Originally Posted by
Ironfish653
I think you’re grossly overestimating the force vectors involved here, and also underestimating the margin of difference in a flush joint -v- 1-2mm off spacers. A flush joint will support the hundreds of pounds you intend to carry, but a >2mm offset will cause the bolt to bend?
If I was installing this rack, I’d either use a couple brass crush washers to fill that gap, or an aluminum sleeve, beveled to match the profile of the fork. I could probably do it in about an hour, with the materials in my garage.
You could also upgrade the mounting bolt itself; instead of the Grade 5 SS typically used, you could get higher strength Grade 8, or source some NAS “Aerospace” grade hardware.
It's been a while since I studied, but I did take some structural engineering classes in college, and I've have no concerns about using a couple of brass crush washers to help conform to the non-flat surface and then a standoff to clear the fork and other bits. There will be two forces acting upon the bolt,
A) the bending force caused by the rack mounting point being offset from the fork mounting point. In this case, the lever arm - even at 8mm - is so short that 40# of downward force will not exceed the yield strength of the bolt.
B) the sheer force caused by the downward weight. Again, the forces at play here are so small that the bolt is not likely to sheer off.
Originally Posted by
smasha
I've got the Tubus Tara rack here, and playing around with it, the fit may not be as bad as I thought.
Pictured, below, is from the Tara installation manual. The red piece is an optional spacer/standoff, 8mm long x 8mm OD.
Based on this, an 8mm spacer with an 8mm OD should be fine... If it sits flush with the face around the eyelet.
So now I'm reconsidering a counterbore around the eyelet. I think that part of the dropout, below the weld, is solid. If that's the case, I don't think an 9-10mm ⌀ counterbore, about 2mm at the deepest point, would put me in any danger.
This would leave the rack close very the QR hardware, but not touching.
This would also put the rack a little off level, but I've seen much worse.
I'm not sure about adding braze-on eyelets now, as the flat-mount brake is kind of where I'd want to put eyelets, on that side. It may be feasible, it may not be.
As always, looking for constructive thoughts and opinions from everyone else here.
I have a Tubus Tara front rack and use those spacers without issue. I don't think that a counterbore is necessary. If anything, if the counterbore has any sharp edges - either at the bottom of the bore or at the top - that becomes a stress concentrator. A couple of soft brass crush washers should be sufficient.