Luggage racks, aluminium vs steel and tube diameter calculations

Nice stuff, those PH 17-4/17-7stainless steels. Also known as AISI 630/631. Strength and hardness after 900F precipitation hardening is truly amazing. That would certainly allow some light and thin-walled tubing to be used.

But so far it seems that these steels are unobtainium in anything but solid round bar stock. Oh well, I didn't expect anything else from a material with keywords like 'aerospace', 'defence', 'missile components' etc. in the datasheet.

BTW: the more I read about it, the more I begin to like carbon/aramid/glass/epoxy composite for this particular problem. The weakness is compressive strength, and especially compressive strength vs impact resistance. Carbon+epoxy is the strongest overall, but it shatters on impact. Aramid+epoxy has good impact resistance, but compressive strength is far lower than aluminium and it's modulus of elasticity is not that good either.

But fortunately it is easy to modify the thickness and composition of the laminate based on the stresses it sees. 'Loops' made out off aramid and glass to survive impacts, supporting structure aramid and carbon for additional strength for example.

I first heat treated 17-4 way back in 1960! US Navy would not allow the H900 1 hr. HT on aircraft because of stress-corrosion-cracking/hydrogen embrittlement problems.

BTW, Rolls-Royce went bankrupt because they put all their eggs in one basket, trying to make carbon-fiber fan blades for the RB-211 jet engine. Engines were for the Lockheed L-1011, and RR couldn't deliver (defect examination problem, IIRC). British gov't. had to bail them out, and they ended up making the blades from titanium (6-4 alloy).

I am quite sure that I could buy a shipload full of 17-4PH. The problem is obtaining a few meters of thinwalled tubing, either round or rectangular if round is unavailable. Same problem with 7020 or 7005 aluminium tube which should be stronger as-welded than 6xxx after a bit of natural aging.

It will probably turn out like this:
- If I can get a good deal on heat-threating the aluminium I think that 6060 tubing is the best option. It is available in a lot of quantities/sizes, cheap, and when threated to -T6 temper not that much weaker than 6061.
- If not, option B is composites. I think that they do have potential in this application. Problem: I like welding and grinding a lot more than messing with itchy fibers and sticky epoxy.
- At last option C: steel. Maybe cold-drawn, maybe 4130. Simple and cheap.

Another BTW: did you know that a lot of good stuff was invented in the 60's? Also things you would not guess, like the entire foundation under the computer graphics that make the average Hollywood movie such a visual spectacle for example.

4130 is very old technology materials wise, and far from ideal for making a bike luggage rack. If you are wanting to make a steel rack, I would suggest contacting a good cycle frame builder, who should be able to help with a small amount of tube which is much more suitable.

However a properly designed composite rack, made using wet lay kevlar/epoxy over foam core, is likely to work far better than anything metallic, and will be stronger/lighter and pretty much indestructible unless bike suffers from crash damage!

It would be possible to make composite rack using a one part tool (unless underside appearance is important, and if you can come up with a nice universal modular design, then it may well be possible to sell them for various other bikes.

I had assumed 4130 tube would be readily available in Europe as way down here in little NZ there is a supplier who sells a wide range, incl. small stuff, with a choice of wall thicknesses for most sizes.

I asked the local bicycle frame buider about high strength steel tubing but most everything he gets is preformed or butted, if he needs plain tube he gets 4130 from the racecar engineering supplies place as above. Rotorua may not be a hotbed of bicycle tech though

Composite would be interesting. How about 'moncoque' luggage with the hard bags as a stressed member? Have it clip in to a couple of solid mounting points top rear & bottom front? Have a rail/rack that clips in to the same points for using soft luggage?

Cheers
Clint

Yes, but those tubes are all fairly large diameter, and especially thin in the middle.
And of course there are a lot more suitable materials, but the benefit of old technology is that it's available to the average shade tree mechanic.

so I might be on the right track after all!

It is available, but transport costs for such a low volume of metal would be far higher than the actual material cost itself.

I am using the rack more for strapping all kinds of stuff to it than as the actual carrier of hard luggage. I think the classical fixed rack plus removable luggage would work best for me. Swing soft luggage over it when on a multi-day trip, use small hard cases in the city when needed, make rack-mounted fuel cells to extend range when needed, etcetera. More possibilities than I have time to fabricate :)

However, with composites I do have a bit more freedom to shape cross-sections according to load (by making the tubes oval, by using more material on the outside of the rack to increase resistance to rocks, etc.).

I don't even think a 'real' mold is required; it is easy enough to wrap carbon/aramid/glass tape over a foam or plastic construction.

Tubes for bicycle rear triangle might be ideal for making a rack, and these could be easily joined using TIG brazing, which will greatly reduce the need for any sort of heat treatment, and lessen the chances of failure due to cracking.

However composite will be far superior to anything metal, and you could probably use something like a plastic container of the right dimensions to make a simple mould for the upper part of the rack, and after laminating the top part, foam stringers underneath could be laminated to the upper part, which would result in something very strong and pretty simple to make.....................

Most frame builders that use high grade materials don't use standard lengths of tube which they cut and shape to the size they need. Instead, the tubes are half finished products with more metal at the joints, less in the middle and other optimizations. Makes sense; nobody wants a handcrafted frame that performs less than a top-shelf production frame.

BTW, talking about exotic materials: I also found that grade 2 titanium thin-walled tubing and sheet is available and not that terrible expensive either, especially not when compared to stainless steel. Would allow an even slightly lighter construction than 6061-T6 and from what I heard it is not that hard to weld once you found a way to absolutely keep oxygen away from the hot titanium.

I would not call it 'far superior'. If you have the possibility to optimize a metal construction I think it will be even better. But a fact is: as a private person I am unable to do so against reasonable cost. Paying $300 to get some exotic tubing delivered, filler, blabla and another $400 for the heat threatment to archieve a 3lb weight saving makes no sense at all.

The main advantage of composite materials is that exhibit a good compromise between availability, weight, ease of processing and sturdiness. 'Cheap' is definitely not an advantage of composites containing carbon and/or aramid, BTW.

From the perspective of a private individual, without access to expensive hard to find materials and heat treatment processing, then it seems to me composites laminated around a foam core, seem to provide a far less costly option than metal construction?

Yes, probably. But don't underestimate the cost of a finished composite part.

Carbon and aramid fabric is very expensive. A few square feet of a reasonable lightweight fabric seems not that expensive, but often quite a few layers are needed since one layer results in 0.015" thickness or so. Then add some good epoxy and consumables such as cups, paint brushes, gloves, etcetera, and the total cost rises quickly. Not applicable here, but for many parts you can also add the cost for the plug, mold, peelply, breather fabric, vacuum bagging materials, electricity for the rigged-up oven to post-bake the epoxy (higher strength epoxies often need a post-cure at elevated temperature), etcetera.

Don't be surprised if it adds up to ~$30 per square foot of 0.1" thick composite when using branded materials with decent datasheets.

In this case: the cost of post-HT alone buys me quite a few square feet of composite, and a square foot of composite is a lot of 'tubing'.

Right, it's less costly than metal and also has a much higher strength/stiffness to weight ratio, which is why it's used everywhere.

Oh wait, no, I got that backwards. It's only used in very high performance, high tech applications. Because it's f'ing expensive.

Only somebody who has not worked with composites would think this is a feasible rack material.

Now, Reynolds air-hardening bicycle tubing, on the other hand, is a far superior material, in all applications, for all purposes!

Is it too late to suggest a cut-down milk crate?

$400 for HT? Sounds like they don't need any more work. Another possible way to make a strong light rack, if you had a cheap source of high strength tubing in the sizes you want, is to glue it together. For this to work, you'd need some good-fitting lugs/fittings to slip the tube into. You'd have to use lap joints with a decent amount of shear area.

Then use a high-strength epoxy and bond it together. No HT involved, and you could mix different kinds of tubing together. Some US fighter aircraft had/have control surfaces bonded that way.

Good luck!

For something like a bike rack there is no real need to use costly pre-preg carbon, which also requires an autoclave to process properly.

If the design is right then a combination of normal CSM and cloth laid up over foam cores, using epoxy resin will work perfectly well.

Many home built light aircraft have wings using this method of manufacture, and I would think that only someone with little or no first hand experience of composite work, would feel steel or aluminum tube would be more suitable?

Yes, because building airplane wings and luggage racks are similar undertakings, and have the same requirements from their materials. Do you think that homebuilt airplane guys often strap weights to their wings and then slam them into rocks?

I don't know very much about composites. I only designed and built 3 of the machines in this facility: http://www.prnewswire.com/news-rele...-center-of-excellence-facility-124169874.html

Both need to be very strong and light. Its very easy to make a composite rack, which is an awful lot stronger than the frame its going to bolt onto. Never quite been able to see the point of this type of thing though, as a damaged rack is far easier to repair than a broken or twisted rear subframe.


If you profess to an expert on composites, then perhaps you can outline how to go about making a strong/light part such as a rack, cheaply and easily and not using carbon or kevlar materials?

Thanks for the friendly posting and good explanation why it is such a bad idea.

I have used composites, but not extensively. Only for tank protectors etc.


BTW: before constructing these protectors, I tested impact strength with a hammer and sliding resistance by mounting a test piece under the motorcycle boot to find how how much aramid I woudl need to gieve suitable protection. So I am not totally clueless about what it can and cannot do, but not experienced either.

I don't know 'a guy' and none of these places are close enough to occasionally drop by with a case of beer Friday afternoon. And of course they are not interested in a single rack. Would you?

6060-T66 is common and fairly strong. 6083-T6 might be another possibility.

Many non-weldable aluminium structures are built this way. I built a computer case using a special aluminium glue in one of those mixing pistols years ago.

But now the problem is 'lugs'. Could make them out of steel I suppose. Selecting sizes so a suitable gap remains should not be a big issue either (otherwise the solution is called 'lathe'). But two materials with a too much differing thermal expansion would not work for a long time. Those joints have a 'design life'.

And one needs to fly under a unique set of load conditions, while the other needs to hold weight under a wide range of load scenarios and WILL be smashed into on a regular basis.

Do you really think that airplane manufacturing materials and structures can just be ported directly over to a luggage rack?

Very easy? Why is it that nobody does this, then?

It's not "very easy" to lay glass, and then resin, in a complex shape with load attachment points, and then clean it up after it all cures. Possible, yes. Very easy, no.

Easy. Weld it out of steel tubes.

If it needs to be REALLY racey (as in, you're building a luggage rack for a WSBK team), weld it out of aluminum instead.

For the application, I don't see a lot of sense in doing something super exotic. 10 feet of 3/4 inch OD by 0.060 wall steel only weighs 5 pounds. It's not like we're talking about a lot of weight here (especially if you're going to strap 20-50 pounds of luggage to the thing). Seems like all this talk about composite luggage racks is a bit silly. Just pack a smaller stick of deodorant and one less pair of loafers and you've got the weight problem dialed.

That was not intended to be a dig at you, only twin-shocker, who is somewhere between a comedian and a moron.

Composites are a good (and sexy!) choice for a project like the one you showed, where you can lay it up on an existing continuous surface and just glue it on when you're done. It just is there to add a layer of protection, but it doesn't need to have significant standalone structural properties.

Trying to lay up a rack is a lot harder. It needs to be strong/stiff in several axes, which means you've got to fabricate some sort of beams (angles are easiest, i-beams most effective, tubes perhaps most convenient). It needs to mount to existing surfaces, which generally means you need to bond in some inserts to bolt through. It needs to be impact-resistant, which is generally not its strong suit (note that few motorcycles have body panels made from fiberglass, especially off-road ones). None of this is impossible, but it's far from easy, and it's questionable whether the savings of a couple of pounds are worth the large amount of extra effort and the additional risk.

I'm all about carbon composite structures, I am deeply involved with them in the aerospace industry, and I truly believe they are one of the materials of the future. However, that doesn't mean it makes sense to use them everywhere. Continuous thin panels is a good place to use them, and making something like a rally fender is a good "big" project. Even in the latter case, I believe that most composite rally fenders use a metal subframe for mounting instrumentation, certainly would not tolerate many pounds of luggage, and may not have great crash survivability.

I must say that I find it surprising that someone who claims to an expert on aerospace composites, doesnt have much idea of either the correct materials or production techniques required to make something as simple as a composite m/c luggage rack!

Essentially a rack can be laminated very easily, using a suitably sized plastic storage container as a mould for the upper part, CSM, Diolen cloth, and epoxy or VE resin system, with foam stringers laminated in to provide the strength, and mounting points made using knurled aluminum bar, drilled to suit mounting bolts, and then laminated into the correct position.

If it were possible to come up with a nicely designed composite rack, with a suitable quick release mounting system, then I would think there would be a good prospect of selling this, as there is no reason why they shouldnt be strong, light, and cost less to produce that metal racks.