Mea Culpa Posts

Mea Culpa

My Significant Mistakes. I am sure there will be more:

Cutting a mitre upside down. On my framebuilding course I managed to cut the second mitre on the top tube upside down. Fortunately I had started with the tube too long so just had enough length left to reverse it. Old adage, measure twice, cut once.

Brake bridge creep. On my second frame I fitted the brake bridge in what I thought was the correct place for long drop caliper brakes. After painting I found I had to send for some EXTRA long drop calipers. Lesson, check the brake bridge position before painting. I understand bridges can creep upwards or downwards during brazing but mine have always crept upwards. On my course I simply brazed the bridge with frame upside down an the seat stays compressed together with a toe strap. Now I would always secure them either with an improvised or manufactured rod securing the bridge to the axle, or clamps above and below the bridge either side during tacking.

Inserting the bottom bracket the wrong way round. At the time I felt this was a terrible mistake but I at least learnt how to rectify it. I had used for the first time a non-ported bottom bracket with no pre cut holes and simply did not think to check the thread direction before brazing it in. I only noticed when I came to chase out the threads. I rectified it by thoroughly de-greasing and cleaning the threads, smearing in some brass flux and melting in some Sif No2 brass which is quite hard and seems suitable for cutting threads in. It was not in the end difficult and neither was re-cutting the thread in the opposite direction. Lets hope it lasts.

Making a fork too short. I still do not know how I managed to do this, I wish I had taken a picture. After completing a fork I thought I had measured correctly, upon inserting the wheel with tyre as a check, found there was no clearance above the tyre. Fortunately I had silver soldered the fork crown so I was able to reheat and remove it without too much difficulty and re-use the fork blades in a new crown with greater depth. I had to scrap the original crown, an expensive mistake. Old adage, measure three times, cut once.

Buckling Chain Stays during cold setting. On this occasion I had fitted chain stay gussets for the first time to strengthen the chain stays. when one of the stays contracted, which seems to happen a lot, levering out the stay caused it to buckle as shown:
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It seems obvious in retrospect that the gusset concentrates the force at the tip. The picture looks worse than the reality but I tried various ways to reduce it without success so will not describe them. Probably thin walled chain stays contributed to this effect but I would be wary of using gussets again. Making sure the cold setting lever is a close fit along the chain stay, or perhaps use hot setting instead. My rear axle jig spacing is 1mm over the intended either side, but there is the alternative of oversizing the rear axle more on the basis bending the chain stays inwards slightly must be less likely to cause buckling. In the end I filled the defect with silver solder to restore the appearance.

Telescoping of the Top Tube into the Seat Tube. This phenomenon caught me completely unawares as I have never heard of it happening or been warned about it. As the photograph shows there is a donut shaped object visible in the seat tube which is the top tube projecting into it.
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This can only have been due to softening of the seat tube metal during the fillet brazing process combined with some tightness of the top tube in the jig, though I did not think this was an issue at the time. Also this was silver soldered so the temperature used was less than brass brazing and I used a heat sink. The tubing was XCR and the seat tube is only 0.5mm thick so this must also have been a factor. In future I would have to aim for zero tension across the top tube before brazing such thin walled tubing. The frame was fortunately salvaged because the seat tube is oversized and required a shim in any case which could be modified to accommodate the small projection.

Failure of Chain Stay to Bottom Bracket Fillet during Cold Setting.
I am not sure this qualified as cold setting, more brute force. On checking the chain stay rear spacing on my “Child’s First Pedal Bike” I decided it need a slight tweak. As there was no room in such a small frame to introduce my usual levers I decided to put a foot on it. This was a mistake as you can see.
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Initially I was shocked this had happened but on reflection considered the reasons why. Usually I am sure that remedial bending occurs in the tube rather than at the joint. In this case very short and wide (22.2mm along the whole length) chain stays are probably unlikely to bend so the force was highly concentrated at the joint. Using my foot and the whole weight of my body was probably too much force. The Right hand stays went rather than the left which may have been due to the application of the force to that side but I realised that I had forgotten to vent that chain stay but carried on regardless and vented the opposite side before brazing. So I suspect non-penetration of the braze may have been a big factor. Of course better that these things happen before the bike is in use, than after.
I levered the stays back to their original position then used a carbide burr on a rotary tool to evacuate the fillet down to the joint and drilled a vent hole in the bottom bracket.
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Re-brazing the joint hopefully has remedied the initial weakness, and miraculously the rear spacing was spot on afterwards!

CONTRIBUTOR CHARLIE MERIVALE HAS OFFERED SOME OF HIS MISHAPS:

Mitreing the wrong tube. I prepped the seat tube as the TT. Fortunately it was just rectifiable. As it happened what came later meant that any short coming would be lost.

Buckling the Top Tube during cold setting. The frame was built on a very simple, and as it turned out, inaccurate jig. When placed on the new jig (the bicycle academy, a line of beauty) it was out by at least 5mm. I therefore set to and tried a little cold setting, as gently and progressively as possible. Giving it one last tweak, there was a ‘give’. The TT buckled. It’s all a learning curve. I cut out the tube, and discovered the stress within the frame as the now released parts found their own resting point, point well beyond alignment. However at least I could see the lugs, and was pleased to note that my braze had penetrated all the way through. It struck me that Paterek has a point in brazing in sections, thereby allowing cold setting along the way and helping to minimise the accumulation of greater stresses.
Ceeway are supplying the new TT, and I hope to fit it next week.

Brazing brake bosses the wrong way round. In a moment of growing self-congratulation, I brazed the cantilever bosses on the wrong way around. Pride came before the fall. Tomorrow I’ll rectify the error……I hope