britishtrident
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| posted on 27/1/05 at 01:55 PM |
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quote:
Originally posted by NS Dev
that's right, there should definitely be no clearance between the bush tube and the bracket, the tube must be clamped so that the bushes rotate
on the tube, and not the whole lot on the bolt, or the bolt will wear out! (I know this is pointing out the obvious, but I've said it
anyway!)
Very true -- if the bolt through the bushes is not fully tight it is an MOT test failure in the UK -- very easy to spot given away by rust stains
around both the nut and the bolt.
I am using triumph bushes with the correct 3/8" UNF bolts -- if designing from scratch I would probably use 7/16" UNF.
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DarrenW
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| posted on 27/1/05 at 01:56 PM |
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Its all well and good everyone quoting shear stresses etc but no-one really knows for sure what loads the wishbones are subjected to. I would suggest
the starting point would be to look at what the OEM's use in a similar application. Failing that over-engineering to give a good safety margin.
Mine has been built with M12 class 8.8's. Iam in no doubt that they wont fail and im happy with that. I intend to take my wife and daughters out
in the car so believe a good safety margin is wise.
All of the cars are subtely different, and the builders have different intentions as to what their cars will be used for. I guess at the end of the
day the old engineering saying holds true - If it dont look right it probably isnt. M12 are possible overkill. M10's should be OK. M8's
look too small for me IMHO.
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Mix
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| posted on 27/1/05 at 04:33 PM |
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Sorry if I confused,
The bit about the clearance was a 'copy and paste' from RX7s post.
There should, I believe, be no clearance and hence the bolt will be in pure shear, (won't it?)
Applying 'aircraft standards' is perhaps not as far removed from the truth as imagined. Vehicle manufacturers have to allow for abuse and
neglect during the life of their product. In our case I would imagine the level of attention devoted to our creations would far exceed that paid to
our every day driver and as such we can, if desired, be less apprehensive when it comes to stressed areas. Also the amount of miles covered, (and
hence fatigue) would be substantially less.
But back to the original question, 'Is an 8mm bolt a suitable suspension pivot?'
I would say yes, and have shown my reasons for thinking so. Remember there are probably four bolts per wheel each capable of taking a shear load of
over 5 tonnes. How strong do we think our chassis are?
Mick (not grumpy  )
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britishtrident
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| posted on 27/1/05 at 05:02 PM |
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The wishbone pivot crush tube to bracket is a friction joint, exactly the same as a road wheel to hub --- wheel bolts don't shear unless left
slack but they do fail from a combination of tensile and torsional load usually when undoing the nut ---- metric Escort/Cortina wheels studs are
famous for this even when the cars were new.
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12a RX-7
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| posted on 27/1/05 at 06:14 PM |
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you don't need to reverse a load to cause a part to fatigue. Simply aplying a load and removing it is enough. The higher the baseline stress the
less of a fluctuating load is required to cause a fatigue failure after X cycles.
OK so I was wrong on how the bushes work. But the bolts are still not it straight shear. If I am understanding how the bushes are loaded there will be
a uniformly distributed load across the length of the bolt where the bush fits over and two regions of shear stress at either end (very simplified) so
you will have peak bending stress and the centre of the bolt and peak shear stresses at the end.
As has been said above a loose bolt increases the stresses greatly and is one of the main causes of bolt failure.
[Edited on 27/1/05 by 12a RX-7]
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Rorty
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| posted on 27/1/05 at 09:00 PM |
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As with most (if not all) FEA and stress analysis, it's hypothetical. What counts is practical testing and then hard facts.
I normally over-design parts for a safety margin and then once the vehicle is up and running, I reduce the dimensions of some parts, one at a time, to
reach the minimal functional size but still have a safety margin.
My off-road cars weigh between 350kg and 420kg and frequently land on just one wheel coming off a table top or jump.
Over the years, I have settled on M10 Class 8.8 fasteners for the wishbones (semi-trailing arms, when used, are fitted with M12 fasteners), though M8
would be OK for wishbones.
I appreciate Locosts are up to twice as heavy, but I have successfully used M10 fasteners for road cars too.
For no particular reason, and I know it's not in any way scientific, I don't like the idea of M8s for wishbones. If I was trying that hard
to save weight, I'd use 3/8" hollow 4130 fasteners.
My race car is inspected from end to end after each day's racing, but even the Locosts you build are your prides and joy and you will probably
lavish attention on them even after they're built, so any loose fastener won't remain loose for long.
Incidently, if using Nylocs (or similar) and they back off (they actually can and do) then replace them with all-metal locknuts.
I use the all-metal ones everywhere except for the wheel nuts. The silver plated ones are perfect for exhaust headers. Sorry, slightly off-topic.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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craig1410
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| posted on 27/1/05 at 09:43 PM |
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May I sum this lot up by saying the following:
M8's should be fine if good quality parts are used with optimum clearances and design factors. Safety margins are towards the lower end.
M10's should be fine unless you make a complete ar$e of the design or are doing daft things like leaving bolts loose. Safety margins are
moderate to high.
M12/0.5" will also be fine unless you make a complete ar$e of the design and safety margins are very high indeed.
All fasteners should be torqued up and not left loose.
BOLTS (plain shank, threaded end) and NOT set screws (threaded all the way) should be used at all times to ensure a good tight fit between bolt, crush
tube and brackets. Ideally use the aero type bolts, especially if relying on M8's) where the ratio of plain shank to threaded section can be
chosen to suit the application.
Bolts should be coated with anti-corrosive thread sealant or copper grease before fitting to ensure ease of dismantling later.
Cheers,
Craig.
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Jon Ison
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| posted on 29/1/05 at 09:40 AM |
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M8's all round on the isonblade, no probs.
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britishtrident
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| posted on 29/1/05 at 01:04 PM |
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One point to consider is the thread ISO metric coarse is a good especially for threads exposed to dirt and corrosion but an finer thread will be
stronger and less likely to slacken off under vibration one of the reasons I am using UNF for my suspension parts.
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NS Dev
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| posted on 29/1/05 at 01:21 PM |
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quote:
Originally posted by craig1410
May I sum this lot up by saying the following:
M8's should be fine if good quality parts are used with optimum clearances and design factors. Safety margins are towards the lower end.
M10's should be fine unless you make a complete ar$e of the design or are doing daft things like leaving bolts loose. Safety margins are
moderate to high.
M12/0.5" will also be fine unless you make a complete ar$e of the design and safety margins are very high indeed.
All fasteners should be torqued up and not left loose.
BOLTS (plain shank, threaded end) and NOT set screws (threaded all the way) should be used at all times to ensure a good tight fit between bolt, crush
tube and brackets. Ideally use the aero type bolts, especially if relying on M8's) where the ratio of plain shank to threaded section can be
chosen to suit the application.
Bolts should be coated with anti-corrosive thread sealant or copper grease before fitting to ensure ease of dismantling later.
Cheers,
Craig.
Exactly my thoughts as well!
Safety margin with the M12's is pretty much infinite, as unless the bolt is completely faulty, the chassis will rip apart before the bolt
breaks.
[Edited on 29/1/05 by NS Dev]
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NS Dev
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| posted on 29/1/05 at 01:32 PM |
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quote:
Originally posted by britishtrident
One point to consider is the thread ISO metric coarse is a good especially for threads exposed to dirt and corrosion but an finer thread will be
stronger and less likely to slacken off under vibration one of the reasons I am using UNF for my suspension parts.
Also a very good point, UNF are commonly used in high stress applications due to the fine thread pitch, which enables greater greater bolt tension for
less tightening torque, and resists loosening. Metric coarse bolts are ok though to be honest, these are plenty fine enough not to come loose easily,
but using UNF will certainly do no harm, and will make it easier to use rose-joints if such a thing became desirable in the future, as most of these
are much cheaper in imperial sizes (good ol USA!!)
As a small point, in terms of thread strength, fine threads (e.g. UNF) are actually weaker than coarse ones e.g. UNC (at normal thread angles, there
is less cross section of bolt in shear within the threaded section)
HOWEVER, they also come loose more easily, and cannot usually be tightened to their correct clamping pressure without pickup on threads and excessive
torque requirement.
All of which is irrelevant really in this application, using Metric coarse (or metric fine if you want to waste money!!) or UNF will be more than
ample!
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Rorty
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| posted on 29/1/05 at 11:12 PM |
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quote:
Originally posted by britishtrident
One point to consider is the thread ISO metric coarse is a good especially for threads exposed to dirt and corrosion but an finer thread will be
stronger and less likely to slacken off under vibration one of the reasons I am using UNF for my suspension parts.
Excellent point that I ommitted to mention. I always use UNF when using imperial fasteners as they also offer finer adjustment on rod ends etc.
The same goes for metric threads; always use fine threads for adjustment/clamping.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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