Join Date: May 2004
Location: Wilmington, DE
Bikes: 2016 Hong Fu FM-079-F, 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2016 Islabikes Beinn 20 (son's)
Mentioned: 6 Post(s)
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Quoted: 541 Post(s)
Some Shimano cranks use offset chainrings where the teeth are in a different plane than the mounting tabs. In those cases, you are far better off using the same Shimano ring for replacement otherwise you will likely end up with shifting problems. The 105 7 speed crank, from memory, is not one of those cranks though. I am not certain about either of the two chainrings you've asked about. If you can sit them side by side with your current ring and confirm the offset either is not present or matches your current ring (I might be wrong about the offset) then I don't see an issue with using one of those rings. Your 7 speed chain should be fine as well.
Assuming your ring is flat though, you could use a Sugino (or Salsa, or Vuelta) flat chainring and get the 38T size you prefer. See here: http://www.universalcycles.com/shopp...&category=3160
The lack of ramps or pins means nothing on an inner chainring. Your current ring shouldn't have any. Any of the above manufacturer's rings should be cheaper than Shimano too.
With all that said, you'll get a far bigger boost in gearing going with a wider MTB cassette, or my preference, a triple. You could even combine both if your knees/terrain called for it. If your bike has RSX brifters, they should be compatible with a triple crank. If you have friction shifters, you are definitely good to go. If you'd like to explore either path, just say so and we can help you through the swap.