Upgrading the rack bolt to 1/4 20

[zerodish]

Forks generally have a hole drilled through them for a caliper brake even if they have brake bosses or disk brakes. I put a rear rack on the front. If you are going to do this some of them are more level if you install them backwards. The problem I had is the hole is drilled through a 1 1/8 tube. The rack strut was pivoting around the tube giving the rack more sway than I would like. I found some 1/4 20 bolts in the road and was surprised they fit in the hole even on the raised portions without threads. The bolt I was using is 6mm. Ace True Value Lowes and Home Depot have selections of single bolts and even single allen wrenches. Caliper brakes are a high stress part and the bolt off of them should be strong enough for a rack. I could have tightened the bolt harder but that risked breaking the bolt which would drop the rack on the road causing me to crash or worse damaging my computer. I found a 1/4 20 grade 8 bolt with a flange built into the head and 1 1/4 unthreaded portion. Grade 8 bolts are more than using a high strength steel they are carefull to reduce stress risers on the threads and head. Go ahead and buy grade 8 nuts if you want. I chose to use the ones I found which are soft but will strip before I can tighten the bolt enough to break it. I'm going to use the aluminum part from the caliper brake that is curved and fits against the tube but this is going to require much filing and trial and error. I have always used stainless bolts for the dropout eyelets. If you have to use the bottom eyelets on the fork you will eventually bend the bolts. Install the bolt head in towards the wheel and put the rack eyelets on the threads on the opposite side of the eyelets. That way if the bolt breaks you will be able to unscrew it. The metric equivalent to grade 8 is 10.9 but these are harder to find. Bolts stronger than grade 8 or 10.9 are subject to hydrogen embrittlement and are very expnsive. I'm paranoid and the rack also has a shoestring around it in case the bolt breaks. The rear rack has 3 struts.   

[canalligators]

In my experience, a bolt getting loose and falling out was a far more frequent problem than a bolt breaking. Yes, if it appears to be a high stress location like the top mount of a rack, I might go to a larger bolt - if there’s enough metal around it. But what I do with almost all the fasteners on my bike, is to replace them with stainless steel. Rust has been a bigger problem to me. But since most stainless steels have a lower tensile strength than even mild carbon steel, I don’t use it on high stress parts like seatpost bolts or recumbent frame clamps.

By the way, I use blue Loctite on almost every fastener on the bike. It has eliminated bolts coming loose.

I also prefer metric; if I’m tapping a new hole, I will generally go to a metric size, i.e. 6 mm instead of 1/4.  The exception would be if a 6 mm strips out; then go just a hair larger with a quarter inch.