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?

https://cimg6.ibsrv.net/gimg/bikefor...abffbb57f8.jpg
Right side with spacer, no problems
https://cimg5.ibsrv.net/gimg/bikefor...214808802b.jpg
Left side, bigger gap (and the zip ties have lasted longer than the bolts)

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?

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.

Are you using grade 8 bolts.
Check them frequently for looseness?

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.

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

Sure, it's an older Planet Bike KOKO rack (non disk), and I have the exact same rack on my steel bike

https://cimg6.ibsrv.net/gimg/bikefor...eb6a350fe1.jpg

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.

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."

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

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.

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.

https://cimg3.ibsrv.net/gimg/bikefor...8376f1e3e1.jpg

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

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?

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?

...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.

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... :foo:

https://www.laxzo.com/ekmps/shops/js...le-20132-p.jpg

https://www.laxzo.com/bicycle-bike-a...le-20132-p.asp

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.

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.

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.

I think it would make servicing the wheel (e.g. for a flat) harder. Also, the skewer does not have as much of a cross section as many bolts.

So, first, pretty much all steels have similar stiffness, as defined by the modulus of elasticity. That is, how much does the steel deform (fractionally) per unit of stress applied.

But softness? I think that might probably relate to yield strength. That is, the amount of stress that can be applied before deformation takes place. That varies between alloys, and even between the same alloy in different states of heat treatment. Generally if you go to higher tensile strength you'll have higher yield strength to resist bending. And the stress that the bolt can take repeatedly without breaking (fatigue point) is higher too.

The right alloy, heat treated, will take more stress before deforming or breaking. SAE grade 8 bolts are alloy steel bolts heat treated to give a minimum of 150,000 psi tensile strength (proofed to 120,000). This is similar to metric grade 10.9. There's an even higher-strength standard for metric bolts, that is 12.9. That's supposed to be around 175,000psi tensile strength! A key point is that a lot of the generic fastener hardware you get at Home Depot is not made to these standards, and is made in China with leftover bean curd and rice husks. And even if the fastener has the right markings to indicate grade 8 or 10.9, it may not be.

Get SS bolts. Where did you get the idea that they were susceptible to brittle fracture?

I'm actually surprised that the hollow skewer-type axles hold up under all the weight and impacts like they do. I have had to replace a couple of bent ones, but never had one come in broken. Back in the day - when I was a kid with a paper route my rear carrier mounted to the rear axle. I would sling two heavy canvas newspaper bags over the rear carrier. No problem with the carrier mounts, but the heavy load would snap the axle inside the hub 2 or 3 times a summer. I kept a spare part on hand and got pretty good at rebuilding the coaster brake assembly.

For bending, there are engineering reports indicating the hollow tube is nearly as strong as a solid bar.

A lot more of the strength of the QR axle vs solid axle depends on the materials and heat treating.

I have broken one QR skewer. It was an older Campy NR skewer. I was towing a trailer (chainstay hitch). But, moving the trailer around a bike rack, somehow I caught the skewer clamp end, and twisted it while clamped tight, breaking the skewer.

Otherwise, so far no problems with the skewers, including some trailer towing using a skewer hitch.