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Discussion Starter · #1 ·
What size down tube is typically needed for a size 19" mountain bike with a 23.5" top tube and a 470mm fork length?

I am trying to get started with frame building and went ahead and ordered three tube sets from NOVA.

Tubes for the NOVA OS MTB come in the following lengths.
DT 650mm, TT 635mm, ST 520mm*, CS 440mm, SS 600mm, HT 200mm

The NOVA OS MTB tubes are the same with the exception of the DT which is 750mm.

The rub is that the down tube length of 650mm that comes from with the NOVA MTB seems too short for building anything other then a small bike.

The down tube length for the NOVA OS MTB was plenty long until I made a bone headed move on my rough cut and accidentally cut it to short.
 

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Building using tubesets is a pretty bad idea. Since you know the length of each tube you need, you just order a tube that confoms to your design goals.

I ordered a tubeset once and I will never make that foolish mistake again.
 

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+1

I agree with Pete. If you don't have a drawing or similar design specs figured out in advance, you should not be buying any tubes yet. And you almost certainly should buy them as individual tubes, because A) the cost is not much greater and B) you will almost certainly end up with something you didn't really want with any of the pre-selected sets.

-Walt
 

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Not enough information.
Do you just want to know length or are you wanting to know diameter and wall thickness as well? If you just need the miter to miter length, you will need to know where the DT joins the HT, angles, etc. If you want opinions on wall thickness and diameter, you will need to give information on rider weight, riding style and conditions, and so on. I am sure PVD uses different tubes for aggressive Marin County single track than for those riding on rail trails.

On a side note, I order full tube sets all the time. I might not always use all the tubes from the same set on a single bike, but I eventually use them for something. For me it makes sense and it is nice to have the "extras" around in case of a tube replacement, etc.
 

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Keep in mind that with butted tubing 'long enough' means not only it's overall length, but where the butts are, and how much. You need to draw the frame, then get a rough idea what tubes you want, and look at the specs for each tube to see where the butts are. You don't want to cut off the butts or leave them too short. The printed specs are probably good enough for you (just starting?), but eventually you'll want to actually measure where the butts are b/c there can be discrepancies.
A quick and dirty approach is if the tube just barely fits the drawing, you'll have enough butt (but not a lot of tube to play with for tweaking miters), and if it's way too long, watch out b/c you'll be cutting off the thick ends. Some upper end tubes have long and short butt options... I'd stay with long for you first number of frames.

-Schmitty-
 

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Discussion Starter · #6 · (Edited)
So I will admit that I am a bit of an idiot and am math challenged. The length of the seat tube is easy to figure out. The top tube and downtubes are slightly harder to figure out due to the angles. On the NOVA site, I see down tubes and top tubes in various sizes but no information about the lengths of butts. I am a bit frustrated by the ordering process although I am more frustrated with myself vs. NOVA. That said it would be nice for a newbie if the tubeset order page described the minimum and maximum sized bicycle that could be built with the set.

For a size 19" mountain bike with a 470mm fork length and a 3.5" head tube what length tubes should I be using for the DT and TT? Geometry on the bike is basic 71 head tube angle and 73 seat tube angle. This is a "plain vanilla" build. I will try to figure this out myself but I fear that I may be too stupid at this point in time. Basically this should be simple math but I don't yet know the rules about how much is typically cut from the tube ends. Using my jig and eye balling by holding up the tubes I think I can use the 635mm top tubes on the bike and will need something like a 700mm down tube.

My guess is that I am going to need to send the tubes back and/or eat them (does anyone want to buy pre-owned used NOVA 31.8mm 650mm down tubes and/or trade them for some slightly longer ones).
 

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Taking your angles, and assuming a top tube effective length of 23.5"(597mm) and a 100mm head tube, 15mm HT extension below the DT, etc, I get a down tube length of 647mm miter to miter. YMMV. knowing you have a relatively harsh angle at the head tube, you can figure on needing more than a 650mm tube to start with. If one were so inclined to use Columbus Zona, they would look here http://www.columbustubi.com/eng/4_4_7.htm. 35mm DT @ 670mm with 125mm butts and in .8/.5/.8 wall might just fit the bill. Again, YMMV. I don't do much with True Temper, but I know all the tube configurations are listed on Henry James web site. Many here use TT tubes and could help you better if they were so inclined.
Calling Lon at Nova will also get you a wealth of information(often more than you ask for). Nova's site is not the greatest, but I like dealing with them. Sometimes specs are not listed and you have to call. In this particular case though, there are specs. http://www.cycle-frames.com/bicycle-frame-tubing/29er-DT-35-x-8-5-8-x-750.html
750 tube with 110/150mm butts. That is plenty of butt length for a 647mm down tube.
I am rambling(purposefully) hoping this will shed some light on how to look at tubes from a length perspective. More information is needed to determine diameter and wall thickness. I hope this helps....somewhat ;-)
 

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+1 for Schmitty

Don't be intimidated by the math - it's nothing more complicated than what you learned (yes, you did, assuming you managed to graduate) in high school.

And if you don't want to do any math, and don't know how to use a full CAD program, there are programs like BikeCAD, RattleCAD, tubemiter, and others that will calculate all the miter lengths and angles for you.

I'll say it again: there is no reason to buy tubes until you have figured out what tubes you need. Design your bike, then build it. Don't put the cart before the horse and try to make inappropriate tubes fit your application because you're lazy or cheap - because the bottom line is that if you're lazy and cheap, you are way out of your league building a bike frame.

-Walt
 
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