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Old 10-19-10, 06:30 PM   #1
whiteroad
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CRank Sizes?

Good day to all fellow cyclist, i would like to get some help or better say information on the following issue, i own a 30/42/52T 170ml crankset that is bent and needs replacement, my question is the following the shifter is for triple can i integrate a double into the shifter. also can chain ring tooth change say to 50t - 39t - 30t and still work whether its triple or double, also will it matter that i use downtube shifters am just trying to see what my options are, my bike its an 8 speed with sunrace r90 grupset that work and shifts flawlessly besides my brother runing over the crank with the car thanks god it was only the crank
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Old 10-19-10, 09:09 PM   #2
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I think what crankset and chainrings you can use depends on the front derailleur, your shifters and the bottom bracket.

You have to worry about the front derailleur handling the different gears, the chainline (distance between the bottom bracket and the position of the chain), and the shifters. When going from a triple to a double I think the shifter is less of an issue, because you can usually adjust it so that both gears are usable, but getting all three chainrings to shift properly on a triple is harder if the parts aren't well-suited-to-each-other.

I went from 52/42/30, to 44/32/22 on my bike. But this was fine for my derailleur (it does 22 tooth difference from big to small and 12 tooth difference from big to middle) but too insane for my shifters, and I had to switch to non-indexed shifters for the front.

Downtube shifters make things easier because they are non-indexed (at least in the front, I think it is possible to have then indexed for the rear) so you can fiddle with the derailleur and set it into whatever position works, whereas with indexed shifters you're stuck with the three positions the shifter is designed with (more or less).
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Old 10-20-10, 12:26 AM   #3
Chris_W
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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.
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Old 10-20-10, 05:20 PM   #4
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thank you all for the information its exactly what i needed and my bike was backed into as it was mounted on my car and not laying in the driveway thanks for the info
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