Originally Posted by
vane171
Thanks, that's what I thought - changing the whole crankset. I asked about pro racing only out of curiosity, I ride only for pleasure from speed and exertion and to keep in shape.
I ask because I got this secondhand bike with 53/39 chainrings and 172.5 mm cranks with 12-25 nice smooth gearing cassette and I don't want to change the cassette. I am looking at the same crankset on eBay with 46/38 rings but a tad longer 175 mm cranks.
On my old bike (frame harking back to late seventies, rest has been upgraded over time) I am stuck with 51/42 (or maybe 53/42) combined with 12-26 cassette. The cassette is actually 12-28T but I had to set the RD so it won't shift to that last sprocket because it would hit wheel spokes. Anyway I find that gearing a tad steep for where I ride.
Now this 'new' bike has this gear setup that I think is just about OK on the low side, the 39-25, but on the high side, 53-12 I don't really need that. Only time I pedal downhill in serious fashion is when I want to see how fast I can go or if I am on a short ride only and want to get the most exercise out of it. Also I gather, 53 is nowadays considered thing of the past, somewhat like the move away from 700x23 tires. While I still like the tire size, I think I should get this crankset from eBay with 46/38 chainrings.
While it is pretty much settled here that 172.5 vs 175 is 'no difference', if I can have a choice, I'd rather go with the current 172.5 cranks the new bike has and transfer the 46/38 rings to it. I believe it is the same BCD from pictures (it is the one data the seller didn't post, I think I will inquire before buying to make sure). The 53/39 rings I would put on the longer cranks, makes sense anyway, and then I could swap the whole crankset if I'd want to change riding area.
Depending which direction I go out, I can have rolling coutryside with medium to small hills, but I can also have roller coaster like up and down ride with longish and fairly steep hills. Currently with my old bike, I avoid those, or else I have to walk the bike (indian style, switch walking and riding). Which is why I think the 53/39 is kinda steep gearing.
I could also just replace the 53 ring with the 46T one, but that would likely mess up switching gears on FD - the big 53 rings has stamped on it 'Use with 39T only'. I guess it is always best to stick with the two rings that manufacturer mated together (for want of better expression).
I wouldn't say it's "no difference." I'm sure many folks here would tell you that either will work just fine on the bike, but if one or other doesn't fit you just fine, that's a big difference. I just swapped out a pair of 170s for a pair of 172.5s and I noticed a difference. Both sets are 39-53, incidentally, paired with a 12-29 cassette.
I think some pro teams run "Spider" chainrings; Cannondale sell their SiSl chainrings in a one-piece unit that attaches to the crank at the BB interface. But I would think when swapping from a flat stage to a mountain stage they would generally pull off the right crank with the 39-53, and stick on the 36-52 instead.
I remember reading that Marcel Kittel's whole Giant-Shimano team were riding 55t chainrings for one particular Tour stage which was due to have a tailwind sprint finish. Presumably the mechanics just changed out the big ring on each bike the night before.