I have a 2cv City Chassis, bought before I knew any better and fitted by the company that put my car back together after it was written off, 7 years ago.

The 2cv City chassis uses bolts to attach the shock absorbers to the chassis. As I'm sure you know both SLC and original tooling chassis' use proper Citroen mounting studs.

The bolts for a 2cv City chassis are M12x75 but are only 8.8 tensile strengthiness.


I've had a number fail which is quite irritating, so I'm thinking of replacing them with these socket heads at 12.9 strengthiness:



From these people http://www.westfieldfasteners.co.uk/ScrewBolt_M.html

Will increasing the tensile strength of the bolts help prevent them shearing? I vaguely get that tensile strength and shear strength are different things?

Sam,I would think that they would be the ones to use,but I am sure the tech heads will prove me wrong.

A pointless post really.

The bolt may have a little more shear strength as they probably increased the tensile strength by making it out of a higher quality metal or heat treating them etc... but, as you said, they are different things, so it might not be that different.

Either way, I'd be surprised if it was worse than your previous ones so out of the two, I'd go for them

The first digit is the bolt material's minimal ultimate tensile strenght (in MPa when multiplied by 100), the other one is a multiplier which tells you about the minimal yield strength, which is normally the limiting factor, especially for shear. For example, for 8.8 grade bolts the values are 800 MPa and 800 * 0.8 = 640 MPa. For 12.9 grade, it's respectively 1200 and 1080. This can be achieved by use of a different material, various types of plastic forming, hardening etc., but that doesn't matter much to you, I suppose.
It means that, theoretically, a 12.9 bolt of the same size is 1,6875 times stronger

I have 10.9 bolts machined to make studs in mine, which managed to tear off the whole front attachment points clear off the crappy aftermarket chassis of mine, with the bolts intact. Twice. I suspect crappy mount design and possibly placing them too far backwards (making the shock too short) were the cause, I think I finally fixed that last year with my own design Anyway, you should ask for, and take some distance measurements from the front suspension arm to the chassis to verify if this is not part of your problem.

its not just a single shear problem as its a rubber eye there is a degree of bending which the 12.9 might not take, ive asked some eingineery metallurgist types and we'll have a variety of answers soon

Actually, most the load would be transferred by friction between bushing/chassis and bushing/nut (washer), not the bushing pressing on the bolt, as it is not a precise or press fit. It also keeps the rubber bush flexing, instead of rotating around the bolt (just like a rubber suspension bush). But the material is stronger anyway, and some surface hardening would make it even more resistant to bending (because bending strain gets stronger from the middle towards the surface, and hardening pushes the yield point even further back, the downside is that there would be no partial failure by elongation, but a cracking of the surface after reaching the yield point).

no - i understand what your saying but its not a rotational stress on the bolt more that the rubber deforms in the shock so the head of the bolt is being pushed further than the base of it - the perfect way to snap a bolt off. if i gave you a pair of pliers and said make that bolt shorter youd grip the head of the bolt and bend it backwards and forwards- not try to twist it off.

but as you say id wory that the 12,9s would snap like glass ( catastrophic failure).

its been a looooong day involving drunk people and a strange old man without trousers im probably not making any sense anymore

goodnight

I shan't ask! Thank you for the BB unit Beer is owed!

That bending results rather from friction pulling the bolt at the length of the bush, under the nut and washer (not at the center of the bush pushing on the bolt itself, although that too, because the bush is rather thin). Also another part of it is "taken up" by the chassis body where the other mating surface of the bush rests and is held by roughly the same amount of friction, distributing the load.
It would fail catastrophically, but still under a much, much higher load. At yield point a non-hardened bolt would elongate/bend permanently, losing tension, and be severely weakened, which is the last step before a complete failure anyway. A hardened one will crack, but that is assuming that it is in fact hardened. Black colored higher grade bolts are almost definitely heat treated and cooled in oil (which gives them the colour), but the galvanised ones may just be a neat combination of a stronger steel alloy and forming process. We'll probably never know, and that's where the 8.8, 12.9 etc values come in - this ensures that the bolts are made up to a certain standard and meet some minimal strength requirements.