November 12th, 2012
I have the pleasure to introduce to you one of my favorite builds in recent memory, a road bike for Patricia:
Let us first note that some Conti Classic tires are destined for those white Velocity rims, fear not! This is just what was on hand.
So the most refreshing thing about this is all the lovely silver, am I right? This is Campy Veloce 10, great for anyone who thinks that 10 is the proper maximum number of cogs on a cassette, and just easy on the eyes as well as working flawlessly. The rims are Velocity Aeroheads in white, and a Thomson seatpost and stem and Ritchey Classic bars round it out.
Before I talk about the challenges of this geometry, some more pics of the frame:
Ok, so Patricia came to us with a Davidson custom frame that fit well, but she wanted some tweaking and more to the point, she wanted lugs. Conventional lugs are cast with a conventional geometry in mind – 73 degree seat and head tubes, 60 degrees between the DT/ST & DT/HT. Well. Not so much here. We were shooting for a traditional look, so didn’t want to go past a 2 degree slop on the top tube; and we wanted to do a slack head tube and plenty of rake in fork for toe clearance. All of that helps to use off the shelf lugs, but it left us with an untenable 57 degrees between the down tube and seat tube, which effectively ruled out any available lugged BB shell. So, we chose to weld all the BB joints, and I just promised to keep it soft so it won’t look too out of place.
Patricia chose the colors and layout, and all I can say is I approve.
Now, that was the start of a small frame – let’s look at one from back in the archives…
We don’t have the budget to do a lot of destructive testing, which is why we tend to stick with what we know. Fortunately, we have friends who are badass racer-types and every once in a while, one of these friends gets in a scrape. Liane was in the middle of a pileup on her nearly ten-year-old Cirlce A (pre-stainless head badge!) and this was the result:
Most important to note, not only did she walk away, she attempted to ride away. Then her front wheel hit the down tube… well. At first blush, you might think this is a terrible thing for a builder to see. But actually, it’s a great study in frame building, stress risers, and why we love steel.
First, let’s note that this was a serious, catastrophic impact. Clearly the front triangle has buckled in several places, but perhaps not clear from the pictures is that the fork is bent as well; the blades are bent slightly back, and the steerer is bent a touch at the crown. But what’s really interesting is what happened to the frame.
For the builder, the best thing is that Liane walked away. The second best thing is that it didn’t break at a joint. In other words, the forces applied to the frame were transferred from the wheel, through the fork and into the front triangle of the frame – which is what happens every time you get on a bike – and in this extreme situation, when those forces achieved a moment that was beyond what the frame could handle. it sort of broke all over.
It’s a study in stress risers; see how the top tube buckled at the cable stop, and the down tube at the shifter boss and the BB lug. Then you have the buckling in the down tube – exactly in the middle of the tube. In other words, it was trying to bend, but eventually the center could not hold.
This is what happens with well made, lightweight bikes. They can break. But they will usually not break at a lug (even a minimalist, bikini lug as seen here), at a fillet or at a weld. And, unlike aluminum or carbon, we can fix it. As I do here.
With a silver brazed frame, relatively little heat is used to join the lugs and tubes. So, you generally have the option of heating the joint and pulling the tube out, or cutting and grinding it out. In the case of a full front triangle, I generally will pull the top tube out of the seat lug, but I like to grind out the dt from the bb. This is mostly because there is so much mass at the BB, it takes a lot of heat on all four joints there. In general we like to keep the heat cycles down, but this is more important in a high stress area like the bb as opposed to the seat collar. When was the last time you saw a bike break at the seat lug? Ok, so here’s work on the BB shell:
For the seat lug, I cut the center of the tube out and use a bungee cord to pull it straight out. This helps keep the seat lug in shape; pulling by hand or trying to twist it out can distort the hot metal.
Note the shadow of silver on the top tube in the shape of the lug. This is what we in the frame building industry refer to as full penetration. In other industries this might mean something else.
Anyway, here’s the (re) finished product:
So, we hope this sort of thing never happens to you. But if it does, we can help.
Next: Jeff’s fixie rando! I know, what? And 29er fever takes hold!
This entry was posted on Monday, November 12th, 2012 at 11:09 pm and is filed under Bike Repairs. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.