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What would you do? TT to ST tube diameters

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What would you do? TT to ST tube diameters

Old 09-16-20, 02:36 AM
  #101  
guy153
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That looks really good! But are those bottle cage boss holes on the back of the ST in the second picture? They can be filled in but I hope the BB shell is the right way round with the left-tighty side on the right. If it's not you'll actually be fine, just throw a bit of loctite on there. Have so far avoided that particular mistake myself but a friend got it on the wrong way around on a bamboo frame and I fortunately remembered his pain

Angle iron is not the best reference surface but it's good enough especially for a first frame. It's not like one's going to be riding the bike on a road made out of Blanchard-ground steel plates.
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Old 09-17-20, 02:08 AM
  #102  
Aldatroid
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Originally Posted by guy153 View Post
That looks really good! But are those bottle cage boss holes on the back of the ST in the second picture? They can be filled in but I hope the BB shell is the right way round with the left-tighty side on the right. If it's not you'll actually be fine, just throw a bit of loctite on there. Have so far avoided that particular mistake myself but a friend got it on the wrong way around on a bamboo frame and I fortunately remembered his pain

Angle iron is not the best reference surface but it's good enough especially for a first frame. It's not like one's going to be riding the bike on a road made out of Blanchard-ground steel plates.
Haha good catch on the bottle cage boss holes!! The second after I had the holes drilled I realized I'd match the DT and everything to the wrong side of the ST... I was pulling my hair for a minute there but though "**** it, I'll just plug them and continue". It's correctly oriented now

The angle iron indeed sucks. Wasn't planning on using it at first but didn't have any other ideas on how to hold the tubes etc. The BB needs to be angled slightly when it's standing on it's side so I used a square washer for the base with a threaded rod going through, pushing a corner of the washer, angling the BB in the direction of the ST. Looks pretty good now but I'm sure it's off so need to check alignment properly after I've faced the BB shell etc. Created a "Not so fancy alignment gauge" based on Paul Brodies "Fancy Alignment Gauge" and checked the ST and DT that way and they're OK. Haven't been able to check twist though.
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Old 09-17-20, 11:19 AM
  #103  
guy153
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I should also make myself a redneck version of Brodie's gauge. But I'm not sure it's so necessary to align the front triangle apart from twist, and I've never had it twist, at least not measurably by me.

Basically I get the BB shell as square as I can onto the bottom of the ST using a combination square with the ruler taken out. Then I get the dropouts and BB shell parallel using the laser and angle iron bolted to the table. Then I jig it up for the front triangle with everything laser-vertical. Then I make sure the ST is centered between a bit of string wrapped around the HT and the dropouts. After welding the front triangle the string can be recentered if necessary a small amount just by moving the chainstays both left or right together.

Compared to bending the front triangle this will leave the dropouts slightly misaligned and the BB shell slightly rotated around the vertical from where it should be. But the BB shell is only 68mm long compared to the 1m or so over which we're measuring the alignment with the string. So it's only a tiny change. All of the bikes I've made have actually ended up with squarer BBs than any of the commercially-made bikes I've owned.

A higher priority tool for me would be a precision "wheel" consisting of an axle and a perfectly square and centred rod to sweep up and down between the stays perhaps with an adjustable thingy that you could set to just brush against them on both sides. Currently I rely on a true and properly dished wheel which is usually a brand new one I've just made for the bike. But there's lots of agonizing and flipping of the wheel.

I think the best way to make precision tools without machining capability is to make things that can be strategically twisted and bent a bit so you can make them, check them with a laser, and tweak them as required. I also have some things a bit like yours with bolts that can be used to push things in and out a bit.
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