britishtrident
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| posted on 22/5/04 at 12:59 PM |
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Cast Alloy Wishbone
Re recent thread on wishbone failure, it strikes me the Locost market id big enough to support the manufacture of cast alloy wishbones. The
pattern wouldn't be difficult and made in heatreated LM25 or similar they could be made more than strong enough without looking crude.
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Browser
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| posted on 23/5/04 at 12:04 AM |
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I'm no expert but wouldn't they be heavier than tubular jobies? Also, heat treated or no I think casting tend to snap in crashes rather
than bending (& thus absorbing the impact), don't they?
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crbrlfrost
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| posted on 23/5/04 at 02:00 AM |
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My primary consideration would be the cyclic fatigue of aluminium control arms. Granted many manufacturers use them these days, but they also have
vast testing budgets. Seems you could design closer the the limit, and thus lighter with steel, whihc below a certain load has pretty much infinite
fatigue strength. But if its within your capabilities, more power to you and best of luck! Cheers!
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NS Dev
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| posted on 23/5/04 at 08:07 AM |
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Fair point, the materials are rather different in fatigue, alloy a bit less predictable in this application.
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britishtrident
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| posted on 23/5/04 at 11:00 AM |
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We use aluminium alloys for wheels without worry -- Although I have seem fatigue cracks in a wheels -- ancient Brabham single seater wheels that
had done a lot of racing miles on a car heavier than it was designd for.
I mentioned LM25 because it is heat treatable -- and perhaps the best cast alloy where toughness is required, and the fatigue life should be very
long provided there is proper attention to detail particularly finish and corner radi
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Viper
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| posted on 23/5/04 at 02:05 PM |
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Didnt the e type have cast alloy wishbones?
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NS Dev
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| posted on 23/5/04 at 02:53 PM |
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All manner of cars have had alloy wishbones (Corvette, Subaru Impreza etc etc ) but they are the same weight as well designed steel ones, if not
heavier. Same with wheels, alloy ones with the same strength as steel ones are heavier than the steel ones! Always puzzled me, the "light
alloy" wheel!! (I know there are many light ones, but most cast alloy road car wheels weigh a ton!)
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britishtrident
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| posted on 23/5/04 at 09:46 PM |
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quote:
Originally posted by NS Dev
All manner of cars have had alloy wishbones (Corvette, Subaru Impreza etc etc ) but they are the same weight as well designed steel ones, if not
heavier. Same with wheels, alloy ones with the same strength as steel ones are heavier than the steel ones! Always puzzled me, the "light
alloy" wheel!! (I know there are many light ones, but most cast alloy road car wheels weigh a ton!)
Surprisingly the other way about because of the increase in thickness. Compare a 4.5 mm thick alumium alloy sheet with a 1.6 mm steel sheet are
both roughly the same weight -- which is stiffer in bending -- the alloy one the 3fold increase in thickness gives a much greater increase in 2nd
moment of area which has a greater effect than the lower mechanical stiffness and strength properties of the alumium alloy.
I worked it out a few years back modeling a wheel rim as a beam in bending- the alumium alloy rim was a lot stiffer and stronger than the steel for
the same weight and a magnesium alloy wheel even more so. Of course the strain at the outer surfaces will be greater for any given deflection.
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flak monkey
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| posted on 23/5/04 at 09:58 PM |
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I would tend to agree with your arguments about it being stronger in bending due to the higher I (second moment of area) value dominating the
equations....
By making it 3 times thicker you increase I by a factor of almost 30 times! (In sheet anyways) This more than accounts for the 3 times reduction in
youngs modulus between aluminium and steel! But then you have wishbones that weigh the same as steel ones and if they were cast you cant make them
hollow...they would be solid so would weigh even more!....what thickness would these ali wishbones be?
You could make some welded ali wishbones from 3mm wall tube maybe? (they would be lighter and stiffer) Or does the welding have too much of an effect
on the structure and properties of the material to do this?
But i would be dubious about the fatigue properties of aluminium, that would be my only concern.
Cheers,
David
[Edited on 23/5/04 by flak monkey]
Sera
http://www.motosera.com
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Cita
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| posted on 24/5/04 at 06:38 AM |
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I cannot imagine that a 3 mm wall Al. tube wishbone would be as "strong",whatever that may mean,as a mild steel tube of 2 mm wall.
With proper (heat) treatment welded Al. can be brought "close" to it's original condition.
It is possible to cast hollow sections...but not at a locost price!
Titanium was develloped to overcome the "misleading weight" advantages of Al. in structural parts and even that is not the
"magic" material!
I believe Titanium has about the same elasticy modulus as mild steel so that's not something to bragg with aldo the permanent distortion factor
is much higher...i think
For the homebuilder, to many unpredictable factors are involved in the developement of cast wishbones so i would not spend money/time in such a
thing,but...who am i?
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NS Dev
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| posted on 24/5/04 at 05:14 PM |
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You are quite right, that's who you are!!
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Aloupol
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| posted on 25/5/04 at 08:23 PM |
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quote:
Originally posted by flak monkey
...if they were cast you cant make them hollow...they would be solid so would weigh even more!....
With an I-shaped section it would be possible to increase quadratic moment related to section area without making a hollow section.
The fatigue concern can be managed too.
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flak monkey
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| posted on 25/5/04 at 08:39 PM |
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Yes what you say is very true and something i hadnt really considered....
But to be honest I would rather stick to steel wishbones that have been tried and tested and save weight elsewhere with alloy components. (Uprights,
floor etc)
Just IMO...i see nothing wrong with the idea however...
Cheers,
David 
Sera
http://www.motosera.com
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crbrlfrost
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| posted on 25/5/04 at 09:18 PM |
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Very good note Aloupol, excellent suggestion. Thinking back, I believe a manufacturer or two does that exact thing. Cheers!
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Cita
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| posted on 26/5/04 at 05:59 AM |
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When dismanteling the rear end for my diff/halfshafts/uprights from the Mercedes in noticed that there were some short sort of trailing arms in steel
shaped in I form.
Those things are stroooooooong!!!!
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Aloupol
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| posted on 27/5/04 at 05:40 PM |
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For the flexion I-shaped section is perfect. Poor in torsion but there's no torsion here. For buckling nothing is beter than a rould tube..
I think welded steel tubes can do it perfectly but why not..
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