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Caterham A Frames - check them as part of your annual maintenance


Croc
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This is applicable to S3 and S5 Caterhams’ with de dion suspension.

 

The A frame is a triangular construction of powdered coated tubular steel than anchors the de dion tube and rear axle to prevent lateral movement of rear suspension.

 

I discovered yesterday that these are under more load from torsion forces than I originally appreciated, and that it is not uncommon that they break, when under load.

 

Yesterday at Croft Circuit, 4th gear, 100mph, going through the first left kink on the back straight, mine broke. Freshly repaved circuit, so no bumps and no warning.

 

When it snapped, it strongly stepped out right ending up as a lurid drift. At the same time, the driveshaft hub popped off the driveshaft producing a sound like trying to mistime changing gears without a clutch and which did not go away when you push the clutch in. At this point my steering corrections forced the hub back on the driveshaft thereby shredding the three peg hub and all its springs and bearings within the hub. After stopping to check, I could drive slowly back to the pits with an unusual level of rear steer waywardness.

 

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The cause was likely a rock had impacted the A frame at some point in its past (March 2018 kit build). There was evidence of fresh steel in the fracture except for 5% of the fracture. This would have been evident as a dent or abrasion of the powder coating under close inspection and then over time as the metal flexes it gives rise to fatigue and fracture.

 

Given the pathetic state of our roads and/or frequency of off track excursions (e.g. by spinmeisters Simon and Mike amongst others), it’s not unreasonable to think someone else may experience this.

 

I was surprised to understand how much twisting force is transmitted through to this A frame tubing. It’s a cheaply constructed part with thin wall tubing.

 

Consider inspecting this as part of your winter mechanical inspection/service exercise.

 

Did not stop me from enjoying the day - held me off track for 2 hours then back on. Needed an underwear change though....

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Mike,

Where does it look like the crack originated relative to the weld?

 

At least the driveshaft didn't start chopping off parts. When I was involved in racing it was not uncommon to see them fail, the shaft get free and remove the suspension rather rapidly.

 

Happy you were not hurt!

 

Andy

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I saw this A-arm issue listed as a known failure area by Super7Driver in the current BAT auction of the '83Caterham. So I added it to my list of things to check on my Cat. Now I have even more reason to do so! Given my history, I am surprised mine hasn't broken yet....

 

Andy-that sounds pretty ominous. Even more reason to check this part!

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Mike,

Where does it look like the crack originated relative to the weld?

 

The crack definitely started off weld - almost opposite it. At the weld point it was all fresh metal fracture. So I and the professional mechanics present believe the weld was not a contributing factor - tube failed not weld. There was a minute touch of corrosion at the point of fail too.

 

Agree with you on the drivershaft - that would be horrific for damage potential.

Edited by Croc
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Andy - its a stock 420R SV. Stock bushings from the factory. No modifications from standard that would otherwise stress out the suspension. Standard Avon ZZS 13 inch tires/Caterham wheels.

 

A critical piece of the fact pattern was from discussing with the Bookatrack mechanics what the failure rate is on these A frames. They called it relatively common - they keep spares in the truck for replacement. More common on race cars with slicks. They had a few there waiting for people like me to come along. They were scathing of the A frame part - called it poor quality. I tend to agree having studied it closely for the first time and seen the thickness of the tube and how flimsy the powder coating was.

 

A failure at normal driving speeds, normal conditions would likely not give rise to any issues. You could drive slowly home like I managed to drive back to the pits in my example. But because these are performance cars, it is not always guaranteed the A frame will fail at low speeds in a controllable way.

 

The driveshaft bearing destruction was unusual - the mechanics had not seen that before. I suspect it was because of the way the suspension was loaded when it failed.

Edited by Croc
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It's a little disturbing that the Bookatrack mechanics call it relatively common. Are you considering having a replacement made from thicker material?

 

-John

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Breezy - Just for clarity, the articles you link are for the de Dion tube. While this had a bad batch series of failures a little while back, it is not the A frame part that failed for me. The A frame bolts up to the de Dion tube but is not the same part.

 

 

John - I am still thinking. A stronger part will negatively alter the handling characteristics since the part needs to actively flex (unlike the de Dion) - so I don't want this. A standard part has gone back in for now while I work out the real risk. Just because one group of people have witnessed a relatively common failure does not mean it is representative. I don't know anyone in the US or on this forum that has had this problem (other than very early live axle Cats). While I am still working through the technical forum at Blatchat, it seems it is present but low level. So this gets me to the BaT guys experience being somewhat unrepresentative because they see cars driven very hard all the time on track. So that logically leads to this is more an issue for those with certain driving patterns - track work, slicks/wide wheels, rough roads, spins/off track excursions, etc. If so then making it a more frequent inspection item is a suitable response as a failure takes time for metal fatigue to occur. This only affects my UK 420R as the US CSR is IRS and does not have an A frame (or deDion).

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And courtesy of Simon450 on this forum who checked on this with Caterham UK this morning, they recommend replacing the A frame and radius arms each year if you do a lot of track work.

 

For the record my 420R has 3000 track miles on it since built March 2018.

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breezy - just for clarity, the articles you link are for the de dion tube. While this had a bad batch series of failures a little while back, it is not the a frame part that failed for me. The a frame bolts up to the de dion tube but is not the same part.

 

doh!

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Since I have had one wheel bearing break and one half shaft snap, in maybe 2,000 miles of track work, I'd add them to the list. Maybe not yearly but every few years or "X" number of miles. I was able to drive onto the trailer with the bad wheel bearing, the snapped half shaft was catastrophic (on track, so zero control since I only had 3 wheels, surrounded by traffic at high speed, damage to disc, wheel, hub, radius arms). And many of those parts require a long wait-this was Oct, and the last of the parts are now arriving).

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  • 2 years later...

I am wondering if anyone on this forum besides Croc has experienced this failure. Has anyone replaced the part because they identified a problem?  

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