Sunday, July 19, 2009

Michael's broken Brew...

Speaking of projects that have lingered, Michael (IF52) has been very patient in awaiting some progress on his wife's Brew racing frame, like a year and a half patient. Coming off a couple of short nights, I was feeling pretty fuzzy in the head and really did not want to tackle anything of consequence, so I pulled this guy off the wall...it's already broken, even if I'm not 100% I can only make it better, right?

This repair highlights a couple of issues that are important to pass on;

1.) For new builders, you need to understand that most seat tube stock IS NOT prepared to be slotted into a frame and built off of. The majority of standard tubes are too light in wall thickness to support the stress of a leveraging seat post and the mechanical weakening that comes with slotting. You must address the problem by one of three options; using an externally butted seat tube designed with a thicker wall for the post insertion butt, using an external sleeve that is brazed over the seat tube and then built off of ala early Groves/Ritcheys etc, or turning down an internal sleeve that is pressed into the seat tube to provide additional strength. Without it, you will most certainly develop a lateral crack extending from the slot around the tube much like this example.

2.) If you are going to perform a repair, do it right. When this frame cracked, a repair was attempted to correct the situation, obviously by a builder who has little experience/understanding of mechanical stresses and how to address them. In this case, the lateral crack from the seat tube slot was simply brazed over top of with silver, a band aid to please the customer for the moment and obviously not for the long term as no attempt was made to limit the extension of the problem or to support it for future use.

With the paint off, here you can see the crack has extended through the silver patch...

The game plan for this repair is to remove the poor silver patch, clean up the tube, fabricate an internal sleeve to improve the tubes ability to sustain the stresses of riding, and then to limit and clean up the cracked area.

I started off by cutting off the seat tube top as close to the top tube as I was comfortable with, keeping the cut straight and parallel. I also removed all the silver patch that was applied and drilled out the end of the crack to stop it's forward progression. In progress in this pic...
Using a machinists square and a file, I then worked around the tube insuring all aspects were as flat as possible.

I then cleaned up the inside of the tubing with a bit of a barrel sander, lightly applying pressure so as to keep the ID constant.
I then got to work on the new sleeve insert, cutting the selected tubing in the cold saw. As this repair will rely on an internal sleeve, I has to choose a piece of tubing that would allow me to keep the OD the same as the original seat tube, have a great enough wall thickness to turn down the wall for the insertion portion so that it still adds strength to the section, and have an ID that will accept a current seat post size...quite a few variables to consider.
Once cut to rough length, the sleeve is squared and deburred in the lathe, then I began to turn down the OD to fit the ID of the seat tube...
Here you can see a quick dry fit to insure I have the tolerances correct before pressing into place...
The piece is then taken out, everything chemically cleaned then pressed into place...

A thorough cleaning to get rid of the balance of the mill scale, a wipe with acetone again, and it's time to weld the two pieces together.

I then drilled and slotted the front aspect of the extension for the compression duties. I am a firm believer in slotting the front of the tube. Why you ask? Well, it never made any sense to me to place the weakest point of the tube, the slot, at the rear of the tube where a majority of the stress from the seat post leverage was concentrated. By moving it around to the front, it allows a uniform dispersal of stress in the rear of the tube.
Some of you may ask why I did not slot the extension prior to pressing it into the tube, an act that would have been easier outside of the frame. Well, I've been caught once or twice having the insert turn just a skoosh when pressing into the frame and then been stuck with aligning the slot, no small task with the pressure fit of the sleeve.
Something to watch out for if attempting this type of repair, you must have good heat control with your tig torch so as not to burn through and cause irregularities inside the seat tube that will cause interference with your seatpost. Tig bead is a bear to try and ream out, so control is the name of the game. Here you can see just a slight heat shadow on the inside...no distortion present.
With the sleeve in place, some cosmetic work is next. I had ground off the silver and the slot reinforcement. There was, however, too much residual silver to fill it with the tig torch without contamination, so I then laid down some 56%, allowing it to float and smooth over the top as well as drawing it down into the crack and in between the tube and the insert, further fortifying the area while making it look visually presentable.
The silver and sleeve was then filed smooth so that the repair becomes invisible, looking original once under paint.
This guy is getting a nice metallic lapis illusion powder coat, then it'll be off to Michael for hanging of parts.
I'll be out of the shop Monday and Tuesday for some personal reasons, looking to be back at ya on Wednesday.
cheers,
rody

7 comments:

Anonymous said...

Rody

Your repair work is by far the most enjoyable aspect of your blog for me. I would amagine it's like teaching...you really have to have a great understanding of your craft to be able to teach or properly do repair works such as this.

I do have one question about sleeves. Does True Temper make a seat tube that doesn't require sleeves. I've never really like the look of sleeves...

Thanks
Larry

IF52 said...

YAY!!! Rody, you rule!

Check your e-mail before you send to powder.

rmb said...

I love to read about the repairs too.

Anonymous said...

What's 'Lapis Illusion' Powdercoat?

IF52 said...

A shimmery medium/dark blue metallic powder coat finish. And powder coat, if that is what you are asking, is an electrostatically applied finish that goes on dry and is baked to form a really durable instead of going on wet and curing.

AdamB said...

Great post and pics- so informative and helpful. What externally butted tubes do you suggest when looking to run a 27.2mm seat post diameter?
Also, what tool are you using to slot the front of the seat tube after it is welded into the frame?

Thanks, AdamB

Rody said...

Larry and Adam,

I'm not familiar with the TT line, but there are many externally butted tubes available from Columbus, Dedda, and others.

rody