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Aluminum Triple Clamp Pinch Bolts??


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Hello all,

I have a question for some tech guys/mechanics. I have M8x40 Hex Flange bolts that are 7075 Aluminum. My triple clamps have 3 pinch bolts on the lower clamp and two pinch bolts on the upper clamp. My question is, are these aluminum bolts sufficient enough in this application....are they strong enough if torqued down to regular specs and loc-tite applied? I suspect they will be just fine, but would like some explanation if available. Thanks.

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General rule of thumb on fasteners is you only need 2 diameters of thread engagement to equal shaft strength, if both threads are of the same material. 7075 is stronger than the cast aluminum triples so if you have 2 diameters of thread engagement you will have clamping tension that equals the strength of the threads.

I think most triple use more than 2 diameters of thread engagment with the steel bolts but the ultimate strength could be the aluminum threads. 3 diameters with aluminum bolts and the bolts will be the weak link. Torque values are for steel fasteners and will possibly fail an aluminum bolt.

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modern bikes call for about 15ftlbs top, 17 bottom and the 7075 bolt is stronger than the cast

aluminum also was a huge problem with gualding for lack of lubrication, and fusing with another metal when in one place for a long time

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Well... I tend towards 13lb/ft for lower clamps to avoid squeezing the fork tubes out of round, but I wouldn't replace a steel fastener with an aluminium one in any structural / safety critical application. The "aluminium frame" thread answers all the questions, but the most pertinent to me are the questions of lower strength, fatigue resistance and possible low corrosion resistance.

If money is no object then titanium would be good.

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the bolts will be 1/3 as stiff - joint will be more flexible, less than 1/2 as strong - 75,000 psi ultimate vs. ~180,000 psi for high grade steel bolts, i would worry about galling as someone mentioned too.

the tapped holes on the triple clamps are not super high strength, but internal threads have more shear stress area than an external thread (bolt). that's why they can reliably use high strength bolt joints to hold the jug on and lots of other places, steel bolt/stud in aluminum threads.

why risk it? you'll save an insignificant amount of weight, but put yourself at risk for a major structural failure. all the moto companies use very high quality hardware on motox bikes. they also put lots of effort into making the bikes lighter, so i would guess those clamp bolts are already as stressed as is reasonable.

when i worked as a bicycle mechanic the aluminum bolt fad int he 90's was a nightmare of squeaky cranks, slipped cables, twisted off heads and broken bolts, and it just didn't save much weight. titanium is better than aluminum for bolts, but still has lots of pitfalls.

Mike

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be sure to put some anti-seize on those Ti bolts. permatex nickel is recommended for ti, but i think even the regular auto parts store anti seize will work. also need to let up on the torque somewhat to account for the extra lube of the anti-seize. maybe check with the bolt manufacturer.

good luck,

Mike

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