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Old 05-18-2012, 12:46 AM   #1
DaBit OP
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Luggage racks, aluminium vs steel and tube diameter calculations

OK, so I am considering selling the heavy and wide TT Zega plus rack setup, and make my own luggage rack just like I did with the LC4. Allows me to make a less wide setup with the weight lower and more forward. Soft luggage for trips, small hard cases for daily use.

I would like to keep the rack light. It is used only 10% of the time, and the other 90% of the time it is only dead weight. Keeping it light can be done by using light materials or larger diameter tubing with thinner walls.

Now, the most practical materials for building the rack is either steel, stainless steel and aluminium tubing. I don't want stainless; very expensive and fairly hard to come by.
Steel is always a good option, but again: obtaining large diameter thin walled steel tubing is not easy also, and steel rusts.
And then there is aluminium. Easy to process, easy to obtain tubing in smaller quantities, does not rust, not as expensive as stainless. But, less stiff and strong (especially as-welded), so larger diameter tubing and larger wall thicknesses required.

Questions:
1) Is there a good reason why aluminium is seldom used as luggage-rack material, apart form the 'middle of nowhere repairability problem'?

2) Say I have a construction made of steel pipe with diameter X and wall thickess Y.
Now, if I want to make the same construction with the same stiffness using aluminium tubing with wall thickness 1.5 * Y and I keep the dimensions of the construction the same except for tube diameter, how much larger would I have to chose the the diameter of the tubing?

I don't think alloy choices affect the outcome much, but if you need numbers: regular steel vs extruded 606x-T4 tubing. Steel pipe diameter 16mm x 2mm wall thickness (approx 5/8" x 0.08"), aluminium 3mm wall thickness.
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Old 05-18-2012, 04:45 AM   #2
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Three of the following four statements are true

1.Aluminium fatiges fast when subject to vibration.
2.The end of your bike is a high vibration area.
3.Luggage racks break.
4.Every farmer has a MIG welder in the barn set up with alloy wire and a fresh bottle of argon.
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Old 05-18-2012, 05:43 AM   #3
DaBit OP
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If metal fatigue was such a big problem, then why do they make subframes out of aluminium?
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Old 05-18-2012, 05:49 AM   #4
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]1.Aluminium fatiges fast when subject to vibration. Depends on the aluminum alloy
2.The end of your bike is a high vibration area. Usually true
3.Luggage racks break. When they are pieces of shit poorly made with no consideration to load and stress
4.Every farmer has a MIG welder in the barn set up with alloy wire and a fresh bottle of argon. As long as the daughter is hawt who cares?
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Old 05-18-2012, 06:25 AM   #5
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Quote:
Originally Posted by DaBit View Post
Questions:
1) Is there a good reason why aluminium is seldom used as luggage-rack material, apart form the 'middle of nowhere repairability problem'?

2) Say I have a construction made of steel pipe with diameter X and wall thickess Y.
Now, if I want to make the same construction with the same stiffness using aluminium tubing with wall thickness 1.5 * Y and I keep the dimensions of the construction the same except for tube diameter, how much larger would I have to chose the the diameter of the tubing?

I don't think alloy choices affect the outcome much, but if you need numbers: regular steel vs extruded 606x-T4 tubing. Steel pipe diameter 16mm x 2mm wall thickness (approx 5/8" x 0.08"), aluminium 3mm wall thickness.
1. I built my rack from 1/2" X 1/8" wall steel tubing, bought a small tube bender and welded it up with MIG .023 wire with C25 gas. Yes it is heavy but two weeks ago after a fluke dust devil knocked my bike over at work and nothing was broken, just had to straighten out a bark buster and repaint 2" spot on rack, I think the added weight of steel was worth it.

2. Can't help with question two except to suggest repost in Garage-Welding Questions thread or post here:
http://weldingweb.com/
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Old 05-18-2012, 06:27 AM   #6
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Quote:
Originally Posted by DaBit View Post
If metal fatigue was such a big problem, then why do they make subframes out of aluminium?
To save weight.

Anything I can think of with an alloy subframe has nothing hanging off it but a rear fender and a tailight and by the time you've owned it 12 months or 500 laps it's just so last season it's time to get a new one.

What did you have in mind for an example of an alloy subframe?



]
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Old 05-18-2012, 06:36 AM   #7
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Quote:
Originally Posted by LuciferMutt View Post
]1.Aluminium fatiges fast when subject to vibration. Depends on the aluminum alloy
2.The end of your bike is a high vibration area. Usually true
3.Luggage racks break. When they are pieces of shit poorly made with no consideration to load and stress
4.Every farmer has a MIG welder in the barn set up with alloy wire and a fresh bottle of argon. As long as the daughter is hawt who cares?

So three usually true and one doesn't matter?

What grade of aluminium is vibration resistant.
The stuff I pick up at boatbuilders is OK on the plate & section but the tube is only good for edging.
I've made some reliable alloy bits but they are moncoque rather than spaceframe.
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Old 05-18-2012, 10:30 AM   #8
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I want to see where this goes. I want to build a rack for my S10 and have been asking the same question
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Old 05-18-2012, 12:12 PM   #9
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I built my racks from steel for a number of reasons. Cost of the tubing was not an issue but I was having trouble finding a suppler of 1" tubing that was bendable. I tried 3/4" 6063 pipe but that is 1.05" od and my bending die did not like that. A new die is a few hundred.

I also chose steel so I could weld it permanent to my subframe thus keep ing the weight down from lack of bolts etc. prob not an option for most people though.

Steel is just more forgiving, cheaper, stronger and easily available. Yes there is a weight penalty but it's not that much, and the commercial ones are powder coated for cheap in bulk but you also should factor that in.

I didn't know much about alloys and was hesitant to drop a bunch on tubing that would crack when I bent it. I will say if I were to do it from aluminum, I would use 6063 3/4" pipe and buy the proper die. It bends nicely, strong, light etc
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Old 05-18-2012, 12:46 PM   #10
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Quote:
Originally Posted by sailah View Post

I didn't know much about alloys and was hesitant to drop a bunch on tubing that would crack when I bent it. I will say if I were to do it from aluminum, I would use 6063 3/4" pipe and buy the proper die. It bends nicely, strong, light etc
Why would you use pipe rather than tube? Wouldn't cutting and welding be stronger than bending Aluminum?
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Old 05-18-2012, 06:10 PM   #11
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More engineering than you probably want to hear

The stiffness of a beam is proportional to the Moment of Inertia and the Modulus of Elasticity of the material . For tubing the Moment of Intertia .049 X (D^4 - d^4). http://www.engineeringtoolbox.com/ar...ia-d_1328.html The Modulus of steel is about 3 times the modulus of aluminum http://en.wikipedia.org/wiki/Young%27s_modulus

Also you asked about stiffness rather than strength. The strength is a function of the stress in the material and the yield strength of the material. The stress is a function of the section modulus instead of the moment of inertia. the section modulus is proportional to the moment of intertia divided by the outer diameter, so that will change the relative tubing diameters with strength rather than stiffness as the criteria.

Of course it is never as simple as those equations because of problems of denting tubes and fatigue, and heat treatment, etc.


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Old 05-18-2012, 06:28 PM   #12
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Quote:
Originally Posted by norexit View Post
Why would you use pipe rather than tube? Wouldn't cutting and welding be stronger than bending Aluminum?
I couldn't find tubing in a bendable alloy locally. I could find 6061 in tubing, but all my local aluminum supplier could get was 6063 in pipe sizes. I was told that 6061 doesn't take to bending very well but that 6063 does.

I bent up some 6063 and it bent very well.

I just checked mcmaster and I guess you can get it in tubing sizes. Good to know for next time, however shipping it gets expensive quick.

As for the cutting and welding vs bending, I much prefer to bend aluminum to make a corner than to cut and weld. I never find the joints to be as strong esp with my TIG welding skills. Plus the aesthetics of a nice bent radius looks so much nicer than a square corner imo.
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Old 05-18-2012, 06:29 PM   #13
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When you run out of things to do try picking the fly shit out of a pile of pepper!! GH
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Old 05-18-2012, 08:33 PM   #14
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Bugger aluminium, go steel, if a welder is available at all, it will be set up for steel as a minimum.

Build it up out of triangles, and it will be light AND strong.





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Old 05-18-2012, 09:40 PM   #15
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That my welder friend, is truly amazing.
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