Has anyone seen anything like this?

[BDA]

I was putting the wheels back on this afternoon and found this crack in my radius arm! The yellow is where I rubbed a yellow tire crayon against the crack to accentuate it. There is a similar crack on the other radius arm.

I checked the Jensen Competition Manual (http://lotus-europa.com/manuals/misc/jensen.pdf) for any suggestions to beef up that area but found nothing. In fact, they cut a large chunk out of the radius arm to make room for cantilever slicks (see picture 3 - I presume they closed that cut-out by welding sheet metal there but it isn't specifically mentioned). I did a quick Knowledgebase search and didn't see anything about cracking radius arms (too many hits to read each one but I've been a member of the the lotuseuropa list for decades and while I don't read every post, I do watch for posts that interest me and I don't think I've ever noticed anybody else discussing this).

Any thoughts on the cause?

[4129R]

Have you taken the paint off back to the bare metal to make sure it is not cracking in the paint finish?

Cracking of that nature in sheet metal is inexplicable.

[BDA]

I haven't taken it down to bare metal but I would assume that the paint is more flexible than the steel. Your absolutely right  that that is the next step and it very well could be that the paint is more brittle than the steel. Thank you for the reminder! I was starting to get overly concerned.

I'll post what I find out.

[jbcollier]

Get both sides stripped and crack tested.

Notching the radius arm to provide clearance for wider tires is fine for racing where parts are regularly crack tested and replaced at set intervals regardless.  For a street car?  Not a good idea in my books.

[BDA]

I only mentioned Jensen's cutting away the trailing arm to show that rather than build up the trailing arm they were able to cut some of it away without problem. I didn't cut on my trailing arm. It's completely stock.

[GavinT]

Haven’t ever seen anything like that reported.

The crack seems to follow (at a distance) the welded on factory doubler. In my mind I’m wondering about the cause, particularly given it’s on both sides.
The only thing I can think of is that the trailing arm might have been too tight a fit over the upright initiating a stress crack.

I’d agree, you’ll need to scrape the paint off to find out.
If I recall, the trailing arms are made of 1.2 mm steel whereas the rest of the chassis is 1.6mm.

[BDA]

I wire brushed the area of one of the cracks and the steel underneath was pristine. There was no hint of any issue. It appears that the paint is what cracked, though that does sound like a strange thing to happen. Given the lack of history of anything like this and the fact that the Jensen folks didn't raise any red flags about racing with a weaker part, I think I'm ok. If I find somebody with some die penetrant, I'll get it checked for cracks but for now I'm feeling much better. 

[BDA]

UPDATE:
At the risk of  I heard back from an engineering classmate of mine (he stayed an engineer and I went to the dark side of computers). He suggested that the cracks are likely because of the heat from the disk. The hot disk heats the paint. It expands and cracks.

I think he's got it right.

[4129R]

That is what a forum is for. Open discussion to help each other.

Sometimes you cannot see the wood for the trees.

Alex in Norfolk (where we have pine forests).