brake disc removal

[mike wilson]

I don't see any marks on the original button head bolts at all. Am I missing something?

 

Probably not. Motor manufacturers can buy in sufficient quantities to be able to define their own cosmetic requirements.

 

Although stainless is usually dis-specified (is that a viable desctription?) for high strain settings, it seems to me that bits of alloy casting would be more liable to give way than steel bolts. This is certainly true in the case of caliper fixings, where two steel bolts are surrounded by no more than an equivalent thickness of alloy. Maybe on disc fixings the veto on stainless is fair. In that case, I would be heading for the company (whose name escapes me for the moment*) that can provide stainless in 10.9 shear strength.

 

*Bumax/Unbrako http://www.unbrako.com.au/bumax.htm

[docc]

Reviewing the shop manual (and admitting that it has been sorely lacking in some cases), there is no specification for bolt hardness or even the recommendation to fit new bolts upon reassembly. Only to apply Loctite 270.

[raz]

Just a warning while we're talking bolts, I'm pretty sure the rear disc is fitted with special shape bolts allowing some float, that ones I wouldn't replace with standard bolts. The front bolts, which I think we're discussing here, seems to be standard bolts though.

[mike wilson]

Dunno f'er absolute certain, Docc, -- especially on the rotor carriers -- because I replaced my caliper bolts many years back. But I've got four suspicious looking identical bolts in the fastener bin on my Guzzi operating tray of the exact size that're marked 8.8, and I'll bet these are the original caliper bolts I took out.

 

Grade 8.8 is rated at 93K PSI yield (shear) strength.

 

A4-80 Stainless would appear to fall short of stock issue by not much -- 6K PSI -- hardly enough to get overly excited about.

 

But then, wye not make it an upgrade rather than a small downgrade, long as y'er at it?

 

 

On the Unbrako site above, in the catalogue pdf (page 48) it specifies testing methodology. Carbon steel is tested on a test piece with a test length 5x diameter. Stainless is tested on _finished product_ with a test length of from 2.5 x diameter upwards.

 

Given the foci for failure on a threaded piece versus a test piece, I am suprised that stainless does so well. Stainless is probably much tougher than carbon steel.

[Guest ratchethack]

Stainless is probably much tougher than carbon steel.

Mike, I'd be a little careful here with this statement. This is where I've seen many get into trouble.

 

I took a look at the Unbrako site. For safety's sake, at the risk of repeating some of the stuff previously mentioned, I believe it's well worth emphasizing that WRT brake applications such as calipers and rotors, as mentioned above, the important strength parameter for fasteners is yield strength. Tensile strength of such bolts as these is not important. Typical engine bolts, on the other hand, have exactly the opposite requirements of brake fasteners. In the case of cylinder head bolts, for example, it's tensile, and NOT yield (shear) strength that's important. Many applications elsewhere on motorcycles have spec reqirements for combinations of both yield and tensile strength.

 

When swapping out fasteners, it's all too common, though occasionally very dangerous, as in the case of brake components, to go down-rate on original fastener spec's, and there's NO downside to up-rate. But it's important to have an awareness of what KIND of strength is required in every critical application.

 

A4-80 Stainless, as described on the Unbrako site, has a tensile strength rating identical to Metric Class 8.8 carbon steel (880 N/mm^2). However, it's yield strength is 6K PSI below Metric Class 8.8 carbon steel.

 

Depending on grade, IMHO it's risky to make a statement that stainless is "probably much tougher" than carbon steel. Again, Metric Class 12.9 grade alloy steel has nearly double the yield strength of A4-80 Stainless, and more than half again greater tensile strength.

 

Many's the rider who discovered the hard way that his blinged-out stainless steel festooned parade float was actually held together in the wrong places with shiny new stainless steel "pre-broken" fasteners!

 

From the Unbrako "Fastener Facts" PDF:

 

The user of metric socket screws must be wary of the strength level of the fasteners he or

she is buying. Purchasing by the simple description metric socket head cap screw can

result in one of many strength levels being received. This can result in installation

problems or undesirable product performance. Metric alloy steel socket head cap screws

manufactured in accordance with international standards can be different strengths and

materials.

[Josh]

What is shear strength?

Mike, I'd be a little careful here with this statement. This is where I've seen many get into trouble.

 

I took a look at the Unbrako site. For safety's sake, at the risk of repeating some of the stuff previously mentioned, I believe it's well worth emphasizing that WRT brake applications such as calipers and rotors, as mentioned above, the important strength parameter for fasteners is yield (shear) strength. Tensile strength of such bolts as these is not important. Typical engine bolts, on the other hand, have exactly the opposite requirements of brake fasteners. In the case of cylinder head bolts, for example, it's tensile, and NOT yield (shear) strength that's important. Many applications elsewhere on motorcycles have spec reqirements for combinations of both yield and tensile strength.

 

When swapping out fasteners, it's all too common, though occasionally very dangerous, as in the case of brake components, to go down-rate on original fastener spec's, and there's NO downside to up-rate. But it's important to have an awareness of what KIND of strength is required in every critical application.

 

A4-80 Stainless, as described on the Unbrako site, has a tensile strength rating identical to Metric Class 8.8 carbon steel (880 N/mm^2). However, it's yield (shear) strength is 6K PSI below Metric Class 8.8 carbon steel.

 

Depending on grade, IMHO it's risky to make a statement that stainless is "probably much tougher" than carbon steel. Again, Metric Class 12.9 grade alloy steel has nearly double the yield (shear) strength of A4-80 Stainless, and more than half again greater tensile strength.

 

Many's the rider who discovered the hard way that his blinged-out stainless steel festooned parade float was actually held together in the wrong places with shiny new stainless steel "pre-broken" fasteners!

 

From the Unbrako "Fastener Facts" PDF:

 

The user of metric socket screws must be wary of the strength level of the fasteners he or

she is buying. Purchasing by the simple description metric socket head cap screw can

result in one of many strength levels being received. This can result in installation

problems or undesirable product performance. Metric alloy steel socket head cap screws

manufactured in accordance with international standards can be different strengths and

materials.

[dlaing]

What is shear strength?

probably the most critical strength for bolts holding on brake disks.

Shear force is what the bolts must resist when braking.

Tensile force is what the bolts must resist if you grabbed the brake disk and tried to pull it away from the wheel. If your disk was warped this might become critical, but otherwise, shear strength is most critical.

 

From wikipedia

Shear stress is caused when a force is applied to produce a sliding failure of a material along a plane that is parallel to the direction of the applied force. An example is cutting paper with scissors.

 

Tensile

 

 

Compressive

----->

 

 

Shear

----->

[mike wilson]

Mike, I'd be a little careful here with this statement. This is where I've seen many get into trouble.

 

I need to be more careful in my writing, never mind anything else. I meant to write "possibly", not "probably".

 

Some interesting information on the relationship between tensile and shear strength hereabout alloy and wrought steel (which _possibly_ includes high tensile) and here about stainless.

 

It seems from the above that stainless' tensile strength is reduced in capacity compared to carbon steel. All conjecture but the second site lists a military spec handbook. Anyone able to get an online view if it?

[docc]

If A70 stainless bolts shear at 65,000 psi, and all 12 would have to shear simultaneously for failure, then over 245,000 pounds of force will have to be applied.

 

I wonder what kind of front tire would still make traction? That's 300 g's for an 800 pound machine with rider. I think I'd pass out first.

[Guest ratchethack]

If A70 stainless bolts shear at 65,000 psi, and all 12 would have to shear simultaneously for failure, then over 245,000 pounds of force will have to be applied.

 

I wonder what kind of front tire would still make traction? That's 300 g's for an 800 pound machine with rider. I think I'd pass out first.

Careful, Docc. That's PSI, (pressure in lbs./square inch), not to be confused with pounds torque (force in lbs./linear foot). How many square inches of significant surface area in the direction of shear d'you think the 12 bolts add up to? NOTE: The smaller the surface area, the higher the PSI (yield) requirement.

 

All I'm saying is best not dissmiss brake fasteners as unimportant by down-rating them, my friend. Second-guessing the Engineers isn't something most of us are qualified to do. They typically spec these things very carefully, and though the rest of the Guzzi may be significantly over-engineered (to the tune of about 150 lbs greater than some of its market equivalents by displacement), IMHO the boys at Brembo ain't as likely to be so overly generous by providing strength and weight (and cost) that's not actually needed. IMHO the potential consequences for under-provision here are just too high.

 

BAA, TJM & YMMV

[docc]

I'm not convinced the A70 is down rated since there appears to be no clarified rating on the original fasteners.

 

And you're right, I used the bolt diameter, 8mm, for the calculation and should have used the area, 50 mm sq. converted to inch measure.

 

Refiguring, I get just over 5,000 pounds per bolt. 60,840 pounds, or 76 g's, to shear them all.

[Guest ratchethack]

Docc, I'm not exactly the best qualified to do the force conversions here. I could probably figure it out by keeping track of the units of measure, but Mechanical Engineers could whip this out PDQ. Somehow it seems to me that 2 1/2 tons per bolt is a mite out o' the ballpark for an 8 mm dia bolt, and seems unlikely to me that Brembo would spec anything close to 76 g's shear strength unless they're building landing gear for the Space Shuttle (and anticipating landings with a full load!).

 

BAA, TJM & YMMV

[docc]

It seems pretty simple to calculate the area of an 8mm bolt, convert that the square inches and multiply times the psi figure you provided for the A70.

 

Tired of shooting in the dark, I eMailed Brembo NA for their specifications on the fastener and an opinion on the stainless replacements.

 

Now, whether they pass the inquiry on to engineering and technical or over to legal . . .

[Guest ratchethack]

Used to be simple enough f'er me also, but we're dealing with measurements of different force parameters here. It was always semi-tricky stuff in Physics (at least I thought so until we got out o' Newton and into Einstein, with Time Dilation and Lorentz Transformations ) and I've forgotten more'n I ever knew. Lately, it's easy f'er me to get tripped up.

 

Good on ya for asking Brembo for a spec.

 

But then, if legal gets ahold of it, you might expect a terabyte or 2 of legalese in response.

[jrt]

So, you can easily calculate the area of a cylinder...but what's the area used in the calculation of shear strength? This seems a good application of calculus