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Old 06-01-09, 12:28 PM   #1
hernick
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Bolt classes and grades - how hard should the steel be?

Bolts come in all kinds of grades and classes. From what I've read, imperial-sized steel bolts are given grades, like grade 5 or grade 8, while metric-sized steel*bolts are given classes, like 8.8, 10.9 or 12.9. These grades seem indicate how hard and how tough the bolts are.

Now, most accessory bolts on a bike seem to be either metric M5 or M6. Those are made in aluminum alloy, in stainless steel and in steel of different classes. I understand that aluminum bolts are the weakest, followed by stainless steel which are stronger but still weaker than even class 8.8 steel bolts. Then, class 10.9 steel bolts are stronger, but not as strong as class 12.9 steel bolts.

So I was wondering, how strong is strong enough? I suppose using a class 12.9 bolt for water bottle carriers may be overkill - maybe such hard bolts would be more liable to strip the threads off the frame before breaking...

So, what class of bolts should I use on a bike? To attach water bottle carriers, rear and front racks, a stack of stuff on the fork crown (fender, rack, canti cable stop hanger...).

I've had one LBS sell me class 8.8 bolts, and I've had another sell me class 12.9 bolts, and I've seen stainless bolts, and aluminum bolts, and I've seen bolts rusted in place and bolts with their heads stripped, and bolts holes with their threads stripped...

Too soft, too weak, too rusty, too hard or too strong? What's the proper balance?
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Old 06-01-09, 12:45 PM   #2
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Every application is different.

Since you appear to be touring and riding in bad weather, stainless steel should be good.
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Old 06-01-09, 01:41 PM   #3
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Anyone correct me if I am ill-advised.

I can really only think of two fasteners on a bike that perhaps should be higher than standard or pedestrian grade- crank arm bolts.

As far as aluminum bolts are concerned, where are these used on a bike? I can't think of any off hand. Aluminum just does not seem very practical since it is relatively weak.
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Old 06-01-09, 02:22 PM   #4
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Anyone correct me if I am ill-advised.

I can really only think of two fasteners on a bike that perhaps should be higher than standard or pedestrian grade- crank arm bolts.

As far as aluminum bolts are concerned, where are these used on a bike? I can't think of any off hand. Aluminum just does not seem very practical since it is relatively weak.
Apparently some of the new expensive road bike brakes are using aluminum torx drive bolts for things like installing the brake shoes. This was noted in a brakes test in the current Velo News.

Other than crank arm bolts the only other area where highest strength bolts might be worthwhile IMO is for rack mounting bolts on heavily loaded racks for touring or cargo bikes. They need to be painted to prevent rust though.

I prefer stainless steel fasteners for bike use but even stainless fasteners can vary in material quality. Smallparts.com has 18-8 stainless metric socket cap screws in a lot of sizes. Some of the stuff I have received with far eastern made bike accessories has been pretty soft and with rough threading so has been replaced with smallparts.com stainless fasteners.
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Old 06-01-09, 04:00 PM   #5
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Not unusual for racks to come with aluminum bolts.
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Old 06-01-09, 04:56 PM   #6
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Not unusual for racks to come with aluminum bolts.
Yeah, completely ********.
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Old 06-01-09, 05:09 PM   #7
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IMO, about the only suitable place for Al bolts is to mount a water bottle cage. Stainless steel is the material of choice for most other applications if you are buying bolts. Otherwise go with what the manufacturer supplies with stems, brakes, derailleurs, etc.

One very POOR application for Al bolts is crank mounting bolts. For a while FSA supplied both steel and Al bolts with their cranks. You were supposed to torque the crank to spec with the steel bolts, then remove and replace them with the Al bolts. Bad idea. The Al bolts tended to loosen and more than one crank was ruined when they backed out.
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Old 06-01-09, 05:54 PM   #8
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One very POOR application for Al bolts is crank mounting bolts. For a while FSA supplied both steel and Al bolts with their cranks. You were supposed to torque the crank to spec with the steel bolts, then remove and replace them with the Al bolts. Bad idea. The Al bolts tended to loosen and more than one crank was ruined when they backed out.
Both the Truvativ Elita ISIS and Stronglight Pulsion ISIS cranks that I have came with aluminum bolts. They came heavily greased presumably to avoid galling which FSA tried to prevent by using steel bolts for the installation. I don't have tons of miles on either crankset but I don't see how the aluminum bolt would be any more likely to loosen up than the crank arm itself (which is aluminum or has an aluminum insert). My '86 Porsche 944 turbo came with aluminum lug nuts so it's not like bicycle crankset manufacturers are alone in their choice of materials. Aluminum can be used for fasteners but it does need to be torqued properly, just like steel fasteners. I'm guessing the same people who had problems with their aluminum crank bolts would be likely to have problems with square taper cranksets with steel bolts for the same reasons.
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Old 06-02-09, 05:49 AM   #9
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Both the Truvativ Elita ISIS and Stronglight Pulsion ISIS cranks that I have came with aluminum bolts. They came heavily greased presumably to avoid galling which FSA tried to prevent by using steel bolts for the installation. I don't have tons of miles on either crankset but I don't see how the aluminum bolt would be any more likely to loosen up than the crank arm itself (which is aluminum or has an aluminum insert). My '86 Porsche 944 turbo came with aluminum lug nuts so it's not like bicycle crankset manufacturers are alone in their choice of materials. Aluminum can be used for fasteners but it does need to be torqued properly, just like steel fasteners. I'm guessing the same people who had problems with their aluminum crank bolts would be likely to have problems with square taper cranksets with steel bolts for the same reasons.
You make a good point but I know personally of at least two cases where the Al bolts did loosen AFTER the crank was installed with steel bolts and torqued to spec. Perhaps the owner was afraid to torque the Al bolts tight enough for fear of stripping them. In any event the Al crank bolts were not on the market very long and, IIRC, FSA had a recall on them and replaced them with steel bolts.

BTW, did your 944 have all Al lug nuts or Al nuts with steel thread inserts?
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Old 06-02-09, 11:32 AM   #10
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You make a good point but I know personally of at least two cases where the Al bolts did loosen AFTER the crank was installed with steel bolts and torqued to spec. Perhaps the owner was afraid to torque the Al bolts tight enough for fear of stripping them. In any event the Al crank bolts were not on the market very long and, IIRC, FSA had a recall on them and replaced them with steel bolts.
I'm happy you brought this up as I will certainly add retorquing the crank bolts to my checklist. I haven't torqued either crank since the initial installation so I'm definitely curious now to see if they move at all. I'll eat some crow if they are loose

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BTW, did your 944 have all Al lug nuts or Al nuts with steel thread inserts?
All aluminum. I forget what the claimed weight savings was over steel lug nuts. Biggest problem with them was mechanics (unknowingly) ruining them by using impact tools on them.
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Old 06-02-09, 12:08 PM   #11
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there are Ti bolts for stems, seatpost collars, seat post rail clamps and pedal spindles.

aside from pedal spindles, the rest aren't high load...
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Old 06-02-09, 01:03 PM   #12
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Any GRADED bolt will work fine on a bicycle.Hell, hardware quality bolts will work fine on a bicycle.A 1/4x20, ungraded hardware bolt has a sheer strenght of about 1500lbs and a clamping force of 60lbs.A grade 8 bolt has almost 3 times that,grade 9/10 bolts have even more.I don't think your going to be snapping that anytime soon.
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