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Help with frame alignment

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Help with frame alignment

Old 10-02-22, 02:37 PM
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TiHabanero
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Help with frame alignment

Have a frame where the front triangle is pulled to the drive side 4mm. I need to bend the top and bottom tubes to the non-drive side 2mm, but cannot figure out how to go about it. The head tube does not have any twist. Any ideas on how to go about this? I do not have a flat table from which to work, therefore I am improvising. Help me improvise a method to align this thing.
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Old 10-02-22, 02:46 PM
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unterhausen
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How are you measuring the 4mm? Tbh, 4mm is not a lot. It's not internet bragging rights straight, but ridable.

Everything is relative. I would not want to align a front triangle in a completed frame. But I'm trying to imagine how this is a front triangle problem since there is no twist. Unless you are measuring from the bb shell, in which case the bb shell is on there crooked. BB shells can be straightened with a big enough lever. Or is the twist evident at the seat tube. It's much easier to bend seat tubes if the seat stays are not attached. If it's just in relation to the rear triangle, bend that, it's much easier.

Edited to add: if you are measuring from the bb shell, I hope it's faced. It's not a great place to measure from, but it's a totally worthless place to measure from once you've brazed the bike and not faced it after.
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Old 10-02-22, 03:01 PM
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TiHabanero
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I am indeed measuring using the BB shell. The shell is faced. The seat tube is mostly in line to the BB shell, off by .5mm. The rear axle is in line with the BB shell. Somehow I got the top and down tubes off to one side. I know it is no big deal as the bike rides really well, but I'm having an obsession about it.
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Old 10-02-22, 05:11 PM
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dsaul
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Originally Posted by TiHabanero View Post
I am indeed measuring using the BB shell. The shell is faced. The seat tube is mostly in line to the BB shell, off by .5mm. The rear axle is in line with the BB shell. Somehow I got the top and down tubes off to one side. I know it is no big deal as the bike rides really well, but I'm having an obsession about it.
Flip it over and measure from the other face of the BB shell. I'd bet that you come up with a different result. BB facing tools don't produce a perfectly perpendicular face and thousandths at the BB face could be millimeters at the head tube. Its also pretty unlikely that the BB is perfectly aligned with the centerline of the frame. It may be centered on the centerline, but even a slight tilt off axis will result in what appears to be a misalignment based on the BB face.
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Old 10-02-22, 06:56 PM
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unterhausen
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You need to free your mind and realize that it's the bb shell and the rear triangle that are off. Okay, it doesn't really matter what is off, but it does matter what you can do about it. On most better tubing, it's not really possible to bend the front triangle. I don't know what you would be bending. The downtube socket of the bb shell will definitely bend. The bend needed at that point of the frame is not actually that much. By my calculations it's .006 degrees. Or, for the degrees, minutes seconds folks, 0 0' 25.2". The chain stays will follow the bb shell. But I think you will have problems caused by the seat stays. If you put the bb shell on a post and use a big lever to move the front triangle the way you need to go, you will be bending the bb shell. At the top of the seat tube it's going to go whichever way it wants to go. You may be able to tweak it there, I have no experience.

I saw a picture of the Alex Singer shop and near their alignment table there was a 2 meter long rod. You need to get something long enough to give you leverage to move the front triangle relative to the bb shell. Something will move. Hopefully nothing buckles. You might want to spread the load on the head tube so it doesn't get dented. It takes a lot of force on a built frame. Check these things before you put the seat stays on.

I have made the mistake of only measuring from one side of the bb shell. I really need to set up my bench centers so I can measure from the head tube. Once a frame is built, I have no interest in its alignment unless there is a problem.
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Old 10-04-22, 06:09 PM
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TiHabanero
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Good input, Interhausen. I have measured from both sides of the BB. As you point out the TT and DT at their source, seat lug and BB shell, are only off by a fraction of a mm, and at the HT are off by many mm as mentioned in my first post. This makes sense as the ST is off by .5mm at the seat lug.
I have measured up the rear triangle for alignment with the ST/HT and BB. It is square with the BB shell, and off by 1.5mm with the ST. I have used 32" rods to untwist a HT a few years ago. Tubing was True Temper standard diameter road tubing (R2 or something like that). Took some effort, but not a big deal with standard diameter tubing.

It has been decided to call it, and reassemble the.bike. The thing rides rides like a dream as it is, therefore being picky is not going to achieve anything other than stoke my ego. Got a great bunch of people here, thanks for all the help.
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