markyb
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| posted on 8/8/12 at 12:04 PM |
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One for the engineers
I see the F1 cars use carbon fibre push rods (obviously too expensive to manufacture for a kit car) and my MNR has steel with nuts welded on. I was
wondering why steel is used in preference to aluminium with the ends threaded internally for the rod ends ?
Is it cost, weight , strength ?
Would the rod itself be strong enough but where the internal threads would be cut for the rod ends be the weak part ?
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ashg
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| posted on 8/8/12 at 12:14 PM |
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i have seen steel ones bend after going over rough roads so personally wouldn't chance aluminium. may work on a race car but is a poor material
choice for a road car unless you have a method of doing the equations to design it properly like a major manufacturer would.
at the end of the day gram pinching on anything other than a race car will hardly make a difference to the drive and feel of the vehicle unless its a
significant weight loss in a critical area
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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ashg
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| posted on 8/8/12 at 12:19 PM |
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btw im not saying dont try and reduce the weight on your car just avoid doing it on level where safety may be compromised.
if you do it on a track vehicle and it fails everyone on the track, in stadium etc has accepted the risk of motorsport. on the public highway people
are not expecting wheels to fall off 
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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balidey
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| posted on 8/8/12 at 12:35 PM |
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quote:
Originally posted by markyb
Would the rod itself be strong enough
If you degisned it to be strong enough... yes.
You can't just replace steel with ally.
Also ally is very prone to fatigue stress unless you know how to work with it.
Dutch bears have terrible skin due to their clogged paws
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designer
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| posted on 8/8/12 at 12:56 PM |
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I am using steel for my suspension arms, with a wall thickness to allow the rod end threads to be cut into the tube.
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DIY Si
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| posted on 8/8/12 at 01:13 PM |
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Ally fatigues too easily to make it as easy to use as steel. As has been said, it can be used safely, and is regularly done so. However, ally
doesn't bend as well as steel, so steel is a safer bet as many failures won't be critical, more of a pain in the arse. A bent rod
won't pitch you into the hedge the way a snapped one will.
The easiest option is what designer has done, get steel tube thick enough to just cut into. It's not usually the lightest option, but if you
can't weld it's easier than welding threaded inserts in.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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britishtrident
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| posted on 8/8/12 at 02:38 PM |
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No point in making it solid bar as the primary issue in the design calcs would be ressistance to buckling.
Although the load is cyclic fatigue is not the most major consideration as being a push rod it isn't really subject to much n the way of load
reversal.
Compared to a steel tube diameter would need to increase, wall thickness would either have to stay the same or increase.
Years ago Rootes Group used alumnium alloy push rods in the valve gear of the higher performance versions of the Minx/Hunter engines, they were
simple alloy tubes with steel ends heat shrunk in, as it is 30 years or so since I last say one of these I am not sure but the design may also have
used a steel band round the outside of the ends. The diameter was about 2.5 to 3.5 times larger than the solid steel version, it was noticeably
lighter.
[Edited on 8/8/12 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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TAZZMAXX
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| posted on 8/8/12 at 02:49 PM |
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You need to remember that suspension components on race cars are treated as consumable items and are right on the limits. For road car or track cars
where you're not planning to swap them out after every session, durability has to be factored in, hence lightweight steel tube being the
material of choice, in most cases.
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britishtrident
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| posted on 8/8/12 at 07:16 PM |
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The calculations are relatively simple, just a matter of doing some basic compressive stress calcs then picking suitable candidate alloy
tube sections and plugging numbers into the Euler equation for buckling of slender struts with pinned ends, then crunching numbers to check fatigue
issues.
The most suitable way to do the ends would be to get steel inserts made which have a step which buts on the end of the alloy tube. The steel
inserts can be heat (cold) shrunk or bonded with a suitable Loctite grade. I would also heat shrink a steel ferrule round the outside of the end
of the tube.
The most likely problem that would be encountered is the ends of the alloy spread because they are effective getting hammered. The steel ferrules
round the ends of the tubes and careful choice of alloy grade show alleviate this.
But is it worthwhile doing compared with a steel tube ? I don't think so the increased outer diameter of tube will have bad effects on aero
and it will look ugly. Unsprung weight saving will be pretty marginal anyway.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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markyb
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| posted on 8/8/12 at 07:38 PM |
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thanks everyone for their input
have to say I do like designers method and might go with that in the future if needed (making sure that it was made by an engineer who knew what he
was doing i.e not made by me)
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david knight
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| posted on 9/8/12 at 10:14 PM |
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"(obviously too expensive to manufacture for a kit car)"
Or is it?
Wishbones and push roads could be made out of Kevlar/carbon quite easily. Instead of making a negative mould make a positive mould that remains within
the wishbone structure and forms the inner core. You could use a medium density foam cut into a solid circular sections, these would push upto a solid
plastic circular section that will form the mould for the inboard wishbone bushes (this tube is removed after the mould has set. The ball joints and
pushrod attachments between the wishbone to the uprights and shocks would need metal fastenings that are integral to the mould and Obviously
The mould would have to be held together accurately in a jig. After all this you would simply lay up the necessary amount of carbon. I would suggest a
layer of unidirectional fibres in the direction of the rod followed by a layer wrapped around the rod at 45deg, a layer of unidirectional and a layer
at 45deg in the opposite direction at the ends of tube the fibres should wrap around the plastic bearing tube. Then pack it with a wicking material to
suck off the excess resin and pop the finished item into a vacuum bag whilst it’s curing. You will have a rough finish on the out side but if you’re
neat with the layup it can still look quite good.
The calculations are not that different from steel, just consider it as a tube but of carbon not steel (I would be pessimistic and only consider the
unidirectional fibres in the direction of the tube)
Materials are readily available and not that expensive, it sounds like a lot of work but if your welding up your own wishbones you will need a jig and
its not a small job. Just a thought!
I would tust this more than my welding, but you haven't seen my welding!
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chillis
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| posted on 9/8/12 at 11:16 PM |
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quote:
Originally posted by markyb
I see the F1 cars use carbon fibre push rods (obviously too expensive to manufacture for a kit car) and my MNR has steel with nuts welded on. I was
wondering why steel is used in preference to aluminium with the ends threaded internally for the rod ends ?
Is it cost, weight , strength ?
Would the rod itself be strong enough but where the internal threads would be cut for the rod ends be the weak part ?
Just threading into the ally wont be nearly good enough, cutting internal threads is a non-starter unless rose joint will do a joint with a 1"
Acme thread
Alternatively machine bosses for each end to accept spherical joints and weld onto the tube. This will require the weld to be xrayed afterwards and in
all probability the whole piece heat treated, so cost to manufacture is running into 4 figures. Weight, they look likely to end up just as heavy or
heavier with no gaurentees on fatigue life (strength)
The weight comes from the wall thickness and is really to resist buckling.
So to sum up - not all that strong, wont last long, very expensive and no real saving in weight. TBH there are easier and much cheaper ways to lose
weight if thats really what you need.
BTW if you are building for the track check up on what the rule book says as suspension parts are subject to all sorts of rules that may prevent you
from using your chosen material.
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