Suttree

So I heard all of this buzz about how long pointed lugs
on the underside of the down tube concentrate stress and
in rare instances lead to failure. So I took my bike
and filed down the lug on the underside of the down tube
to be very thin around the stress concentrating point--
and then sanded it. Anyway--I am an amateur at this sort
of thing but I enjoy this frame so I thought I would file down the lug.
I doubt in this instance, per the frame builder's description of the butting
and strength--that it would fail--this is mostly an aesthetic issue.
But here ya go. Thoughts comments, critique
is appreciated.

Torque1st

Why did you do that? Now you have destroyed the continuity of the paint film which will allow corrosion to set in.

nomoneysonny

Did they mean to file down the top of the lug or file off the point on the lug? Is this supposed to allow the joint to flex? I guess I really don't get the reasoning behind this mod.

Suttree

I'm going to put automotive touch up paint on it to keep out rust until I
get it powder coated.

The point of the lug on the bottom of the down tube concentrates lateral stress
created when you brake and sprint. That lug point in that particular location can lead
to failures on some thin-wall tubing because the point concentrates stress onto the tube.
There are two ways around this--either use a lug with a spoon-shaped point or
file down the point so that it tapers down to a very thin thickness.

Nessism

You did fine...assuming you were planning on painting anyway. The risk of tube failure was very small but your method of thinning the lug is a viable technique.

Suttree

Thanks Nessim. I'm interested in frame building and the various
ways people approach it. I have an old lugged Miyata 912--
which has similar but shorter points. Out of curiousity I flipped
her over and looked at the lug on the bottom of the d-tube.
It was filed down to almost paper thinness at the point--
pretty nice attention to detail for a production bike.

Six jours

Has anyone ever actually seen this kind of "can opener" effect, ie. a lug point penetrating through a tube?

I haven't, and while what I haven't seen is a lot, the whole deal sounds a little fishy. I wonder if maybe the custom builders aren't making things up in an attempt to explain why their frame are "better" than lower end lugged frames.

Nessism

Back when investment cast lugs hit the market, some old time builders said they were too "stiff" and could lead to can-opener effect. Don't think that ever really became an issue but there have been some very high quality builders that made this claim.

Fast forward to modern times and I can say that many of the tubesets on the market have super short butts which make it very difficult to fit the entire lug on the butted section of the tube after mitering. In a case like this, the thinned lug is not a bad idea at all. I don't think there would be an issue under normal usage but if you really stressed it, such as in a crash, it may make a difference.

NoReg

"The point of the lug on the bottom of the down tube concentrates lateral stress
created when you brake and sprint. That lug point in that particular location can lead
to failures on some thin-wall tubing because the point concentrates stress onto the tube."

Well of course they pointed it partly to taper those loads out in the first place. Then some wag comes along and says they created a stress riser...

On the other hand tapering them out for thickenss, as you have done, is what is supposed to be done anyway. Paper thin is about 3 thou and that seems quite a bit too little to me. Also in the first picture it looks as though you flattened it a little in the process, rather than compounding it down. Could just be the lighting though.

Homebrew01

Shhhhhhhhh. You're not supoosed to tell !! Reminds me of a builder who put a little cross brace inside the bottom of the steerer tube to "stiffen" it. I prefer builders who differentiate themselves a little more credibly: distinctive style for example.

Suttree

One manufacturer I talked to showed me frames that had failed at that exact
location. I saw them with myself. It is not common--but it is possible.
0.9 m wall tubing. Thin.

Homebrew01

In this case it was at the base, where the steerer tube is surrounded by the fork crown. More like .25 inches at that point.

Nessism

I've never seen a 1/4" thick frame tube. Even butted steerer tubes, on the thick bottom portion, are thinner than that. What are you referring to?

Suttree

No--the butting profile is 9-7-9 which refers to
0.9 mil etc.

Six jours

He's talking about steerer tube thickness at the butt. I just measured a 531 steerer I've got laying around. Just about .09" or 2.28 mm. Sticking an Everest crown on brings it to about .21" or a little over 5 mm.

Also FWIW, I don't know too many people who consider SL (.9/.6/.9) to be "thin", unless it's being compared to something sold by Sears. Especially in the era of OS tubing, .9 seems on the thick side to me.

Regardless of the exact measurements, I would have a really hard time believing that the base of the butted portion of the steerer is a failure point. And regardless of all of this latest nitpick, I'd still be interested in pics of a tube that was "can opened" by an unfiled lug. I'm really starting to believe that it's urban legend.

Homebrew01

The fork crown is brazed onto the steerer tube, so combining the two results in a pretty massive piece of metal - probably the least in need of reinforcement.

dy123

I don't really understand why you would do this on a finished frame but if you are experimenting what the heck. Go for it.

The real issue is with thin tubing and 9/7/9 is not considered thin. If you have some 7/5/7 and the frame is small then there could be some concern. The lug point could be in an area where some of the butt is cut off and you are in the transition between .7mm and .5mm. That is where the talk of the can opener effect comes in.

I've never seen it, but I always round the sharp points on my lugs anyway.

Enter Spirit for lugs or Pego-Richie New high performance tubes with longer butts to account for lugs.