Niles H.

Eyelets sometimes break, especially with heavy loads over long distances on rough roads and trails.

What are some approaches to ensuring maximum strength for these eyelets?

What are the strongest available eyelets?

Butterthebean

I don't think I've ever seen an eyelet break. Now, I can see the eyelet break off if not brazed on properly. Before brazing on the eyelet, file a flat on one side of the eyelet to give more surface contact area.

Nova has these heavy duty eyelets. I suspect that your rack mounting hardware will break before these eyelets will break.

Live Wire 

These are the strongest rack mounts out there https://www.novacycles.com/catalog/pr...851950b1e91f92
I've used them on a couple of mtb frames intended for ultra-long races (think GDR) and off road tours where you have to carry everything. Obviously, these are for mounting on the stays and not the drops- good for getting enough rack clearance over a 29" wheel.

NoReg

It depends whether you want to use regular size ones, or go all custom, or even bypass conventional mounting systems altogether.

Probably the simplest way of getting stronger eyelet, would be to go to a better quality drop. Most drops are cast, which is very low quality at best, find someone who laser cuts their drops, and it should be a big leap forward, or go to a better quality supplier, like Paragon (haven't personally tried their stuff) but they are legend, if not in the touring field). If you are just making a bike for yourself, it isn't too big a hassle to make your own drops. Another option is to just drill and thread a piece of bar, and then cut a shallow hollow in a beefy drop, and braze the barrel nut in place.

It isn't that uncommon to break the eyelets, to the extent that among super long distance cyclists the use of eyelets based on 6mm bolts is an occasionally heard recommended upgrade. This takes bolt breakage out of the picture also. Here is a typical statement on the MEC.ca website:

"1 Millimetre
We started threading our bikes' braze-ons to fit them with 6mm bolts rather than the standard 5mm. Since then, our bolts have stopped breaking. We also use heavy duty galvanised steel collars to attach the top part of the racks to the seat stays. We find it easier to replace a broken collar than to extract a decapitated bolt from an eyelet."

The p-collar recommendation segues nicely into the idea of alternative attachment methods. At one extreme you have the idea of welding directly to the frame, as with Tout Terrain (an old idea though). On the front, some people drill through the fork blade, into the meat of the axle plate, and braze in a water bottle mount. Others braze the upper mounts low on the fork blades. More radical options would be to sort out completely different methods that might rely on beefy fittings. These kinds of options mean custom frames and racks. If one goes for stuff like that it is probably better to also have the conventional eyelets, just in case.

northboundtrain

I've been making my own drop-outs from 3/16" 4130. I just use the drill press, bandsaw, and grinder. If I want eyelets, then I integrate them into the drop-outs and go with 6mm x 1.00mm holes instead of the more common 5mm x .80mm.

unterhausen

where do you get 4130 sheet?

NoReg

https://www.aircraftspruce.com/catalo.../4130sheet.php

Though I asume that considering most drops are some kind of casting, one could make drops out of CR also, if one stuck to the beefier sections. 3/16 4130 would be nice for dressing with stainless faces.

JohnDThompson 

Even better, take a round file and file the dropout to accept the eyelet. You can get contact over half the circumference of the eyelet that way, plus once painted, it will look like the dropout was forged with the eyelets in place.

NoReg

"I don't know if eyelet failure is considered a common problem in loaded touring circles."

It does happen though failure of the bolts is more comon. I can remember some threads. I don't think one should expect failures in normal light loaded touring use, but there is a wide range of use.