brake disc removal

[docc]

Wouldn't it be the area of the shaft o the bolt. Technically it would not be based on the diameter of the bolt but the diameter at the depth of the threads, no?

[Paul Minnaert]

you can not calculate material strenght to the finished product, the way the tread is made deformes material, and changes stucture.

[mike wilson]

But, allegedly, that is how stainless parts are tested as opposed to carbon steel and you are therefore comparing apples to oranges.

 

Two other thoughts:

 

if you have forward calipers, whatever steel you use is probably stronger than the alloy lug surrounding it.

 

if you have rearward calipers, the fork stanchions will retain the calipers if the bolts shear.

[Guest ratchethack]

But, allegedly, that is how stainless parts are tested as opposed to carbon steel and you are therefore comparing apples to oranges.

Not sure I follow, Mike. Standards are standards. If they weren't accepted universally, they wouldn't be standards. As such, the testing parameters that define standards don't vary with the material tested. The standards for tensile and yield (shear) strength are the same for all fasteners, regardless of material -- carbon steel, stainless steel, alloy steel, phosphor bronze, etc.

if you have forward calipers, whatever steel you use is probably stronger than the alloy lug surrounding it.

"Probably stronger" is a description that I b'lieve Mechanical Engineers would frown upon. These guys tend to be pretty anal with this stuff -- and I figure that's a GOOD THING. "Whatever steel" is certainly not something I'd rely upon for retaining bolts on forward calipers, though many have, and have also managed to live to regret it. . .

 

It's pretty unusual to find riders replacing caliper carriers, though I've seen it done a few times. The Pro's who know what they're doing over-build them, often using billet stock to ensure strength by over-designing them when they lack the R&D tools of the typical OEM Engineers. It's the "when in doubt, overbuild" philosophy.

 

The weak point in brake assemblies is most often INTRODUCED by replacement of retaining bolts, something much more likely to be done by the amateur hobbyist -- typically one looking to "spiff up" his ride cosmetically, without regard to standards. This is wot I've run into many times. It's the basis for all my blather on this thread -- IMHO it's something wise to avoid, but impossible to avoid without an understanding of the underlying principles.

if you have rearward calipers, the fork stanchions will retain the calipers if the bolts shear.

If you consider sheared caliper bolts and a loose caliper drifting around on a disk, held in place (however briefly, as may well be the case ) only by its fluid line against the back side of a fork (lets just say at high speed, approaching a hairpin in the mountains with a sheer drop-off and no shoulder ), It's a scenario that I doubt any Engineer would specify as a valid "backup" should the caliper bolts fail -- but that's just me.

[docc]

you can not calculate material strenght to the finished product, the way the tread is made deformes material, and changes stucture.

 

Certainly an 8mm shaft would have greater strength before it is threaded. I'm not sure of the thread depth on an 8mm bolt with 1.25 mm pitch, but I would suppose the strength would then be more like a 6mm shaft.

 

 

I believe the yield strength standards Ratchet cited are for the finished bolt, not just the material, yes?

[jrt]

Finished or unfinished bolt is a whole different issue. I was thinking about the shearing force itself. It can't be evenly distributed over the entire (or around the entire) bolt. It's concentrated at a point or distributed over a sector of the cylinder, no?

[docc]

I thought a bolt was a finished thing. Seems like the force would be focused across the bolt encompassing it's area. You think it would be spread across the area for the entire thickness of the rotor?

 

It's more engineering than I know. I do have a resource with ME background. I'll try him since I haven't heard back from Brembo yet. Hopefully their engineering and technical people can give the definitive response.

 

Otherwise, I'm mentally preparing myself for the instant at which both my rotors shear off simultaneously. I think my only hope will be to keep them clutched firmly in the calipers while gaulding the absolute nuts off my splines engine braking like a madman.

[mike wilson]

Not sure I follow, Mike. Standards are standards. If they weren't accepted universally, they wouldn't be standards. As such, the testing parameters that define standards don't vary with the material tested. The standards for tensile and yield (shear) strength are the same for all fasteners, regardless of material -- carbon steel, stainless steel, alloy steel, phosphor bronze, etc.

 

Carbon steel is tested by using test pieces. These are formed and finished to produce the very best result. No sudden changes of profile, highly polished surface. If stainless is tested by stressing the finished product, say a socket headed cap screw with its dramatic changes in profile and superb stress raiser threaded part, then there is no feasible way of comparing results. The stainless will lose every time. As, I suspect, the carbon steel would if the situation was reversed.

[Guest ratchethack]

Carbon steel is tested by using test pieces. These are formed and finished to produce the very best result. No sudden changes of profile, highly polished surface. If stainless is tested by stressing the finished product, say a socket headed cap screw with its dramatic changes in profile and superb stress raiser threaded part, then there is no feasible way of comparing results. The stainless will lose every time. As, I suspect, the carbon steel would if the situation was reversed.

Mike, I read a bit more and believe I better understand your previous point about comparing "apples and oranges" -- particularly in Europe. Here in the US we seem to have pretty good "common use" standards, such as the ones I've cited previously.

 

I b'lieve both Docc and Jason's queries above are addressed here (note last sentence in this post).

 

But what of the case where (for example) an Italian made motorcycle is shipped all over the world? What standards for fasteners apply, and how to make valid comparisons for fastener strength on critical parts?

 

It seems that here in the US we have Fastener Standards according to both American Society for Testing and Materials (ASTM) and the American Society of Metallurgists (ASM).

 

In Europe, we find DIN, BS, ANSI, and ISO standards.

 

Then we have both Property Class and Strength Grade.

 

Now this is where the water gets deep quick, and I figure this is (in part) where the ME's have well earned their degrees. A vacant expression works its way across my face. The more I read the more confusing it gets.

 

Some of these standards seem to run into the engineering equivalent of a Tower of Babel. . .

 

I figure the boys at Bolt Science, both here and in the UK (see link below) are a valid source of info, upon which anyone interested might depend at some point, and thankfully so.

 

http://www.boltscience.com/index.htm

 

My approach continues as before: When replacing fasteners on the Guzzi (or car, or what have you) best go up-rate on any fastener. Best ALSO be careful to know wot standards are important for the application, and not assume the same spec standards are used for every fastener regardless of source.

 

Bolt Science FAQ’s

 

http://www.boltscience.com/pages/faq.htm

 

exerpts pertinent to this thread:

Some details on conversion guidance between metric and inch based strength grades is given in section 3.4 of the standard SAE J1199 (Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners).

 

Metric fastener strength is denoted by a property class which is equivalent to a strength grade. Briefly:

 

Class 4.6 is approximately equivalent to SAE J429 Grade 1 and ASTM A307 Grade A

Class 5.8 is approximately equivalent to SAE J429 Grade 2

Class 8.8 is approximately equivalent to SAE J429 Grade 5 and ASTM A449

Class 9.8 is approximately 9% stronger than equivalent to SAE J429 Grade 5 and ASTM A449

 

 

Shear Strength

 

Bolted shear joints can be designed as friction grip or direct shear. With friction grip joints you must ensure that the friction force developed by the bolts is sufficient to prevent slip between the plates comprising the joint. Friction grip joints are preferred if the load is dynamic since it prevents fretting.

 

With direct shear joints the shank of the bolts sustain the shear force directly giving rise to a shear stress in the bolt. The shear strength of a steel fastener is about 0.6 times the tensile strength. This ratio is largely independent of the tensile strength. The shear plane should go through the unthreaded shank of a bolt if not than the root area of the thread must be used in the calculation.

[dlaing]

 

Bolt Science FAQ’s

 

http://www.boltscience.com/pages/faq.htm

 

exerpts pertinent to this thread:

snip

Bolted shear joints can be designed as friction grip or direct shear. With friction grip joints you must ensure that the friction force developed by the bolts is sufficient to prevent slip between the plates comprising the joint. Friction grip joints are preferred if the load is dynamic since it prevents fretting.

 

With direct shear joints the shank of the bolts sustain the shear force directly giving rise to a shear stress in the bolt. The shear strength of a steel fastener is about 0.6 times the tensile strength. This ratio is largely independent of the tensile strength. The shear plane should go through the unthreaded shank of a bolt if not than the root area of the thread must be used in the calculation. [/indent]

It seems like ideally the brake disks should interlock with the wheel rather than rely on friction grip.

I wonder if the shear plane goes through the unthreaded shank of the bolts?

I guess that would require the threads starting enough below the shear plane that the shoulder of the bolt could take the shear rather than the threaded part of the bolt.

I suppose it could be re-engineered if Guzzi got it wrong, but at a compromise with the amount of threading.

 

I guess radial brake calipers are designed to take some of the shearing forces away from the bolts.

[Guest ratchethack]

I guess radial brake calipers are designed to take some of the shearing forces away from the bolts.

Not a good guess. Radial brakes have the same retaining bolt requirements as standard brakes WRT shear loads, since both designs employ retaining bolts with axial orientation 90 degrees to the force of braking.

[docc]

There is no unthreaded shank on these bolts. Perhaps that is testimony that the shear is not direct but friction grip.

 

I'm more certain now that I can't calculate the force that is created in braking. Last night's sleeplessness had me pondering the torque moment between the axle and caliper as divided by the friction surface. I feel that this multiplies the force applied to the braking system. Jason's assertion that calculus is required is well taken.

[docc]

I've reached Brembo NA, but they have no engineering department. I have emailed Brembo for the third time. I intend to be relentless on getting a response.

 

Does the Guzzi parts guide have a specification for the 8mmx 1.25x 20mm button head bolts?

 

(I did notice the carrier bolts on the Ducati monster have a Torx drive. Seems smart considering how easily these tear up.)

[dlaing]

Not a good guess. Radial brakes have the same retaining bolt requirements as standard brakes WRT shear loads, since both designs employ retaining bolts with axial orientation 90 degrees to the force of braking.

Not a good answer, BAA, thanks for trying.

Standard brakes' bolts are at a near perfect 90 degree angle.

Radial brakes' bolts are slightly off from 90°.

If the bolts were placed between the pistons in this image, they would be at 90° to the tangent, but the bolts are placed so that they are about 75° to the force.

[dlaing]

Does the Guzzi parts guide have a specification for the 8mmx 1.25x 20mm button head bolts?

Just a part number: "98 23 08 20"

and description: "screw"