britishtrident
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| posted on 1/12/12 at 10:48 AM |
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Actually if you take time to do a mental calculation the loads normally experience by the top ball joint including under braking are not that
enormous.
The loads a rod end will survive under combined single shear and bending will always be considerably lower than under pure axial tension but it
just a matter of doing simple calcs and using Mhor's circle to select a joint to give an adequate FoS and ensuring the design ensures
a minimum length of the shank of the rod end is exposed to minimise the peak bending moment. Of course it easier just to follow the proven
path and follow the example of a M18 thread Transit drag link.
In the 40 odd years I have been around racing cars I have never seen a rod end on a top wishbone fail unless under major impact with the Armco ,
in a surprising number of those the ball joint shank bent rather than snapped.
It is a different matter putting loads into a road end that could pop the ball out out of the socket.
[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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matt_gsxr
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| posted on 1/12/12 at 11:11 AM |
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Isn't this the time that someone says that they used to be a F1 engineer with 20years of experience and all their kids design rally cars for
subaru, or have I missed that bit.
Surely someone. What is this forum coming to.
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ffrgtm
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| posted on 1/12/12 at 11:24 AM |
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While I see where you're coming from, I argue that there are too many exceptions to the rule to maintain that stance in the long term. In
fact... if we move away from SLA suspension the torque reaction at the upper outboard mount could be much larger than the lower (for example in a
trailing link setup).
I would also argue that mohr's circle is going to give you a far less than complete picture. Like I said, this is a situation where stress
concentrations leading to crack growth in the thread roots in the primary concern. Even a modified goodman diagram is going to be pretty far off the
mark as dynamic deflections in the individual components are going to change the loading geometries.
If you want to design an optimized vehicle that uses a rod end in a bending location you'd need to use multiphysics simulation... and probably
magnaflux on a regular basis. At that point anyone sane would just us an encapsulated spherical in the first place.
On a broader level... using a high factor of safety to account for incorrect loading is sort of against the very essence of the car in question 
(as I cross the fine line)
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britishtrident
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| posted on 1/12/12 at 01:00 PM |
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The provided the joint is correctly sized material used for forged rod end has a more than sufficient notch fracture toughness to cope with the
load cases in all but the most extreme impact scenarios,
[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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CNHSS1
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| posted on 1/12/12 at 01:21 PM |
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imho its the use of appropriate technology.
Formula Fords which have been the stalwart of motorsport for 40years use rosejoints in the method thats being slandered here, but they have a
different set of criteria than some home built road car. There are no potholes, icy roads, tractors pulling out on circuits, and an FF doesnt weigh
1500-2500kg like most modern road cars, and generally 30%-50% of a 7Type
in an FF, the rosejoint breaking in a shunt often dissipates the energy of a crash actually reducing the risk to the intrepid pilot. It also allows a
half hour change of a damaged corner, rather than ripping lumps out of the chassis.
Larger heavier race cars, often use spherical bearings in housings welded onto the wishbones. This removes the risk of the threads breaking (there are
none in this scenario) but also the simple camber/castor adjustment. The adjustment is usually added at the inboard mounting end my adding shims
between the wishbone mounts and the chassis. Bit more fabrication, bu easily replicated on a 7type.
by using 'road car' balljoints (maxi, cortina etc) the loads dont go away, they just move to the next weak point, often the design or
welding of the wishbone its attached to, there are plenty of horror stories on here of wishbones shearing etc.
if you dont like a particular method, dont use it on your car, but ensure that youve thought about the route that the 'breakage' will
follow. A minor shunt on a race car that didnt use a rosejoint as a mechanical 'fuse' could lead to the wishbone shearing and intruding
into the cabin with nasty consequences for the driver.
if the items are specced correctly, or over specced to move the failure elsewhere as is necessary, then i dont see the issue. As to which of us is
qualified to make that specificatrion call, well thats another 20 page thread isnt it?
one last point, the irony of us all worried about the load paths and relative safety of a 60year old design 7type built in sheds by amatuers isnt lost
on me!
if you want safe, drive the Euro-box thats almost certainly on all of our drives anyway and dont think of getting into 30metres of tube and GRP...
CNH
defo not an engineer...
[Edited on 1/12/12 by CNHSS1]
"Racing is life, everything else, before or after, is just waiting"---Steve McQueen
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vipe
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| posted on 5/12/12 at 10:33 PM |
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Years ago there was an article in racecar engineering about this.... basically they fail when used wrong. Otherwise they would not be used, as they
are, in critical parts of various machinery.
Seen a few fail in formula student. to be honest, most were no way big enough (top joint, 1/4" single shear...).
not used on road much because of cost (there is a reason good ones are the price they are) and they last about no time at all even on track. never
mind on potholes and hitting curbs,etc.
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phelpsa
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| posted on 5/12/12 at 11:56 PM |
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They fail if they are not specified correctly. If specified correctly they do not fail.
It doesn't matter if they are in bending, shear, tension, compression or what application they are in. You could use them to hang paintings if
you want, if it does the job then there is no problem.
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britishtrident
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| posted on 6/12/12 at 07:52 AM |
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The reasons why they are not used on production cars is (1) They are not self adjusting. (2) Even with dust boots they are not sealed against
water, grit and road salt. Any ball joint exposed to the hostile road environment would last a matter of weeks.
Before the 1930s the track rod ends used were pretty crude adjustable rod ends that had to be greased every 100 miles. In the 1930s with the
gradual introduction of independent suspension and fully enveloping bodywork more modern looking non-adjustable track rod ends appeared properly
sealed with dust boots to seal out road grit, however these still needed greased every 500 to 1500 miles. Just after WW2 GM used self-adjusting
track rod ends and ball joints which did not require regular greasing and over the next 12 years or so these completely replaced the non-adjusting
type and led to the gradual disappearance of king pins and trunnions from passenger car suspension.
A self-adjusting rod end or ball joint has a stiff diaphragm spring sandwiched between the bottom cup of the ball joint and the lower cap.
Occasionally you can see the effects of the diaphragm spring when checking a cars steering for free play you will find track rod end that has
little or no lost motion but exhibits vertical motion in that the ball jumps up and down in its' socket as the steering is moved.
[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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