Article: Why do some De-Dion tube’s need to be changed? PistonHeads article

Tuesday 6th August 2013

Well… changed the old rotting spax rear dampers on the back of the car for some new AVOs fitted with stiffer 150lb springs.

Took the car round the block, ride was beautiful, seemed great.
Took it for an extended test ride, it was going beautifully, riding well, then as the speed increased it got twitchier and twitchier form the rear, nearly binned into the scenery on a fast sweeper, felt like the from wanted to tuck under the car.

Pulled over, kicked tyres, checked wheel nuts were on properly, checked the bearings for play… nothing.

Drove home very carefully, car felt like the rear was on a skid pan, wanting to shoot out either way, noticed both rears were squealing when going straight ahead, gave me my first clue, I’ve somehow altered the toe.

Got home, rears were red hot (driving at 30mph), definitely a load of toe . Sure enough, got my metre rule out checked the alignment, they were toed maybe 5-10 degrees :0.

Wheels off, checked id bolted everything together right, all nuts and washers on tight. No problem…. hmmm.

Wheels on drive down the road, felt like the car wanted to kill me, like the rear was on flat tyres on ice. Got 100 yard turned around, wheels off….

AHHHHH, socking great bend in the dedion tube, which I hadn’t noticed… obviously, recent work on the rear, new stiffer springs etc. had been a push too far for the fatigued tube and it bending was causing the massive toe in!
An unlucky coincidence for the new AVO dampers, which otherwise seem very good. smile

Anyway did a bit of research and found this fantastic thread.

http://www.blatchat.com/t.asp?id=94283&pn=1&am…

If like me you were previously blissfully unaware of de-dion failures, give yours a check… I believe I was quite lucky that I didn’t bin it.

Apologies as I am sure it had been discussed before but a bit of awareness for people newer to caterhams such as myself is not a bad thing.

Tuesday 6th August 2013

After reading about problem online, could have well been that thread, I checked mine. One of the mounting points for the rear suspension was slowly ripping itself out of the tube. Luckily I spotted it and then stopped using the car. A new tube was ordered and fitted at the several hundred pounds that they cost. The new one is beefier and gusseted more.

Tuesday 6th August 2013

Just checked mine and its cracked by the weld in the middle. I also noticed that the rear end was rather lively in a straight line and couldn’t initially find anything wrong, even tried different tyre pressures.

Having looked at the photos on the BlatChat forum as I was able to locate the fault. Thanks for the heads-up.

Tuesday 6th August 2013

Blimey, you were lucky to catch that… ordered a new one this afternoon, they are very expensive for what they are I think, but we needed one. :/

Will post up pictures where it went when the new one gets here and I take the old one off.

Tuesday 6th August 2013

How expensive is expensive?

Mine is just a small crack in the centre section as far as I can see without removing the tube, almost tempted to weld it until the winter lay up….

Tuesday 6th August 2013

Mine is just a small crack in the centre section as far as I can see without removing the tube, almost tempted to weld it until the winter lay up….

£306-318 plus delivery depending on Caterham (SV, etc).

Saturday 10th August 2013

New tube fitted and its a completely different car.

The old tube had split in two different places either side of the centre bracket, adjacent to existing welds. New tube is definitely a lot more substantial than the old one.

Saturday 10th August 2013
The new ones are 3mm CDS compared to what IIRC was 16 SWG/1.6mm for the old ones. Much longer lasting.

Saturday 10th August 2013

You can weld the tube up and plate it in the known weak areas for extra strength.

So far as I am concerned I would never trust the under-slung damper mounts. For me they were a cost cutting exercise too far.

Sunday 11th August 2013

Having looked at how far out the geometry goes when the tube cracks in the centre, I don’t think welding is a realistic option although I agree it looks possible.

With the cracks shown in the photograph it was almost impossible to push the car because of the change in rear wheel geometry.

Sunday 11th August 2013
Just a comment why would you risk your safety for £300 and try and weld if its cracked in one place you won’t know if other area’s are corroded or compromised its a high performance car…….it may be simple in its design but its just not worth the risk not replacing the whole tube. One owner I know said the tube had looked like it had been on the bottom of the sea so decided to paint it and was surprised when it failed the blamed the manufacturer for a poor design! rather than take responsibility for not getting it checked!!!
Sunday 11th August 2013

You would obviously set it back to straight before welding, although plating it has a tendency to distort due to the heat.

Nothing shimming the ears can’t sort though.

Sunday 11th August 2013

Just for those who haven’t read the excellent thread on Blatchat about De Dion tube failures.

The earlier De Dion tubes that tend to fail have different designs, but one thing in common, they use 12 gauge steel tube.
Current tubes are 10 gauge and consequently much stronger and heavier. I don’t know when they swapped to the heavier gauge steel but it was many years ago.

If the tube has failed, and is of the earlier thin wall type, it seems a bad idea to even consider welding it up.
Even adding plates is just going to move the stresses along to where the thinner steel starts again.

When building my Seven there was early surface corrosion on the unpainted inner surface, I sprayed cavity wax in from both ends, but I am sure most tubes don’t get this. A bit of condensation in there and the tube will corrode from the inside out. If I had an older Seven with the thin wall tube, I would order a new one now and fit it. £300 for peace of mind seems a small price to pay.

Sunday 11th August 2013
My comments were with reference to making the 3mm ones stronger. They also break.

Sunday 11th August 2013My comments were with reference to making the 3mm ones stronger. They also break.

Thanks for the warning, I had assumed my 2010 De Dion should be OK (as the thread says no known failures of version 5 or later), but I will keep a closer eye on it when servicing in the future.
Sunday 11th August 2013

Just to be clear, I believe that failures of the de-dion tube where the shock absorber brackets are located may be caused or aggravated by corrosion.

Failures around the middle section as shown on my photograph (above) are probably the result of fractures resulting from stresses caused in the original welding process. There was no sign of corrosion at the point of fracture on my car.

And yes, the new tube is a lot more substantial than the original version.

 

via De-dion tube fun (and a warning for folks like me) – Page 1 – Caterham – PistonHeads

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