Last night after working a 6 day week, I got to do a little of mechanical problem solving brain dump.
When I got the Raleigh Mtn Trials, I knew it had a fork bent at the crown race. This only happens in serious sideswipe accidents, so I knew to check the rest of the frame. The head tube and seat tube are in-plane so thats good, but the rear triangle was out of alignment. A month or so ago, I did a cursory alignment and spacing to true my new rear wheel with the Eno eccentric hub. Wheel was centered and dishes according to the chainstays but off to the left at the seatstays. So I put that knowledge in my back pocket and moved on to other things.
Fast forward to last night and I had about an hour of free time to do some serious measurements, string tests and bending of metal. I love telling people that “cold setting” is simply bending metal, but using some tools to measure the amount of deflection. Love it.
I knew that I needed to move the rear triangle to the right at the upper stay bridge, so I stuck an axle spaced to 130mm in the dropouts then set the right rear dropout (right side of the frame) down on a 2x4 and the head tube on a wooden raised portion of the floor in the basement.
I hit the left seat stay a few times with a heavy rubber mallet and measured again; a few more whacks, measure, whack, measure. Back in the stand, a new string test showed the rear triangle was out to the left, and a straightedge showed the stays were almost perfectly aligned in-plane and extremely close to isosceles when using the seat tube as an apex reference point.
Knowing that I had an isolated but aligned rear triangle that was 5mm off to the left, I set the frame back on the floor, this time on the left side, then simply stepped on the seat tube to apply some even downward pressure on the whole frame to move the triangle towards the right.
After a few repeats and measurements, I got the rear triangle aligned within 2mm, which is not enough for me. It’s close enough to ride the bike hands-free but not enough for my perfectionism.
So it hangs in the stand now with 2 strings on it which don’t have the precision I need. I’ll be ordering some 5lb test fishing line to really get this done precisely this week.
Everyone would be surprised how much crude bending goes into the alignment of their bike frames, even from right out of the jig after welding. Heat deforms the tubes and final set needs to be done with rulers, calipers, and “gentle persuasion “.
The fork is ready to be threaded then installed into a Tioga MTB sealed headset (alloy version of the BMX Beartrap).
I don’t yet have the 26 tooth granny ring drilled for 56BCD but that will happen soon enough. I have the original SR FX crankset with 26/36 rings that I may run using a chain tensioner but the whole reason to be crazy about the 4-tooth difference is to avoid a chain tensioner. I need to clean up the roller cam brake and bottle brush out the seat tube again. I’ll patch the paint chips and clean up the burned patch, then blast frame saver throughout and put the whole thing together.
Phil