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In time trial races, I see people are using these big chainrings. Is 50/11 combination doesn't give enough top speed? What's the real advantage? I can only think of the negative, which is it makes the speed difference between two cogs wider.

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Usually the pro's are using 52T or 53T chain rings. I don't do time trials (and wouldn't be quite at that level), so I'm just giving a theory: it's to fine-tune the gearing to the terrain and conditions. One more tooth on the chain ring with the same cluster gives about 2% higher gearing. If the course is flat and/or with a bit of a tail wind that 2% can be of benefit. Gearing adjustments can make the difference between pushing a higher gear efficiently and not. – andy256 Jun 12 '14 at 3:39
Really you don't understand 54 will give more speed than a 50 if you you can drive it? – Paparazzi Jun 12 '14 at 3:43
Sprinters don't need them; they need lower gearing to accelerate (that's what all the waggling is about), and then they push whatever gear they're in to a high cadence. I'm not a sprinter and can push 150 for 100m or so. Most sprinters push up to 180, and beyond. – andy256 Jun 12 '14 at 5:09
@imel96 sprinters are about peak power regardless of heat and oxygen debt, so they spin fast and suck up the pain afterwards. Most/all people have their peak power at much higher rpm than their max aerobic or peak sustained power, it's just that they can only sustain it for 10-60 seconds. A sprint, in other words. – Móż Jun 12 '14 at 9:20
Indurain used 63-14 in some hour récord attempts, then 59 -14. There's a limit to how small the rear COG can be, 11 teeth being very close to the minimum, but limit is not so close regarding more teeth in the front ring (ser those motor Paced records) – gaurwraith Jul 23 '15 at 14:32
up vote 11 down vote accepted

UCI time trials are a class race contested in the tiny fractions of a percent, tiny differences in equipment become very important. Riders practice in wind tunnels to tweak everything from clothing to pedalling style, because a shift that gives an overall gain of 0.01% in speed can result in a win.

Considering more than just the top gear, the jump from 50/11 - 50/12 (the two highest gears) and 54/12 - 54/13 is noticeably bigger (9% vs 8%). That may well explain the change, even if it results is slightly lower peak speed, since it means that on slight slopes the rider has better gear options available. Note that on downhill sections the 54/12 rider has the 54/11 ratio available, where the 50/11 rider has no choice but to spin faster.

The difference between gearing 50/11 and 54/12 is about 1% (4.545454... vs 4.5). The larger chainring is bigger and heavier so it has more aero drag but lower mechanical losses (especially the loss difference between 11T and 12T is significant... but that loss should be less than 2% of the total power output). I suspect the human power output difference between those two at a given speed is less important than the comfort factor of being able to choose a gear that feels right.

It does not even have to translate directly into better average or total power output from the rider, as long as it gives a better time. Also possible is that it makes the rider feel faster, and that directly affects performance.

edit in response to imel96's question in comments: explain more about gear ratio selection (sorry, can't do tables here so you're seeing a spreadsheet turned into preformatted text)

When riding people care about pedal rpm, which is determined by gearing. Humans have a power/speed curve. The further from their optimum cadence they are, the less efficient they are (and in a time trial efficiency is what matters). The closer the gear ratios are, the closer to the right gear they can get so the closer to their peak efficiency point they can stay.

This wee table shows what's likely to be the next 4 cogs lower than the original question on the two cassettes we're discussing. The question I'm looking at here is "going up a slight rise, what gear options are available".

54/12 top gear  
cassette cogs   ratio to next gear  Development (m)
12          9.05
13  92.31%  8.35
14  92.86%  7.76
15  93.33%  7.24
16  93.75%  6.79
average 93.06%  

50/11 top gear  
11          9.14
12  91.67%  8.38
13  92.31%  7.73
14  92.86%  7.18
15  93.33%  6.70
average 92.54%  

The immediate answer is that the 54T rider can make a slightly smaller shift. Ratios let us ignore the actual gear and focus on the size of the shift. The 54T rider shifts to a gear 92.3% of their current one, the 50T rider drops to 91.67%. And that happens every time - on average the next lower gear is 93% for the 54T rider, and 92.54% for the 50T.

(it's possible that the 54/12 rider will keep the 11T small cog so they have a downhill gear, but we can ignore that for this comparison because in that case the 50/11 rider is out of options).

That sounds really minor, but remember that those riders are fighting it out in the fractions of a percentage point.

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There's also the psychology factor: to psych the others out :-) – andy256 Jun 12 '14 at 5:01
For that I prefer spooky eyes on the bum of my bike shorts... – Móż Jun 12 '14 at 6:00
Lol. For some time my brother was wearing knicks with the brand name IN prominently displayed ... it was a bit disconcerting! – andy256 Jun 12 '14 at 6:21
It's not obvious to me, so you're saying using bigger chainring allows selecting lower cogs -> closer ratios. – imel96 Jun 13 '14 at 0:00
Keep in mind, too, that most time trialists who run a 54T (or higher) have replaced the 39T little ring with something like a 42T or 44T (ex. Phinney runs a 56/44). This "smooths" the transition between shifts, so you can have more gradual gearing options for the TT. – altomnr Jun 24 at 16:58

Time trials are about generating the maximum sustained power possible. Not many people can do that at 100+ rpm. Time trial gearing needs to match the rpm that the rider generates maximum power vs the terrain. A bigger chainring provides closer gear ratios.

After all these events are won by seconds over an hours effort. Even the tiniest percent improvement in efficiency matters.

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"A bigger chainring provides closer gear ratios." I think this has a lot to do with it. I changed from a 11-32 cassette to a 12-23 cassette (8 speed), and it's amazing how much difference having very fine gear selection helps. Using bigger front cogs makes the difference between individual gears even smaller, allowing the rider to be in a more efficient gear. – Kibbee Jun 12 '14 at 18:02
@Kibbee bigger chainring by itself makes gear ratios wider, not smaller. This is easy to check by watching speed difference between gears when in small and big chainring. – imel96 Jun 12 '14 at 18:41
@imel96 I'm going to edit my answer to explain this in more detail (and delete my earlier comment so we don't get long discussions). – Móż Jun 13 '14 at 1:02

You might also ask why racers are using the 11 cog at all, it is a device causing a lot of friction, typically 6-8 percent, and even over 10 percent at low loads and high cadence. Those are figures similar to a bad hub gear.

I try to avoid the 12 cog as well, ending up with a 61 cog on my 700c with 13 cog as smallest. A positive side effect is that gear ratios are closer. It is on paper marginally slower than a 53/11, but noticably easier to crank around on top gear at speeds over 50 kmh.

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Could you edit this to explain where the friction comes from and what you mean by "a 61 cog?" I think that would make it a much more useful answer. – dlu Jul 23 '15 at 1:12
This makes much more sense now after I think of it. Even when 10 cog became available, it only sees specific uses because it's so inefficient. So, bigger cogs are more efficient (as long as they don't require_very_ high cadence). Unfortunately, compacts killed cassettes like 12-25. – imel96 Nov 10 '15 at 0:09

There is something to be said for us lesser riders having an ability push the bike to a higher speed on flats or descents. Remember, in TT racing every second counts, even though I can only drive my 54x12 at 85rpm on the flats, I CAN drive it at 95 - 105rpm or more on even a little bit of a down so I can usually squeeze a few more MPH, often much more, out of the fast bits. Acceleration is not the point, smooth, super consistent, barely sustainable, painful power is the point. I make up for it with a few more teeth on my back end for the uphills, but the limitation there is my strength to weight on a given

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The very simple answer is that top ranked pros are simply that much stronger than the rest of us. They can push a 54t ring 80+ rpm over a flat course for an hour or more. Most people struggle pushing a 50x12 big ring on flat ground for anything more than a few minutes (I gave up when I stopped racing and now use a 48t big ring!) You can look at wattage data all day long and watch every stage of the Tour but until you actually go out and ride with some of these pros it's simply impossible to comprehend just how much faster they are than the rest of us!

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It's not just pros, a lot of amateur triathletes also have big ring on their tt bikes, despite that they ride slower than pros on standard road bike. – imel96 Jul 23 '15 at 23:15

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