Rack bolts bent - assume the worst?

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badams
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Rack bolts bent - assume the worst?

Postby badams » 19 Dec 2017, 1:15pm

I'm building up a new (second-hand) bike for my wife, mainly using parts from her existing one which is now in limbo after she somehow bent one of its forks almost completely straight. On disassembling the rear rack, I discovered that several of the mounting bolts were slightly bent. They wouldn't have been when I'd originally installed it, so I figure this must mean the rack has been overloaded (?) - and if so, that it might have been compromised, or at least that the same or worse could happen if I install it again. The 'or worse' option is the failure of the bolts when our child seat is on the back....

Has anyone else had this happen, and might there be any other reason for it other than overloading? I used stainless allen bolts, nothing special.
The rack which is a generic Bor Yueh alu one we've had for ages, on and off both our bikes, which we've kept mainly because it fits the child seat where no other sturdy-enough racks seem to, as the graveyard of almost-new racks in our shed attests to.

Roadster
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Re: Rack bolts bent - assume the worst?

Postby Roadster » 19 Dec 2017, 2:11pm

Do the rack stay and the mudguard stay both share the same eyelet on the rear drop out?
Is the rack stay mounted on the inside (next to the eyelet) or on the outside (next to the head of the mounting bolt)?
If the heavily loaded rack stay is fitted outside of the mudguard stay, it will exert more leverage on the shank of the bolt and bend it.

I didn't know there were any alloy racks capable of safely supporting a child seat...

nsew
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Joined: 14 Dec 2017, 12:38pm

Re: Rack bolts bent - assume the worst?

Postby nsew » 19 Dec 2017, 2:14pm

I’d be concerned about the receiving threads, they may need tapping. Have you run a good bolt through these to check? Rack bolts do work loose if unattended and this plus load would bend them a little. Has anyone else been using the bike and offering lifts?

badams
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Re: Rack bolts bent - assume the worst?

Postby badams » 19 Dec 2017, 2:22pm

Thanks both of you. In answer to Roadster's questions, the rack was mounted on its own eyelets - there are separate ones for the mudguards. We used to have a different rack, I'm fairly sure alloy too, which came with a child seat - a Beto I think - so I figure they are used for that purpose (in any case, if you try to look for steel racks to do the job, such as Tubus, they advise you against it as their tubes are hollow and can't withstand the clamping stresses apparently - what to do?)

Nsew, that's a good point. I'll do that just to check. One was less tight than I'd expected on removal, but the others all seemed pretty snug - and no, no-one else has used it.

With this all in mind, does anyone have a good suggestion for a rack which CAN handle a child seat? To complicate things, we use a GMG 911 seat, now sadly discontinued, which is great but turns out has pretty specific requirements in terms of the placement of the horizontal struts on the top of the rack and the width of the rack itself. I'd want a rack rated at 25kg at least, preferably more. The only contender I could think of which seems like it must be strong enough and isn't a weird shape / closed at the top is the Thorn expedition rack, or whatever it's called, but it does look like a piece of Meccano. Too bad the Tubus racks can't support a child seat, even if I could afford them.

Bowedw
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Re: Rack bolts bent - assume the worst?

Postby Bowedw » 19 Dec 2017, 2:22pm

Personally I would use hi tensile steel bolts as probably stronger than stainless and if possible put a washer and nut on the back. Depending on the frame and rack, it may be possible to drill and tap threads for larger diameter bolts. Only do this if the components have enough metal around the holes or you may increase the danger risk. it's not unusual to find that bolts under load bend a little, they may have stretched and gone a bit slack over time giving a bit of allowance to bend. Be careful of overtightening as well which could result in sudden failure down the line.
Handlebar stems often have the same diameter bolts and are usually steel and I have been tempted to replace with stainless, that is until the thought of the consequences of bolt failure comes to mind.

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horizon
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Re: Rack bolts bent - assume the worst?

Postby horizon » 19 Dec 2017, 2:46pm

I remember this quote from CJ's review of the Spa tourer but i suppose that bending is somewhat different from shearing off:

Kudos to Spa for 6mm threads in the bottom rear carrier eyes. They’re
much less prone to loosen and strip under the weight of a touring load. The
usual 5mm are adequate for the upper fixings and lighter front panniers.
I have two doctors, my left leg and my right leg. (G. M. Trevelyan)
PS I always wondered why the YHA HQ was called Trevelyan House. :)

badams
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Re: Rack bolts bent - assume the worst?

Postby badams » 19 Dec 2017, 2:49pm

Ah, that's interesting. I'm clueless with metallurgy so it's good to know that stainless is not as strong as hi-ten (as the name suggests I guess). When I sort the fork situation with the donor bike I'll look into tapping the threads out. I'm sure there will be room to do that, and it's a good idea, as the review you've quote, horizon, suggests. Many thanks.

Brucey
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Re: Rack bolts bent - assume the worst?

Postby Brucey » 19 Dec 2017, 3:15pm

if the rack is on separate eyes at the dropout, the chances are good that a bolt can be put through the dropout from the inside, without the head (filed down a bit if necessary) interfering with the chain etc. If the rack is then mounted with a well-tightened nylock nut on the outside, it is probably as secure as it needs to be.

The load in such a joint is transferred in good part by friction, and you only get friction if the fastener still has a decent tension in it. The most common causes of tension loss are

a) that there is paint on the dropout and/or
b) that the fastener starts to back out.

Once the bolt loses tension it sees a different load in such a joint and will bend more easily. If you have not removed the paint from the dropout, do so; powder coating in particular is notorious for slowly oozing out of such joints and allowing fasteners to lose tension.

I'd advise caution if you are thinking about drilling out to M6; both the rack and the frameset may not have enough wall thickness remaining for adequate strength.

I would also advise caution if you are thinking about using 12.9 grade capheads; these bolts are ostensibly stronger, but are also prone to corrosion, and tend to fail by brittle fracture. By contrast a stainless steel bolt will let you know if it is struggling by bending slightly, well before it is in any danger of actually breaking.

If you use a stainless steel bolt as described above (i.e. as a protruding stud, with a nut on the outside) the nut can safely be ('over'-) tightened several times in such a way that the bolt yields slightly, without any risk to the threads in the frame. Typical stainless bolts (with rolled threads) have a very generous elongation to failure.

Remember, the more tension there is in the bolt, the more friction in the joint there is, and the bending/shearing loads on the fastener itself are actually reduced.

If all else fails, there are plenty of other options including

1) that you make up some adapter plates (that bolt onto the dropouts in as many places as possible) and then the rack bolts on to the adapter plates.
2) that the bottom section of the rack leg is replaced by a length of thin-walled steel tube, that is bonded as a sleeve joint to the rack leg using epoxy resin, for a length of at least 1". The flattened end of the tube should be stronger and tougher than the aluminium part.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Brucey
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Re: Rack bolts bent - assume the worst?

Postby Brucey » 19 Dec 2017, 3:29pm

BTW an M5 bolt (8.8 or A2.70 stainless) will happily take at least 3/4 of a tonne in tension. Unless the joint has an exceptionally low coefficient of friction in shear, and/or the fastener loses tension, it won't be the bolt itself that is the weak link.

M6 bolts may be stronger, but not all racks will even accept them. Simply by pushing more the paint out of the way when they are tightened, M6 bolts provide an improvement, but this is no substitute for removing the paint from the joint (like you should do) beforehand.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

badams
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Re: Rack bolts bent - assume the worst?

Postby badams » 19 Dec 2017, 3:40pm

Thanks Brucey, that's all very useful info. And fascinating to read, too. Thanks.

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RickH
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Re: Rack bolts bent - assume the worst?

Postby RickH » 20 Dec 2017, 5:41pm

Roadster wrote:I didn't know there were any alloy racks capable of safely supporting a child seat...

When I got a Co-Pilot Limo child seat a few years ago it was supplied with an alloy Blackburn EX1 rack.

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Gattonero
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Re: Rack bolts bent - assume the worst?

Postby Gattonero » 24 Dec 2017, 11:49am

badams wrote:I'm building up a new (second-hand) bike for my wife, mainly using parts from her existing one which is now in limbo after she somehow bent one of its forks almost completely straight. On disassembling the rear rack, I discovered that several of the mounting bolts were slightly bent. They wouldn't have been when I'd originally installed it, so I figure this must mean the rack has been overloaded (?) - and if so, that it might have been compromised, or at least that the same or worse could happen if I install it again. The 'or worse' option is the failure of the bolts when our child seat is on the back....

Has anyone else had this happen, and might there be any other reason for it other than overloading? I used stainless allen bolts, nothing special.
The rack which is a generic Bor Yueh alu one we've had for ages, on and off both our bikes, which we've kept mainly because it fits the child seat where no other sturdy-enough racks seem to, as the graveyard of almost-new racks in our shed attests to.


In a nutshell: it all sounds like the bolts weren't good quality.
A class 10 bolt in M5 size is quite resistant to shear force, is more likely you will have the rack damaged (or at least, fatigued, as been some alluminium alloy) than the bolt shearing off.
Keep in mind that the brazed-on bosses are not made of hard steel, if the rack was often loaded with bolts kept loose, this does wear out the female thread and it's not a good situation.

To make things easy, use some medium-strength Loctite and class 10.9 bolts of correct length.
Very important also to check the rack struts and bottom mounts (the latter are less likely, tho) to join the braze-ons properly flat. Often I see struts that join the frame with an angle and this is not a good coupling for obvious reasons
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...