Are 8mm bolts ok for the bones?
Pat...
most people use bigger bolts, you can read into that whatever you like! is there a need to use 8mm bolts? i would, myself, follow the crowd on this
one unless there was a good reason to deviate. obviously high tensile would be the way to go anyway...
I think 8mm high tensile bolts should be fine but I'm using M10 high tensile (8.8) bolts myself. I think many people are using 1/2" UNF to
fit the commonly available poly bushes.
Hope this helps,
Craig.
Plain shank M12 on the MK Indy IIRC
Indy does indeed use M12 bolts and the Tiger recomend 1/2" UNF for the Avon.
I'm not sure I'd be comfortable using M8 bolts.
Nick
You might of course be better with M8. In an accident the bolt is more likely to snap than an M12 and therefore sacrificing the wishbone but saving
the chassis!
James
I think most of us just use whatever fits our bushes 
Using 8mm high tensile bolts wouldn't concern me.
Mick
it'd concern me 
IMO of course
tom
also pat, the sierra uprights have M12 pinch bolts standard, not M10. are you just trying to save weight?? get some hollow bolts
Triumph wishbones bushes designed for were 3/8" unf bolts.
This is approx 9.5 mm --- regard this as the minimum useable size.
Ive used M12 8.8's including diff mount and engine mounts.
Just done a quick bit of research, I was bored.
Grade 8 bolt, 8mm diameter has a shear strength of 3,181kg.
This is in single shear. I believe I'm correct in thinking that in our application the bolt is in double shear and hence would have a shear
strength of 6,362kg.
So based on those figures I would be happy with 8mm bolts if that was what my bushes required.
Mick
I quite agree, M8 grade 8.8 or better would be fine, I'd personally use grade 10.9 or 12.9 m8 capheads to get the best quality bolt in there but see no problem with m8's at all. My car (ST) uses 1/2" unf to suit the bushes but this is ridiculously OTT!!! It's just that they fit!! If I were being fastidious I'd drill them out through their centrs to lighten them!!
I find higher grade bolts tend to be more brittle. Instead of bending they shatter under stress so they are not necessarily better in all applications.
I feel able to comment on this as I work for a steel manufacturer!
Yes, they will be more "brittle" but then for any sort of shock load experienced on a car they will be fine.
"Brittleness" is measured by impact properties, hardness by hardness test and tensile by tensile test, but in general, all are
linked.....the harder steels are more brittle and have higher tensiles, in fact you can ascertain tensile strength by cross reference from hardnesses,
a good way to check a bolt for a critical application before using it is to harnedd test the centre of it's head and cross reference the tensile
strength............................... however, a grade 12.9 Caphead bolt still has VERY good impact properties, much much much!!!!! stronger than
the welds holding the wishbone brackets to your chassis!! Welds are almost always the weakest point of any structure, unless it has been fully stress
relieved.
So, yes they are more brittle than a grade 8,8 bolt, but when fitted to a bushing made of rubber or plastic, and to a bracket welded to a chassis made
of 1.6mm mild steel, the bolt will be the last bit to let go, rest assured, and then the 8.8 one will break before the 12.9, whatever the nature of
the loading!!!!!!!!!!!!!!!!!!!!!!!!
quote:
Originally posted by Mix
Just done a quick bit of research, I was bored.
Grade 8 bolt, 8mm diameter has a shear strength of 3,181kg.
This is in single shear. I believe I'm correct in thinking that in our application the bolt is in double shear and hence would have a shear strength of 6,362kg.
So based on those figures I would be happy with 8mm bolts if that was what my bushes required.
Mick
i'm an aircraft engineer and its not the size of the bolt (within reason!) which is the question. its the quality.
you will be amazed at the size of the bolts they use on aircraft.
for example, a pfcu (hydraulic ram) is connected with a 1/4" bolt at either end and that has to cope with tensile stress and shear loads at
3500psi of pressure. if that bolt fails, the aircraft would go down!!!
oh, and a harrier gr7's (jumpjet) complete wing is held on by six 3/8" to 1/2" bolts!!!
think of that when u are all flying off to sunny spain in august!
NIce one Mattplace.
It's something I have to keep reminding myself when designing (posh word for scraps of paper!!) car stuff, keep aeroplanes and Colin Chapman
firmly in mind. Apparently he always quoted something about what a 1/4" bolt could lift, and if you look at a lot of circuit race cars they are
held together by VERY skinny bolts.
I'm not saying that we shouldn't build a decent safety margin into locosts, and I would suggest erring on the larger size for bolts when
unsure, but it'sa always worth considering the stresses involved and what other parts of the structure they are fed through. Often the bolt is
disproportionally strong compared to the rest of the structure.
Bear in mind that a grade 10.9 bolt has more tensile strength than any steel that you or I can buy in the un heat-treated condition, so it's
strength when compared cross-section to cross section to a weld, for example, is huge!
Mix and NS Dev, there's no such thing as M8 Grade bolts. You're confusing two entirely different standards.
The reason I mention this is there are real differences and the grading systems are not interchangeable.
The term "Grade" is used for imperial (SAE etc.) fasteners and the distinctions are Grades 5, 8 etc.
Metric fasteners are distinguished by "Class" such as Class 8.8, 10.9 and 12.9.
A common mistake is the substitution of Class 8.8 fasteners when Grade 8 is specified; people see the “8.8” marking and think it’s Grade 8. The
difference in tensile strength is 25% and could potentially spell disaster if they are mixed up.
Grade 5 is roughly the equivalent of Class 8.8 and Grade 8 is roughly the equivalent of Class 10.9.
I often use 1/2" fasteners for wishbones purely for convenience when using over the counter bushes etc. If I'm making my own bushes
I'll step the M16 rod ends on the front uprights down to 10mm and use Class 10.9 10mm fasteners throughout.
I only use Class 12.9 for the CV screws.
Rescued attachment impmetric_comparison.gif
Rorty
Thanks for pointing that out.
My results were arrived at by taking the properties of SAE grade 8 steel, (tensile strength 150,00 psi) and converting them to a bolt of 8mm cross
section. I see from your table that this equates to a class 10.9 M8 bolt.
I still would have no reservations about using M8 bolts as suspension pivots however.
British Trident
Yes I know the bolt would also be in tension but as shear strength is calculated as 60 percent of tensile strength I didn't think it relevent.
There is also the frictional force due to 'clamp up' of the bracket / bush tube, (runs for cover) which acts in favour of the shear
strength, (if you get what I mean).
Mick
PS That means an M8 class 8.8 bolt in double shear will fail at over 5 tonnes, disregarding the clamp up friction.
Don't think that's going to be the weakest point.
[Edited on 27/1/05 by Mix]
I never said there was such thing as an M8 grade bolt!!!!!!!!!!!!!!!!!
Please read my post properly!!!!
I do know a fair amount as I work for a company tha tmanufactures engineering alloyed steels!!
M8 is spec for dimension, I said GRADE 8.8, 10.9 or 12.9, as in Ultimate Tensile Strength grades, I did not say GRADE 8!!! I am not completely
stupid!!!!!!!!!!!!!!!!..........read properly!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
If you check that your bolt has 8.8, 10.9 or 12.9 stamped on it then that's what you have got!!!!!!!!!!!!!!!
[Edited on 27/1/05 by NS Dev]
quote:
Originally posted by NS Dev
I never said there was such thing as an M8 grade bolt!!!!!!!!!!!!!!!!!
quote:
Originally posted by NS Dev
I quite agree, M8 grade 8.8 or better would be fine...
Don't forget you need to account for fatigue. How many cycles do you want your bolt to last ? for a car I would suggest aiming for an infinite
life as even a full on clubman racer is going to find it difficult to justify the weight saved by using an M8 bolt that needs to be changed after so
many hours running over an M12 which will last the full life.
That is the reason a military jet can run with seemingly small bolts for high loadings, they are maintained without fail and at very strict
intervals.
A bolt is almost never in straight shear either, if there is a clearance (the wishbone moves so there must be) you are also putting a bending stress
into the bolt across the clearance. This can result in significant reduction in bolt failure load, there is also the bolt tension to consider from
torqe tightening.
What load does a mass of 6800 odd Kg equal then ? are we using gravity to determine the force in Newtons ? don't forget that although a locost
may only have a mass 600Kg a 10G or more shockload through the suspension will not be an unlikely scenario.
quote:
Originally posted by 12a RX-7
Don't forget you need to account for fatigue. How many cycles do you want your bolt to last ? for a car I would suggest aiming for an infinite life as even a full on clubman racer is going to find it difficult to justify the weight saved by using an M8 bolt that needs to be changed after so many hours running over an M12 which will last the full life.
That is the reason a military jet can run with seemingly small bolts for high loadings, they are maintained without fail and at very strict intervals.
A bolt is almost never in straight shear either, if there is a clearance (the wishbone moves so there must be) you are also putting a bending stress into the bolt across the clearance. This can result in significant reduction in bolt failure load, there is also the bolt tension to consider from torqe tightening.
What load does a mass of 6800 odd Kg equal then ? are we using gravity to determine the force in Newtons ? don't forget that although a locost may only have a mass 600Kg a 10G or more shockload through the suspension will not be an unlikely scenario.
A bolt is almost never in straight shear either, if there is a clearance (the wishbone moves so there must be) you are also putting a bending stress
into the bolt across the clearance.
Most believe that there should no clearance between the bracket and crush tube.
Mick
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!)
As you say Mix, for the wishbone to pivot there must be some clearance and thus some bending load, but not a great deal.
I don;t think anybody is advocating loading bolts to the sort of levels seen on aircraft, certainly this wouldn't be wise as the quality levels
are a bit different!! but nonetheless, it's just worth bearing in mind what sort of size bolt is actually needed for an application. (says me who
has 1/2" UNF wishbone bolts..................one wishbone bolt would pick up my car, two passengers, and another one chained on to it too!!!!)
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!)
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.
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 )
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.
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]
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.
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.
M8's all round on the isonblade, no probs.
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.
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.
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.
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.