Gear Combo Guru
Join Date: May 2007
Bikes: Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
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I'm impressed that your brother was able to only drive over your crank and nothing else. You should check that this hasn't caused any problems anywhere else, especially in the bottom bracket area, including whether the BB axle is still straight.
Going from 52-42-30 to 50-39-30 will not require a change in FD, just move the FD down by a few mm to line up with the top of the new big chainring.
You could also go with a double chainring and still use the same derailleur without much trouble, but triples are more versatile, so I would stick with that.
The one problem that you might have is that many the new cranks are designed for a narrower 10-speed chain, so the rings are placed slightly closer together. If you use a wider 8-speed chain on them then the chain may rub on the outer rings when in the smaller cogs in the back. If you go for a Sora or Tiagra crankset, then those are at least designed for a 9-speed chain so this should reduce this problem.
Instead of replacing the whole crankset and chainrings, you might consider just replacing the one crank that got bent, if that is the only damage. You probably have a traditional square-taper crank. Left-hand cranks should be pretty cheap, and right hand cranks are also not bad, and you can just put your existing chainrings onto the new crank. However, if your rings are well worn anyway then a complete new crankset might be the best way to go.