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Keep breaking rack bolts

Old 03-11-20, 11:25 AM
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arsprod 
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Keep breaking rack bolts

I've now broken left side rack bolts three times on my aluminum winter commuter. The bolts are the black steel ones and before I drill out the piece that's in there and bolt it again I'm wondering if there's a better way to mount the rack. I have the exact same rack on my regular commuter/touring bike but it's steel and this isn't an issue. I was looking at the Tubus QR adapter but that costs more than the rack. I carry panniers on both sides, with clothes and laptop I'm guess it's still less than 25 lbs. Thoughts?


Right side with spacer, no problems

Left side, bigger gap (and the zip ties have lasted longer than the bolts)
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Old 03-11-20, 11:32 AM
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The spacer causes bending loads in the bolt. The bolt has much less fatigue resistance that way. Find a way to mount the rack stay flush with the frame. That will keep the bolt loaded mostly in shear.

And the corrosive environment doesn't help. Maybe also switch to stainless steel bolts.

Last edited by AnkleWork; 03-11-20 at 11:38 AM.
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Old 03-11-20, 11:56 AM
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Originally Posted by AnkleWork View Post
The spacer causes bending loads in the bolt. The bolt has much less fatigue resistance that way. Find a way to mount the rack stay flush with the frame. That will keep the bolt loaded mostly in shear.

And the corrosive environment doesn't help. Maybe also switch to stainless steel bolts.
Understood - isn't stainless more brittle?
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Old 03-11-20, 12:05 PM
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Agree that bending forces contribute to broken bolts. There are disk brake specific racks that have a built in "stand off" to provide strut/caliper clearances. These reduce the bending forces and make the bolt work in shear more then a bending way. While SS can be weaker then alloy steel it's when corrosion resistance is needed that SS is best. Although I admit to having to deal with fewer "rusted in place" rack bolts then "allowed to be loose and bent till failure" bolts. Having both happen at the same time really suggests that the bike's maintenance is lacking, for the manor the bike is used.

25 lbs on a rack 5 or 6 days a week for months/years adds up to a lot of stress cycles and not a small stress at that. Can you mention the rack brand/model and whether it has any triangulation in it's lateral structure. (As example some racks have a pair of struts angle inward to the top to better brace against side sway). I see bikes with flexy racks and big loads too often. A flexy race will stress the mounting hardware worse then a stiff rack does.
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Old 03-11-20, 12:07 PM
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Are you using grade 8 bolts.
Check them frequently for looseness?
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Old 03-11-20, 12:07 PM
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Originally Posted by arsprod View Post
Understood - isn't stainless more brittle?
Impossible to say in general -- you'd have to compare specific alloys & treatments. Most bicycle stainless bolts are Austenitic and not particularly brittle. Many black steel allen bolts are through-hardened for use in tooling, etc. and could experience brittle fracture under extreme impact (you would see the fracture plane rotated ~45 degrees). Instead, it appears that you are dealing with fatigue failure possibly accelerated by corrosion.

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Old 03-11-20, 12:18 PM
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I see far more problems (not just with racks) that have the rider's lack of doing periodic maintenance as the root issue that if done differently (as in taken better care of their bikes) would have avoided the problems either all together or at a rate that's only wear related.

We have a term for the rider's not noticing a budding problem, they only know of it when something stops working as it should. JRA, as in I was just riding along.

The point some of try to make often here is that a bike that's paid attention to and gets periodic cleaning/servicing is a far more reliable, safe and fun bike then one that's only dealt with when something breaks. Andy
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Old 03-11-20, 12:22 PM
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Originally Posted by Andrew R Stewart View Post
Agree that bending forces contribute to broken bolts. There are disk brake specific racks that have a built in "stand off" to provide strut/caliper clearances. These reduce the bending forces and make the bolt work in shear more then a bending way. While SS can be weaker then alloy steel it's when corrosion resistance is needed that SS is best. Although I admit to having to deal with fewer "rusted in place" rack bolts then "allowed to be loose and bent till failure" bolts. Having both happen at the same time really suggests that the bike's maintenance is lacking, for the manor the bike is used.

25 lbs on a rack 5 or 6 days a week for months/years adds up to a lot of stress cycles and not a small stress at that. Can you mention the rack brand/model and whether it has any triangulation in it's lateral structure. (As example some racks have a pair of struts angle inward to the top to better brace against side sway). I see bikes with flexy racks and big loads too often. A flexy race will stress the mounting hardware worse then a stiff rack does.
Sure, it's an older Planet Bike KOKO rack (non disk), and I have the exact same rack on my steel bike

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Old 03-11-20, 12:34 PM
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If more brute strength is required, tapping the frame to a larger bolt size is an option. In any case a straight force-path is best.

BTW it doesn't look like the bolt was loose.
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Old 03-11-20, 12:41 PM
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I'd think it sheared from being too soft a bolt as opposed to too hard and brittle. In this small size it's common to find ungraded bolts and most soft and cheap. You might find some graded bolts in good selection at the auto parts store hanging up on a display rack.
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Old 03-11-20, 12:51 PM
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Originally Posted by Iride01 View Post
I'd think it sheared from being too soft a bolt as opposed to too hard and brittle. In this small size it's common to find ungraded bolts and most soft and cheap. You might find some graded bolts in good selection at the auto parts store hanging up on a display rack.
Hmmm, I've never seen "softness" mentioned as an engineering property for bolt materials. Perhaps you seek the property called "strength."
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Old 03-11-20, 01:10 PM
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Originally Posted by AnkleWork View Post
If more brute strength is required, tapping the frame to a larger bolt size is an option. In any case a straight force-path is best.

BTW it doesn't look like the bolt was loose.
Referring to your post about maintenance I admit I can get lax, but I'm pretty sure you're right and this wasn't loose. It was only on there for about a month and I used loctite! I'm getting the general sense it's the stress related to length of bolt and I'm not sure harder bolts are the long term answer. Then again, it's almost spring and I'm more than ready to get off this studded tire bike
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Old 03-11-20, 01:42 PM
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Let's back up: why are the spacers necessary on this bike?
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Old 03-11-20, 03:12 PM
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Originally Posted by ThermionicScott View Post
Let's back up: why are the spacers necessary on this bike?
+1, I would suggest flexing/bending the rack stay to eliminate the need for the spacer, which is acting as a lever and adding a bending moment. You might need to bend it further up.to bring the end close then bend the end parallel to the dropout. Then the bolt will be loaded in shear and will be much stronger. Use grease as a minimum, anti-corrosion agent like Tef-Gel preferably, on the threads to avoid the bolt seizing. Properly-designed and torqued bolted joints generally should not require use of thread lockers.
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Old 03-11-20, 03:51 PM
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Why are you using spacers?
This is my recently purchased Topeak 30kg rack mounted on my 87 Rockhopper. NO SPACERS! I'll carry five 12 packs of soda at times.
I'd inadvertently purchased the disc brake version which have spacers built in to the rack.


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Old 03-11-20, 04:22 PM
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arsprod,
Have you considered using a larger diameter spacer for your rack? The larger outside diameter of the spacer gives the rack a bit more support and lessens the flexing of the bolt in its fixture. More of the stress is transferred to the frame over a slightly larger space than the thin one pictured.
Admittedly, there is not much frame space to work with and the amount of stress from the rack is higher because of the bolt size, but a larger area for the rack to grip is an advantage. Just my thoughts, MH
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Old 03-11-20, 04:38 PM
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Originally Posted by Bill Kapaun View Post
Why are you using spacers?
This is my recently purchased Topeak 30kg rack mounted on my 87 Rockhopper. NO SPACERS! I'll carry five 12 packs of soda at times.
I'd inadvertently purchased the disc brake version which have spacers built in to the rack.
Disk brakes
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Old 03-11-20, 05:07 PM
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Originally Posted by arsprod View Post
Disk brakes
Totally missed that- Duh!

My disc rack had the spacers welded to the rack.
Your problem is exacerbated by loose spacers.
You are using Grade 8 bolts aren't you?
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Old 03-11-20, 06:47 PM
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Originally Posted by AnkleWork View Post
Hmmm, I've never seen "softness" mentioned as an engineering property for bolt materials. Perhaps you seek the property called "strength."
Didn't know I had to use engineering lingo here. This isn't going to be like the discussion going on in the nutrition forum because someone called corona virus a flu is it?
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Old 03-11-20, 08:15 PM
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Originally Posted by Iride01 View Post
Originally Posted by AnkleWork View Post
Hmmm, I've never seen "softness" mentioned as an engineering property for bolt materials. Perhaps you seek the property called "strength."
Didn't know I had to use engineering lingo here. This isn't going to be like the discussion going on in the nutrition forum because someone called corona virus a flu is it?
OK, non-engineering: you may as well have referred to the color of the aura of the bolt being wrong. "Soft" just isn't meaningful in any lingo. And to remove any incorrect impression I may have conveyed, you have my permission to not have to do anything. Happy?

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Old 03-11-20, 08:39 PM
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Originally Posted by AnkleWork View Post
If more brute strength is required, tapping the frame to a larger bolt size is an option. In any case a straight force-path is best.
...if this were me, and I was experiencing this problem, I would probably try this first. I would also probably through bolt the larger replacement, and nut it on the inside of the dropout, if possible. If I could find a bolt I could adapt so that the part going through the dropout was not threaded, but only the end where I nutted it was, I would do that, too. Finally, I might spooge in some kind of anti-seize in the hope that the sacrificial metals in it might preserve my bolt for longer service.

Doing all this will require some fiddling, but you're gonna have to fiddle with it to drill out that bolt remainder anyway.
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Old 03-11-20, 09:41 PM
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For the vintage style on top of seat stay brakes, why not try a disc specific rack.

I'm not quite sure what is out there...



https://www.laxzo.com/bicycle-bike-a...le-20132-p.asp
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Old 03-11-20, 09:49 PM
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Originally Posted by arsprod View Post
Referring to your post about maintenance I admit I can get lax, but I'm pretty sure you're right and this wasn't loose. It was only on there for about a month and I used loctite! I'm getting the general sense it's the stress related to length of bolt and I'm not sure harder bolts are the long term answer. Then again, it's almost spring and I'm more than ready to get off this studded tire bike
They are a lot better answer than you are getting now.
Grade 8 bolts have a much higher shear strength than lesser grades.
That's why you never see them recommended in "shear pin" use such as in snow blower & roto tiller blades.
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Old 03-12-20, 07:08 AM
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Originally Posted by 3alarmer View Post
...if this were me, and I was experiencing this problem, I would probably try this first. I would also probably through bolt the larger replacement, and nut it on the inside of the dropout, if possible. If I could find a bolt I could adapt so that the part going through the dropout was not threaded, but only the end where I nutted it was, I would do that, too. Finally, I might spooge in some kind of anti-seize in the hope that the sacrificial metals in it might preserve my bolt for longer service.

Doing all this will require some fiddling, but you're gonna have to fiddle with it to drill out that bolt remainder anyway.
I drilled out the old one last night and had to retap the hole (note to self, don't use loctite). Unfortunately, there's really no room behind the stay for much of a nut.

I'm curious why more racks don't use the wheel skewer? Seems like a pretty robust piece.
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Old 03-12-20, 07:44 AM
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Originally Posted by arsprod View Post
I'm curious why more racks don't use the wheel skewer? Seems like a pretty robust piece.
This is really only an issue with first generation disc brakes that were mounted on the seatstays. The issue didn't occur with rim brakes or current discs mounted on the chainstays.

I do like axle/skewer mounted trailer hitches.

However, I would imagine the rack mounted on the axle would be a hassle if there were better options available.
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