Fastener strength for motorcycle brakes
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    Default Fastener strength for motorcycle brakes

    I bought some fasteners to replace my brake rotor bolts on my motorcycle, I need to space out the rotor for some new brakes. The existing bolts were flat head socket screws, 1/4-20, I know none of the other specs for the originals. What I picked up are 1/4-20 flat head socket screws, they are grade 8 alloy steel, the rockwell hardness is c32, they have a 120,000 psi tensile strength, ASME : B18.3 spec. I am unable to find the shear strength. Does anyone on here with more knowledge of steel and fasteners know if I need to be worried about these bolts shearing off. There are 5 of them holding on my front brake rotor, all the load placed on the fasteners will be applied directly to the side. Thanks in advance, Chris

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    Tires slide at less than 2G. So if your bike had all it's weight on the front end in a panic stop, Call it 1000# (500X2)

    1000# at 24" (tire diameter) would be 2000# at 12" (brake rotor)

    2000# divided by 5 is.........400# each fastener .

    Shear is always more than 1/2 tensile, but give yourself a safety factor.

    You are GOOD!

    eta

    Clamped up tight, those fasteners will never see any shear!

    Like lug bolts on an car wheel.

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    Those are alloy steel fasteners, and unless zinc plated, will degrade with exposure to water, salt, and other corrodents from the roads. From a "new" standpoint I agree they're fine, but it's not a bad idea to treat them as consumables and replace them when they show signs of rust.

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    Calg, thanks for your help, that little piece of info that shear is more than half tensile is real good to know, Milland: I'm in Maine, rust and corrosion is my world, fortunately the bike gets put up by the time the salt trucks come out, I'll definitely keep an eye on them though

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    If your worried about corrosion on the threads you could use a light duty thread locker which should seal the threads in the bore and maybe also under the head from creeping corrosion.

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    Quote Originally Posted by Chris.Mitchell View Post
    Calg, thanks for your help, that little piece of info that shear is more than half tensile is real good to know, Milland: I'm in Maine, rust and corrosion is my world, fortunately the bike gets put up by the time the salt trucks come out, I'll definitely keep an eye on them though

    I'm in Vermont, A mile off Pavement. I repalced the brake rotor fixing bolts on the Morini with Titanium aviation hardware just because I had them available.

    I don't worry about corrosion, the bike goes up for the winter long before the salt trucks get sent out . ;-)

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    Hub's aluminum, right ? The threads in the aluminum will never be as strong as the bolts. You could run grade 5 and not have a problem with the bolts.

    Shear doesn't matter, unless you leave them loose. The load is never "applied to the side."

    What's more of a problem with those flathead allens is the socket goes deep into the center of the bolt right where the underhead angles of the flathead make it thin. Sorry, bad verbal description but draw it out, you'll see a really thin wall at the bottom of the hex. That's where they pop the heads off. I really don't like them, for that reason. If you can fit buttonheads in there, it's at least a little better.

    Must be a light bike for 1/4-20 ? I've always seen 3/8.

    CalG, AMA banned titanium axles a hunnerd years ago because they liked to snap the threads off. So since they were banned, we'd bore out the end, stick a piece of something ferrous in there, plug it, face the end so you couldn't see, and end up with an axle that weighed almost as much as steel, took a lot more effort and cost to make, had the liabilities of ti, but passed tech ! Ain't no danged AMA gonna tell us what to do !

    Mert used aluminum bolts to mount the shocks .... 286 pound XR, wet.

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    Unbrako catalog lists 4710# single shear for a 1/4 fhcs, for what it's worth

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    Quote Originally Posted by CalG View Post
    Tires slide at less than 2G. So if your bike had all it's weight on the front end in a panic stop, Call it 1000# (500X2)

    1000# at 24" (tire diameter) would be 2000# at 12" (brake rotor)

    2000# divided by 5 is.........400# each fastener .

    Shear is always more than 1/2 tensile, but give yourself a safety factor.

    You are GOOD!

    eta

    Clamped up tight, those fasteners will never see any shear!

    Like lug bolts on an car wheel.

    5x1/4" grade 8 fasteners can have about 15000 lbs clamp load total. With 0.15 friction coefficient that would be 2250 lbs before slipping.
    typical? 12" disk could have 6" diameter mounting hole pattern, using your 24" wheel size and 1000# that would be 4000lbs at the bolt circle.
    Still ok for shear but not pure clamping/friction joint.

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    Quote Originally Posted by MattiJ View Post
    5x1/4" grade 8 fasteners can have about 15000 lbs clamp load ...
    That's why I said I've never seen 1/4" bolts on a hub. 5/16" and 3/8" yes, but never 1/4". And 8 mm or bigger on Jap stuff. Maybe a very light bike ?

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    Quote Originally Posted by EmanuelGoldstein View Post
    Hub's aluminum, right ? The threads in the aluminum will never be as strong as the bolts. You could run grade 5 and not have a problem with the bolts.

    Shear doesn't matter, unless you leave them loose. The load is never "applied to the side."

    What's more of a problem with those flathead allens is the socket goes deep into the center of the bolt right where the underhead angles of the flathead make it thin. Sorry, bad verbal description but draw it out, you'll see a really thin wall at the bottom of the hex. That's where they pop the heads off. I really don't like them, for that reason. If you can fit buttonheads in there, it's at least a little better.

    Must be a light bike for 1/4-20 ? I've always seen 3/8.

    CalG, AMA banned titanium axles a hunnerd years ago because they liked to snap the threads off. So since they were banned, we'd bore out the end, stick a piece of something ferrous in there, plug it, face the end so you couldn't see, and end up with an axle that weighed almost as much as steel, took a lot more effort and cost to make, had the liabilities of ti, but passed tech ! Ain't no danged AMA gonna tell us what to do !

    Mert used aluminum bolts to mount the shocks .... 286 pound XR, wet.
    Maybe I'm confused, are we comparing socket cap screws to button head screws? Because in my experience the button head screws have the thinner portions, though not that it matters since you'll never get enough torque on those tiny hexes to shear the head.

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    IF the brake rotor fastening system includes nuts (it's unclear to me) I strongly advise you not use Nyloc type self lockers ......... albeit sprocket mounting on drum brakes, I've known the plastic get hot enough to melt a come loose.

    Go for a full metal Simmons or deform nut.

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    Quote Originally Posted by CalG View Post
    I'm in Vermont, A mile off Pavement. I repalced the brake rotor fixing bolts on the Morini with Titanium aviation hardware just because I had them available.

    I don't worry about corrosion, the bike goes up for the winter long before the salt trucks get sent out . ;-)
    Which Morini do you have Cal?

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    are you going to grease the heads? dry static friction is more like 0.6-0.8.

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    Quote Originally Posted by Strostkovy View Post
    Maybe I'm confused, are we comparing socket cap screws to button head screws?
    Maybe you're right, sorry, I thought he was using the flat head ones. We used both the flat head and button head, had better luck with buttons. Plus they don't seize in the seat, which sucks.

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    Quote Originally Posted by MattiJ View Post
    5x1/4" grade 8 fasteners can have about 15000 lbs clamp load total. With 0.15 friction coefficient that would be 2250 lbs before slipping.
    typical? 12" disk could have 6" diameter mounting hole pattern, using your 24" wheel size and 1000# that would be 4000lbs at the bolt circle.
    Still ok for shear but not pure clamping/friction joint.
    If we go by this chart: Coefficient of Friction Equation and Table Chart - Engineers Edge static friction should be around .4 to .5 or so.

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    Quote Originally Posted by Chris.Mitchell View Post
    I bought some fasteners to replace my brake rotor bolts on my motorcycle, I need to space out the rotor for some new brakes. The existing bolts were flat head socket screws, 1/4-20, I know none of the other specs for the originals. What I picked up are 1/4-20 flat head socket screws, they are grade 8 alloy steel, the rockwell hardness is c32, they have a 120,000 psi tensile strength, ASME : B18.3 spec. I am unable to find the shear strength. Does anyone on here with more knowledge of steel and fasteners know if I need to be worried about these bolts shearing off. There are 5 of them holding on my front brake rotor, all the load placed on the fasteners will be applied directly to the side. Thanks in advance, Chris
    And if the free advice you have received is wrong ?
    We don't care, we aren't the ones getting wrapped around a power pole.....

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    Quote Originally Posted by digger doug View Post
    And if the free advice you have received is wrong ?
    We don't care, we aren't the ones getting wrapped around a power pole.....
    It's the same value of all our free advice - zip, nada, none-a-rino, etc. But it gives the OP a chance to review some thoughts and come to their own conclusion after additional research.

    Which brings up the point - if you're questioning the value of anything this site provides, what the heck are you doing here? Isn't there a cat stuck in a tree somewhere you could rescue? Find something useful to do with your life...

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    What brand of motorcycle uses 1/4 x 20 screws to hold the brake rotor? For that matter, what motorcycle with disk brakes uses any SAE fastners?

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    Most Brake parts dont have super hard bolts like grade 8 and above, they become brittle and prone to shear if the forces are right (like they come loose.

    blue lock-tite on the threads and you will be perfectly fine. factory bolts are only grade 5 most of the time, it gives the bolt a bit of stretch when the rotor gets hot and expands. too hard and the bolt head pops off.


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