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frame geometry question concerning down tube angle

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frame geometry question concerning down tube angle

Old 02-06-19, 10:14 AM
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mstateglfr
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frame geometry question concerning down tube angle

When a frame is fillet or tig'd, how is the down tube angle determined?
With lugs, the angle is pretty much set based on the bottom bracket and head tube lug sockets needing to line up. Straight forward.

Since that constraint doesnt apply to fillet or tig, what determines the angle? Along this line of question- is there a downside to having 1 or 2" of head tube below the down tube joint? Wasnt sure if that increases lateral flex or something negative like that.


**I am referring to a typical drop bar frame where there are no issue to work around for individual rider fit.
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Old 02-06-19, 12:07 PM
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Don't know the right answer but on the filleted frame I built I didn't even consider the angle just cut the DT to fit once the HT angle and TT length were determined. As far as flex Nova sells a HT reinforcing ring, I did install these top and bottom on the HT.

https://www.cycle-frames.com/bicycle...NFORCING-RING/

Good luck

Steve
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Old 02-06-19, 12:22 PM
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why would you want a lot of extra space under the DT? It could possibly look goofy. I suggest learing how to use a frame specific cad program. I use bikecad, and it's great. There is also rattlecad, which seems to be really nice too. So far the big advantage of bikecad is the number of pre-defined parts.

This will not really affect stiffness in a way that matters while riding. The top/down tube spacing can affect the stiffness against braking forces if it's too small. On a small frame, you want to avoid overlapping the top tube and down tube. Bikes like that have a higher failure rate from what I have seen.
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Old 02-06-19, 01:40 PM
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Originally Posted by unterhausen View Post
why would you want a lot of extra space under the DT? It could possibly look goofy. I suggest learing how to use a frame specific cad program. I use bikecad, and it's great. There is also rattlecad, which seems to be really nice too. So far the big advantage of bikecad is the number of pre-defined parts.

This will not really affect stiffness in a way that matters while riding. The top/down tube spacing can affect the stiffness against braking forces if it's too small. On a small frame, you want to avoid overlapping the top tube and down tube. Bikes like that have a higher failure rate from what I have seen.
This question actually came from me using bikecad as you can choose how much past the top tube and down tube you want the head tube to extend. I agree it could look goofy if it goes to an extreme.
Just wasnt sure if some here do something like always have 15mm of space below the down tube or always have 25mm of space- etc etc, and why they decided on that.
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Old 02-06-19, 02:03 PM
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I have enough room so that I don't face into the fillet. 10mm should be enough.
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Old 02-06-19, 02:29 PM
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It might be worth checking your fork crown doesn't hit the down tube.
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Old 02-06-19, 02:37 PM
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I run with BikeCad too and use 15mm as my bottom edge of head tube to down tube lower contact point. I simply measured a few lugs and copied the average. Like Steve I let the DT/HT angle fall where it might otherwise. (But I don't build bike with suspensive forks, those who do might need more clearance).

A side bar story. When I worked at Cyclery North, Chicago in 1985, they premitered all the DT for a 60* HT angle. This was what we set the jig at too. This was rather astounding to me as I was then using the trig sheets I had gotten during the Eisentraut class and also had a Vernier protractor, I knew DT/HT angles varied either side of 60* all the time. When I asked Eddy, the boss and frame designer, why was the angle always at 60* he replied "because that's what every good handling bike I ever rode had". It took a couple of months and a few frames for me to convince Tommy (the other builder) that Eddy was designing frames that didn't make dimensional sense and that the trig sheets I used were based on laws of mathematics and not misconceived opinions. We kept this to out selves, taking Eddy's critical design elements and running through the trig came up with the real numbers. When after the season I moved from Chicago I left a set of the trig sheets with Tommy. Andy
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Old 02-06-19, 02:59 PM
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Here is a pic of my filleted frame with 1 inch of the HT sticking past the TT and DT. I did it because that is what I felt like doing.
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Old 02-06-19, 08:10 PM
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The down tube angle is the result of a number of other frame measurements like HTA, axle to crown height, bottom bracket drop, downtube length, head tube length, etc. It is not a number that you choose, it is simply the result of all of those other frame design choices.

In terms of how much head tube to leave below the down tube, I like to leave at least 20mm. I like the look of it and it keeps me from having to weld to the thickest part of the Paragon head tubes.
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