Luggage racks, aluminium vs steel and tube diameter calculations

Discussion in 'The Garage' started by DaBit, May 18, 2012.

  1. DaBit

    DaBit Been here awhile

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    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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  2. Rucksta

    Rucksta SS Blowhard

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    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.
    #2
  3. DaBit

    DaBit Been here awhile

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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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  4. LuciferMutt

    LuciferMutt Rides slow bike slow

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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?
    #4
  5. KS Jay

    KS Jay Been here awhile

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    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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  6. Rucksta

    Rucksta SS Blowhard

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    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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  7. Rucksta

    Rucksta SS Blowhard

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    So three usually true and one doesn't matter? :D

    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.
    #7
  8. nanisx

    nanisx Been here awhile

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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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  9. sailah

    sailah Lampin' it

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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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  10. norexit

    norexit Adventurer

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    Why would you use pipe rather than tube? Wouldn't cutting and welding be stronger than bending Aluminum?
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  11. GSWayne

    GSWayne Old Guy nOOb

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    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/area-moment-inertia-d_1328.html The Modulus of steel is about 3 times the modulus of aluminum http://en.wikipedia.org/wiki/Young's_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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  12. sailah

    sailah Lampin' it

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    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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  13. GH41

    GH41 Been here awhile

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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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  14. Andy-Gadget

    Andy-Gadget Any bike can go anywere

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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.
    [​IMG]
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  15. Ironwood

    Ironwood Friday Harbor, WA

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    That my welder friend, is truly amazing.
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  16. Ironwood

    Ironwood Friday Harbor, WA

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    And just for the record, you can bend 6061 after heating it in the bend area to a very slight yellow cast. Maybe 900f. You can see it take a slight yellow. After it cools you can bend it for an hour or so. It will re-temper over several days to near a T-4 hardness. The same thing happens when you weld it.
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  17. villageidiot

    villageidiot Long timer

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    I just built a tail rack from 1/2" 16ga tubing. It was cheap, strong, easily bendable, welded up nice. Bought 20 feet of it for less than 10 bucks.

    I like aluminum too, but damn steel is just so easy
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  18. DaBit

    DaBit Been here awhile

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    Examples that come to mind:

    BMW G650X has an allow subframe. Subframe has to carry rider and possibly some light luggage.
    KTM 950 Adventure has an alloy subframe. Carries rider, pillion and luggage.
    SV650 uses aluminium for the frame.

    Quite a bit of aluminium used, and those constructions carry a lot more than 15-20kg of luggage.

    And yes, those subframes tend to break when loaded heavily. But 5kg in a tailbag does more harm than 10kg on a loop of tube which is supported at 3-5 spots.

    I converted my TIG-welder from DC to AC/DC for only one reason: the good local availablility of aluminium extrustions in all sizes and shapes and lower quantities.

    Steel is also available, but mainly in 'construction sizes', thus heavy walled and high diameter. The sizes I want must be ordered and transported, so minimum quantity and cost inclusing shipping is far higher than aluminium. And then there is hydraulic pipe, which is also expensive and heavy galvanised (yuk).

    'Strength' is a function of construction dimensions and amount of metal used. For the same amount of strength and stiffness an aluminium construction needs larger dimensions and more metal volume. The one million dollar question is 'will it still save weight then?'

    I am trying to figure out what that weight penalty would be.

    English is not my native language, so would you care to explain the difference between pipe and tube?

    OK, I will do a bit of studying on this. But I suppose that the 'to the power of four' in the equations would quickly outweigh the extra metal used for larger dimensions.

    Yes, I did. A luggage rack consists of quite a bit of metal and carries only a fairly light load. When it is stiff enough, it is probably strong enough i suppose.

    Of course. The dents-in-tubes problem is also why I cannot use for example 1" x 0.030" steel tube. An aluminium 1" x 0.1" tube is just as heavy but won't dent as easily.

    :eek1. Not exactly a light construction. Beautiful work though.
    But you can (and should) use triangles with aluminium also ;)
    #18
  19. sailah

    sailah Lampin' it

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    The difference in pipe vs tube is really just how they are measured.

    Tubing is measured in outside diameter, od. And then wall thickness

    Pipe is measured in inside diameter and also wall thickness sometimes expressed as schedule. Schedule 10, 40, 80 get thicker as you go up.

    1" tubing is 1" od, but 3/4" pipe is 1.05" od

    I might have some if my specs wrong, I'm just a hobbyist recalling from memory.

    I have had excellent luck with my jd2 bender, works beautifully to put nice radii in any type of tubing. The bender isn't terribly expensive, it's the dies that add up as each set is a couple hundred bucks. The harbor freight option is terrible though all that does is kink it.

    I'd be very interested if someone actually knew about bending aluminum tubing. All my effluence is in 6063 pipe and since I don't have a die set for that, haven't bent much.

    I have a 1" die with a 3.5" clr, anyone know if 6061 will take in like a t4 temper? I'd prefer not to have to heat it first
    #19
  20. GSWayne

    GSWayne Old Guy nOOb

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    Did your grandfather work on the Eiffel tower :rofl

    Nice work, a space frame is a very efficient design technique. I bet that rack is much stronger than the rest of the motorcycle. Unless those are steel tubes, it does probably weigh few pounds though. Ducati likes triangulated frames.
    #20