Anyone know what grade of bolt I should use for attaching hats to rotors? (5/16 UNF).

I would use a Wilwood or AP bolt kit but they both supply Torx head and I have to use a socket cap for clearance.

I'm leaning towards 12.9 but would this be too brittle, is grade 8 or 10.9 a better choice?

10.9 socket cap seems impossible to get hold of so if 12.9 is safe I'll use that.

The torque between disc and bell should be transmitted by friction alone so as long as the bolts are properly tight the bolt grade shouldn't matter that much.
8.8 would do the job as would 12.9.
If you use 12.9 just be careful not to over torque.

Thanks BT.

Wilwood specify a torque of 25 ft-lbs, which is what you would use for a 12.9 bolt. And I've just spoken to Hi-Spec who told me they use grade 8.8...

Decisions... now I'm leaning more towards 8.8 to avoid the brittleness concern.

Won't 12.9 rust a lot quicker?

Superficially, probably 12.9 will rust a bit, not a big concern though.

I've found some M8 bolts which are grade 8.8 - that's 0.06 mm bigger than 5/16", can't see any reason why I shouldn't use these instead? (the hat and rotor are unthreaded, I will be using an all-metal locking nut on the other side).

Metric and UNF thread pitch is different

DON'T BODGE BRAKES

Yes, obviously - I'm not suggesting using a UNF nut on a metric bolt!

I mean using an M8 nut & bolt through the unthreaded holes in the hat & rotor.

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Originally posted by rodgling
Yes, obviously - I'm not suggesting using a UNF nut on a metric bolt!

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Sure, but I can calculate/look up the correct torque setting easily enough. The only thing I can see would be having 0.06 mm less hole clearance around the bolt, which doesn't strike me as an issue?

Well, in the end I was able to find some 5/16" grade 8.8 bolts so job done.

Unfortunately they are fully threaded (no shank) which is not ideal but given that HiSpec use 8.8 M6 bolts and mine have a minor diameter of more than 6mm I think this will be fine.

[Edited on 22/3/13 by rodgling]

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Originally posted by rodgling
....in the end I was able to find some 5/16" grade 8.8 bolts....

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Well, they are sold as such. TBH a lot of UNF socket caps sold in the UK seems to be sold as class 12.9 rather than grade whatever

High Tensile Cap Head Bolt 5/16"' UNF Thread 1"' Long Fasteners Workshop Car | eBay

sold as maybe - but whats actually writen on the bolt ? (all bolts must have the grade and maker marked on them)

but I guess that someone could be making weird hybrid bolts with metric material and inch sizes


alot of bolt suppliers seem to know less than they should about the grades
we had one try and tell us that a grade 8.8 was the same steel as S355 and the old BS916 bolts
when you look it up BS916 is not that far off a S355 (still not the same mind) but then S355 isn't a bolt grade at all !
and the nearest ISO898 bolt material is somewhere between a 4.6 and a 4.8

Is it very unusual for imperial sized bolts to be made with metric material specifications? It doesn't sound unreasonable?

Don't know what's marked on the bolt, I will check when they arrive.

Rotor ? thought this was a thread about the new Locost helicopter for a minute there.

Just read the receipt and they're rebranded Demon Tweeks bolts (at twice the price), grade 12.9. Bugger. Back to square one.

Can't really tell why you wouldn't want to use 12.9 instead of the 8.8s ? I know the 8.8 is more prone to bending, but the 12.9 needs such a lot more force before breaking than a 8.8 that is should always be a better choice, no?

Should probably also not be that difficult to calculate the actual load on these things? Take the brake torque (for example assuming 70% bias on the front, 1.1g deceleration and 1.0 friction coefficient of tyres). Along with the disc bolt radius you will find a force between hat and disc, which probably is loading 2 bolts at most?

shear strength of the different bolt grades is well known.

Marc: I asked Hi-Spec about 8.8 vs 12.9, and they thought that 12.9 would be more brittle (which was my concern too) - apparently they use M6 8.8, even on much larger systems. So I think it's very safe to say that M8 8.8 will be strong enough. I suspect as BT said, either would be fine.

I have to use socket caps for clearance reasons (otherwise I'd use Wilwood/AP/etc supplied bolts). In the UK, these are only available in ungraded or 12.9. I've ended up ordering some class 8.8 bolts from Ebay in Germany - this seems to be the easiest answer.

Also, it turns out that in M8, I can get a bolt with a shank of a suitable length, whereas in 5/16 I can't. So M8 it is.

In principle the hat and rotor won't move relative to each other, so there can't be any shear load at all? Or is my lack of mechanical engineering background showing...?

To comment on a few misconceptions in this thread:

I don't understand the impression that 12.9 bolts are brittle. They are inherently ductile.

The strength of a fastener is primarily controlled by the heat treatment, not the composition, so an 8.8 grade bolt could be made from the same raw material as a 12.9 bolt.

The corrosion resistance of the bolt is not affected by the strength of the fastener.

Socket head cap screws are generally made to 12.9 grade as the reduced cross section of the head leads to a weaker design. It's rare to find anything else.

[Edited on 25/3/13 by Grimsdale]

I'm no expert and please correct me if I'm wrong - but my understanding is that higher grade bolts are stronger (12.9 is about 50% stronger than 8.8) but more brittle.

From the Steel Designer's Handbook:

http://books.google.co.uk/books?id=df7R9rT3nmUC&pg=PA200&lpg=PA200&dq=8.8+12.9+brittle&source=bl&ots=GZ9e9dct8E&sig=Bdwz1 mnsItEmU_besMCLThvy-qU&hl=en&sa=X&ei=RlhQUYDDCai70QX7joDYDA&ved=0CDYQ6AEwATgK#v=onepage&q=8.8%2012.9%20brittle&f=false

You're sort of right, but it's more a case of 'less ductile' rather than 'more brittle'. The bolt is still ductile and as such will plastically deform before overload failure.

The link you've provided relates to hydrogen embrittlement. This can occur with 10.9 and 12.9 grade bolts that have been poorly electroplated, without a satisfactory post-plating baking step, which is needed to drive off hydrogen which can get absorbed by the steel.
It can happen, but it's a manufacturing defect, not an inherent property of the material. If you're really worried about it, use an unplated bolt.

Andy

Oke, just a quick sanity check;

Stopping your car at say 800kg, 70% at the front, 1.1g deceleration gives a brake torque of about 500Nm per front wheel.
Bolt radius for the disc is about 80mm, so that's 6.25kN if it were a single bolt

Single shear strength of M8 8.8 (or 5/16" grade 5)
25kN on the shank, about 16kN on the thread

For 10.9 / grade 8:
33kN on the shank, 21 kN on the thread.

Base on these figures it really makes sense to buy the bolts with shanks (shouldered bolts, is that correct, we'd call them borstbout). Then the load applied on max braking is 25% of the breaking strength of M8 8.8 and should be fine.

Dave, as far as I can see, on these discs the bolt is the only connection between disc and hat. Therefore the full brake torque / force is transferred through these bolts (in single shear) from disc to hat.

MarcV

A bolt has a shank and a threaded region. A screw is a fully threaded bolt. A shouldered bolt has a shank of larger diameter than the thread.

Clear as mud, but hope it helps (plus most people get it wrong anyway).

Andy

[Edited on 25/3/13 by Grimsdale]

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Originally posted by MarcVBase on these figures it really makes sense to buy the bolts with shanks (shouldered bolts, is that correct, we'd call them borstbout). Then the load applied on max braking is 25% of the breaking strength of M8 8.8 and should be fine.

Dave, as far as I can see, on these discs the bolt is the only connection between disc and hat. Therefore the full brake torque / force is transferred through these bolts (in single shear) from disc to hat.

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Andy,

Even more confusing are the difference between languages. For us a bolt (bout) is defined as a threaded fastener which is torqued from the outside of the head (ie hex head), where a screw (schroef) is torqued from the inside (ie torx, philipshead).

Dave,

With the braking forces estimated now you could well be right. M8 8.8 with 17kN preload should resist about 10kN of braking force before the disc will slide relative to the hat. Ofcourse the preload changes over temperature considerably, but if it wouldn't hold, the bolts should still survive well.

Overall no reason to foresee problems with 8.8 and these are best available (at least overhere)...

Tradditionally in the UK we called a fully threaded bolt a "Set Screw" but the term has fallen out of use.

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Originally posted by rodglingAndy: I take your point about brittleness, looks like self-coloured 12.9 is the best choice?

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