Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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Almost all modern spindles share the same taper, but the starting width at the end varies. That means that while the taper fit is OK, mis-matched cranks and spindles will have different depths of engagement. On some the spindle will come all the way through the arm and cannot be tightened, on others the opposite where the spindle will only enter the crank to less than half the depth.
Ideally you want the spindle to reach to within 2-3mm of the end of the taper, allowing maximum contact area, while leaving room to be drawn on as they tighten.
If you're having constant issues of loose cranks on well matched parts, I suspect the mechanic and rider both contributed. Odds are the crank was never properly tightened onto the taper, and continued to be ridden after it began to loosen. It's also possible that the crank was shaved by the forward edge of the spindle during installation. I've seen many spindles with sharp corners at the leading edge, and make it a policy to chamfer all four sides with a grinder or belt sander before using. Spindles from bigger companies rarely need this step and it's done in production.
In summary, get a properly matched spindle and crank, install it to proper tightness, and ask the rider to notify you at the first sign of looseness, evidenced by clicking once or twice per crank revolution.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions”
- Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance