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  1. #1
    Gammal cyklist Reynolds's Avatar
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    5mm frame rack bolts

    Who set this standard? 6mm is much more adequate IMO. I've modified some frames/racks to 6mm, but some have so thin walls that I didn't dare to. What's your opinion?

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    Quote Originally Posted by Reynolds View Post
    . . . much more adequate IMO. . .
    Would not 10mm be yet "more adequate"??

  3. #3
    Gammal cyklist Reynolds's Avatar
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    Quote Originally Posted by AnkleWork View Post
    Would not 10mm be yet "more adequate"??
    Nothing against 10mm, what would be the problem, 20 grams more?
    Those 5mm bolts are the weak link, sooner or later they'll loose/break if you carry some substantial weight on bumpy roads.

  4. #4
    coprolite fietsbob's Avatar
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    When I wanted the rear rack to be secure and reliable I went with a 5mm bolt made of high strength Steel

    you may find those , like I did, at an Industrial Fastener outlet , the 8mm Hex head is Marked with an 8.8 ..

    they held fast for 6 months , as they were installed with threadlock compound too .. and are still solidly there 15 years later.



    Who set this standard?
    the metric standard came out of the French Revolution .. 1799. http://en.wikipedia.org/wiki/Metric_system
    Last edited by fietsbob; 04-24-14 at 02:17 PM.

  5. #5
    Senior Member Bill Kapaun's Avatar
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    I've wondered the same thing, since at times, I'll have close to 60 lbs. on my rear rack. (50 lbs. lawn food + 2 Wald folding baskets)
    When I'm going to haul a heavy load, I always check to make sure they're tight!

    If I understand it correctly, a 5mm bolt (and others in that size range) would have .5mm thread depth, basically resulting in a 4mm "pin" to support the weight.
    Going to 6mm would be a 5mm pin.
    Cross sectional area would be a ratio of 16:25 or 56% stronger.
    Maybe my thread depth is off, but the concept remains the same.

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    Senior Member SquidPuppet's Avatar
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    5mm seems very adequate to me. Buy high strength heat treated black bolts and nylon lock nuts or use some Locktite. My handlebars are held in place with 5mm bolts and they are subjected to a helluva lot more weight, leverage, and jarring than a basket or rack.

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    Senior Member Bill Kapaun's Avatar
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    Quote Originally Posted by AnkleWork View Post
    Would not 10mm be yet "more adequate"??
    Without thicker drop outs to support the extra diameter, the gain wouldn't be proportional.
    There's a point of being adequate, better and absurd.

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    Quote Originally Posted by Bill Kapaun View Post
    ...
    There's a point of being adequate, better and absurd.
    funny! that's how i refer to my first three wives.

  9. #9
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by Bill Kapaun View Post
    I've wondered the same thing, since at times, I'll have close to 60 lbs. on my rear rack. (50 lbs. lawn food + 2 Wald folding baskets)
    When I'm going to haul a heavy load, I always check to make sure they're tight!

    If I understand it correctly, a 5mm bolt (and others in that size range) would have .5mm thread depth, basically resulting in a 4mm "pin" to support the weight.
    Going to 6mm would be a 5mm pin.
    Cross sectional area would be a ratio of 16:25 or 56% stronger.
    Maybe my thread depth is off, but the concept remains the same.
    With the proviso that "it depends on the rack", I've not found an M5 (or 10-32) bolt to any worse than an M6. I've never had an M5 fail. I have had some issues with the larger diameter bolt in that most racks have to be drilled out to use them. If the rack is aluminum, the limiting factor becomes the metal on the rack leg, not the bolt. If the rack were a steel, this would be less of an issue. If the rack were a Tubus, it wouldn't matter at all because the mounting system is so different.
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    Here's a chart for metric bolt strength ratings. It's sometimes hard to find specs for allen head bolts but hex head types will have the ratings marked on the head. If you've been using stainless you can see that most of the SS fasteners available at shops and hardware stores are much weaker than any of the non-SS available. Bolt Depot - Bolt Grade Markings and Strength Chart

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    Quote Originally Posted by Reynolds View Post
    Who set this standard? 6mm is much more adequate IMO. I've modified some frames/racks to 6mm, but some have so thin walls that I didn't dare to. What's your opinion?
    Your rack system is only as strong as your weakest link, usually the rack itself not the screws. By modifying your racks you've probably made your system weaker by weakening the weakest link.

  12. #12
    Ding! Bandera's Avatar
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    Quote Originally Posted by Reynolds View Post
    Who set this standard?
    John D. Thompson will know w/ his strong dropout-history-FU.

    That being said I've never seen a 5mm fixing bolt fail in rack or mudguard service, although a heat treated bolt, a compression washer & blue locktight is a very good idea.

    This is an answer seeking a question that I never asked "back when" I saw laden trans-con tourists seeking service for other issues, not rack fixing bolt failure.

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    Last edited by Bandera; 04-24-14 at 07:02 PM.
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    Quote Originally Posted by Bezalel View Post
    Your rack system is only as strong as your weakest link, usually the rack itself not the screws. By modifying your racks you've probably made your system weaker by weakening the weakest link.
    A single M5 bolt, grade 8.8, has a shear strength of something about 600 kg, if it's properly installed in something at least as strong as it. I'm pretty sure I've got two holding my racks on....

    The rack post or the dropout is almost certainly the limiting factor.

  14. #14
    Gammal cyklist Reynolds's Avatar
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    Quote Originally Posted by Bezalel View Post
    Your rack system is only as strong as your weakest link, usually the rack itself not the screws. By modifying your racks you've probably made your system weaker by weakening the weakest link.
    Quote Originally Posted by dscheidt View Post
    A single M5 bolt, grade 8.8, has a shear strength of something about 600 kg, if it's properly installed in something at least as strong as it. I'm pretty sure I've got two holding my racks on....

    The rack post or the dropout is almost certainly the limiting factor.
    Quote Originally Posted by cyccommute View Post
    With the proviso that "it depends on the rack", I've not found an M5 (or 10-32) bolt to any worse than an M6. I've never had an M5 fail. I have had some issues with the larger diameter bolt in that most racks have to be drilled out to use them. If the rack is aluminum, the limiting factor becomes the metal on the rack leg, not the bolt. If the rack were a steel, this would be less of an issue. If the rack were a Tubus, it wouldn't matter at all because the mounting system is so different.
    That's why I didn't modify all racks, only those with enough material to do so.
    I use black Allen bolts, thick washers, nylocks/Loctite and haven't had problems myself. But I've seen many who did, esp. on gravel/washboard roads.

  15. #15
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    Quote Originally Posted by Crankycrank View Post
    If you've been using stainless you can see that most of the SS fasteners available at shops and hardware stores are much weaker than any of the non-SS available. Bolt Depot - Bolt Grade Markings and Strength Chart
    The stainless steel bolts shown in the link are AISI Grade 304 (aka 18/8 stainless) and are chosen for their corrosion resistance, not their strength.

    You can get stainless steel bolts made from "400-series" stainless alloys that are much stronger but give up some corrosion resistance. These are specialty items and not commonly found in bike shops or hardware stores.

  16. #16
    incazzare. lostarchitect's Avatar
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    I've never seen an M5 on a rack fail. I have seen several bent racks, though. I think most racks are weaker than the bolts.
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  17. #17
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    Quote Originally Posted by lostarchitect View Post
    I've never seen an M5 on a rack fail. I have seen several bent racks, though. I think most racks are weaker than the bolts.
    I've never seen a properly installed rack bolt fail either. A problem could arise if a fender stay was installed inboard of the rack leg. That would cantilever the rack load further out on the bolt.

  18. #18
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    Tradition.

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