Bolt size question
 

I was listening to a podcast about bike touring and the fellow,a bike mechanic, said he changed the holes in the rear drop out to M6 vs. M5 as this is a common weak point.

Thoughts?

Which holes are you referring to, the eyelets for rack and mudguard mounting? Some dropouts have M6 eyelets out of the box.

The typical reason why rack bolts fail is from fatigue. A rack flexes with pedal strokes and road surface shocks. If the rack isn't well braced (and many would be surprised at the number of racks with no lateral triangulation) this flex will be worse. If the rack bolts are doing double duty, like fender brace attachment too, then again the bolt's flexing will be worse. If the bolt is allowed to become loose the flex will be greater. And in time the paper clip breaks, so to speak. Increasing the bolt's diameter by about 20% sure extends the life span of poorly dealt with bolts. Note that last bit "poorly dealt with". because for most people's touring a well engineered rack using M5 bolts that are kept tight and don't do any other job are quite strong enough.

it's when the tour is over lot's of unimproved/unpaved roads and the pack loads are really high (think 75+ lbs) and the consequence of a broken rack/bolt is possible unhealthy (think third/forth world countries, deserts and such) that going to a M6 bolt is really a smart idea.

In my shop experience (and of course my personal touring which last time had a bike and load weight of over 110 lbs) the number of broken rack bolts has been less then the number of eyes damaged by loose bolts, by over tightening or by using a miss matched bolt/screw (like dry wall screws:) ). I even have heard of eyelets breaking off the drop outs from too much flex and stress. On my first Scotland tour the Eclipse front low rider rack strut broke, after it had seen thousands of loaded miles over the 15+ years of it's being in use. But bolt breakage is not too common IME.

Still were I to make a third world touring bike I would strongly consider going with M6 bolts and their bigger eyes and bosses. But those dreams are in my past so M5s are fine for me. Andy (who for his touring bikes followed the one bolt does one job policy)

As long as the bolts are tight, 5mm is more than enough. I imagine if you have them cantilevered out to hold up fenders as well as a rack and let them get a little loose 5mm might not be enough. I recommend not letting them get loose.

When I had a rack and fenders on the same eyelet, I ran the bolt from the inside, put the rack on first, used a nut, then a washer then the fenders, then a washer and then another nut. Seemed to work well and there was much less chance the bolt would loosen up and get hammered by the rack.

I agree with Eric. We see quite a number of rack and fender equipped bikes at the shop and many that have only one eyelet per side to use. Most have the fender braces inside the rack's. Most of these also have no large (yes, a "fender" washer) washer on both sides of the common "V" bent fender brace. So not only does the single bolt see the rack's vertical loads far from the eye (thus a bending instead of a shear stress) but the fender brace can open up and cause the bolt to not snug the rack up as much. I believe this is done because of a few reasons. First critical thinking and basic mechanical concepts are not well taught any longer in school. Second most can't or won't figure out how to bend the fender brace so it wraps around the rack brace on it's way to the top of the rear fender. And third is that bike accessory companies rarely work together to make more then their product fit well in many situations. A simple solution would be if rack manufactures included a fender eye behind and to the rear of their rear most brace. That Eclipse low rider rack I mentioned included this (yes on a front rack) way back in the late 1970s. But I suppose some rack brand's liability lawyer would not like this... Andy

Thanks for your replies!
I like the idea of running the bolt from the inside. Going to take a look at our racks before our next tour from Wyoming back to Ohio.

Wouldn't hurt to use some blue threadlocker

Blue will clash with my frame. I have to find another color!😁😁

You aren't supposed to see the threadlocker.

Red will probably work fine in this application

Wow, one of my favorite issues. Another reason rack bolts fail is when tourists ride off road and the jolting when you hit bumps can shear water bottle cage bolts right off. When the bolts shear off you are faced with what could be a difficult task (out in the field), of removing the broken piece in the eyelet. My solution is to mount the bolts from the drop out inside and attach nuts from outside the rack, so when they should shear off you just need to unthread them with the still intact allen head, and replace. You may need to use a button head type of bolt on the right side, so the chain avoids jamming in the small cog.

The load limit in shear of a 5mm bolt is over 1000 pounds. The fatigue life of a bolt is reduced markedly if the bolt is improperly preloaded, i.e. it's loose.

I just did this on a transportation bicycle I'm putting back together for when my daughter visits. I hadn't thought of doing it this way before. My brain was just programmed to screw in the bolt from the outside. I did exactly as Eric said, I screwed the bolt in the dropout with the head on the inside. After I put the rack on, I tightened it up with a nut, then washers on both sides of the fender struts and finally a nylon insert lock nut on the outside. An added bonus was that it was easier to assemble the various units with the head on the inside. I could put the struts on the end of the bolts already sticking out instead of putting the bolt through the struts 1st and then trying to locate the threaded hole with the interference of the weight of the accessories. I am sure it is much less likely to come loose doing it this way.

This transportation frame has had an interesting history. I made it as an example of the kind of frame we wanted the XB3 bicycle company in Kharkov, Ukraine (near the Russian border) to make for our charity bicycle project. They used to make the majority of bicycles for the former USSR. It then spent time in the city of Cherkassy, Ukraine at the home of one of the owners of the laser cutting company that makes our main triangle fixtures. Eventually I brought it back to the States with me where it was stolen one rainy night when I hadn't brought it back inside. One of my framebuilding class students spotted riding by while he was waiting at a red light. Eventually the guy was arrested and I got it back. I repaired the repainted the damaged paint and now I am putting it back together again with the bolts in the dropout eyelets pointing out instead of in.

It's great you got your bike back.

The other bonus of doing it inside out is that you don't have to trim the bolt so carefully, it's not going to run into anything like it will if it's a little too long and the head is on the outside.

The other bonus is that if for whatever reason you lose the threads cut into the frame (I'm always nervous about threads cut into permanent fixtures) you can just throw a nut on the outside.