I am correctly finishing my RSR style strut brace and I am curious as to which alloy aluminum tube would best suit my application.
Curt and the other Midwest Essers might get a chuckle as I showed some of them my predicament. I ended up putting a bow into the current “hardware store” alloy I am currently using.
I am using tube that is 1”OD and 7/8” ID. Please see this McMaster guide that I have been looking at. I am thinking 6061 or 5052-O.
Opinions welcome….
I found a section of structural tubing. I'm now looking the item below (specs)
Part Number: 1968T376
Material
Aluminum Alloy 2024
Shape
Structural Tubes
Structural Tube Type
Plain Round
Structural Tube Tolerance
Oversize
Outside Dia.
1"
Outside Dia. Tolerance
±.009"
Inside Dia.
.87"
Wall Thickness
.065"
Wall Thickness Tolerance
±.007"
Straightness Tolerance
.01" per foot
Metal Finish
Unpolished (Mill)
Operating Temperature Range
-320º to +300º F
Metal Bendability
Not Bendable
Brinell
120
Temper
Hard T3
Yield Strength
42,000 psi
Metal Flareability
Not Flareable
Specifications Met
American Society for Testing and Materials (ASTM)
ASTM Specification
ASTM B210
Length
6'
Length Tolerance
+1"
I would use a 6061 as a good general purpose medium strength alloy with very good corrosion resistance and readily weldable.
If you can buy it in a T6 Temper which will provide optimum strength without affecting other properties.
The Young's Modulus of all the Aluminium Alloys is around 70 GPa and will only vary about 2% for the higher strength 2000 Series and 7000 Series Alloys so the stiffness of the bar will be vitrtually identical.
2000 Series alloys are a bit stronger but the high Copper contect does make them a bit more prone to corrosion, which may not be a big issue but the surface may discolour more quickly and they are not readily weldable and generally unsuitable for anodising.
Strength shouldn't really be an issue as the function of the bar is to add stiffness and it shouldn't be operating anywhere close to the materials Yield point or it will be too flexible.
The factory RSR bars did not have adjustiable ends, just a hole for a bolt. I am not sure if they were steel or alloy
That is correct.Originally Posted by mike curnow
Chris, thanks for your input. It sounds like the 6061 would be the ticket, I am still undecided about welding the ends in; currently they are just closely toleranced.
If I go with the 6061; I may weld the ends and powdercoat both bars black.
6061 is quite weldable and shouldn't give any problems unlike a 2000 or 7000 Series alloy which will tend to overage in the Heat Affected Zone and may fatigue.
I think that the factory RSR bars were steel and I would agree that they had pinned ends rather that spherical joints.
A steel bar with the same dimensions would be 3 times stiffer than aluminium and also 3 x the weight.
So to get the same 'stiffness' in a steel bar as a one inch aluminium bar what is the calculated diameter and weight difference of that bar as compared to the aluminium one? I am sure the answer will not be "1/3 of an inch" and "same weight". Now where are my old science tables.......
Regards
Mike
RS#1551(sold)
67S
73E (home after 25 years) and sold again
Early S reg. #681
Mike,
The Modulus of steel is 3 times that of aluminium to within a couple of perecent.
70 GPa for aluminium and 207 GPa for steel.
The density of aluminium is virtully one third of steel.
If you make the bars the same size then 3 times the stiffness and 3 times the weight is about right.
Hence Modulus to density ratio of aluminium and steel are about the same and interestingly Titanium has around the same modulus to density ratio as well.
If you mess with the geometry then things will change due the the moment of inertia but that wasn't the point I was making.
Next project, a special rear sway bar... I have read that a high carbon content steel is generally used; Can anyone please point me in the right direction. Chris or Mike..?
Thanks,
It is quite common for stock sway bars to be made from a simple high carbon steel such as a 1085 or similar but they will have been designed so that the stresses will alwasy be below the endurance limit of the steel being used so that fatigue failures are avoided.
Stock bars tend to be heat treated after forming as materials such as 1085 are not too ductile.
I would tend to use a Chromoly such as 4340 as I think that this type of material has good resiliance and better fatigue resistance than carbon steels.
Depending on the design you may need to heat treat after manufacture to optimise properties and without knowing a little more about consrtuction it is difficult to gine more advice.
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