(MBBO#05 Class 2) Flashback GT - Done!

tjwilson

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Jan 15, 2016
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It really gorgeous... I do think it'll triangulate well with the seat pan and other bars.. a few gussets and it'll displace effectively..
I'd be more interested in how the stays deal with all those dimples.. from my understanding of the draft, it seems like there would be a great deal of torque going on down there at all times, I don't have experience with how they would perform because the wall structure in my mind is "compromised". I'm no used to building at this scale..
On a Harley that would be a fatigue point so it's a no no..
But it's a bike and not vibrating itself to death constantly also :bigsmile:..
Regardless I'm in awe:)... It's a beauty
Thanks.

To be honest I've been so concerned about the strength up and down that I haven't really considered side to side forces. It seems like that is were there might be an issue with the dimpled tubes. Not sure, I'm not an engineer and am just building by best guess. True though that dimpling of tubes for clearance on factory frames are (always?) horizontal along the tube instead of vertical? Once I have the frame complete I plan to take some measurements and run the bike through a few cautious test rides. I'll remeasure to see where and if there is any movement and then if required plan additional reinforcements. The good thing is that there is plenty of room on the inside of the stays to add material. The bad thing, it could potentially be a very short test ride!
 
Jan 9, 2013
1,251
1,541
Los Angeles
Thanks.

To be honest I've been so concerned about the strength up and down that I haven't really considered side to side forces. It seems like that is were there might be an issue with the dimpled tubes. Not sure, I'm not an engineer and am just building by best guess. True though that dimpling of tubes for clearance on factory frames are (always?) horizontal along the tube instead of vertical? Once I have the frame complete I plan to take some measurements and run the bike through a few cautious test rides. I'll remeasure to see where and if there is any movement and then if required plan additional reinforcements. The good thing is that there is plenty of room on the inside of the stays to add material. The bad thing, it could potentially be a very short test ride!
After the rest of bars are there I'm sure you'll be able to even torque test it to see with the clearances u have going on :).
I just checked my cranbrook.. the seat stays have no dimples... They are carrying the weight and load to the rear axle, top brace is a bit past the dimples of the chain stay.... In your case it's more like a fixed swing arm as far as how load will transfer.
 
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tjwilson

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Latest progress, bending the side stays. Symmetry is hard! They ended up closer to each other than the image of the tubing in the bender would indicate... close enough anyway. I added a plug to one end, packed the tubes with sand, plugged the other end, and welded the tubes together. Then I ran the twin tubes through the bender. (Probably important to note that I made sure that the plugs didn't seal the ends completely. This prevented pressure from building up in the tubes as they were welded.)

tubingBenderAndSides.jpg


Bottom end reinforcement. I'll weld this in place after cleaning up the previous welds.

bottomEndReinforcement01.jpg


bottomEndReinforcement02.jpg


bottomEndReinforcement03.jpg


Side stays in-process and in position. I plan to make some adjustments. The rear end of the side stays will be trimmed and the sides rotated a bit. The upper ends will be trimmed and tube extensions added to follow what will be the bottom edge of the seat. I'm also thinking of tapering the tube from 3/4" to 1/2" along that edge and shortening the seat length a bit. These are the handlebars that are the winning choice... so far.

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rear3qtr.jpg


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sideDrive.jpg
 
Dec 23, 2014
292
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Portland Oregon
Latest progress, bending the side stays. Symmetry is hard! They ended up closer to each other than the image of the tubing in the bender would indicate... close enough anyway. I added a plug to one end, packed the tubes with sand, plugged the other end, and welded the tubes together. Then I ran the twin tubes through the bender. (Probably important to note that I made sure that the plugs didn't seal the ends completely. This prevented pressure from building up in the tubes as they were welded.)

View attachment 35116

Bottom end reinforcement. I'll weld this in place after cleaning up the previous welds.

View attachment 35117

View attachment 35118

View attachment 35119

Side stays in-process and in position. I plan to make some adjustments. The rear end of the side stays will be trimmed and the sides rotated a bit. The upper ends will be trimmed and tube extensions added to follow what will be the bottom edge of the seat. I'm also thinking of tapering the tube from 3/4" to 1/2" along that edge and shortening the seat length a bit. These are the handlebars that are the winning choice... so far.

View attachment 35120

View attachment 35121

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View attachment 35123

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Love the flow of this bicycle it is unique for sure. Looks nimble
 

tjwilson

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Jan 15, 2016
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Hey Carl...

Not a lot to update, was out of town this past weekend. Did get a start on prepping some of the pieces that will probably make their way into the side / seat stays. Also worked up a digital mock-up to experiment with how much I want to shorten the seat and refine how the seat flows into the side stays.

Here's a pict of the pieces and parts. The tapered pieces are donor chain stays, .75" to .5". Really thick wall, .120" x .5", tubing was used for the tight bends. These will be for the back of the seat, maybe. Might try for a tighter radius. The two longer bent pieces are .049" x .5". These were filled with sand, plugged, welded together, and sent through the bender.

seatStayPieces.jpg


This side view has the pieces in approximate position. For comparison the left side shows the pieces in mock-up, right shows the existing frame end digitally removed and the intended seat profile added.

seatShorten.jpg


No picts but, pulled everything apart last night and cleaned up the welds that will be hard to get to once the side stays and bottom brace are welded on. I've got time set aside this coming weekend so hopefully there will be more to show soon.
 

tjwilson

Pro Member
Jan 15, 2016
523
2,197
58

tjwilson

Pro Member
Jan 15, 2016
523
2,197
58
A little weld clean up. Most of the tubing is relatively thin .049" wall so I was a little concerned about doing too much smoothing. Just enough to keep the filler from having to be too thick.

weldCleanup01.jpg


weldCleanup02.jpg


Most of the weekend was spent working on the seat/side stays. I started by making a cardboard template to find the angle of the curved sides extending back to the end of the seat. Then I tacked the pieces and parts previously bent together using the template as reference.

seat01.jpg


seat02.jpg


seat03.jpg


seat04.jpg


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The sides were trimmed and solid rod inserts were used between the sides and the new seat base.

seat08.jpg


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seat10.jpg


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The frame backbone was trimmed and new pieces bent and cut to tie the end of the seat to the frame.

seat11.jpg


Next up will be final welding the tacks!