Member Question: Chainring Replacement

author : FitWerx

comments : 0

Is gear overlap from the big to small ring a consideration? Will the ratio increment of the compact setup be very beneficial to my riding?

Question from blackmelas
I'm getting ready to replace my 53/39 chainrings which I currently run with 27-12 cassette (mostly flat where I ride with a few steep hills in my area and with 250m to 500m ascent on most of my rides). I've been reading a little about compact cranks and thought about upgrading from Shimano 105 cranks to Ultegra 50/34 and a 23-11 cassette. The gear ratios are very similar for both options, though the overlap in ratios from the big to little chainring will be much less with the compact crankset.

Is gear overlap from the big to small ring a consideration (currently when shifting from my big to small ring, I will up shift two or three gears in the rear but with a compact it could be 5 in the rear)? Will the ratio increment of the compact setup be very beneficial to my riding? What other pros and cons are there to the compact setup? Lastly I am running SRAM force derailleurs. Will these be OK with the compact chainrings?
 
Answer
This is a great question. First, you are correct in that the 50/34 chainrings paired with the 23-11 cassette will give you a wide range of gearing options with less overlap of gears compared to your existing 53/39 and 27-12 cassette. There are pros and cons of having less overlap of gears. 
 
Less overlap can be beneficial when you’re ascending a hill and will have more gears to choose from. This will allow you to shift more often and keep your cadence in a tighter range. Less gearing overlap is also beneficial when you’re descending a hill or pedaling on a downhill grade and will have more gears to choose from. 
 
The downside of less gearing overlap occurs when you’re spending a great deal of time on flats or in the 17-23 mph range. This is where you may find it inconvenient to have to shift back and forth between the chainrings more frequently than with your existing setup. Every chainring shift will usually be combined with the need to shift 3-4 cogs in the rear cassette as well. (Picture spinning out in the 34 chainring, but shifting over to the 50 chainring will require a quick 3-4 shifts in order to maintain cadence. Shortly after, you need to shift back for the same reason). 
 
Our usual recommendation is to combine the 50/34 compact crankset with a 25-12 or 27-12 cassette. Adding an extra 1-2 overlapping gears will greatly reduce the shifts between chainrings while riding on flatter terrain. Of course if you live and ride in a hilly area, then choosing the 23-11 cassette with less gearing overlap may better suit your needs. 
 
Lastly, assuming you’re running SRAM shifters with your SRAM Force derailleurs you’ll be fine with the Shimano compact crankset.

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date: November 5, 2008