FACT or FICTION: Differences in tubing and lug flexibility can cause cracks in paint

[timto]

Envision cracks in paint along the shoreline if you will...

Any opinions on this? Background is yes I've seen this first hand but am suspicious as to the explanation I was presented when asking in detail about it.

FACT or FICTION?!?!

Cheers

Tim

[Dazza]

Envision cracks in paint along the shoreline if you will...

Any opinions on this? Background is yes I've seen this first hand but am suspicious as to the explanation I was presented when asking in detail about it.

FACT or FICTION?!?!

Cheers

Tim

Millions of lugged frames have been made properly
millions of these frames have been painted properly
paint does not crack along the lug shore line unless some this is amiss in the processes
such as fault in the frame or faulty paint

Not speaking about Alloy lug to carbon tube.

[e-RICHIE]

Envision cracks in paint along the shoreline if you will...

Any opinions on this? Background is yes I've seen this first hand but am suspicious as to the explanation I was presented when asking in detail about it.

and the explanation presented was atmo?

[timto]

Background is I have a frame (not built by me) where cracks appeared in the paint along the shoreline of the DT/BB socket. I consulted some people very experienced in the world of frame repair and the discussion settled on differences in flexibility between the tube and lug being the cause. Strange one n'est pas? Darrel summed up what I was feeling on the issue but wanted to check in with the telai-istas who would have seem many more frames up close, done repairs, painted etc etc than myself.

[EricKeller]

if we were talking about cracks in the tube, it would be one thing, but it makes no sense with regards to the paint. The thicker bb shell is constraining the tube from moving at the shoreline. And a decent paint will take a heck of a lot more deformation than you put it through by riding. I'm just putting the finishing touches on repairs to a bike with a dent in the tt; it looks like it was painted that way.

[Scheisserad61]

Millions if not trillions of Euro lugged bikes were painted with relatively brittle acrylic lacquers and older stove enamels and they don't have any such problems around their thick stamped lug or BB sockets WRT paint loss.

Sounds to me like your people "very experienced in the world of frame repair" are also very experienced in the art of spreading fertilizer.

(I'm curious if they are the ones who painted said frame in question and if this is simply a CYA explanation ?)

If there is a huge difference in flexibility between the tube and the lug socket then tube breakage is something that can occur. This is why pointed and tapered lug sockets came into being. The very old designs that had a straight 90 degree cut to the socket caused the relatively flexible tubing to break / crack at the edges. Cutting away at the edges of the socket and thinning it a bit relieves the "notch" effect.

So unless your frame is from the early part of the twentieth century, when thick heavy castings were still available, it seems like quite a stretch of the imagination to attribute paint cracking to differences in flex.

Perhaps there was just a poor job done WRT the priming or sanding around the bb ?

[timto]

Here's an image - thanks all for the feedback. The crack in paint is on the upper edge of the lug shore and not where the tube goes into the lug. You're looking at the dt entering the dt/bb port.

You guys basically echo the same sentiment I feel and thanks for that. I'm content to leave the situation as is.

[gltapley]

From you picture it looks like a bit of corrosion on the shore line of the lug. Where spots are is right on the edge of the lug, above where the tube goes into the lug and the rod fills the joint. If it were a crack in would be low on the shore line not high like this one is. The paint tends to be a bit thinner on the upper edge of the shore line, and thicker on the lower edge.

[Dancingbear]

From you picture it looks like a bit of corrosion on the shore line of the lug. Where spots are is right on the edge of the lug, above where the tube goes into the lug and the rod fills the joint. If it were a crack in would be low on the shore line not high like this one is. The paint tends to be a bit thinner on the upper edge of the shore line, and thicker on the lower edge.

Although I agree with much of your post, I'm going to differ regarding whether it makes a difference where on the shore line the paint cracked. A shore line is a cosmetic fillet of sorts, some folks do it fuller than others, but the strength of a lugged joint is the shear within the socket. The filler material is weaker than either the shell or the DT. To my eye this looks as though the "fillet" material in the shore line stayed on the DT, and the crack (yes I think it is cracked) manifested between the filler and the BB shell. The jagged nature of the upper part of the paint crack says movement to me. And +1 to the camp that believes that a crack in the paint indicates failure in the joint.

[EricKeller]

I don't see any evidence of a crack in the metal. Looks like bad adhesion to me. Is that a massive run under the dt, or an optical illusion?

[Pi Guy]

I am curious if you all can point me to what I am not seeing on the pic. I just see a slightly dirty BB with a red line drawn on the pic. Are you guys referring to the stuff that appears to be on the edge of the lug just to the right of the red line?

-Jason

P.S. Oops. Now I see it. It does look like more of a crack then dirt.

[zank]

Strip the paint and end the speculation.

[timto]

Bike was actually stripped down to chrome - was sent in for this same reason - cracking in paint!. the repair guys couldn't see any imperfections in the chrome so to them - couldn't be movement in the tube/lug causing cracking and suggested not diving deeper into the issue. i went with their suggestion to be disappointed the the same issue has returned. A stumper for sure.

If the bike didn't have chrome (no experience with it) I would have stripped, and added some more brass to what I felt was very poor penetration when peering inside the bb (doesn't look like your stuff zank!)

Anyhoo it's beyond the effort / cost line now ... thanks for all who chimed in

Tim