When you get a True Temper, Supertherm top tube; you get a tube that is 1-1/4" in diameter, and 675mm long. It is also double butted, that means it is thicker at the ends, than it is in the middle. In this case it's 1.0mm at the ends, and tapers to 0.7mm in the middle. They design it that way to save weight. When you get the spec sheet, they will tell you the dia, Length,thickness, then the butt lengths. example; 100/75/225/75/200. So it's 1.0 for the first 100mm, then tapers from 1.0 to 0.7 in the next 75mm, then is 0.7mm for 225mm, then tapers from 0.7 to 1.0 for 75mm, then is 1.0 for 200mm.
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You will notice it is not symetrical. The designer now has to determine where to place the butts. I need to cut the tube down to 520mm long. If I want the tube symetrical, I'd cut it so I'd have 72.5mm of 1.0mm thickness on both ends. So it would be 72.5/75/225/75/72.5.
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So now you have a symetrical tube, but the stress on the tube is not symetrical. Most of the stress will be at the toptube/headtube joint. So you want to cut the tube so the butt at the top/headtube joint is longer than the one at the top/seattube joint. I'm sure there is a long list of engineers out there who can come up with a formula, but I'm not one of them. Looking at the numbers, I made the long butt 10% (72.5x 0.20) longer than the short butt. 87/75/225/75/58.
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You might even go longer than 20% on a BMX frame. I say that because there is a lot more stress put on the handlebars on a BMX frame vs. a road frame.
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If you notice the top tube has a larger diameter than the seat tube. So now if you cut a hole for the seat tube, you will get something that looks like this.
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If you don't have one of these, you got to get one of these. It measures as accurate as 0.1 degrees. You can't see it, but at the other end is a level. The level is sensitive to 0.1 degrees. It will get you close enough.
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Now we are jigged up.
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You will notice it is not symetrical. The designer now has to determine where to place the butts. I need to cut the tube down to 520mm long. If I want the tube symetrical, I'd cut it so I'd have 72.5mm of 1.0mm thickness on both ends. So it would be 72.5/75/225/75/72.5.
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So now you have a symetrical tube, but the stress on the tube is not symetrical. Most of the stress will be at the toptube/headtube joint. So you want to cut the tube so the butt at the top/headtube joint is longer than the one at the top/seattube joint. I'm sure there is a long list of engineers out there who can come up with a formula, but I'm not one of them. Looking at the numbers, I made the long butt 10% (72.5x 0.20) longer than the short butt. 87/75/225/75/58.
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You might even go longer than 20% on a BMX frame. I say that because there is a lot more stress put on the handlebars on a BMX frame vs. a road frame.
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If you notice the top tube has a larger diameter than the seat tube. So now if you cut a hole for the seat tube, you will get something that looks like this.
Some people will cut a hole throught the top tube, and do it like the old GT, and there is nothing wrong with that, I just didn't design my frame like that
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With a vise and a couple wood wedges the problem is quickly fixed. This way you can squeeze the tube down 1/8", and give it a gentle taper.-
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If you don't have one of these, you got to get one of these. It measures as accurate as 0.1 degrees. You can't see it, but at the other end is a level. The level is sensitive to 0.1 degrees. It will get you close enough.
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Now we are jigged up.
1 comment:
Glad to see you're on your way with another frame. Lookin' good!
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