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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 at 3:39
    
Really you don't understand 54 will give more speed than a 50 if you you can drive it? –  Blam Jun 12 at 3:43
    
@Blam can anyone drive it that fast? 105 rpm cadence 50x11 already gives ~60 kph. How much faster do you want to get and why sprinters aren't having them? –  imel96 Jun 12 at 4:07
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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 at 5:09
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@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. –  Mσᶎ Jun 12 at 9:20

2 Answers 2

up vote 5 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 at 5:01
    
For that I prefer spooky eyes on the bum of my bike shorts... –  Mσᶎ Jun 12 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 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 at 0:00

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 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 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). –  Mσᶎ Jun 13 at 1:02

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