# Motorcycle Case Trunk Mount



## CalgaryPT (Jan 13, 2018)

I don't ride but a friend of mine is hard core and travels to Alaska and NWT on his bike. His rear luggage case mount broke (stress fracture). The design of it seemed flawed to both of us. In an effort to allow maximum flexibility for straps and other accessories (as well as reduce weight), all the mounting holes and cutouts reduced the metal at its key points, leaving a lot of the case over hanging the rear of the bike with lots of weight on it. Plus, the mounting holes line (sadly) up exactly with the stress points on the design, which is exactly where it broke. Duhh. Field testing should have caught this methinks.

This is the replacement mount I built. Original was 0.125, but some lower weight alloy that seemed not as magnetic as you'd think. It sure wasn't stainless.  Replacement is 0.1875 A36 with angle iron reinforcement bracing. If you measure the cutouts WRT to the length, almost half the strength of the original plate is lost. Granted, our new design is heavier (6 lbs 1 oz vs. 3 lbs 8 oz), but we kept almost all the metal at key stress points.

If I had to do over, which I may, I'd modify my clamping procedure and welding order on the angle iron. I erroneously welded on the ends first which caused warping in the plate in excess of 0.125. Had I welded the centers first, some of this could have been avoided. I don't think it will effect the mounting enough to have to redo it, but I felt dumb for not thinking this through first.

On a related note, because I don't have a heavy knee mill like many of you, I can only countersink using a drill press or hand drill. Like others I can never get a smooth countersink this way. I've tried changing speeds, different flutes on the countersinks, light pressure vs high pressure, RapidTap vs no fluid, horizontal vs. vertical attack, etc. All result in a choppy/patterned countersink. So if anyone has other tips, please let me know.


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## Dabbler (Jan 13, 2018)

It is faster to use a 6X48 belt sander to do the chamfers, using a 20 or 40 grit belt.


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## PeterT (Jan 13, 2018)

Supposedly these euro odd number 3 flute countersinks are good but I haven't tried myself. They get spendy in the bigger sizes.
http://www.sowatool.com/Product/4/262/2888

I know exactly what you mean though. I have some decent 6-flute USA brand that cut well most of the time. Then every once in a while they develop kind of a harmonic, sometimes you can feel a slight buzz. then they develop that knurly looking pattern vs. a shiny conical cut. I think it is a speed feed combo thing, at least for me. If you either back off the cutting pressure or over-feed it too hard,  it can start the pattern willies. The annular cutters I use on the mill are exact same deal, you have to feed very careful on certain materials. Usually when I stop to shoot some cutting fluid in there, you kind of have to re-enter the cut with some down feed vs. just kissing the bottom.

Try & see if you get better results on mill with table locked & tight vise vs. kind of free floating material on drill press. It might be favoring more solid setup, or better chuck run-out etc. Stainless can be a bugger for work hardening with any kind of machining, that I know.


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## CalgaryPT (Jan 13, 2018)

Will try all your suggestions. Thanks.


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## John Conroy (Jan 14, 2018)

I have had good results from these.

https://www.kbctools.ca/products/CUTTING TOOLS/DRILLS/CENTER DRILLS/141.aspx


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## CalgaryPT (Jan 14, 2018)

Thanks John...will look at these too.


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## PeterT (Jan 14, 2018)

Good point John. I don't know why I had in my mind they don't come in 82-deg for flat head screws, but they do. Maybe the pilot holes don't match clearance holes? I'll have to check on why I overlooked them.


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## CalgaryPT (Jan 15, 2018)

Hummm. I have no doubt these will work better. I do have some smaller center drills I've used before. The complication I face is that I use an ironworker to punch most of my holes now so I don't use a pilot hole. Maybe I'll try chucking the countersink/center drill in my mag drill next time to reduce the rattle factor and see it that helps.....

Thanks everyone.


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