Luggage racks, aluminium vs steel and tube diameter calculations

[Ironwood]

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.

[villageidiot]

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

[DaBit]

Originally Posted by Rucksta

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

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.

Originally Posted by sailah

Steel is just more forgiving, cheaper, stronger and easily available.

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?'

Yes there is a weight penalty but it's not that much

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

Originally Posted by norexit

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

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

Originally Posted by GSWayne

For tubing the Moment of Intertia .049 X (D^4 - d^4).

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.

Also you asked about stiffness rather than strength.

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

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.

Originally Posted by Andy-Gadget

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.

. Not exactly a light construction. Beautiful work though.
But you can (and should) use triangles with aluminium also ;)

[sailah]

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

[GSWayne]

Originally Posted by Andy-Gadget

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.

Did your grandfather work on the Eiffel tower

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.

[GSWayne]

the answer to the OP question is 20.4 mm outer diameter Al tube with 3 mm wall thickness should be as stiff as 16 mm steel with 2 mm wall.

[Ironwood]

Bending 6061-T6 is not going to work well without preheating. It is just too hard. what are your objections?

[dirtdiver]

[QUOTE=Andy-Gadget;18719935]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.



Wow!!!

[dirtdiver]

I just think of aluminum as having fatigue cracking due to vibration. Particularly around the heat affected zone of the welds.

I thought it was interesting that steel has a curve of when it will fail due to number of cycles of vibration versus stress. With steel you can get under the curve so that if you are under the stress amount the steel will last for an infinite amount of cycles of vibration and never fail.

Aluminum, however, if you give it enough cycles of vibration will fail no matter what the stress level.

So, if you go with aluminum I would expect that you would need to over design the rack and then expect some cracks in the welds over time regardless.

I think that cast or machined aluminum allows you to stay away from the heat affected zone due to welds.

DD

[Benesesso]

What I'd do:

Buy some 6061-0 tubing, bend (easily) and weld it. When you're finished, take it to a heat treating co. and tell them to HT it to the T6 condition. Also tell them to bend it back to the original shape after the SHT of the 1st HT step.

You'll have a strong and light rack if you do your homework on the tubing sizes. Forget about leaving 6061 as welded. If you don't want to/cannot do the post-weld HT, make the rack from 4130 steel tubing. Preheat each weld area to ~300 deg F. before you weld. For added insurance, use a torch and reheat the welded areas to a very dull red after the weld area has cooled to room temp.

[DaBit]

Originally Posted by GSWayne

the answer to the OP question is 20.4 mm outer diameter Al tube with 3 mm wall thickness should be as stiff as 16 mm steel with 2 mm wall.

Thanks! Saves a bit of calculations.
That also means that the rack made from aluminium tube is 35% lighter compared to steel. Hmm, might be worth the hassle. We are all spending a lot of money on aftermarket exhaust cans because they are lighter than the OEM cans, and then we throw the weight savings away by adding an overdesigned luggage rack made of heavywalled steel tubing.

BTW: I found this link. Might be useful for peopel with the same questions.

Originally Posted by Benesesso

Forget about leaving 6061 as welded.

Because of the threefold reduction in strength around the HAZ, or other reasons?

I'll check what post-HT would cost me for such a small rack.

[Benesesso]

Originally Posted by DaBit

Thanks! Saves a bit of calculations.
That also means that the rack made from aluminium tube is 35% lighter compared to steel. Hmm, might be worth the hassle. We are all spending a lot of money on aftermarket exhaust cans because they are lighter than the OEM cans, and then we throw the weight savings away by adding an overdesigned luggage rack made of heavywalled steel tubing.

BTW: I found this link. Might be useful for peopel with the same questions.



Because of the threefold reduction in strength around the HAZ, or other reasons?

I'll check what post-HT would cost me for such a small rack.

Yes. Alcoa Aluminum's advice is to use 11ksi for the as-welded yield strength of 6061 heat-affected zones (HAZs). They are fully aware of the room temp T4 condition which is stronger. There are many reports of as-welded 6061 HAZs failing during use.

You can expect ~35-40 ksi YS after HT, nearly the same for as-welded mild steel (low carbon cheap stuff). Your rack could therefore weigh ~1/3 of a soft steel one. If you use 4130 and leave it as-welded, your 6061-T6 rack would weigh ~1/2 of it.

If you decide to HT the 6061, I wouldn't worry about the lack of a fatigue limit. Not too many airplane wings fall off, even after all those many miles of buffeting winds that make the wings flap up and down so much.

[DaBit]

I am not too concerned about the fatigue limit. After all: how many hours in their life are these racks loaded? 2000 miles a year? Thus, let's say, 60 hours? The rest of the year they only have to carry their own weight.
And even if they break, big deal. Things do break, that's part of life. If it happens close to home I'll get there with some tiedown straps, ducttape and tiewraps. If it happens far away: it is amazing what one can do with some epoxy resin and glassfiber band.

Now the 'bad' news: 6061 aluminium tubing is another ´hard-to-obtain' item. Locally available aluminium tube turns out to be almost exclusively 6060-T66 (AlMgSi0.5 F22). Weaker than 6061-T6 (yield stress 160MPa vs 230MPa), but I suppose that resistance to denting and stiffness determines wall thickness and tube OD more than strength does.
There is something in 6082 too, but not in the required dimensions (exclusively large OD large wall thickness tubing).

I also checked the availability of 7020 or 7005 alloy round or square tubes due to their good as-welded strength, but those are unobtainable in small quantities.

[Twin-shocker]

Make the rack out of ERW tube.........some of the tubes can be lighter gauge than main supporting parts. Its going to weigh a little more than the expensive exotica suggested here, but will be cheap and easy to make, and can easily be repaired on the road if there are any problems.

[Benesesso]

Originally Posted by Twin-shocker

Make the rack out of ERW tube...

Quite a difference from your usual recommendation for "modern" Reynolds whatever.