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measuring tubes for c-c length

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measuring tubes for c-c length

Old 05-27-19, 07:56 PM
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chrism2352
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measuring tubes for c-c length

I'm building my first frame, using paper miter templates and am having some trouble figuring out where to miter the tubes based on my intended c-c lengths, ie: I want a 670mm downtube (31.7 mm & 73ļ HT angle). If my HT is 36mm and the BB is 40mm, how do I figure out where to miter the tube.

I can subtract the 18mm of the HT and 20mm of the BB from the total length, for 632mm DT length, but how do I account for the increased distance to the center of the HT caused by the angle of the DT to the HT? Also, since the DT length is based on the center of the DT, how do I figure out the length of either the top or bottom of the DT?
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Old 05-27-19, 08:11 PM
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Andrew R Stewart 
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Originally Posted by chrism2352 View Post
I'm building my first frame, using paper miter templates and am having some trouble figuring out where to miter the tubes based on my intended c-c lengths, ie: I want a 670mm downtube (31.7 mm & 73ļ HT angle). If my HT is 36mm and the BB is 40mm, how do I figure out where to miter the tube.

I can subtract the 18mm of the HT and 20mm of the BB from the total length, for 632mm DT length, but how do I account for the increased distance to the center of the HT caused by the angle of the DT to the HT? Also, since the DT length is based on the center of the DT, how do I figure out the length of either the top or bottom of the DT?

Trig. Do the math and find the miter edge to miter edge on either the top or bottom of the tube in question. Andy
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Old 05-27-19, 08:37 PM
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unterhausen
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Bikecad or rattlecad. It's not a hard trig problem, but why risk screwing up when someone else has done the hard work already?
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Old 05-27-19, 09:49 PM
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When I build a frame I make a drawing on a large sheet of paper. I first use a ruler and compass to draw out the frameís C to C measurements. Then I simply use a ruler and straight edge to draw in the tubing widths. Itís a painstaking process but isnít all that bad in the grand scheme of a frame build.

I use the dimensions from this drawing to determine where I need to place the miter templates.

Tips:

Obviously the drawing needs needs to be quite accurate. Spend time on it.

I bought a large compass specifically for making this drawing off of some math oriented website. It was only $10-15. Small compasses like the ones you used in high school arenít nearly accurate enough in my opinion. They are just too small with too closely spaced degree markings to work well for a full-scale bike frame drawing.

For free paper that works great go to your local Home Depot. They have a large roll of paper you can use to cover the inside of your car if you bought something that can get your car dirty. Itís on the same cart that you get that plastic cord to tie large items to the top of your car. Just tear a few feet of that off, it works great. Obviously buy something while you are there or else itís the same as stealing.
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Old 05-28-19, 05:31 AM
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Rattlecad is free. Design your frame there and pull all of the measurements you need from the multiple drawings it creates.

Where did you obtain the center to center measurements? I design my frames using fit points like saddle height, saddle setback and bar reach with the desired stem length. Frame angles are whatever they need to be to achieve the desired fit, bottom bracket drop and trail number for the intended use. The tube lengths are just resultant figures from that design and not a length that I choose in advance.
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Old 05-28-19, 06:34 AM
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A full size drawing is very useful. As Andy alludes, once you have this you can measure the edge to edge distances. I used to make a drawing myself on paper which is not difficult, but for my most recent project I used Rattlecad which is even easier. I'm lucky and have access to a full size printer at work but if you don't there are blueprint services which should be able to make one for you. Just be sure to verify the dimensions because I found some scaling issues when printing the drawing for my frame.
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Old 05-28-19, 07:02 AM
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what's your process for getting a properly scaled full-size drawing from rattlecad? Does it output a pdf or some better format?
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Old 05-28-19, 07:13 AM
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Thanks for all the quick feedback. Unfortunately it's been 25 years since I graced the inside of a trig classroom, so those formulas are sitting somewhere in the bottom of the dustbins of memory. I'm more than happy to brush up if someone has a good resource for these calculations. In the meantime I'll see if I can make sense of Rattlecad. Thanks again
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Old 05-28-19, 09:35 AM
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I have a Microsoft Excel spreadsheet that will do the math for you. It converts a tube’s center-to-center measurement to the distance from the edge of one tube to the edge of the other. If you email me a request I can send it to you. My email address is my name in all lower case letters without a space between my 1st and last name. After the symbol you get by using the shift key and the number 2, the rest of the address is qtm.net.

This spreadsheet was done for me by one of my framebuilding class students many years ago that took engineering at nearby Notre Dame University. It has a page for a top tube and another for the down tube. You have to fill figures in 6 columns: What is the length of your tube? What is the head angle? What is the seat angle? What is the head tube diameter? What is the down tube diameter? What is the seat tube diameter? Once you fill in those dimensions, the spreadsheet in column B will fill in the top edge-to-edge distance and in column C the bottom edge-to-edge distance. Of course you will get your original center-to-center dimensions by measuring them off of your full-scale drawing.

In recent years I haven’t used this spreadsheet since Bikecad and rattlecad became available but it is still a good option for those that don’t have those applications.
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Old 05-28-19, 11:01 AM
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Originally Posted by Nessism View Post
A full size drawing is very useful. As Andy alludes, once you have this you can measure the edge to edge distances. I used to make a drawing myself on paper which is not difficult, but for my most recent project I used Rattlecad which is even easier. I'm lucky and have access to a full size printer at work but if you don't there are blueprint services which should be able to make one for you. Just be sure to verify the dimensions because I found some scaling issues when printing the drawing for my frame.
I think Rattlecad will output into .dwg format which is easily translated into a drawing using solidworks or similar.
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Old 05-28-19, 01:00 PM
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Email sent. Thanks, Doug!
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Old 05-29-19, 06:58 PM
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As a hobby builder and one trained in mechanical drawing before the wide spread use of CAD, I use a large full scale drawing of the frame and measure against the drawing plus lay the frame to the drawing to check it. Yes, I am also able to run CAD as we worked in AutoCAD a long time ago, but no longer have a copy of it, and I really like the old world method of drawing it out by hand.
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