Bolt specs

[AM400]

Recently brought a bolt kit for the rear suspension on my 400 from a very well known independant. On fitting the bolts I have noticed that they are "R" rated as opposed to the original "S" or 8.8. Having checked out the spec it would appear that R is 45-50 ton inch 2 and S and 8.8 50-55.

Seems odd that on upgrading the suspension I have effectively down graded the bolts!! Am I looking to deep into this? did they fit the higher tensile bolts for a good reason or are the R bolts good enough?

Cheers
Andy

[FrenchTVR]

This could be a can of worms.

Where are the bolts fititng too? For instance if the bolt is going throught the trailing link bush, then it only anchors on a couple of bits of 2 to 3mm plate, in which case the bolt is tens of times stronger than the metal it is attached too.
There are not amny places IMHO on a car suspension set-up whrere the limits of a 4.6 bolt could be approached.
A lot of the forces applied are more likely to try and bend the bolt, in which case it will make very little difference whether it is 4.6 or 8.8 grade.
As it's name suggests a high "tensile" bolt is designed to have a greater resistance to tightening torque and elongation under load.
Hope this helps.

[jchase]

So what happens to these flavours of bolt in single sheer? It was my understanding the hard bit on high tensile bolts was only a couple of mm deep anyway. I would guess in sheer the softer bolt might actually be tougher, as opposed to stronger; with the hard one being susceptible to fatigue failure after less load cycles. But then I know nothing! Anyone broken one? was it fatigue or fast fracture? I fink I got 10.4 on mine, and ya got me worried now...

jim

[shpub]

I crashed tested my Wedge into a tyre wall several years ago and bent the rear hub carrier bolts. No breakage just a 20 degree bend in them.
I would not worry.

[FrenchTVR]

jchase said:

So what happens to these flavours of bolt in single sheer?

In single shear, with the thread in the shear plane a grade 4.6 M12 bolt will withstand 13.5kN (1.37tonnes), whilst a grade 8.8 M12 bolt will withstand 31.6kN (3.16 tonnes). However as I stated earlier, on a car, generally the steel the bolt goes through will tear or deform long before the bolt shears, unless of course the bolt is corroded etc. The thickness of the plate also has an effect on the load capabilities of the bolt.

As Steve said, generally things will bend first.

[jchase]

I agree FrenchTVR, the ones in the mild steel plates are fine. Ah, but the one that worries me is the damper bolt. It is inside a very rigid tube, with a bending moment applied via the damper, so you get a moment and sheer. Still whats the worst than can happen if it fatigues?

[wedg1e]

At my Transit's recent service, the techies noted that one of the U-bolts that clamps the rear axle to the leaf springs ( is this the 19th or 21st century?) had snapped.
They didn't even seem to think it was serious enough for me not to keep driving...!

Perhaps the 850Kg of lead blocks I had in there a few weeks back had something to do with it...

[AM400]

Thanks for the replies,

I have spoken to the supplier today and he assures me that they have been using this spec of bolt for years with no problems so I will go along with that.

Andy

[FrenchTVR]

jchase said:

Ah, but the one that worries me is the damper bolt. It is inside a very rigid tube, with a bending moment applied via the damper, so you get a moment and sheer.

From a calculation/engineering point of view, as the damper butts up against the boss that the bolt goes through, the moment would be so small as to be ignored

Worst that can happen if it does snap is that you get a very bouncy ride.