vane171

In pro races when bike mechanics are changing gears on the front for next day hilly race, do they swap just chainrings, or is it the whole assembly, crank arms with chainrings on them? Maybe some more serious riders around here also sometimes swap front gears once in a while?

If it is all the same which method you choose, I'd rather use the rings swapping, because then I would stick with the shorter, 172.5 mm cranks.

Swapping just the chainrings makes me worry about ring alignment but maybe these 'new' technology rings keep 'true' unless they are bent like in an accident?

topflightpro

It probably depends on what a pro team has in stock. But changing a crank is a lot quicker and easier than chainrings. And in some cases, one has to remove the crank to remove the chainrings.

WhyFi

Why couldn't you stick with the same length crank arms regardless?

As TFP mentions, it's a lot faster to swap out a crankset.

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).

Litespud

In my experience, the impact of "mismatching" chain rings is overstated. I used a standard Campagnolo 53/39 10 sp crank, but I replaced the 53t with a 50t TA ring with no obvious reduction in front shifting speed or reliability. The TA ring has the requisite pins and ramps, but it's not "matched" with any specific small ring. The 50t put me in the middle of the straight run on my 12/23 cassette (12-19, 21, 23), whereas the 53t had me spending more time in the larger sprockets, and the smaller ones were going to waste. My biggest gear is a 50/12, but if I spin out on that (which happens on descents sometimes) I'm going fast enough. If I spin out 50/12 on the flat, I'm a god

vane171

Or you could say, you are going too fast for your own good . I do that too, but I am aware I am playing the devil. Big gear like that is useful if you draft behind somebody who is much better than you and pedals downhill to boot.

Drafting or being hidden inside peloton or just a group or riders makes a huge difference in what gears you need or can utilize. Sometimes I quite can't believe the speed the world tour riders go at, but riding inside a pack makes a big difference as we all know.

Leinster

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.

delbiker1 

Make sure it is in fact the same BCD and the two cranksets are compatible with the same BB.

vane171

I sent the seller a message to inquire about BCD, even sent him this table where it says, he should measure 76.4 mm between bolt centers or from bolt edge to the other bolt edge for 130 BSD crank. But I think in this case, only other option is it might be 110 BCD. Otherwise it is Bontrager hollow spindle crankset, exactly the same what I have, except smaller rings, and hopefully, the same BCD.



I wish more technical info would be given as part of the world tour races. I didn't think of that when I posted, that some cranksets are one piece with chainring(s) and even can be detached separately on one side. My crank is exactly like the one I might buy, except I have those 53/39 rings.


53/39T 172.5 mm I have on my new bike



46/38T 175 mm I want to buy

vane171

I just checked and my old bike has 39T, same as the above crank and that is no good. I can live with high gears if I have to since I don't need to shift into them (even if that is far from ideal) but on low end, the situation is quite different. I know from my old bike that I lack lower gears on bigger hills where I often ride.

I found two cranks with triple rings but have no idea if they would fit. There may not be enough space next to the bike frame and also, the existing Ultegra RD may not be able to take up chain slack in that 30T gear. It is not that kind with the long double jockey wheel arm. One seller of this crank says it came from his Trek 7.7 FX which is not TT bike frame at all.







This is the Trek TTX 9.5 I'd like to put the tripple ring crank on.