very minor design change can cause unforeseen problems.
• The standard ball joint used probably increases the distance between the bend in the rod and the position where the rubber bushed eye end had been formerly.
• The rod might have been bent slightly sharper to allow clearance, adding these two things would put extra stain concentrated on the rod adjacent to the lock nut.
• A one-piece rod can flex over its entire length but fitting the screwed-on ball joint then stiffens the rod and again concentrates the strain to where it fractured.
• In an overloaded case, a sharply cut thread can start a stress fracture and cause a premature failure and, given that a solid rod has a central fulcrum, with the bend further out, was it of sufficient diameter for the modified part?
• Finally, if this rod had been heated in order to bend it, what metal was used? If somebody did the wrong thing with heating and quenching the metal could become brittle.
Many years ago I experienced an occurrence where a manufacturer made a slight change in the design of a component and also changed the weld type, this combination of changes caused problems. Luckily I picked up on some minor cracks before any failure had occurred and I contacted them. They sent me the design loading, they couldn’t see that two long gussets were weaker than four short, they had calculated loading in one direction as being several times the expected load in use, but there was a twisting force as well that they hadn’t taken into consideration. An incorrectly placed gusset moved the stresses to a weld with poor penetration, where a creeping crack began. They promptly started to look into the problem and found that I wasn’t the only one to experience it. As a result they supplied a brand-new front axle that had been welded differently, alleviating the problem. A very minor design change can cause unforeseen problems.