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bolt strength
foskid - 23/5/11 at 04:32 PM

Hi folks,
At the moment my suspension is assembled using 8.8 grade bolts during the build, are these sufficient or do I need to upgrade them for final assembly ?

Cheers, John.

omega0684 - 23/5/11 at 04:38 PM

8.8 will do nicely!

Neville Jones - 23/5/11 at 04:55 PM

If they're the size most use, 4.6's will be more than enough!

iank - 23/5/11 at 05:22 PM

It's been argued that for suspension 8.8 are a better choice than 12.9 due to harder bolts failing by snapping rather than bending when hitting a curb hard.

Same reason crane hooks aren't made of high tensile steel (or so I was told at school) since if overloaded you want them to bend slowly rather than snap.

mcerd1 - 23/5/11 at 05:57 PM

quote:
Originally posted by iank
It's been argued that for suspension 8.8 are a better choice than 12.9 due to harder bolts failing by snapping rather than bending when hitting a curb hard.

sounds about right to me, but I can't see it making that much difference in real life



there are all kinds of odd grades out there (4.6, 8.8, 10.9 and 12.9 being the most common)

the first number is related to th tensile strength (higher the better)

the secnond is how tough it is (lower numbers will bend more before breaking)



I can dig out the stuff to tell you how strong / tough they are if anyones really bothered...


[Edited on 23/5/2011 by mcerd1]

snapper - 23/5/11 at 06:12 PM

Most suspension bolts on production cars are 10.9
Certainly the Sierra diff bolts are 10.9

mcerd1 - 23/5/11 at 06:20 PM

quote:
Originally posted by snapper
Most suspension bolts on production cars are 10.9
Certainly the Sierra diff bolts are 10.9

but ford use 10.9 for almost everything, and the sierra is twice the weight

[edit]I know its not really as simple as that....

[Edited on 23/5/2011 by mcerd1]

flak monkey - 23/5/11 at 06:26 PM

I worked this all out ages ago.

An 8.8 grade bolt has a shear strength of about 375N/mm^2. Which works out to be 3.15 tonnes for an M12 8.8 bolt in single shear

All suspension bolts in a seven are in double shear... so 6.3tonnes more or less.

Thats a lot more than you could hang from the suspension brackets.

Mild steel rods would be strong enough at that diameter

Infact you would be better if designing from scratch to use M10 bolts rather than M12...but who am I to argue with convention?

mcerd1 - 23/5/11 at 06:34 PM

^^ but if you use too small a diamiter of bolt you may end up with a bearing failure rather than sear, especial on a thin suspenssion bracket




[Edited on 23/5/2011 by mcerd1]

flak monkey - 23/5/11 at 06:49 PM

I worked that out too. Even M8 is plenty big enough for any forces one single bolt could forsee... Bearing in mind that you have 4 times that strength at the front and a bit more than 4x at the rear...

mad4x4 - 23/5/11 at 07:12 PM

quote:
Originally posted by Neville Jones
If they're the size most use, 4.6's will be more than enough!



Errr no they won't - neither will A2 or A4 Stainless, Suspension can take a lot of loading. Yes I know that this is in the horizontal plane compared to a tensile pull on a bolt but I'd always had a strong bolt over a piece of cheese. I would expect Mr. IVA would also say no if you used unstamped or 4.6 grade bolts. For the sake of a few pence use 8.8's

>>>"Grabs FLAC Jacket and Runs for Cover..>>>"

flak monkey - 23/5/11 at 07:22 PM

The reason not to use stainless is different. A4-70 is technically the same as 8.8 to make no difference...

4.6 bolts ARE more than strong enough, but would fail IVA... but there we go...

Like I said before the brackets will give out way before the bolts will...

Liam - 23/5/11 at 09:57 PM

quote:
Originally posted by mcerd1
^^ but if you use too small a diamiter of bolt you may end up with a bearing failure rather than sear, especial on a thin suspenssion bracket




[Edited on 23/5/2011 by mcerd1]


You shouldn't - because the bolt should not be used as a bearing!

mcerd1 - 23/5/11 at 10:11 PM

quote:
Originally posted by Liam
quote:
Originally posted by mcerd1
^^ but if you use too small a diamiter of bolt you may end up with a bearing failure rather than sear, especial on a thin suspenssion bracket




[Edited on 23/5/2011 by mcerd1]


You shouldn't - because the bolt should not be used as a bearing!

I mean crushing or ovaling of the bolt or the part that the bolt goes through

foskid - 24/5/11 at 07:49 AM

OK, thanks for the advice gents, I guess I'll stick to 8.8 bolts

Cheers, John.

Doctor Derek Doctors - 24/5/11 at 12:25 PM

To be honest how the bolts are used is far more critical than the material they are made of. If the positioning and mating is well designed then 8.8's are no problem.