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Old 05-21-2012, 09:42 AM   #31
sailah
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Quite a difference from your usual recommendation for "modern" Reynolds whatever.
Don't do it, resist the urge
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Old 05-21-2012, 09:59 AM   #32
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Don't do it, resist the urge
Good advice! I guess I should delete that----.

But I won't, because I enjoy double-shocks witty replies. See below.
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Benesesso screwed with this post 05-21-2012 at 02:50 PM
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Old 05-21-2012, 11:49 AM   #33
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Quite a difference from your usual recommendation for "modern" Reynolds whatever.
Seems weird to recommend producing something as simple as a luggage rack using costly and hard to find materials, that need to be TIG welded and also need complex and expensive post production heat treatment, but suggest a competition space frame chassis should be made using outdated and inappropriate materials, that also require complex and costly post production heat treatment?
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Old 05-21-2012, 07:41 PM   #34
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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.
But not many people put their airplane wings back in service after they've had a little tweak in a crash. Repeatedly.

The last racks I built were 4130 CrMo & man have they taken a beating. 4130 is also a good deal more corrosion resistant than mild steel since that is a concern. Check out racecar builders or their suppliers for a source of 4130.

Alu would be cool for less demanding duty though & as the guys say, properly HT 6061 is quite strong stuff. T6 is actually pretty easy to do if you have access to a 600(IIRC) deg C oven.

Be sure to post pics!
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Old 05-21-2012, 08:17 PM   #35
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The last racks I built were 4130 CrMo & man have they taken a beating. 4130 is also a good deal more corrosion resistant than mild steel since that is a concern.
Actually it's not. Nowhere near enough Cr to have much effect. Paint/powder coating is the answer.
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Old 05-21-2012, 08:52 PM   #36
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Actually it's not. Nowhere near enough Cr to have much effect. Paint/powder coating is the answer.
Maybe that was something I remembered from the internet rather than an engineering text

The first rack I made was stainless, cos it was what we had lying around, I expected it to crack eventually & it did, but it took a surprising amount of abuse. The biggest issue with most racks is not the material, it's the shitty design, particularly the attachment points, often there's no simple way of doing it right, it takes a funky bracket or something to distribute the load properly:



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Old 05-21-2012, 09:03 PM   #37
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If the OP could get some thin wall 17-4PH or 17-7PH, he could weld it, heat treat it with a simple 900-950 F 4-hour hold time, and let it air cool.

He'd have a super strong, super light, corrosion resistant rack that would resist fatigue cracking. The YS should be at least 150ksi. Makes the 11ksi of as-welded 6061 look like a joke.
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Old 05-21-2012, 11:38 PM   #38
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Using brittle HT materials to make a luggage rack, will certainly mean a rack strong enough to probably take double the amount of weight you would want to carry on the back of a bike. However when something is properly designed, there is little point in making an add on part thats considerably stronger than the frame it bolts onto, as the frame itself will break rather than the rack, if it gets overloaded or suffers crash damage. This is all pretty basic stuff, and seems strange no one understands that the strongest material, in this case is a long way from being the most suitable.................
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Old 05-22-2012, 12:57 AM   #39
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Seems weird to recommend producing something as simple as a luggage rack using costly and hard to find materials
I don't think that aluminium tube qualifies as 'costly' and 'hard to find'. OK, the 6061 variant is hard in the sizes I want, but 6060 is not and it is not that much less strong when treated to -T6 spec.

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that need to be TIG welded
It's the only welder I have, and I like chasing that puddle of liquid metal.

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and also need complex and expensive post production heat treatment
The post-HT is indeed expensive. If I could run the rack with an existing batch it's not that expensive, but if they have to fire the oven just for me, ouch.

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The last racks I built were 4130 CrMo & man have they taken a beating.
I don't need the extra strength 25CrMo4 gives me compared to regular mild steel, I want it to be lighter than average. You can call me nuts because in the end we are talking about a weight saving of 1.5kg/3lb or something. But hey, I am not doing it for a living, I am doing it for fun. When finished I am a little bit better in fabricating stuff, I am a little bit better in welding, I have gained more material knowledge, know a bit better what NOT to do, and had fun doing it.

CrMo doesn't bring me much in the weight savings department since I cannot obtain it with much smaller wall thicknesses than regular steel.
Besides: is 25CrMo4/4130 steel so common in the US? I know of only one supplier in The Netherlands, and that one is not exactly 'local' either.
Sure, the roll cage sizes could probably be sourced from a builder, but small OD thinwalled tube is a different story.

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Alu would be cool for less demanding duty though
Two little kids, work and the GF prevent me from taking large trips the coming few years. So indeed, light duty.
And especially on a large trip I would not want to take much luggage with me anyway. Travel light, with the additional weight as close to the CoG as reasonable, and preferrably all of it between the wheel axles. That's how I like it.

Biggest stress the racks see is the rider using them as a grab handle to pull the rear of the bike out of a ditch or something.

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If the OP could get some thin wall 17-4PH or 17-7PH, he could weld it, heat treat it with a simple 900-950 F 4-hour hold time, and let it air cool.
I suppose 17-4PH and 17-7PH are not just different names for 304/316 stainless? :lol:
I will check out that stuff too. Never heard of it.

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However when something is properly designed, there is little point in making an add on part thats considerably stronger than the frame it bolts onto, as the frame itself will break rather than the rack
If one uses a stronger material, one could use less of it to archieve the same strength. But there are practical limits; an UltraSuperDuperSteel tube with 0.25mm/0.1" wall thickness could provide an acceptable yield strength, but it would dent too easily.

Also: I only see metals specified with 'pull' forces. What happens when compressing a tube instead of extending it? How do aluminium and steel compare when loaded in compression?

Another weird idea I have: leave metal for what it is. Construct the rack from PVC tubing and PVC sheet. Cheap, very easy to process, very light. Eventually subsitute the PVC with polycarbonate when PVC turns out to be too 'floppy'.
Then, when finished, wrap the PVC 'mold' with a glass/aramid/epoxy composite. No carbon, or at least not on the parts that may be subject to a direct impact. Paint to protect the aramid from sunlight or just finish with a layer of black glass. The result should be a light, strong and stiff construction which can take quite an impact also.

DaBit screwed with this post 05-22-2012 at 01:57 AM
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Old 05-22-2012, 02:29 AM   #40
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I suppose 17-4PH and 17-7PH are not just different names for 304/316 stainless? :lol:
I will check out that stuff too. Never heard of it.

Also: I only see metals specified with 'pull' forces. What happens when compressing a tube instead of extending it? How do aluminium and steel compare when loaded in compression?
The PH stainless steels have about the same corrosion resistance as 304, which is a bit less than 316 in chloride exposure. But 304/316 are not hardenable via HT, only via cold-working. Cu is responsible for hardening 17-4, while Al does it for 17-7. Al is also primarily responsible for greatly strengthening jet engine turbine blades, which operate at nearly twice the melting temp. of Al.

Metals have about the same strength in compression as tension, but buckling must be avoided. There are equations to figure out the OD vs length in order to avoid it.
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Old 05-22-2012, 04:59 AM   #41
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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.

DaBit screwed with this post 05-22-2012 at 05:41 AM
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Old 05-22-2012, 06:35 AM   #42
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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).
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Old 05-22-2012, 09:11 AM   #43
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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.
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Old 05-22-2012, 11:06 AM   #44
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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.
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Old 05-22-2012, 01:39 PM   #45
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CrMo doesn't bring me much in the weight savings department since I cannot obtain it with much smaller wall thicknesses than regular steel.
Besides: is 25CrMo4/4130 steel so common in the US? I know of only one supplier in The Netherlands, and that one is not exactly 'local' either.
Sure, the roll cage sizes could probably be sourced from a builder, but small OD thinwalled tube is a different story.
.
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
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