Framebuilders - Brazing some racks
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12-04-09, 07:01 AM
I am looking into building some racks, and want to braze them. Anyone have any input on materials? I am looking at 4130 cromo, but want to know if it really needs to be tubing, or can solid rod work? I know tubing is lighter, I am not always looking at what is lightest, but what will hold up the best after years of heavy use.
One can use solid rod, but it has some disadvantages. The weight part is obvious, but there are other issues. It is harder to braze and sometimes even to weld rod together. The reason is the size of the end vs. the density of the structure. You could make a much larger tube out of the metal required for a solid 1/4" rod, and that tube would fillet inside and out, over a wider diameter, so you could neatly get more braze in there to hold the joint over a wider base. For panniers 1/4" rod either aluminum or stainless works not too badly, but it doesn't compare to 3/8" tube, and the tube will often fit todays fancy hardware in essential ways. On the other hand, by and large 1/4" rod isn't stiff enough for a porteur, where the weight penalty will mount if you use enough of the stuff. Where rod makes sense is with really small stuff like water bottle racks for, say, a bottle of Phil's oil.
Of course one of the great rack innovations of all time was the Blackburn rack, and it is solid aluminum rod, but it needs to be TIG welded. Could probably be gas welded.
Keep in mind that structurally strength increases to the square and stiffness to the cube of diameter. That is why tubing is both lighter and tougher than rod in applications where stiffness is paramount.
rodar y rodar
12-04-09, 08:51 PM
As I see it, advantages to using rod:
Easier to get locally than tubing, easy to MIG weld, easy to vise bend, strong IF you use big enough material.
Disadvantages to using rod:
It`s gonna be one heavy sucker! You said you aren`t very concerned with weight, but you might not realize how much of a boat anchor it`ll turn out to be in order to match the strength of even 5/16 tubing- probably several pounds per rack. If you`re planning to hang panniers from it, you`ll probably need at least 5/16 in order to match with most modern pannier mounting systems. Harder to braze than tubing (maybe even impossible if you`re thinking about MAPP and silver). Those easy vise bends will be really tough to get as nice and even as tubes bent with a hand bender- you could probably get around that with a some blacksmithing skill or some kind of rod bender, though.
What kind of racks are you thinking about? What kind of joining method? Some alternatives that you might consider are angle iron or brake tubing- both relatively easy to get ahold of and stiffer per given weight than solid round rod. I`ve been using hydraulic tubing (8 and 10mm with 1mm wall) and so far had good results- probably a little better than relatively thin walled brake line, but I know that brake line has been used before with acceptable results. I`ve also seen some porter style racks on the internet built with angle iron that looked decent and were probably very strong and I`d guess ligher than solid round rod. Cetma-style racks would probably be a good candidate for angle iron- Google them up if you aren`t familiar with them. If by some chance you have acces to a sheet metal brake, bent sheet metal (mini angle iron) can also be nice and stiff without getting super heavy. Or just use rod, like you asked about to begin with, but be advised that it`s going to be REALLY heavy.
EDIT: Racks are loads of fun to build and even with minimum skills can turn out to be useable. More skill, more prettier, more funner as you progress. If you have a hankering to try it, give it a shot by whatever method you chose- I`m just trying to help you along with a few tips. Good luck with it and enjoy yourself.
12-04-09, 09:43 PM
I'm about to start building racks too. Here's a few things to consider.
3/8" Round Bar Mild Steel - ~.375 lb per foot - about 50 cents a foot
3/8" Round 4130 Bar - ~.375 lb per foot - a little over a dollar a foot
3/8" x .065 wall DOM tubing - ~.19lbs per foot - about $1.20 a foot
3/8" x .065 4130 tubing - ~ .22 lbs per foot - about $3.25 a foot
All taken from OnlineMetals.com, don't ask me why the solids are the same weight, but the tubing isn't.
I'm leaning towards using the mild steel DOM tubing.
Also bending small diameter tubing is cheap and easy with a tool such as this - http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=3755
rodar y rodar
12-04-09, 10:19 PM
JMD, that`s much less weight difference than I thought and I suspected they might have "typo`d" the dia instead of the weights for the solid rod. But I just weighed a 10 ft piece of 3/8 1018 on a very rough scale normally used for weighing in the hundreds of pounds and it came up 4 lb, so I suppose Online Metals`s numbers are right. Also note that 3/8 - .035 4130 tubing (which really should be plenty) is listed at 0.13 lb/lf. Anyway, another material option occured to me since I posted. Welded seem square tubing is usually pretty easy to find at local metal supply houses- Aircraft Spruce lists 3/8 and 1/2 -.049 square Cromo tubing at .22 and .30 lb/lf respectively. The cheap stuff I get from the local supply place is probably similar weight, the main advantage being easy availablity. DOM would probably be a good choice too if you aren`t trying to get thin walls.
12-04-09, 10:54 PM
They sell the DOM in .049 and .035 also. I was just using those for comparison. However the price goes up for the .035 pretty sharply.
12-05-09, 08:27 AM
Thanks for the input. It seems tubing is a better choice to go, more because of the workability than anything. I am not really worried about the size of the tubing fitting off the shelf panniers, I am making my own of those...my wife thinks I am a bit crazy, I agree with her most times. It seems that racks tend to all fail in the same place after loaded touring, right down near the eyelets for the drop outs, and I was thinking perhaps rod would help alleviate some of that.
What about creating a form of a drop out for the racks that would slide an inch or two into the tubing. It would have to be machined to match the inside diameter of the tubing. If you were building a rack with 3/8" tubing, use 3/8" rod and lathe an inch on the end to match the ID of the tube, kind of like a inside lug, then sweat braze it on. Just a thought.
rodar y rodar
12-05-09, 09:17 AM
Nothing wrong with crazy. Racks have certainly been known to break, even good sturdy racks, but unless you`re really carrying a huge load or taking it really hard over the rough stuff it shouldn`t be a chronic problem. I take it you`ve lost a few in a row? What models or what kind of construction? Cheapo racks work out fine for some people, but if that`s what you`ve been breaking maybe you need to up the ante. Or if they already were nice solid racks, maybe they were mounted funny so that one side took all the load until it failed, then left the other side all alone? That`s about the only thing I can think of that might cause a rash of failures, but something is fishy.
12-05-09, 11:44 AM
I can't image a properly engineered and professionally built rack failing at the eyelet attachment point. Here's the detail of my Tubus Luna rear rack attachment.
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