Why use 54T (or bigger) chainring?

Question

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.

EDIT: new info

I found this article One ring to rule them all: Tony Martin's Canyon Speedmax, stating another benefit of using large chainring:

... by running a huge front chainring, even Tony Martin will often have to use a larger rear cog, which will pull the rear derailleur a bit closer into the bike's center and thus slightly out of the wind, further reducing drag.

The chainring in the article is a 58T

Answer 1

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.

Gear ratio selection

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 efficiency is what matters in a time trial). 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.

These tables show 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

cassette cogs	ratio to next gear	Development (m)
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.

Answer 2

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.

http://www.ihpva.org/HParchive/PDF/hp50-2000.pdf

Answer 3

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.

Answer 4

From what I've read, a 54-56 tooth chainring also allows the pro rider to stay in the middle of his cassette where drivetrain drag is at its minimum. Better to be in the middle of the cassette than on the 11 or 12 where the chain angle is more extreme and chain drag forces are much higer.

Answer 5

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 day...so...

Answer 6

It is said larger pully wheels let the chain bend less, which causes less fraction and saves watts. I thought the same applies to cog.

We should avoid use small cogs (11~12T) too much. To match the larger cog (13T+), we have to use larger chainring.

Answer 7

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!

Answer 8

This answer consolidates information scattered across others.

Drivetrain friction was only starting to be understood in 2014. However, all else equal:

1. More cross-chained = higher friction.
2. Less chain articulation, i.e. bigger chainring and bigger cog = less friction.

Those are marginal gains, but they don't want to risk losing a TT by a few seconds. Also, if you check the gearing math, in a 58t cog, 90 rpm with a 28mm tire would get you 60.6 km/h in a 11t cog, and 55.6 km/h in a 12t. Comparing that to winner average speeds, they could actually push a gear that massive on some parts of the course. And it is true that if possible, it would reduce friction if they selected a big ring that allowed them to use a 12t or a 13t cog for the faster sections.