On close inspection, we found that the De Dion tube was cracked almost all the way through on the off-side at the shock absorber mount. The shock absorber is mounted into a tube that is welded onto the bottom of the De Dion, and it looks as though that weld has cracked at the most highly stressed point and grown from there.
I was initially surprised that I hadn't spotted the crack when I was doing the rustproofing in that area, but it's clear from the photos that the powder coat was still intact until the crack grew big, hiding it from view.
So, having being recovered home from Ireland by the RAC (I didn't fancy having my wheel snap off on the motorway!!) and taken the tube off, it's time to contact Caterham.
Clicking the images below will enlarge them. The first three are in situ, the others are with the tube removed. The last image is of the nearside, and shows that that is cracking as well.
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As an Engineer, I'm surprised by the arrangement now that I've studied it in detail. The weld is subject to the full bending load from the shock, yet the weld is the very weakest area, full of chemical impurities, crack initiation sites and surrounded by a brittle heat affected zone from the welding process. The cost of designing this mount safely must have been high!
Craig, There have been isolated incidences of the de dion tubes starting to crack. This is not restricted to 21's (in fact, I think yours is the first - purely because of the low occurence of this and the small number of 21's). We have investigated this on more than one occasion and every time, the result has been that the most likely cause is the damper bolt working loose. It is worth noting that this design has undergone motor industry durability testing and is proven. Therefore, thank you for bringing it to our attention, but in this case further action from us is not required. I hope this information is of interest to you and should you have any queries, please do not hesitate to contact me. Regards SIMON LAMBERT Aftersales Manager
This seems to be a pretty safe line for Caterham to take because there is no way under normal circumstances that the owner can prove that their damper bolts were tight. Mine were done up tight enough that I had to smack a 12" spanner with a 4lb lump hammer to shift them, but I have no way of knowing if they have always been that tight...
And here are some responses from various se7ens mailing lists:
> > > The mounting points for the springs are, or at least they were on Craig's, > > > welded onto the _bottom_ of the tube inset into a slot which has been > > > machined, or perhaps filed, into the bottom of the tube. As such there will > > > be huge tension on these welds and it is not surprising that they crack. > > > What's more, the mounting are threaded, blind, tubes meaning that there is > > > also a twisting effort being applied to the mounting. The old mountings (a > > > tube through the de Dion tube and welded in) which are also on Craig's tube > > > are a thing of beauty in comparison. > > > > > > It's just about (with exception of that Locost 4x4 thing) the > > > worst piece of engineering I've seen on a kit car. > > > > > > The equivalent thing on the de Dion on the back of my Dax is completely > > > different and properly engineered. But, I suppose it has to be because > > > people put 500bhp Cossie engines in them. > > > > To misquote Alan Staniforth, the chassis is essentially a 3D bracket to > > which the suspension is bolted. Making it stiffer is always a good idea. > > > > I too was deeply appalled by the shocking piece of accidental > > mis-engineering that was that Caterham de Dion tube. It's fairly hard to > > think how it could have been designed worse... > > A Vee shaped notch in the centre of the tube?
> > > I heard it said it was because the springs they wanted to use were a > > > bit longer so they welded the tube in the bottom. It does seem > > > extraordinary though... > > > > you ask the spring manufacturer for a spring. they ask: how long, > > what diameter, what rate. simple! > > > > you don't look for a few lying around then bodge the car up to make > > them fit! > > They wanted some progressive springs which were going to be more > expensive to manufacture to their spec....so they bodged the car > instead.